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Re: [CentralTexasGeocachers] Kim Komando

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  • 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 1 of 15 , Feb 16, 2013
    • 0 Attachment
      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 2 of 15 , Feb 16, 2013
      • 0 Attachment
        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 3 of 15 , Feb 16, 2013
        • 0 Attachment
          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 4 of 15 , Feb 17, 2013
          • 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 5 of 15 , Feb 18, 2013
            • 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

               



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


            • 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 6 of 15 , Feb 20, 2013
              • 0 Attachment
                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

                 



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