41214Re: [CentralTexasGeocachers] Kim Komando
- Feb 16, 2013It 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
> 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
> 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
> 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
> 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
> is that the GPSr never updates after the initial time reading until you
> it off; in this case you might have a drift of up to a tens of microseconds
> over the course of a day.
> 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,
>> 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.
>> GPSr receives signals from the satellites, and measures the length of time
>> 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
>> satellite. In the physics world we call this a "time of flight" system.
>> GPSr does this for all satellites it can "see."
>> In order to make use of this information, the GPSr needs to know where
>> satellite was located when it transmitted the data. To do this, each
>> transmits some information about its current orbit. This is called
>> 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"
>> a signal strength bar for each satellite the GPSr can see; the solid bars
>> ones where the unit has received the ephemeris update, while the ones
>> 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
>> 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,
>> think newer GPS units have some way of modeling the evolution of
>> data, so if you turn off your GPSr for ~a few hours, it "locks in" much
>> when you turn it back on. However, if you wait too long (a day or more) or
>> 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
>> 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,
>> want the place in 3-D space where gives you the right distance to all of
>> So where does the error come from? Simple: remember that we multiplied
>> time of flight by speed of light to get the distance? Well, this is just
>> approximation. The speed of light is not a constant when you change
>> it depends (mostly) on matter density. Denser materials have slower speed
>> light. Those clouds overhead? They make light go slower. Those tree
>> 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
>> to get to you. All of this contributes to an error in the estimated
>> ***for each satellite***, and a different one for each satellite, at that.
>> when you go to make that computation of "where do I need to be to make all
>> these distances work out?", you can't get it exactly right. If there were
>> such speed-of-light effects, the position accuracy of GPS would be under
>> Oh, one more thing to mention. The GPSr can't actually compute the time
>> flight until the end of this exercise, because while it knows what time
>> satellite transmitted its signal, the GPSr doesn't know what time it is
>> locally. The computation that gives the position *also* gives the local
>> It's all one big hairy computation. The math is ferocious. At the end of
>> process, the GPSr knows where it is, and also what time it is locally.
>> 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
>> 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
>> long. Mark a spot on the ground with a rock, flag, whatever, and stretch
>> the ropes so all the ends meet at the rock/flag, but point them in
>> directions. Now cheat a little: pull a few of the ropes 8-12 inches away
>> 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
>> are geocachers. Position one kid at the end of each rope. Have them pick
>> the rope end, and tell them that once they pick it up, they must remain
>> that spot until you tell them they can move again." Have all the
>> pull in their ropes completely.
>> Now pick your first geocacher, and give him the end of one of the ropes.
>> him walk away from the satellite until it's slightly tight. Explain to
>> 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,
>> 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:
>> could be anywhere on that circle. Not very useful. But what if we add a
>> 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
>> intersect, because we know the distance to TWO satellites know. Ask them
>> 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
>> location. This time, there is only one place. Point out that the place is
>> exactly on top of the rock. Why not? Because our estimate of the distance
>> the satellite is only that: an estimate. It has some error in it, and
>> error makes for an in where the computed position is. What's worse, with
>> 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
>> After each addition, ask the kids to estimate how big a mistake is
>> 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
>> kids is at making this estimate.
>> That's it. The only thing left to point out is that they did this walking
>> the ground which is a 2D object, but the earth is a 3D object. That means
>> need one more satellite for everything. Four to get a basic position, 5
>> more to be able to estimate accuracy.
>> 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
>> 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"
>> Cc: "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
>> camp. It's super easy, actually conveys more of the nuances of GPS
>> 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!
>> aka Team Landshark
>> On Feb 15, 2013, at 10:46 PM, "bigguy9211116" <bigguy9211116@...>
>>> 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;
>>> This is her explanation of how GPS works and I thought someone might find
>>> helpful and decided to post it. I also hope I did it right!
>> Woman is 53 But Looks 25
>> Mom reveals 1 simple wrinkle trick that has angered doctors...
- << Previous post in topic Next post in topic >>