MnGCA

[A-body]

Everyone that has a 60CSX knows that when you stop moving, they jump around a bit. It takes a while to get used to but is still irritating. Today, I hooked mine to my computer, and cleared everything in it to start over. It has been plugged in for six hours on my desk, turned on, and has traveled for 1.59 miles. Pretty amazing

[bflentje]

While the satellites are considered stationary, I guarantee that they really do move up there in outer space.

[ArcherDragoon]

I get the same with my Oregon...had the same with my Vista and 60CSX...each model was a bit different, but I would suspect it is due partially to the movement of sats as flentje states as well as just the nature of GPS signal and how the signal is received...
_________________
You may only be young once...but I will be immature forever!!!

[Moe the Sleaze]

[ArcherDragoon]

Ok...so Moe said it better than I did...

_________________
You may only be young once...but I will be immature forever!!!

[bflentje]

[ST11]

GPS does not measure distance it measures time. It's the time the signal takes to reach the gps from each satellite that is measured.
To complicate things time moves slower for the satellites as they are future away from the surface of the earth and are moving much faster than the outer layer of earth. It's and Einstein thing.
At any rate the clocks (cpu clock not normal time clock) must be skewed just a bit to compensate for the difference in space/time. If the clocks ran at the same rate then the coordinates would move out of whack 17 feet per day.
At any rate there is a lot of math going on plus radio waves, even microwave travels through different materials at different speeds. For instance one satellite has a clear unobstructed path to earth while another must send it's signal through fog or rain clouds.
So the constant adjustments are compounded with electronic compensations.

In simple words - "Your GPS is goofy"

They say an owner tends to mirror his dog in time so perhaps it's the same with GPS?

[15Tango]

Both my Oregon and 60CSx get jumpy when stationary, but I think that's because they get a little paranoid when they're left alone outside of their natural element, which is guiding me to the next cache.
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There comes a time in every young boy's life when he gets an irresistible urge to seek buried treasure.--Mark Twain

[ArcherDragoon]

[Sky King 36]

I don't mean to be the naysayer here... well, I guess I do... This has nothing at all to do with the satellites moving, and I just couldn't keep my big yapper shut any more... If moving satellites caused 1.59 mile errors, there wouldn't be many non-naval applications for it. They’re always moving. Your receiver even knows how to do very precise doppler corrections, and even knows how to estimate changes in propagation delay as the satellites approach the horizon.

No doubt, if you leave your GPS on all day long, you will see your position drift pretty freely, but will probably spend about 50% of the time within a 30 foot circle, about 25% of the time within a 50 foot circle... and by the time you are getting to "stray" fixes that are 100 feet from the average, you're down to a 3-5% kinda thing.

The various sources of dilution of precision (or DOP) will range over time... There's ADOP (atmospheric DOP) which is caused by different satellite signals having to pass through different thicknesses of atmosphere, and different ionosphere conditions. There's GDOP (geometric DOP) caused by the geometry of the satellites you can "see." You get much better fixes when using satellites that are widely dispersed in the sky. Imagine "reverse" triangulation (technically called resection) using a compass to two known points... If those points are 10 degrees apart from each other, a 1 degree misreading manifests itself in HUGE errors in position. If the two points are 90 degrees apart, a one degree misreading of each azimuth results in a tiny error. That's GDOP.

If you add up all the sources of DOP--there are 3 or 4 big ones and 4 or 5 small ones... AND your ephimeral Keps (the orbital correction data in your GPSr) is at its maximum age, you still can't get anywhere near 1.59 miles. You either have clock capture on enough satellites, with current enough almanac data, or you don't. At 1.59 miles, you are many times outside the largest possible error to still have clock capture. I could delve into a nauseating discourse about how each binary one and zero modulated on to the satellite’s signal has a length in the air, and you are well outside the error window of detecting the rise time and fall time of that binary signal if you are 1.5 miles out of position. But I won’t. I'll just say that one bit of binary data is about 960 feet long in space, shorter in the atmosphere, so your mathematical maximum error is about half that or you don't, by definition, have clock sync.

So what IS happening? It's the software in consumer GPSrs. They do all kinds of things to "smooth" the positions on screen and to make assumptions about where you "should" be during momentary signal lapses... Nuvis do "snap to road" for instance, and it is not unusual for car GPSrs to have "momentum" as a calculation variable: "If you are driving due east at 60mph, I'll just assume that a second from now, you'll about 90 feet east of where you are now, even before I have completed the final calculations on the satellite data."

Aviation and commercial receivers don't do this... they calculate the fix and they either display the fix with confidence, or, they report immediately to the user that they have lost sufficient clock sync to report a fix. They don't "guess" or try to smooth out the position plot to make the user feel better. Consumer GPSrs, however, DO smooth the coords and DO make assumptions. What has happened in your case is that your signal diluted to the point where your GPSr lost useful sync, but in a way that fooled your GPSr firmware into thinking it should still be trying to work up a fix using assumptions, and it failed epically. When you got to a few hundred feet of error, your GPSr already knew that it had flipped out, but it wanted to keep trying and not leave you with nothing. So it thought about where you had been lately and started making more and more erroneous guesses about where you are now. The longer a period of time it was left to guess, the crazier the guesses got.

While I’m here, a note about GDOP and a common misperception among cachers. When you walk into the woods, it is usually not the overhead canopy that is causing your fixes to slip. Remember, the more widely dispersed the satellites, the lower the GDOP and the better the fix. In the woods what you are usually losing is all the satellites on the horizon, which now have many, many tree trunks between you and them. A signal coming from 5 degrees above the horizon could have to pass through literally hundreds of feet of lumber to get to you… Not gonna happen. So the thicker the woods, the smaller the cone of satellites above you left to work with… and as the satellites that are left for you to use are closer together, this has an enormous impact on accuracy, one that cannot be corrected with WAAS.

Phew! I need a nap now!

[A-body]

I guess I knew some of that but SkyKing explained it in more detail than I can coprehent. I don't think I meant to say that my GPS was showing me 1.59 miles from home, but that it traveled 1.59 miles, 30 feet at a time as it bounced around my current location

[Sky King 36]

[ArcherDragoon]

[ST11]

What Sky King was saying is

Abody your gpsr is goofy

[sigilwig44]