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Re: [CentralTexasGeocachers] Kim Komando

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  • Victor Engel
    Another reason for inaccuracy was not mentioned. The military actually scrambles the signal a bit to create the inaccuracy. To get full accuracy you have to
    Message 1 of 15 , Feb 16, 2013
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      Another reason for inaccuracy was not mentioned. The military actually
      scrambles the signal a bit to create the inaccuracy. To get full
      accuracy you have to know how to properly decrypt the signal. That's
      something only the military is intended to have access to.

      It's possible to gain a higher level of accuracy by using two GPS
      units, one at a fixed, known position. The random error can then be
      backed out using this unit as a reference.

      Victor

      On 2/16/13, Dave Read <dave@...> wrote:
      > Hey Esther!
      >
      > The post worked just fine -- great video.
      >
      > If anyone is interested, last summer I developed a hands-on method for
      > explaining GPS to kids so my wife old teach a geocaching class at a Cub
      > Scout camp. It's super easy, actually conveys more of the nuances of GPS
      > (especially the source of the position inaccuracy), and best of all, is
      > totally understandable by kids as young as 9 or 10!
      >
      > Thanks for the link!
      >
      > Dave
      > aka Team Landshark
      >
      >
      > On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...>
      > wrote:
      >
      >> Howdy!
      >> I don't normally post links and I am not very techno savvy but I do listen
      >> to the Digital Goddess, Kim Komando who is.
      >> Today I ran across this;
      >>
      >> http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl&utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f
      >>
      >> This is her explanation of how GPS works and I thought someone might find
      >> it helpful and decided to post it. I also hope I did it right!
      >> Esther/BGTx
      >>
      >>
      >


      --
      Victor Engel
    • Victor Engel
      Kim s video was also wrong in that it is not true that there are always at least 4 satellites in a direct line of sight. It may be true that there are always
      Message 2 of 15 , Feb 16, 2013
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        Kim's video was also wrong in that it is not true that there are
        always at least 4 satellites in a direct line of sight. It may be true
        that there are always at least four above the horizon, but they may be
        blocked by mountains or downtown buildings.

        Last summer, I went to Guatemala and visited, among other places, a
        location for a development that my brother's architecture firm is
        working on. It's nestled in a valley. I recorded our path with my GPS
        but lost the signal completely at about 1000 yards of the building
        site. My GPS wasn't able to reacquire a signal until we had mostly
        left the valley.

        From the building site up to the mountain tops the angle of view was
        about 40 degrees except to the west where it was about 15 or 20
        degrees. I've been meaning to try to figure out how many satellites
        could have been visible, but I bet it was no more than two at the
        time. Maybe three.

        On 2/16/13, Victor Engel <brillig@...> wrote:
        > Another reason for inaccuracy was not mentioned. The military actually
        > scrambles the signal a bit to create the inaccuracy. To get full
        > accuracy you have to know how to properly decrypt the signal. That's
        > something only the military is intended to have access to.
        >
        > It's possible to gain a higher level of accuracy by using two GPS
        > units, one at a fixed, known position. The random error can then be
        > backed out using this unit as a reference.
        >
        > Victor
        >
        > On 2/16/13, Dave Read <dave@...> wrote:
        >> Hey Esther!
        >>
        >> The post worked just fine -- great video.
        >>
        >> If anyone is interested, last summer I developed a hands-on method for
        >> explaining GPS to kids so my wife old teach a geocaching class at a Cub
        >> Scout camp. It's super easy, actually conveys more of the nuances of GPS
        >> (especially the source of the position inaccuracy), and best of all, is
        >> totally understandable by kids as young as 9 or 10!
        >>
        >> Thanks for the link!
        >>
        >> Dave
        >> aka Team Landshark
        >>
        >>
        >> On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...>
        >> wrote:
        >>
        >>> Howdy!
        >>> I don't normally post links and I am not very techno savvy but I do
        >>> listen
        >>> to the Digital Goddess, Kim Komando who is.
        >>> Today I ran across this;
        >>>
        >>> http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl&utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f
        >>>
        >>> This is her explanation of how GPS works and I thought someone might
        >>> find
        >>> it helpful and decided to post it. I also hope I did it right!
        >>> Esther/BGTx
        >>>
        >>>
        >>
        >
        >
        > --
        > Victor Engel
        >


        --
        Victor Engel
      • Dave Read
        OK, since you asked. This is waaaaaay more information than I give kids, but it will set the stage for you to understand how the kid activity works. The
        Message 3 of 15 , Feb 16, 2013
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          OK, "since you asked." This is waaaaaay more information than I give kids, but it will set the stage for you to understand how the kid activity works.

          The position inaccuracy comes from the way GPS positions are computed. Your GPSr receives signals from the satellites, and measures the length of time it took for the signal to travel from the satellite to the GPSr. It then multiplies by the speed of light to compute the distance between you and the satellite. In the physics world we call this a "time of flight" system. The GPSr does this for all satellites it can "see."

          In order to make use of this information, the GPSr needs to know where each satellite was located when it transmitted the data. To do this, each satellite transmits some information about its current orbit. This is called "ephemeris" data. The ephemeris data is transmitted by the satellite only once in a while…IIRC it's about once every 90 seconds. When you turn on your GPSr and it says "looking for satellites," mostly what it is doing is waiting for the ephemeris updates to roll in. The GPSr "sees" the satellites almost immediately, but without the ephemeris data it doesn't know where the satellites are, so it's helpless. On my Oregon 450, the "satellite view" shows a signal strength bar for each satellite the GPSr can see; the solid bars are ones where the unit has received the ephemeris update, while the ones filled in white are ones with no update yet. The reason it can take some GPSrs "forever" to lock in is that if the ephemeris update is garbled due to weak signal, the GPSr has to wait for the satellite to transmit it again. For marginal signals, this process can take a long time to complete. Also FYI, I think newer GPS units have some way of modeling the evolution of ephemeris data, so if you turn off your GPSr for ~a few hours, it "locks in" much faster when you turn it back on. However, if you wait too long (a day or more) or if you get on a plane and go to a completely new location, the ephemeris evolution model breaks down and it has to go back to waiting for ephemeris updates the old-fashioned way.

          OK, so now the GPSr knows where all of its satellites are located. It can compute time of flight and thus distance to the satellites. It's a simple matter to solve for the spot where all the distances converge. That is, you want the place in 3-D space where gives you the right distance to all of the satellites. 

          So where does the error come from? Simple: remember that we multiplied the time of flight by speed of light to get the distance? Well, this is just an approximation. The speed of light is not a constant when you change materials; it depends (mostly) on matter density. Denser materials have slower speed of light. Those clouds overhead? They make light go slower. Those tree leaves overhead? Same thing. Moist air? Same thing. You also get "multi-path" effects, which is the signal bouncing off a building and taking a longer path to get to you. All of this contributes to an error in the estimated distance ***for each satellite***, and a different one for each satellite, at that. So when you go to make that computation of "where do I need to be to make all of these distances work out?", you can't get it exactly right. If there were no such speed-of-light effects, the position accuracy of GPS would be under a foot.

          Oh, one more thing to mention. The GPSr can't actually compute the time of flight until the end of this exercise, because while it knows what time the satellite transmitted its signal, the GPSr doesn't know what time it is locally. The computation that gives the position *also* gives the local time. It's all one big hairy computation. The math is ferocious. At the end of the process, the GPSr knows where it is, and also what time it is locally. This means that on average, your GPSr is the best clock you own — typical time accuracy is around 10-20 nanoseconds. FWIW one nanosecond is almost exactly one foot, so if your unit is reporting 15 foot position accuracy, you can assume that implies (roughly) 15 nanosecond time accuracy.

          Now for the kids exercise. Start with 5-6 ropes, preferably around 10-15 feet long. Mark a spot on the ground with a rock, flag, whatever, and stretch out the ropes so all the ends meet at the rock/flag, but point them in different directions. Now cheat a little: pull a few of the ropes 8-12 inches away from the spot, and make few "overrun" the spot by the same distance.  

          Identify 5-6 kids (one for each rope) to be satellites. The rest of the kids are geocachers. Position one kid at the end of each rope. Have them pick up the rope end, and tell them that once they pick it up, they must remain in that spot until you tell them they can move again." Have all the satellites pull in their ropes completely.

          Now pick your first geocacher, and give him the end of one of the ropes. Have him walk away from the satellite until it's slightly tight. Explain to the kids that the ropes represent the GPSr's estimate of the distance to the satellite. You can explain about the time of flight thing if you want to, but younger kids probably won't get it.

          Tell that first kid that he could be any place on earth that is that same distance from the satellite. Where is he? You may need to prompt him to walk a circle around the satellite, but some kids get it instinctively. Answer: he could be anywhere on that circle. Not very useful. But what if we add a second satellite?

          Set up a second kid the same way as the first, but with the rope from a different satellite. Explain that they could be any place that the two signals intersect, because we know the distance to TWO satellites know. Ask them to find the place. Most kids will figure out quickly that there are two such places. If not, help them find the second place.

          Now add a third kid the same way as the first two, and ask them to find the location. This time, there is only one place. Point out that the place is not exactly on top of the rock. Why not? Because our estimate of the distance to the satellite is only that: an estimate. It has some error in it, and that error makes for an in where the computed position is. What's worse, with only three satellites, you can't even estimate how much of an error you made!

          Add the other satellites, one by one, and repeat. Watch the error get smaller. After each addition, ask the kids to estimate how big a mistake is possible, not by looking at where the rock is, but by looking at how much the ropes overlap or don't touch. You'd be surprised at how good your average group of kids is at making this estimate.

          That's it. The only thing left to point out is that they did this walking on the ground which is a 2D object, but the earth is a 3D object. That means you need one more satellite for everything. Four to get a basic position, 5 or more to be able to estimate accuracy.

          Cheers,
          Dave
          aka Team Landshark

          From: "gumbietygress@..." <gumbietygress@...>
          Reply-To: <CentralTexasGeocachers@yahoogroups.com>
          Date: Saturday, February 16, 2013 7:56 AM
          To: <CentralTexasGeocachers@yahoogroups.com>
          Subject: Re: [CentralTexasGeocachers] Kim Komando

           

          So what is the source of the position inaccuracy? Other than electromagnetic interference from the user. [Hey, some of us can confound a watch.]
           
          BarbJ =ripples in the atmosphere?= Tygress

          ---------- Original Message ----------
          From: Dave Read <dave@...>
          To: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
          Cc: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
          Subject: Re: [CentralTexasGeocachers] Kim Komando
          Date: Sat, 16 Feb 2013 07:02:05 -0600

          Hey Esther!
           
          The post worked just fine -- great video.
           
          If anyone is interested, last summer I developed a hands-on method for explaining GPS to kids so my wife old teach a geocaching class at a Cub Scout camp. It's super easy, actually conveys more of the nuances of GPS (especially the source of the position inaccuracy), and best of all, is totally understandable by kids as young as 9 or 10!  
           
          Thanks for the link!
           
          Dave
          aka Team Landshark


          On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...> wrote:

           

          Howdy!
          I don't normally post links and I am not very techno savvy but I do listen to the Digital Goddess, Kim Komando who is.
          Today I ran across this;

          http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl&utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f

          This is her explanation of how GPS works and I thought someone might find it helpful and decided to post it. I also hope I did it right!
          Esther/BGTx

           



          ____________________________________________________________
          Woman is 53 But Looks 25
          Mom reveals 1 simple wrinkle trick that has angered doctors...
          ConsumerLifestyleMag.com

        • Dave Read
          This is not correct. The fuzzing you are talking about is (was) called selective availability, and was turned off in May 2000 by executive order from Bill
          Message 4 of 15 , Feb 16, 2013
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            This is not correct. The fuzzing you are talking about is (was) called "selective availability," and was turned off in May 2000 by executive order from Bill Clinton. This action, in fact, was the genesis of geocaching in the first place. See http://www.geocaching.com/about/history.aspx for more details.

            Basically, when people starting thinking about the implications of the change from ~100-foot accuracy to ~10-foot accuracy, somebody thought "I could make a fun game out of this!" and the first cache was published less than a month later.

            D

            From: Victor Engel <brillig@...>
            Reply-To: <CentralTexasGeocachers@yahoogroups.com>
            Date: Saturday, February 16, 2013 7:59 AM
            To: <CentralTexasGeocachers@yahoogroups.com>
            Subject: Re: [CentralTexasGeocachers] Kim Komando

             

            Another reason for inaccuracy was not mentioned. The military actually
            scrambles the signal a bit to create the inaccuracy. To get full
            accuracy you have to know how to properly decrypt the signal. That's
            something only the military is intended to have access to.

            It's possible to gain a higher level of accuracy by using two GPS
            units, one at a fixed, known position. The random error can then be
            backed out using this unit as a reference.

            Victor

            On 2/16/13, Dave Read dave@...> wrote:
            > Hey Esther!
            >
            > The post worked just fine -- great video.
            >
            > If anyone is interested, last summer I developed a hands-on method for
            > explaining GPS to kids so my wife old teach a geocaching class at a Cub
            > Scout camp. It's super easy, actually conveys more of the nuances of GPS
            > (especially the source of the position inaccuracy), and best of all, is
            > totally understandable by kids as young as 9 or 10!
            >
            > Thanks for the link!
            >
            > Dave
            > aka Team Landshark
            >
            >
            > On Feb 15, 2013, at 10:46 PM, "bigguy9211116" bigguy9211116@...>
            > wrote:
            >
            >> Howdy!
            >> I don't normally post links and I am not very techno savvy but I do listen
            >> to the Digital Goddess, Kim Komando who is.
            >> Today I ran across this;
            >>
            >> http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl&utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f
            >>
            >> This is her explanation of how GPS works and I thought someone might find
            >> it helpful and decided to post it. I also hope I did it right!
            >> Esther/BGTx
            >>
            >>
            >

            --
            Victor Engel

          • Victor Engel
            That would be an accurate clock if the GPS unit actually used it for time display, which I believe is not the case for most units. Victor Engel ... That would
            Message 5 of 15 , Feb 16, 2013
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              That would be an accurate clock if the GPS unit actually used it for time display, which I believe is not the case for most units.

              Victor Engel





              On Sat, Feb 16, 2013 at 10:03 AM, Dave Read <dave@...> wrote:



              OK, "since you asked." This is waaaaaay more information than I give kids, but it will set the stage for you to understand how the kid activity works.

              The position inaccuracy comes from the way GPS positions are computed. Your GPSr receives signals from the satellites, and measures the length of time it took for the signal to travel from the satellite to the GPSr. It then multiplies by the speed of light to compute the distance between you and the satellite. In the physics world we call this a "time of flight" system. The GPSr does this for all satellites it can "see."

              In order to make use of this information, the GPSr needs to know where each satellite was located when it transmitted the data. To do this, each satellite transmits some information about its current orbit. This is called "ephemeris" data. The ephemeris data is transmitted by the satellite only once in a while…IIRC it's about once every 90 seconds. When you turn on your GPSr and it says "looking for satellites," mostly what it is doing is waiting for the ephemeris updates to roll in. The GPSr "sees" the satellites almost immediately, but without the ephemeris data it doesn't know where the satellites are, so it's helpless. On my Oregon 450, the "satellite view" shows a signal strength bar for each satellite the GPSr can see; the solid bars are ones where the unit has received the ephemeris update, while the ones filled in white are ones with no update yet. The reason it can take some GPSrs "forever" to lock in is that if the ephemeris update is garbled due to weak signal, the GPSr has to wait for the satellite to transmit it again. For marginal signals, this process can take a long time to complete. Also FYI, I think newer GPS units have some way of modeling the evolution of ephemeris data, so if you turn off your GPSr for ~a few hours, it "locks in" much faster when you turn it back on. However, if you wait too long (a day or more) or if you get on a plane and go to a completely new location, the ephemeris evolution model breaks down and it has to go back to waiting for ephemeris updates the old-fashioned way.

              OK, so now the GPSr knows where all of its satellites are located. It can compute time of flight and thus distance to the satellites. It's a simple matter to solve for the spot where all the distances converge. That is, you want the place in 3-D space where gives you the right distance to all of the satellites. 

              So where does the error come from? Simple: remember that we multiplied the time of flight by speed of light to get the distance? Well, this is just an approximation. The speed of light is not a constant when you change materials; it depends (mostly) on matter density. Denser materials have slower speed of light. Those clouds overhead? They make light go slower. Those tree leaves overhead? Same thing. Moist air? Same thing. You also get "multi-path" effects, which is the signal bouncing off a building and taking a longer path to get to you. All of this contributes to an error in the estimated distance ***for each satellite***, and a different one for each satellite, at that. So when you go to make that computation of "where do I need to be to make all of these distances work out?", you can't get it exactly right. If there were no such speed-of-light effects, the position accuracy of GPS would be under a foot.

              Oh, one more thing to mention. The GPSr can't actually compute the time of flight until the end of this exercise, because while it knows what time the satellite transmitted its signal, the GPSr doesn't know what time it is locally. The computation that gives the position *also* gives the local time. It's all one big hairy computation. The math is ferocious. At the end of the process, the GPSr knows where it is, and also what time it is locally. This means that on average, your GPSr is the best clock you own — typical time accuracy is around 10-20 nanoseconds. FWIW one nanosecond is almost exactly one foot, so if your unit is reporting 15 foot position accuracy, you can assume that implies (roughly) 15 nanosecond time accuracy.

              Now for the kids exercise. Start with 5-6 ropes, preferably around 10-15 feet long. Mark a spot on the ground with a rock, flag, whatever, and stretch out the ropes so all the ends meet at the rock/flag, but point them in different directions. Now cheat a little: pull a few of the ropes 8-12 inches away from the spot, and make few "overrun" the spot by the same distance.  

              Identify 5-6 kids (one for each rope) to be satellites. The rest of the kids are geocachers. Position one kid at the end of each rope. Have them pick up the rope end, and tell them that once they pick it up, they must remain in that spot until you tell them they can move again." Have all the satellites pull in their ropes completely.

              Now pick your first geocacher, and give him the end of one of the ropes. Have him walk away from the satellite until it's slightly tight. Explain to the kids that the ropes represent the GPSr's estimate of the distance to the satellite. You can explain about the time of flight thing if you want to, but younger kids probably won't get it.

              Tell that first kid that he could be any place on earth that is that same distance from the satellite. Where is he? You may need to prompt him to walk a circle around the satellite, but some kids get it instinctively. Answer: he could be anywhere on that circle. Not very useful. But what if we add a second satellite?

              Set up a second kid the same way as the first, but with the rope from a different satellite. Explain that they could be any place that the two signals intersect, because we know the distance to TWO satellites know. Ask them to find the place. Most kids will figure out quickly that there are two such places. If not, help them find the second place.

              Now add a third kid the same way as the first two, and ask them to find the location. This time, there is only one place. Point out that the place is not exactly on top of the rock. Why not? Because our estimate of the distance to the satellite is only that: an estimate. It has some error in it, and that error makes for an in where the computed position is. What's worse, with only three satellites, you can't even estimate how much of an error you made!

              Add the other satellites, one by one, and repeat. Watch the error get smaller. After each addition, ask the kids to estimate how big a mistake is possible, not by looking at where the rock is, but by looking at how much the ropes overlap or don't touch. You'd be surprised at how good your average group of kids is at making this estimate.

              That's it. The only thing left to point out is that they did this walking on the ground which is a 2D object, but the earth is a 3D object. That means you need one more satellite for everything. Four to get a basic position, 5 or more to be able to estimate accuracy.

              Cheers,
              Dave
              aka Team Landshark

              From: "gumbietygress@..." <gumbietygress@...>
              Reply-To: <CentralTexasGeocachers@yahoogroups.com>
              Date: Saturday, February 16, 2013 7:56 AM
              To: <CentralTexasGeocachers@yahoogroups.com>
              Subject: Re: [CentralTexasGeocachers] Kim Komando

               

              So what is the source of the position inaccuracy? Other than electromagnetic interference from the user. [Hey, some of us can confound a watch.]
               
              BarbJ =ripples in the atmosphere?= Tygress

              ---------- Original Message ----------
              From: Dave Read <dave@...>
              To: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
              Cc: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
              Subject: Re: [CentralTexasGeocachers] Kim Komando
              Date: Sat, 16 Feb 2013 07:02:05 -0600

              Hey Esther!
               
              The post worked just fine -- great video.
               
              If anyone is interested, last summer I developed a hands-on method for explaining GPS to kids so my wife old teach a geocaching class at a Cub Scout camp. It's super easy, actually conveys more of the nuances of GPS (especially the source of the position inaccuracy), and best of all, is totally understandable by kids as young as 9 or 10!  
               
              Thanks for the link!
               
              Dave
              aka Team Landshark


              On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...> wrote:

               

              Howdy!
              I don't normally post links and I am not very techno savvy but I do listen to the Digital Goddess, Kim Komando who is.
              Today I ran across this;

              http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl&utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f

              This is her explanation of how GPS works and I thought someone might find it helpful and decided to post it. I also hope I did it right!
              Esther/BGTx

               



              ____________________________________________________________
              Woman is 53 But Looks 25
              Mom reveals 1 simple wrinkle trick that has angered doctors...
              ConsumerLifestyleMag.com




            • Jay Bingham
              Dave, What a great simplified explanation and a great activity for the kiddos too. If it is ok with you, I suggest that Greg (geojewett) post it on the
              Message 6 of 15 , Feb 16, 2013
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                Dave,

                 

                What a great simplified explanation and a great activity for the kiddos too.

                If it is ok with you, I suggest that Greg (geojewett) post it on the Geocaching Austin web site.

                 

                --| Jay /\ Bing-GTX
                |+| Georgetown , TX USA
                |-- “What you see depends mainly on what you are looking for.”

                 


                From: CentralTexasGeocachers@yahoogroups.com [mailto:CentralTexasGeocachers@yahoogroups.com] On Behalf Of Dave Read
                Sent: Saturday, February 16, 2013 10:04 AM
                To: CentralTexasGeocachers@yahoogroups.com
                Subject: Re: [CentralTexasGeocachers] Kim Komando

                 

                 

                 

                OK, "since you asked." This is waaaaaay more information than I give kids, but it will set the stage for you to understand how the kid activity works.

                 

                The position inaccuracy comes from the way GPS positions are computed. Your GPSr receives signals from the satellites, and measures the length of time it took for the signal to travel from the satellite to the GPSr. It then multiplies by the speed of light to compute the distance between you and the satellite. In the physics world we call this a "time of flight" system. The GPSr does this for all satellites it can "see."

                 

                In order to make use of this information, the GPSr needs to know where each satellite was located when it transmitted the data. To do this, each satellite transmits some information about its current orbit. This is called "ephemeris" data. The ephemeris data is transmitted by the satellite only once in a while…IIRC it's about once every 90 seconds. When you turn on your GPSr and it says "looking for satellites," mostly what it is doing is waiting for the ephemeris updates to roll in. The GPSr "sees" the satellites almost immediately, but without the ephemeris data it doesn't know where the satellites are, so it's helpless. On my Oregon 450, the "satellite view" shows a signal strength bar for each satellite the GPSr can see; the solid bars are ones where the unit has received the ephemeris update, while the ones filled in white are ones with no update yet. The reason it can take some GPSrs "forever" to lock in is that if the ephemeris update is garbled due to weak signal, the GPSr has to wait for the satellite to transmit it again. For marginal signals, this process can take a long time to complete. Also FYI, I think newer GPS units have some way of modeling the evolution of ephemeris data, so if you turn off your GPSr for ~a few hours, it "locks in" much faster when you turn it back on. However, if you wait too long (a day or more) or if you get on a plane and go to a completely new location, the ephemeris evolution model breaks down and it has to go back to waiting for ephemeris updates the old-fashioned way.

                 

                OK, so now the GPSr knows where all of its satellites are located. It can compute time of flight and thus distance to the satellites. It's a simple matter to solve for the spot where all the distances converge. That is, you want the place in 3-D space where gives you the right distance to all of the satellites. 

                 

                So where does the error come from? Simple: remember that we multiplied the time of flight by speed of light to get the distance? Well, this is just an approximation. The speed of light is not a constant when you change materials; it depends (mostly) on matter density. Denser materials have slower speed of light. Those clouds overhead? They make light go slower. Those tree leaves overhead? Same thing. Moist air? Same thing. You also get "multi-path" effects, which is the signal bouncing off a building and taking a longer path to get to you. All of this contributes to an error in the estimated distance ***for each satellite***, and a different one for each satellite, at that. So when you go to make that computation of "where do I need to be to make all of these distances work out?", you can't get it exactly right. If there were no such speed-of-light effects, the position accuracy of GPS would be under a foot.

                 

                Oh, one more thing to mention. The GPSr can't actually compute the time of flight until the end of this exercise, because while it knows what time the satellite transmitted its signal, the GPSr doesn't know what time it is locally. The computation that gives the position *also* gives the local time. It's all one big hairy computation. The math is ferocious. At the end of the process, the GPSr knows where it is, and also what time it is locally. This means that on average, your GPSr is the best clock you own — typical time accuracy is around 10-20 nanoseconds. FWIW one nanosecond is almost exactly one foot, so if your unit is reporting 15 foot position accuracy, you can assume that implies (roughly) 15 nanosecond time accuracy.

                 

                Now for the kids exercise. Start with 5-6 ropes, preferably around 10-15 feet long. Mark a spot on the ground with a rock, flag, whatever, and stretch out the ropes so all the ends meet at the rock/flag, but point them in different directions. Now cheat a little: pull a few of the ropes 8-12 inches away from the spot, and make few "overrun" the spot by the same distance.  

                 

                Identify 5-6 kids (one for each rope) to be satellites. The rest of the kids are geocachers. Position one kid at the end of each rope. Have them pick up the rope end, and tell them that once they pick it up, they must remain in that spot until you tell them they can move again." Have all the satellites pull in their ropes completely.

                 

                Now pick your first geocacher, and give him the end of one of the ropes. Have him walk away from the satellite until it's slightly tight. Explain to the kids that the ropes represent the GPSr's estimate of the distance to the satellite. You can explain about the time of flight thing if you want to, but younger kids probably won't get it.

                 

                Tell that first kid that he could be any place on earth that is that same distance from the satellite. Where is he? You may need to prompt him to walk a circle around the satellite, but some kids get it instinctively. Answer: he could be anywhere on that circle. Not very useful. But what if we add a second satellite?

                 

                Set up a second kid the same way as the first, but with the rope from a different satellite. Explain that they could be any place that the two signals intersect, because we know the distance to TWO satellites know. Ask them to find the place. Most kids will figure out quickly that there are two such places. If not, help them find the second place.

                 

                Now add a third kid the same way as the first two, and ask them to find the location. This time, there is only one place. Point out that the place is not exactly on top of the rock. Why not? Because our estimate of the distance to the satellite is only that: an estimate. It has some error in it, and that error makes for an in where the computed position is. What's worse, with only three satellites, you can't even estimate how much of an error you made!

                 

                Add the other satellites, one by one, and repeat. Watch the error get smaller. After each addition, ask the kids to estimate how big a mistake is possible, not by looking at where the rock is, but by looking at how much the ropes overlap or don't touch. You'd be surprised at how good your average group of kids is at making this estimate.

                 

                That's it. The only thing left to point out is that they did this walking on the ground which is a 2D object, but the earth is a 3D object. That means you need one more satellite for everything. Four to get a basic position, 5 or more to be able to estimate accuracy.

                 

                Cheers,

                Dave

                aka Team Landshark

                 

                From: "gumbietygress@..." <gumbietygress@...>
                Reply-To: <CentralTexasGeocachers@yahoogroups.com>
                Date: Saturday, February 16, 2013 7:56 AM
                To: <CentralTexasGeocachers@yahoogroups.com>
                Subject: Re: [CentralTexasGeocachers] Kim Komando

                 

                 

                So what is the source of the position inaccuracy? Other than electromagnetic interference from the user. [Hey, some of us can confound a watch.]

                 

                BarbJ =ripples in the atmosphere?= Tygress

                ---------- Original Message ----------
                From: Dave Read <dave@...>
                To: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                Cc: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                Subject: Re: [CentralTexasGeocachers] Kim Komando
                Date: Sat, 16 Feb 2013 07:02:05 -0600

                Hey Esther!

                 

                The post worked just fine -- great video.

                 

                If anyone is interested, last summer I developed a hands-on method for explaining GPS to kids so my wife old teach a geocaching class at a Cub Scout camp. It's super easy, actually conveys more of the nuances of GPS (especially the source of the position inaccuracy), and best of all, is totally understandable by kids as young as 9 or 10!  

                 

                Thanks for the link!

                 

                Dave

                aka Team Landshark



                On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...> wrote:

                 

                Howdy!
                I don't normally post links and I am not very techno savvy but I do listen to the Digital Goddess, Kim Komando who is.
                Today I ran across this;

                http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl&utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f

                This is her explanation of how GPS works and I thought someone might find it helpful and decided to post it. I also hope I did it right!
                Esther/BGTx

                 



                ____________________________________________________________
                Woman is 53 But Looks 25
                Mom reveals 1 simple wrinkle trick that has angered doctors...
                ConsumerLifestyleMag.com

              • Dave Read
                By all means, feel free. I developed this demo/explanation for my wife to use at Webelos Extreme Adventure Camp last summer, and she taught it to about 120
                Message 7 of 15 , Feb 16, 2013
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                  By all means, feel free. I developed this demo/explanation for my wife to use at "Webelos Extreme Adventure Camp" last summer, and she taught it to about 120 kids there. She used it again recently with a group of ~25 adult leaders, and a friend used it when teaching the Geocaching merit badge to his son's Troop just a couple of weeks ago. It's been through a few feedback cycles, so it's pretty polished now.

                  The problem we were trying to address was kids thinking that their GPSr would situate them directly "on top" of the cache, and getting frustrated when they had to search for it. We felt like we needed to explain the accuracy story to them so they would understand that the position was only approximate, and that searching for the cache was half the fun.

                  D

                  From: Jay Bingham <binghamjc@...>
                  Reply-To: <CentralTexasGeocachers@yahoogroups.com>
                  Date: Saturday, February 16, 2013 11:16 AM
                  To: <CentralTexasGeocachers@yahoogroups.com>
                  Subject: RE: [CentralTexasGeocachers] Kim Komando

                   

                  Dave,

                   

                  What a great simplified explanation and a great activity for the kiddos too.

                  If it is ok with you, I suggest that Greg (geojewett) post it on the Geocaching Austin web site.

                   

                  --| Jay /\ Bing-GTX
                  |+| Georgetown , TX USA
                  |-- “What you see depends mainly on what you are looking for.”

                   


                  From: CentralTexasGeocachers@yahoogroups.com[mailto:CentralTexasGeocachers@yahoogroups.com] On Behalf Of Dave Read
                  Sent: Saturday, February 16, 2013 10:04 AM
                  To: CentralTexasGeocachers@yahoogroups.com
                  Subject: Re: [CentralTexasGeocachers] Kim Komando

                   

                   

                   

                  OK, "since you asked." This is waaaaaay more information than I give kids, but it will set the stage for you to understand how the kid activity works.

                   

                  The position inaccuracy comes from the way GPS positions are computed. Your GPSr receives signals from the satellites, and measures the length of time it took for the signal to travel from the satellite to the GPSr. It then multiplies by the speed of light to compute the distance between you and the satellite. In the physics world we call this a "time of flight" system. The GPSr does this for all satellites it can "see."

                   

                  In order to make use of this information, the GPSr needs to know where each satellite was located when it transmitted the data. To do this, each satellite transmits some information about its current orbit. This is called "ephemeris" data. The ephemeris data is transmitted by the satellite only once in a while…IIRC it's about once every 90 seconds. When you turn on your GPSr and it says "looking for satellites," mostly what it is doing is waiting for the ephemeris updates to roll in. The GPSr "sees" the satellites almost immediately, but without the ephemeris data it doesn't know where the satellites are, so it's helpless. On my Oregon 450, the "satellite view" shows a signal strength bar for each satellite the GPSr can see; the solid bars are ones where the unit has received the ephemeris update, while the ones filled in white are ones with no update yet. The reason it can take some GPSrs "forever" to lock in is that if the ephemeris update is garbled due to weak signal, the GPSr has to wait for the satellite to transmit it again. For marginal signals, this process can take a long time to complete. Also FYI, I think newer GPS units have some way ofmodeling the evolution of ephemeris data, so if you turn off your GPSr for ~a few hours, it "locks in" much faster when you turn it back on. However, if you wait too long (a day or more) or if you get on a plane and go to a completely new location, the ephemeris evolution model breaks down and it has to go back to waiting for ephemeris updates the old-fashioned way.

                   

                  OK, so now the GPSr knows where all of its satellites are located. It can compute time of flight and thus distance to the satellites. It's a simple matter to solve for the spot where all the distances converge. That is, youwant the place in 3-D space where gives you the right distance to all of the satellites. 

                   

                  So where does the error come from? Simple: remember that we multiplied the time of flight by speed of light to get the distance? Well, this is just an approximation. The speed of light is not a constant when you change materials; it depends (mostly) on matter density. Denser materials have slower speed of light. Those clouds overhead? They make light go slower. Those tree leaves overhead? Same thing. Moist air? Same thing. You also get "multi-path" effects, which is the signal bouncing off a building and taking a longer path to get to you. All of this contributes to an error in the estimated distance ***for each satellite***, and a different one for each satellite, at that. So when you go to make that computation of "where do I need to be to make all of these distances work out?", you can't get it exactly right. If there were no such speed-of-light effects, the position accuracy of GPS would be under a foot.

                   

                  Oh, one more thing to mention. The GPSr can't actually compute the time of flight until the end of this exercise, because while it knows what time the satellite transmitted its signal, the GPSr doesn't know what time it is locally. The computation that gives the position *also* gives the local time. It's all one big hairy computation. The math is ferocious. At the end of the process, the GPSr knows where it is, and also what time it is locally. Thismeans that on average, your GPSr is the best clock you own — typical time accuracy is around 10-20 nanoseconds. FWIW one nanosecond is almost exactly one foot, so if your unit is reporting 15 foot position accuracy, you can assume that implies (roughly) 15 nanosecond time accuracy.

                   

                  Now for the kids exercise. Start with 5-6 ropes, preferably around10-15 feet long. Mark a spot on the ground with a rock, flag, whatever, andstretch out the ropes so all the ends meet at the rock/flag, but point them in different directions. Now cheat a little: pull a few of the ropes 8-12 inches away from the spot, and make few "overrun" the spot by the same distance.  

                   

                  Identify 5-6 kids (one for each rope) to be satellites. The rest of the kids are geocachers. Position one kid at the end of each rope. Have them pick up the rope end, and tell them that once they pick it up, they must remain in that spot until you tell them they can move again." Have all the satellites pull in their ropes completely.

                   

                  Now pick your first geocacher, and give him the end of one of the ropes. Have him walk away from the satellite until it's slightly tight. Explain to the kids that the ropes represent the GPSr's estimate of the distance to the satellite. You can explain about the time of flight thing if you want to, but younger kids probably won't get it.

                   

                  Tell that first kid that he could be any place on earth that is that same distance from the satellite. Where is he? You may need to prompt him to walk a circle around the satellite, but some kids get it instinctively. Answer: he could be anywhere on that circle. Not very useful. But what if we add a second satellite?

                   

                  Set up a second kid the same way as the first, but with the rope from a different satellite. Explain that they could be any place that the two signals intersect, because we know the distance to TWO satellites know. Ask them tofind the place. Most kids will figure out quickly that there are two such places. If not, help them find the second place.

                   

                  Now add a third kid the same way as the first two, and ask them to find the location. This time, there is only one place. Point out that the place is not exactly on top of the rock. Why not? Because our estimate of the distance to the satellite is only that: an estimate. It has some error in it, and that error makes for an in where the computed position is. What's worse, with only three satellites, you can't even estimate how much of an error you made!

                   

                  Add the other satellites, one by one, and repeat. Watch the error get smaller. After each addition, ask the kids to estimate how big a mistake is possible, not by looking at where the rock is, but by looking at how much the ropes overlap or don't touch. You'd be surprised at how good your average group of kids is at making this estimate.

                   

                  That's it. The only thing left to point out is that they did this walking on the ground which is a 2D object, but the earth is a 3D object. That means you need one more satellite for everything. Four to get a basic position, 5 or more to be able to estimate accuracy.

                   

                  Cheers,

                  Dave

                  aka Team Landshark

                   

                  From: "gumbietygress@..." <gumbietygress@...>
                  Reply-To: <CentralTexasGeocachers@yahoogroups.com>
                  Date: Saturday, February 16, 2013 7:56 AM
                  To: <CentralTexasGeocachers@yahoogroups.com>
                  Subject: Re: [CentralTexasGeocachers] Kim Komando

                   

                   

                  So what is the source of the position inaccuracy? Other than electromagnetic interference from the user. [Hey, some of us canconfound a watch.]

                   

                  BarbJ =ripples in the atmosphere?= Tygress

                  ---------- Original Message ----------
                  From: Dave Read <dave@...>
                  To: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                  Cc: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                  Subject: Re: [CentralTexasGeocachers] Kim Komando
                  Date: Sat, 16 Feb 2013 07:02:05 -0600

                  Hey Esther!

                   

                  The post worked just fine -- great video.

                   

                  If anyone is interested, last summer I developed a hands-on method for explaining GPS to kids so my wife old teach a geocaching class at a Cub Scout camp. It's super easy, actually conveys more of the nuances of GPS (especially the source of the position inaccuracy), and best of all, is totally understandable by kids as young as 9 or 10!  

                   

                  Thanks for the link!

                   

                  Dave

                  aka Team Landshark



                  On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...> wrote:

                   

                  Howdy!
                  I don't normally post links and I am not very techno savvy but I do listen to the Digital Goddess, Kim Komando who is.
                  Today I ran across this;

                  http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl&utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f

                  This is her explanation of how GPS works and I thought someone might find it helpful and decided to post it. I also hope I did it right!
                  Esther/BGTx

                   



                  ____________________________________________________________
                  Woman is 53 ButLooks 25
                  Mom reveals 1 simple wrinkle trick that has angered doctors...
                  ConsumerLifestyleMag.com

                • Dave Read
                  I don t know about your GPSr, but my Oregon 450 doesn t even have a menu choice to set the clock. The only time it has to show me is the one computed by the
                  Message 8 of 15 , Feb 16, 2013
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                    I don't know about your GPSr, but my Oregon 450 doesn't even have a menu choice to set the clock. The only time it has to show me is the one computed by the GPS chip. It even computes which time zone I'm in, although there *is* a way to change that setting to let me enter a time zone manually if I so desired (I don't).

                    FWIW, I worked on a military system a few years back, where we read the time directly from the GPS chip. We then fed this into an NTP server and synced the entire collection of computers to the master clock that came from the GPS. The master system drifted a little relative to our calibrated reference clock, typically 1-2 nanoseconds at any given time.

                    If your GPSr were to set its internal clock with the first valid reading after you turned it on, and then use an onboard clock afterwards, you would still have sub-microsecond accuracy for many minutes afterwards. The easiest case to imagine is that they set the clock from the GPS time every time an ephemeris update comes in, which would be once every 1-2 minutes. Worst case is that the GPSr never updates after the initial time reading until you turn it off; in this case you might have a drift of up to a tens of microseconds over the course of a day.

                    D

                    From: Victor Engel <brillig@...>
                    Reply-To: <CentralTexasGeocachers@yahoogroups.com>
                    Date: Saturday, February 16, 2013 10:51 AM
                    To: <CentralTexasGeocachers@yahoogroups.com>
                    Subject: Re: [CentralTexasGeocachers] Kim Komando

                     

                    That would be an accurate clock if the GPS unit actually used it for time display, which I believe is not the case for most units.

                    Victor Engel





                    On Sat, Feb 16, 2013 at 10:03 AM, Dave Read <dave@...> wrote:



                    OK, "since you asked." This is waaaaaay more information than I give kids, but it will set the stage for you to understand how the kid activity works.

                    The position inaccuracy comes from the way GPS positions are computed. Your GPSr receives signals from the satellites, and measures the length of time it took for the signal to travel from the satellite to the GPSr. It then multiplies by the speed of light to compute the distance between you and the satellite. In the physics world we call this a "time of flight" system. The GPSr does this for all satellites it can "see."

                    In order to make use of this information, the GPSr needs to know where each satellite was located when it transmitted the data. To do this, each satellite transmits some information about its current orbit. This is called "ephemeris" data. The ephemeris data is transmitted by the satellite only once in a while…IIRC it's about once every 90 seconds. When you turn on your GPSr and it says "looking for satellites," mostly what it is doing is waiting for the ephemeris updates to roll in. The GPSr "sees" the satellites almost immediately, but without the ephemeris data it doesn't know where the satellites are, so it's helpless. On my Oregon 450, the "satellite view" shows a signal strength bar for each satellite the GPSr can see; the solid bars are ones where the unit has received the ephemeris update, while the ones filled in white are ones with no update yet. The reason it can take some GPSrs "forever" to lock in is that if the ephemeris update is garbled due to weak signal, the GPSr has to wait for the satellite to transmit it again. For marginal signals, this process can take a long time to complete. Also FYI, I think newer GPS units have some way of modeling the evolution of ephemeris data, so if you turn off your GPSr for ~a few hours, it "locks in" much faster when you turn it back on. However, if you wait too long (a day or more) or if you get on a plane and go to a completely new location, the ephemeris evolution model breaks down and it has to go back to waiting for ephemeris updates the old-fashioned way.

                    OK, so now the GPSr knows where all of its satellites are located. It can compute time of flight and thus distance to the satellites. It's a simple matter to solve for the spot where all the distances converge. That is, you want the place in 3-D space where gives you the right distance to all of the satellites. 

                    So where does the error come from? Simple: remember that we multiplied the time of flight by speed of light to get the distance? Well, this is just an approximation. The speed of light is not a constant when you change materials; it depends (mostly) on matter density. Denser materials have slower speed of light. Those clouds overhead? They make light go slower. Those tree leaves overhead? Same thing. Moist air? Same thing. You also get "multi-path" effects, which is the signal bouncing off a building and taking a longer path to get to you. All of this contributes to an error in the estimated distance ***for each satellite***, and a different one for each satellite, at that. So when you go to make that computation of "where do I need to be to make all of these distances work out?", you can't get it exactly right. If there were no such speed-of-light effects, the position accuracy of GPS would be under a foot.

                    Oh, one more thing to mention. The GPSr can't actually compute the time of flight until the end of this exercise, because while it knows what time the satellite transmitted its signal, the GPSr doesn't know what time it is locally. The computation that gives the position *also* gives the local time. It's all one big hairy computation. The math is ferocious. At the end of the process, the GPSr knows where it is, and also what time it is locally. This means that on average, your GPSr is the best clock you own — typical time accuracy is around 10-20 nanoseconds. FWIW one nanosecond is almost exactly one foot, so if your unit is reporting 15 foot position accuracy, you can assume that implies (roughly) 15 nanosecond time accuracy.

                    Now for the kids exercise. Start with 5-6 ropes, preferably around 10-15 feet long. Mark a spot on the ground with a rock, flag, whatever, and stretch out the ropes so all the ends meet at the rock/flag, but point them in different directions. Now cheat a little: pull a few of the ropes 8-12 inches away from the spot, and make few "overrun" the spot by the same distance.  

                    Identify 5-6 kids (one for each rope) to be satellites. The rest of the kids are geocachers. Position one kid at the end of each rope. Have them pick up the rope end, and tell them that once they pick it up, they must remain in that spot until you tell them they can move again." Have all the satellites pull in their ropes completely.

                    Now pick your first geocacher, and give him the end of one of the ropes. Have him walk away from the satellite until it's slightly tight. Explain to the kids that the ropes represent the GPSr's estimate of the distance to the satellite. You can explain about the time of flight thing if you want to, but younger kids probably won't get it.

                    Tell that first kid that he could be any place on earth that is that same distance from the satellite. Where is he? You may need to prompt him to walk a circle around the satellite, but some kids get it instinctively. Answer: he could be anywhere on that circle. Not very useful. But what if we add a second satellite?

                    Set up a second kid the same way as the first, but with the rope from a different satellite. Explain that they could be any place that the two signals intersect, because we know the distance to TWO satellites know. Ask them to find the place. Most kids will figure out quickly that there are two such places. If not, help them find the second place.

                    Now add a third kid the same way as the first two, and ask them to find the location. This time, there is only one place. Point out that the place is not exactly on top of the rock. Why not? Because our estimate of the distance to the satellite is only that: an estimate. It has some error in it, and that error makes for an in where the computed position is. What's worse, with only three satellites, you can't even estimate how much of an error you made!

                    Add the other satellites, one by one, and repeat. Watch the error get smaller. After each addition, ask the kids to estimate how big a mistake is possible, not by looking at where the rock is, but by looking at how much the ropes overlap or don't touch. You'd be surprised at how good your average group of kids is at making this estimate.

                    That's it. The only thing left to point out is that they did this walking on the ground which is a 2D object, but the earth is a 3D object. That means you need one more satellite for everything. Four to get a basic position, 5 or more to be able to estimate accuracy.

                    Cheers,
                    Dave
                    aka Team Landshark

                    From: "gumbietygress@..." <gumbietygress@...>
                    Reply-To: <CentralTexasGeocachers@yahoogroups.com>
                    Date: Saturday, February 16, 2013 7:56 AM
                    To: <CentralTexasGeocachers@yahoogroups.com>
                    Subject: Re: [CentralTexasGeocachers] Kim Komando

                     

                    So what is the source of the position inaccuracy? Other than electromagnetic interference from the user. [Hey, some of us can confound a watch.]
                     
                    BarbJ =ripples in the atmosphere?= Tygress

                    ---------- Original Message ----------
                    From: Dave Read <dave@...>
                    To: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                    Cc: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                    Subject: Re: [CentralTexasGeocachers] Kim Komando
                    Date: Sat, 16 Feb 2013 07:02:05 -0600

                    Hey Esther!
                     
                    The post worked just fine -- great video.
                     
                    If anyone is interested, last summer I developed a hands-on method for explaining GPS to kids so my wife old teach a geocaching class at a Cub Scout camp. It's super easy, actually conveys more of the nuances of GPS (especially the source of the position inaccuracy), and best of all, is totally understandable by kids as young as 9 or 10!  
                     
                    Thanks for the link!
                     
                    Dave
                    aka Team Landshark


                    On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...> wrote:

                     

                    Howdy!
                    I don't normally post links and I am not very techno savvy but I do listen to the Digital Goddess, Kim Komando who is.
                    Today I ran across this;

                    http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl&utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f

                    This is her explanation of how GPS works and I thought someone might find it helpful and decided to post it. I also hope I did it right!
                    Esther/BGTx

                     



                    ____________________________________________________________
                    Woman is 53 But Looks 25
                    Mom reveals 1 simple wrinkle trick that has angered doctors...
                    ConsumerLifestyleMag.com




                  • Victor Engel
                    It has nothing to do with MY GPS unit. I didn t save where I read the information, but I ll cite it if I run into it again. It seems silly not to use the GPS
                    Message 9 of 15 , Feb 16, 2013
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                      It has nothing to do with MY GPS unit. I didn't save where I read the
                      information, but I'll cite it if I run into it again. It seems silly
                      not to use the GPS data. One need only correct for GPS UTC difference
                      (about 16 seconds now I think) and adjust for timezone.

                      The article I read didn't state what units typically use, just that
                      they don't typically use the GPS time data for time display.

                      Victor

                      On 2/16/13, Dave Read <dave@...> wrote:
                      > I don't know about your GPSr, but my Oregon 450 doesn't even have a menu
                      > choice to set the clock. The only time it has to show me is the one
                      > computed
                      > by the GPS chip. It even computes which time zone I'm in, although there
                      > *is* a way to change that setting to let me enter a time zone manually if I
                      > so desired (I don't).
                      >
                      > FWIW, I worked on a military system a few years back, where we read the
                      > time
                      > directly from the GPS chip. We then fed this into an NTP server and synced
                      > the entire collection of computers to the master clock that came from the
                      > GPS. The master system drifted a little relative to our calibrated
                      > reference
                      > clock, typically 1-2 nanoseconds at any given time.
                      >
                      > If your GPSr were to set its internal clock with the first valid reading
                      > after you turned it on, and then use an onboard clock afterwards, you would
                      > still have sub-microsecond accuracy for many minutes afterwards. The
                      > easiest
                      > case to imagine is that they set the clock from the GPS time every time an
                      > ephemeris update comes in, which would be once every 1-2 minutes. Worst
                      > case
                      > is that the GPSr never updates after the initial time reading until you
                      > turn
                      > it off; in this case you might have a drift of up to a tens of microseconds
                      > over the course of a day.
                      >
                      > D
                      >
                      > From: Victor Engel <brillig@...>
                      > Reply-To: <CentralTexasGeocachers@yahoogroups.com>
                      > Date: Saturday, February 16, 2013 10:51 AM
                      > To: <CentralTexasGeocachers@yahoogroups.com>
                      > Subject: Re: [CentralTexasGeocachers] Kim Komando
                      >
                      >
                      >
                      >
                      >
                      >
                      > That would be an accurate clock if the GPS unit actually used it for time
                      > display, which I believe is not the case for most units.
                      >
                      > Victor Engel
                      >
                      >
                      >
                      >
                      >
                      > On Sat, Feb 16, 2013 at 10:03 AM, Dave Read <dave@...> wrote:
                      >>
                      >>
                      >>
                      >>
                      >> OK, "since you asked." This is waaaaaay more information than I give kids,
                      >> but
                      >> it will set the stage for you to understand how the kid activity works.
                      >>
                      >> The position inaccuracy comes from the way GPS positions are computed.
                      >> Your
                      >> GPSr receives signals from the satellites, and measures the length of time
                      >> it
                      >> took for the signal to travel from the satellite to the GPSr. It then
                      >> multiplies by the speed of light to compute the distance between you and
                      >> the
                      >> satellite. In the physics world we call this a "time of flight" system.
                      >> The
                      >> GPSr does this for all satellites it can "see."
                      >>
                      >> In order to make use of this information, the GPSr needs to know where
                      >> each
                      >> satellite was located when it transmitted the data. To do this, each
                      >> satellite
                      >> transmits some information about its current orbit. This is called
                      >> "ephemeris"
                      >> data. The ephemeris data is transmitted by the satellite only once in a
                      >> whileŠIIRC it's about once every 90 seconds. When you turn on your GPSr
                      >> and it
                      >> says "looking for satellites," mostly what it is doing is waiting for the
                      >> ephemeris updates to roll in. The GPSr "sees" the satellites almost
                      >> immediately, but without the ephemeris data it doesn't know where the
                      >> satellites are, so it's helpless. On my Oregon 450, the "satellite view"
                      >> shows
                      >> a signal strength bar for each satellite the GPSr can see; the solid bars
                      >> are
                      >> ones where the unit has received the ephemeris update, while the ones
                      >> filled
                      >> in white are ones with no update yet. The reason it can take some GPSrs
                      >> "forever" to lock in is that if the ephemeris update is garbled due to
                      >> weak
                      >> signal, the GPSr has to wait for the satellite to transmit it again. For
                      >> marginal signals, this process can take a long time to complete. Also FYI,
                      >> I
                      >> think newer GPS units have some way of modeling the evolution of
                      >> ephemeris
                      >> data, so if you turn off your GPSr for ~a few hours, it "locks in" much
                      >> faster
                      >> when you turn it back on. However, if you wait too long (a day or more) or
                      >> if
                      >> you get on a plane and go to a completely new location, the ephemeris
                      >> evolution model breaks down and it has to go back to waiting for
                      >> ephemeris
                      >> updates the old-fashioned way.
                      >>
                      >> OK, so now the GPSr knows where all of its satellites are located. It can
                      >> compute time of flight and thus distance to the satellites. It's a simple
                      >> matter to solve for the spot where all the distances converge. That is,
                      >> you
                      >> want the place in 3-D space where gives you the right distance to all of
                      >> the
                      >> satellites.
                      >>
                      >> So where does the error come from? Simple: remember that we multiplied
                      >> the
                      >> time of flight by speed of light to get the distance? Well, this is just
                      >> an
                      >> approximation. The speed of light is not a constant when you change
                      >> materials;
                      >> it depends (mostly) on matter density. Denser materials have slower speed
                      >> of
                      >> light. Those clouds overhead? They make light go slower. Those tree
                      >> leaves
                      >> overhead? Same thing. Moist air? Same thing. You also get "multi-path"
                      >> effects, which is the signal bouncing off a building and taking a longer
                      >> path
                      >> to get to you. All of this contributes to an error in the estimated
                      >> distance
                      >> ***for each satellite***, and a different one for each satellite, at that.
                      >> So
                      >> when you go to make that computation of "where do I need to be to make all
                      >> of
                      >> these distances work out?", you can't get it exactly right. If there were
                      >> no
                      >> such speed-of-light effects, the position accuracy of GPS would be under
                      >> a
                      >> foot.
                      >>
                      >> Oh, one more thing to mention. The GPSr can't actually compute the time
                      >> of
                      >> flight until the end of this exercise, because while it knows what time
                      >> the
                      >> satellite transmitted its signal, the GPSr doesn't know what time it is
                      >> locally. The computation that gives the position *also* gives the local
                      >> time.
                      >> It's all one big hairy computation. The math is ferocious. At the end of
                      >> the
                      >> process, the GPSr knows where it is, and also what time it is locally.
                      >> This
                      >> means that on average, your GPSr is the best clock you own ‹ typical time
                      >> accuracy is around 10-20 nanoseconds. FWIW one nanosecond is almost
                      >> exactly
                      >> one foot, so if your unit is reporting 15 foot position accuracy, you can
                      >> assume that implies (roughly) 15 nanosecond time accuracy.
                      >>
                      >> Now for the kids exercise. Start with 5-6 ropes, preferably around 10-15
                      >> feet
                      >> long. Mark a spot on the ground with a rock, flag, whatever, and stretch
                      >> out
                      >> the ropes so all the ends meet at the rock/flag, but point them in
                      >> different
                      >> directions. Now cheat a little: pull a few of the ropes 8-12 inches away
                      >> from
                      >> the spot, and make few "overrun" the spot by the same distance.
                      >>
                      >> Identify 5-6 kids (one for each rope) to be satellites. The rest of the
                      >> kids
                      >> are geocachers. Position one kid at the end of each rope. Have them pick
                      >> up
                      >> the rope end, and tell them that once they pick it up, they must remain
                      >> in
                      >> that spot until you tell them they can move again." Have all the
                      >> satellites
                      >> pull in their ropes completely.
                      >>
                      >> Now pick your first geocacher, and give him the end of one of the ropes.
                      >> Have
                      >> him walk away from the satellite until it's slightly tight. Explain to
                      >> the
                      >> kids that the ropes represent the GPSr's estimate of the distance to the
                      >> satellite. You can explain about the time of flight thing if you want to,
                      >> but
                      >> younger kids probably won't get it.
                      >>
                      >> Tell that first kid that he could be any place on earth that is that same
                      >> distance from the satellite. Where is he? You may need to prompt him to
                      >> walk a
                      >> circle around the satellite, but some kids get it instinctively. Answer:
                      >> he
                      >> could be anywhere on that circle. Not very useful. But what if we add a
                      >> second
                      >> satellite?
                      >>
                      >> Set up a second kid the same way as the first, but with the rope from a
                      >> different satellite. Explain that they could be any place that the two
                      >> signals
                      >> intersect, because we know the distance to TWO satellites know. Ask them
                      >> to
                      >> find the place. Most kids will figure out quickly that there are two such
                      >> places. If not, help them find the second place.
                      >>
                      >> Now add a third kid the same way as the first two, and ask them to find
                      >> the
                      >> location. This time, there is only one place. Point out that the place is
                      >> not
                      >> exactly on top of the rock. Why not? Because our estimate of the distance
                      >> to
                      >> the satellite is only that: an estimate. It has some error in it, and
                      >> that
                      >> error makes for an in where the computed position is. What's worse, with
                      >> only
                      >> three satellites, you can't even estimate how much of an error you made!
                      >>
                      >> Add the other satellites, one by one, and repeat. Watch the error get
                      >> smaller.
                      >> After each addition, ask the kids to estimate how big a mistake is
                      >> possible,
                      >> not by looking at where the rock is, but by looking at how much the ropes
                      >> overlap or don't touch. You'd be surprised at how good your average group
                      >> of
                      >> kids is at making this estimate.
                      >>
                      >> That's it. The only thing left to point out is that they did this walking
                      >> on
                      >> the ground which is a 2D object, but the earth is a 3D object. That means
                      >> you
                      >> need one more satellite for everything. Four to get a basic position, 5
                      >> or
                      >> more to be able to estimate accuracy.
                      >>
                      >> Cheers,
                      >> Dave
                      >> aka Team Landshark
                      >>
                      >> From: "gumbietygress@..." <gumbietygress@...>
                      >> Reply-To: <CentralTexasGeocachers@yahoogroups.com>
                      >> Date: Saturday, February 16, 2013 7:56 AM
                      >> To: <CentralTexasGeocachers@yahoogroups.com>
                      >> Subject: Re: [CentralTexasGeocachers] Kim Komando
                      >>
                      >>
                      >>
                      >>
                      >> So what is the source of the position inaccuracy? Other than
                      >> electromagnetic
                      >> interference from the user. [Hey, some of us can confound a watch.]
                      >>
                      >> BarbJ =ripples in the atmosphere?= Tygress
                      >>
                      >> ---------- Original Message ----------
                      >> From: Dave Read <dave@...>
                      >> To: "CentralTexasGeocachers@yahoogroups.com"
                      >> <CentralTexasGeocachers@yahoogroups.com>
                      >> Cc: "CentralTexasGeocachers@yahoogroups.com"
                      >> <CentralTexasGeocachers@yahoogroups.com>
                      >> Subject: Re: [CentralTexasGeocachers] Kim Komando
                      >> Date: Sat, 16 Feb 2013 07:02:05 -0600
                      >>
                      >>
                      >> Hey Esther!
                      >>
                      >> The post worked just fine -- great video.
                      >>
                      >> If anyone is interested, last summer I developed a hands-on method for
                      >> explaining GPS to kids so my wife old teach a geocaching class at a Cub
                      >> Scout
                      >> camp. It's super easy, actually conveys more of the nuances of GPS
                      >> (especially
                      >> the source of the position inaccuracy), and best of all, is totally
                      >> understandable by kids as young as 9 or 10!
                      >>
                      >> Thanks for the link!
                      >>
                      >> Dave
                      >> aka Team Landshark
                      >>
                      >>
                      >> On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...>
                      >> wrote:
                      >>
                      >>>
                      >>> Howdy!
                      >>> I don't normally post links and I am not very techno savvy but I do
                      >>> listen to
                      >>> the Digital Goddess, Kim Komando who is.
                      >>> Today I ran across this;
                      >>>
                      >>> http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl
                      >>> &utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f
                      >>>
                      >>> This is her explanation of how GPS works and I thought someone might find
                      >>> it
                      >>> helpful and decided to post it. I also hope I did it right!
                      >>> Esther/BGTx
                      >>>
                      >>
                      >>
                      >>
                      >> ____________________________________________________________
                      >> Woman is 53 But Looks 25
                      >> Mom reveals 1 simple wrinkle trick that has angered doctors...
                      >> <http://thirdpartyoffers.juno.com/TGL3142/511f9031a9480103120a7st04vuc>
                      >> ConsumerLifestyleMag.com
                      >> <http://thirdpartyoffers.juno.com/TGL3142/511f9031a9480103120a7st04vuc>
                      >>
                      >>
                      >>
                      >>
                      >
                      >
                      >
                      >
                      >
                      >
                      >
                      >
                      >


                      --
                      Victor Engel
                    • gumbietygress@juno.com
                      And then there s bounce ; those times you re moving faster than the math/update (sometimes holding still is the best answer, but, looking at the results from
                      Message 10 of 15 , Feb 17, 2013
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                      • 0 Attachment
                        And then there's 'bounce'; those times you're moving faster than the math/update (sometimes holding still is the best answer, but, looking at the results from Howard's GPS event contest, that's not always the answer); signal interference from who knows what (my 60 would get very odd in cemeteries -- and there ARE spots on the earth where even your mechanical compass can't be trusted); and sometimes, your unit just gets notional: watching my Oregon count 'up' from a cache (I was about 20 feet away and not moving -- it counted up to 250, then back down to 195... I've discovered that when some brands of batteries get under 30% charge (down enough for the 'no backlight message), the unit cops an attitude; won't hold satellites, etc. etc. etc.).
                        And I'm seriously not beyond also tossing in personal electromagnetic field (some people call them auras) interference.
                        Very complex stuff.
                        But LOVE the way you illustrate it. Good show -- nothing like a good hands on picture for the kiddos.


                        ---------- Original Message ----------
                        From: Dave Read <dave@...>
                        To: <CentralTexasGeocachers@yahoogroups.com>
                        Subject: Re: [CentralTexasGeocachers] Kim Komando
                        Date: Sat, 16 Feb 2013 10:03:30 -0600

                         
                        OK, "since you asked." This is waaaaaay more information than I give kids, but it will set the stage for you to understand how the kid activity works.
                         
                        The position inaccuracy comes from the way GPS positions are computed. Your GPSr receives signals from the satellites, and measures the length of time it took for the signal to travel from the satellite to the GPSr. It then multiplies by the speed of light to compute the distance between you and the satellite. In the physics world we call this a "time of flight" system. The GPSr does this for all satellites it can "see."
                         
                        In order to make use of this information, the GPSr needs to know where each satellite was located when it transmitted the data. To do this, each satellite transmits some information about its current orbit. This is called "ephemeris" data. The ephemeris data is transmitted by the satellite only once in a while…IIRC it's about once every 90 seconds. When you turn on your GPSr and it says "looking for satellites," mostly what it is doing is waiting for the ephemeris updates to roll in. The GPSr "sees" the satellites almost immediately, but without the ephemeris data it doesn't know where the satellites are, so it's helpless. On my Oregon 450, the "satellite view" shows a signal strength bar for each satellite the GPSr can see; the solid bars are ones where the unit has received the ephemeris update, while the ones filled in white are ones with no update yet. The reason it can take some GPSrs "forever" to lock in is that if the ephemeris update is garbled due to weak signal, the GPSr has to wait for the satellite to transmit it again. For marginal signals, this process can take a long time to complete. Also FYI, I think newer GPS units have some way of modeling the evolution of ephemeris data, so if you turn off your GPSr for ~a few hours, it "locks in" much faster when you turn it back on. However, if you wait too long (a day or more) or if you get on a plane and go to a completely new location, the ephemeris evolution model breaks down and it has to go back to waiting for ephemeris updates the old-fashioned way.
                         
                        OK, so now the GPSr knows where all of its satellites are located. It can compute time of flight and thus distance to the satellites. It's a simple matter to solve for the spot where all the distances converge. That is, you want the place in 3-D space where gives you the right distance to all of the satellites. 
                         
                        So where does the error come from? Simple: remember that we multiplied the time of flight by speed of light to get the distance? Well, this is just an approximation. The speed of light is not a constant when you change materials; it depends (mostly) on matter density. Denser materials have slower speed of light. Those clouds overhead? They make light go slower. Those tree leaves overhead? Same thing. Moist air? Same thing. You also get "multi-path" effects, which is the signal bouncing off a building and taking a longer path to get to you. All of this contributes to an error in the estimated distance ***for each satellite***, and a different one for each satellite, at that. So when you go to make that computation of "where do I need to be to make all of these distances work out?", you can't get it exactly right. If there were no such speed-of-light effects, the position accuracy of GPS would be under a foot.
                         
                        Oh, one more thing to mention. The GPSr can't actually compute the time of flight until the end of this exercise, because while it knows what time the satellite transmitted its signal, the GPSr doesn't know what time it is locally. The computation that gives the position *also* gives the local time. It's all one big hairy computation. The math is ferocious. At the end of the process, the GPSr knows where it is, and also what time it is locally. This means that on average, your GPSr is the best clock you own — typical time accuracy is around 10-20 nanoseconds. FWIW one nanosecond is almost exactly one foot, so if your unit is reporting 15 foot position accuracy, you can assume that implies (roughly) 15 nanosecond time accuracy.
                         
                        Now for the kids exercise. Start with 5-6 ropes, preferably around 10-15 feet long. Mark a spot on the ground with a rock, flag, whatever, and stretch out the ropes so all the ends meet at the rock/flag, but point them in different directions. Now cheat a little: pull a few of the ropes 8-12 inches away from the spot, and make few "overrun" the spot by the same distance.  
                         
                        Identify 5-6 kids (one for each rope) to be satellites. The rest of the kids are geocachers. Position one kid at the end of each rope. Have them pick up the rope end, and tell them that once they pick it up, they must remain in that spot until you tell them they can move again." Have all the satellites pull in their ropes completely.
                         
                        Now pick your first geocacher, and give him the end of one of the ropes. Have him walk away from the satellite until it's slightly tight. Explain to the kids that the ropes represent the GPSr's estimate of the distance to the satellite. You can explain about the time of flight thing if you want to, but younger kids probably won't get it.
                         
                        Tell that first kid that he could be any place on earth that is that same distance from the satellite. Where is he? You may need to prompt him to walk a circle around the satellite, but some kids get it instinctively. Answer: he could be anywhere on that circle. Not very useful. But what if we add a second satellite?
                         
                        Set up a second kid the same way as the first, but with the rope from a different satellite. Explain that they could be any place that the two signals intersect, because we know the distance to TWO satellites know. Ask them to find the place. Most kids will figure out quickly that there are two such places. If not, help them find the second place.
                         
                        Now add a third kid the same way as the first two, and ask them to find the location. This time, there is only one place. Point out that the place is not exactly on top of the rock. Why not? Because our estimate of the distance to the satellite is only that: an estimate. It has some error in it, and that error makes for an in where the computed position is. What's worse, with only three satellites, you can't even estimate how much of an error you made!
                         
                        Add the other satellites, one by one, and repeat. Watch the error get smaller. After each addition, ask the kids to estimate how big a mistake is possible, not by looking at where the rock is, but by looking at how much the ropes overlap or don't touch. You'd be surprised at how good your average group of kids is at making this estimate.
                         
                        That's it. The only thing left to point out is that they did this walking on the ground which is a 2D object, but the earth is a 3D object. That means you need one more satellite for everything. Four to get a basic position, 5 or more to be able to estimate accuracy.
                         
                        Cheers,
                        Dave
                        aka Team Landshark
                         
                        From: "gumbietygress@..." <gumbietygress@...>
                        Reply-To: <CentralTexasGeocachers@yahoogroups.com>
                        Date: Saturday, February 16, 2013 7:56 AM
                        To: <CentralTexasGeocachers@yahoogroups.com>
                        Subject: Re: [CentralTexasGeocachers] Kim Komando
                         
                         

                         

                        So what is the source of the position inaccuracy? Other than electromagnetic interference from the user. [Hey, some of us can confound a watch.]
                         
                        BarbJ =ripples in the atmosphere?= Tygress

                        ---------- Original Message ----------
                        From: Dave Read <dave@...>
                        To: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                        Cc: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                        Subject: Re: [CentralTexasGeocachers] Kim Komando
                        Date: Sat, 16 Feb 2013 07:02:05 -0600

                         

                        Hey Esther!
                         
                        The post worked just fine -- great video.
                         
                        If anyone is interested, last summer I developed a hands-on method for explaining GPS to kids so my wife old teach a geocaching class at a Cub Scout camp. It's super easy, actually conveys more of the nuances of GPS (especially the source of the position inaccuracy), and best of all, is totally understandable by kids as young as 9 or 10!  
                         
                        Thanks for the link!
                         
                        Dave
                        aka Team Landshark


                        On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...> wrote:

                         

                        Howdy!
                        I don't normally post links and I am not very techno savvy but I do listen to the Digital Goddess, Kim Komando who is.
                        Today I ran across this;

                        http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl&utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f

                        This is her explanation of how GPS works and I thought someone might find it helpful and decided to post it. I also hope I did it right!
                        Esther/BGTx

                         

                         



                        ____________________________________________________________
                        Woman is 53 But Looks 25
                        Mom reveals 1 simple wrinkle trick that has angered doctors...
                        ConsumerLifestyleMag.com

                         

                         



                        ____________________________________________________________
                        How to Sleep Like a Rock
                        Obey this one natural trick to fall asleep and stay asleep all night.
                        peaklife.com
                      • J Crozier
                        Wow! That s an AWESOME explanation! Footnotes coach is a teacher, and he said that was EXACTLY the type of lesson he d love to use in his classes (if he
                        Message 11 of 15 , Feb 18, 2013
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                        • 0 Attachment
                          Wow!  That's an AWESOME explanation!  Footnotes coach is a teacher, and he said that was EXACTLY the type of lesson he'd love to use in his classes (if he still had any).  Nicely done!
                          Julie
                          Mrs. Captain Picard


                          On Sat, Feb 16, 2013 at 10:03 AM, Dave Read <dave@...> wrote:
                           


                          OK, "since you asked." This is waaaaaay more information than I give kids, but it will set the stage for you to understand how the kid activity works.

                          The position inaccuracy comes from the way GPS positions are computed. Your GPSr receives signals from the satellites, and measures the length of time it took for the signal to travel from the satellite to the GPSr. It then multiplies by the speed of light to compute the distance between you and the satellite. In the physics world we call this a "time of flight" system. The GPSr does this for all satellites it can "see."

                          In order to make use of this information, the GPSr needs to know where each satellite was located when it transmitted the data. To do this, each satellite transmits some information about its current orbit. This is called "ephemeris" data. The ephemeris data is transmitted by the satellite only once in a while…IIRC it's about once every 90 seconds. When you turn on your GPSr and it says "looking for satellites," mostly what it is doing is waiting for the ephemeris updates to roll in. The GPSr "sees" the satellites almost immediately, but without the ephemeris data it doesn't know where the satellites are, so it's helpless. On my Oregon 450, the "satellite view" shows a signal strength bar for each satellite the GPSr can see; the solid bars are ones where the unit has received the ephemeris update, while the ones filled in white are ones with no update yet. The reason it can take some GPSrs "forever" to lock in is that if the ephemeris update is garbled due to weak signal, the GPSr has to wait for the satellite to transmit it again. For marginal signals, this process can take a long time to complete. Also FYI, I think newer GPS units have some way of modeling the evolution of ephemeris data, so if you turn off your GPSr for ~a few hours, it "locks in" much faster when you turn it back on. However, if you wait too long (a day or more) or if you get on a plane and go to a completely new location, the ephemeris evolution model breaks down and it has to go back to waiting for ephemeris updates the old-fashioned way.

                          OK, so now the GPSr knows where all of its satellites are located. It can compute time of flight and thus distance to the satellites. It's a simple matter to solve for the spot where all the distances converge. That is, you want the place in 3-D space where gives you the right distance to all of the satellites. 

                          So where does the error come from? Simple: remember that we multiplied the time of flight by speed of light to get the distance? Well, this is just an approximation. The speed of light is not a constant when you change materials; it depends (mostly) on matter density. Denser materials have slower speed of light. Those clouds overhead? They make light go slower. Those tree leaves overhead? Same thing. Moist air? Same thing. You also get "multi-path" effects, which is the signal bouncing off a building and taking a longer path to get to you. All of this contributes to an error in the estimated distance ***for each satellite***, and a different one for each satellite, at that. So when you go to make that computation of "where do I need to be to make all of these distances work out?", you can't get it exactly right. If there were no such speed-of-light effects, the position accuracy of GPS would be under a foot.

                          Oh, one more thing to mention. The GPSr can't actually compute the time of flight until the end of this exercise, because while it knows what time the satellite transmitted its signal, the GPSr doesn't know what time it is locally. The computation that gives the position *also* gives the local time. It's all one big hairy computation. The math is ferocious. At the end of the process, the GPSr knows where it is, and also what time it is locally. This means that on average, your GPSr is the best clock you own — typical time accuracy is around 10-20 nanoseconds. FWIW one nanosecond is almost exactly one foot, so if your unit is reporting 15 foot position accuracy, you can assume that implies (roughly) 15 nanosecond time accuracy.

                          Now for the kids exercise. Start with 5-6 ropes, preferably around 10-15 feet long. Mark a spot on the ground with a rock, flag, whatever, and stretch out the ropes so all the ends meet at the rock/flag, but point them in different directions. Now cheat a little: pull a few of the ropes 8-12 inches away from the spot, and make few "overrun" the spot by the same distance.  

                          Identify 5-6 kids (one for each rope) to be satellites. The rest of the kids are geocachers. Position one kid at the end of each rope. Have them pick up the rope end, and tell them that once they pick it up, they must remain in that spot until you tell them they can move again." Have all the satellites pull in their ropes completely.

                          Now pick your first geocacher, and give him the end of one of the ropes. Have him walk away from the satellite until it's slightly tight. Explain to the kids that the ropes represent the GPSr's estimate of the distance to the satellite. You can explain about the time of flight thing if you want to, but younger kids probably won't get it.

                          Tell that first kid that he could be any place on earth that is that same distance from the satellite. Where is he? You may need to prompt him to walk a circle around the satellite, but some kids get it instinctively. Answer: he could be anywhere on that circle. Not very useful. But what if we add a second satellite?

                          Set up a second kid the same way as the first, but with the rope from a different satellite. Explain that they could be any place that the two signals intersect, because we know the distance to TWO satellites know. Ask them to find the place. Most kids will figure out quickly that there are two such places. If not, help them find the second place.

                          Now add a third kid the same way as the first two, and ask them to find the location. This time, there is only one place. Point out that the place is not exactly on top of the rock. Why not? Because our estimate of the distance to the satellite is only that: an estimate. It has some error in it, and that error makes for an in where the computed position is. What's worse, with only three satellites, you can't even estimate how much of an error you made!

                          Add the other satellites, one by one, and repeat. Watch the error get smaller. After each addition, ask the kids to estimate how big a mistake is possible, not by looking at where the rock is, but by looking at how much the ropes overlap or don't touch. You'd be surprised at how good your average group of kids is at making this estimate.

                          That's it. The only thing left to point out is that they did this walking on the ground which is a 2D object, but the earth is a 3D object. That means you need one more satellite for everything. Four to get a basic position, 5 or more to be able to estimate accuracy.

                          Cheers,
                          Dave
                          aka Team Landshark

                          From: "gumbietygress@..." <gumbietygress@...>
                          Reply-To: <CentralTexasGeocachers@yahoogroups.com>
                          Date: Saturday, February 16, 2013 7:56 AM
                          To: <CentralTexasGeocachers@yahoogroups.com>

                          Subject: Re: [CentralTexasGeocachers] Kim Komando

                           

                          So what is the source of the position inaccuracy? Other than electromagnetic interference from the user. [Hey, some of us can confound a watch.]
                           
                          BarbJ =ripples in the atmosphere?= Tygress

                          ---------- Original Message ----------
                          From: Dave Read <dave@...>
                          To: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                          Cc: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                          Subject: Re: [CentralTexasGeocachers] Kim Komando
                          Date: Sat, 16 Feb 2013 07:02:05 -0600

                          Hey Esther!
                           
                          The post worked just fine -- great video.
                           
                          If anyone is interested, last summer I developed a hands-on method for explaining GPS to kids so my wife old teach a geocaching class at a Cub Scout camp. It's super easy, actually conveys more of the nuances of GPS (especially the source of the position inaccuracy), and best of all, is totally understandable by kids as young as 9 or 10!  
                           
                          Thanks for the link!
                           
                          Dave
                          aka Team Landshark


                          On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...> wrote:

                           

                          Howdy!
                          I don't normally post links and I am not very techno savvy but I do listen to the Digital Goddess, Kim Komando who is.
                          Today I ran across this;

                          http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl&utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f

                          This is her explanation of how GPS works and I thought someone might find it helpful and decided to post it. I also hope I did it right!
                          Esther/BGTx

                           



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                        • Greg Jewett
                          Jay - Thanks for the referral! I would love to post this to the website. Dave.. I wrote you personally on this before I saw this message. I am just catching
                          Message 12 of 15 , Feb 20, 2013
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                            Jay - Thanks for the referral!  I would love to post this to the website. 

                            Dave.. I wrote you personally on this before I saw this message.  I am just catching the website up.  



                            Greg Jewett
                            Geocache/Munzee Handle:  GeoJewett
                            (512) 627-7290
                            geojewett (at) geocachingaustin.com
                            http://geocaching.ejewett.com/
                            http://www.geocachingaustin.com/
                            http://www.munzee.com/m/GeoJewett/
                            Don't print this e-mail unless it's necessary. Save a tree






                            On Feb 16, 2013, at 11:16 AM, Jay Bingham wrote:

                             

                            Dave,

                             

                            What a great simplified explanation and a great activity for the kiddos too.

                            If it is ok with you, I suggest that Greg (geojewett) post it on the Geocaching Austin web site.

                             

                            --| Jay /\ Bing-GTX
                            |+| Georgetown , TX USA
                            |-- “What you see depends mainly on what you are looking for.”

                             


                            From: CentralTexasGeocachers@yahoogroups.com [mailto:CentralTexasGeocachers@yahoogroups.com] On Behalf Of Dave Read
                            Sent: Saturday, February 16, 2013 10:04 AM
                            To: CentralTexasGeocachers@yahoogroups.com
                            Subject: Re: [CentralTexasGeocachers] Kim Komando

                             

                             

                             

                            OK, "since you asked." This is waaaaaay more information than I give kids, but it will set the stage for you to understand how the kid activity works.

                             

                            The position inaccuracy comes from the way GPS positions are computed. Your GPSr receives signals from the satellites, and measures the length of time it took for the signal to travel from the satellite to the GPSr. It then multiplies by the speed of light to compute the distance between you and the satellite. In the physics world we call this a "time of flight" system. The GPSr does this for all satellites it can "see."

                             

                            In order to make use of this information, the GPSr needs to know where each satellite was located when it transmitted the data. To do this, each satellite transmits some information about its current orbit. This is called "ephemeris" data. The ephemeris data is transmitted by the satellite only once in a while…IIRC it's about once every 90 seconds. When you turn on your GPSr and it says "looking for satellites," mostly what it is doing is waiting for the ephemeris updates to roll in. The GPSr "sees" the satellites almost immediately, but without the ephemeris data it doesn't know where the satellites are, so it's helpless. On my Oregon 450, the "satellite view" shows a signal strength bar for each satellite the GPSr can see; the solid bars are ones where the unit has received the ephemeris update, while the ones filled in white are ones with no update yet. The reason it can take some GPSrs "forever" to lock in is that if the ephemeris update is garbled due to weak signal, the GPSr has to wait for the satellite to transmit it again. For marginal signals, this process can take a long time to complete. Also FYI, I think newer GPS units have some way of modeling the evolution of ephemeris data, so if you turn off your GPSr for ~a few hours, it "locks in" much faster when you turn it back on. However, if you wait too long (a day or more) or if you get on a plane and go to a completely new location, the ephemeris evolution model breaks down and it has to go back to waiting for ephemeris updates the old-fashioned way.

                             

                            OK, so now the GPSr knows where all of its satellites are located. It can compute time of flight and thus distance to the satellites. It's a simple matter to solve for the spot where all the distances converge. That is, you want the place in 3-D space where gives you the right distance to all of the satellites. 

                             

                            So where does the error come from? Simple: remember that we multiplied the time of flight by speed of light to get the distance? Well, this is just an approximation. The speed of light is not a constant when you change materials; it depends (mostly) on matter density. Denser materials have slower speed of light. Those clouds overhead? They make light go slower. Those tree leaves overhead? Same thing. Moist air? Same thing. You also get "multi-path" effects, which is the signal bouncing off a building and taking a longer path to get to you. All of this contributes to an error in the estimated distance ***for each satellite***, and a different one for each satellite, at that. So when you go to make that computation of "where do I need to be to make all of these distances work out?", you can't get it exactly right. If there were no such speed-of-light effects, the position accuracy of GPS would be under a foot.

                             

                            Oh, one more thing to mention. The GPSr can't actually compute the time of flight until the end of this exercise, because while it knows what time the satellite transmitted its signal, the GPSr doesn't know what time it is locally. The computation that gives the position *also* gives the local time. It's all one big hairy computation. The math is ferocious. At the end of the process, the GPSr knows where it is, and also what time it is locally. This means that on average, your GPSr is the best clock you own — typical time accuracy is around 10-20 nanoseconds. FWIW one nanosecond is almost exactly one foot, so if your unit is reporting 15 foot position accuracy, you can assume that implies (roughly) 15 nanosecond time accuracy.

                             

                            Now for the kids exercise. Start with 5-6 ropes, preferably around 10-15 feet long. Mark a spot on the ground with a rock, flag, whatever, and stretch out the ropes so all the ends meet at the rock/flag, but point them in different directions. Now cheat a little: pull a few of the ropes 8-12 inches away from the spot, and make few "overrun" the spot by the same distance.  

                             

                            Identify 5-6 kids (one for each rope) to be satellites. The rest of the kids are geocachers. Position one kid at the end of each rope. Have them pick up the rope end, and tell them that once they pick it up, they must remain in that spot until you tell them they can move again." Have all the satellites pull in their ropes completely.

                             

                            Now pick your first geocacher, and give him the end of one of the ropes. Have him walk away from the satellite until it's slightly tight. Explain to the kids that the ropes represent the GPSr's estimate of the distance to the satellite. You can explain about the time of flight thing if you want to, but younger kids probably won't get it.

                             

                            Tell that first kid that he could be any place on earth that is that same distance from the satellite. Where is he? You may need to prompt him to walk a circle around the satellite, but some kids get it instinctively. Answer: he could be anywhere on that circle. Not very useful. But what if we add a second satellite?

                             

                            Set up a second kid the same way as the first, but with the rope from a different satellite. Explain that they could be any place that the two signals intersect, because we know the distance to TWO satellites know. Ask them to find the place. Most kids will figure out quickly that there are two such places. If not, help them find the second place.

                             

                            Now add a third kid the same way as the first two, and ask them to find the location. This time, there is only one place. Point out that the place is not exactly on top of the rock. Why not? Because our estimate of the distance to the satellite is only that: an estimate. It has some error in it, and that error makes for an in where the computed position is. What's worse, with only three satellites, you can't even estimate how much of an error you made!

                             

                            Add the other satellites, one by one, and repeat. Watch the error get smaller. After each addition, ask the kids to estimate how big a mistake is possible, not by looking at where the rock is, but by looking at how much the ropes overlap or don't touch. You'd be surprised at how good your average group of kids is at making this estimate.

                             

                            That's it. The only thing left to point out is that they did this walking on the ground which is a 2D object, but the earth is a 3D object. That means you need one more satellite for everything. Four to get a basic position, 5 or more to be able to estimate accuracy.

                             

                            Cheers,

                            Dave

                            aka Team Landshark

                             

                            From: "gumbietygress@..." <gumbietygress@...>
                            Reply-To: <CentralTexasGeocachers@yahoogroups.com>
                            Date: Saturday, February 16, 2013 7:56 AM
                            To: <CentralTexasGeocachers@yahoogroups.com>
                            Subject: Re: [CentralTexasGeocachers] Kim Komando

                             

                             

                            So what is the source of the position inaccuracy? Other than electromagnetic interference from the user. [Hey, some of us can confound a watch.]

                             

                            BarbJ =ripples in the atmosphere?= Tygress

                            ---------- Original Message ----------
                            From: Dave Read <dave@...>
                            To: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                            Cc: "CentralTexasGeocachers@yahoogroups.com" <CentralTexasGeocachers@yahoogroups.com>
                            Subject: Re: [CentralTexasGeocachers] Kim Komando
                            Date: Sat, 16 Feb 2013 07:02:05 -0600

                            Hey Esther!

                             

                            The post worked just fine -- great video.

                             

                            If anyone is interested, last summer I developed a hands-on method for explaining GPS to kids so my wife old teach a geocaching class at a Cub Scout camp. It's super easy, actually conveys more of the nuances of GPS (especially the source of the position inaccuracy), and best of all, is totally understandable by kids as young as 9 or 10!  

                             

                            Thanks for the link!

                             

                            Dave

                            aka Team Landshark



                            On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...> wrote:

                             

                            Howdy!
                            I don't normally post links and I am not very techno savvy but I do listen to the Digital Goddess, Kim Komando who is.
                            Today I ran across this;

                            http://www.tvkim.com/watch/2790/kim-on-komand-how-does-gps-work?utm_medium=nl&utm_source=tvkim&utm_content=2013-02-15-article-screen-shot-f

                            This is her explanation of how GPS works and I thought someone might find it helpful and decided to post it. I also hope I did it right!
                            Esther/BGTx

                             



                            ____________________________________________________________
                            Woman is 53 But Looks 25
                            Mom reveals 1 simple wrinkle trick that has angered doctors...
                            ConsumerLifestyleMag.com



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