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41214Re: [CentralTexasGeocachers] Kim Komando

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  • Victor Engel
    Feb 16, 2013
      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.


      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
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      Victor Engel
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