100 things....

[wallace]

According to that manual, this thing isn't very accurate.
From the manual.

"On average, the Module has a +/-5 meter position accuracy and a +/-0.1 meter per second velocity accuracy.

Primarily due to satellite geometry, measuring ParallaxGPS Receiver Module* Revision 11 altitude using GPS may introduce an accuracy error of 1.5 times the receiver?s position accuracy (in the case of our GPS Receiver Module, this corresponds to
about +/-20 meters in the vertical direction)."

5 meters is about 16.5 feet, 0.1 meter is about 3.9 inches.  I think this is typical civilian accuracy.  I know the military can put a missile into any selected window in a building from a couple thousand miles away. But I think it takes some special techniques to be able to measure a drift of 2-3/4 inches over a years time.

Dale, I think those position accuracies are for autonomous positions, the kind you get from a single receiver. More accuracy will come from using two receivers and post-processing software. Those could be used differentially (more than one receiver) with the right data collection and software.

Its true that the elevation fix is always weaker.

[wallace]

My understanding is that originally GPS was military and not available to the public.
Then it went public but the military degraded its accuracy so other powers could not use it well.
Then they got their own maybe and the military let the accuracy get better. Maybe the stuff w\Wallace sauidx about increasing the accuracy was involved in that too.

I have a bunch of precise gps receivers I use and there is a rebar that I drove in the ground 10 years ago out behind the house. If I let these cook on that nail for a few hours it will collect enough satellite data to figure out the plate movement. I forget what the shift vector is around here, maybe 2mm per year or so. Relative to what??

Wow those GPS's are cool.
I would guess the zero point would be Greenich England since lots of times are set in relation to it.

Interestingly, GPS calculations have to use general relativity to accurately calculate the location of the satellites which in turn are used to accurately calculate the locations on earth. Without the special relativity calculations, the times and locations of the satellites would be off and everything would be inaccurate.

I think that GPS and/or satellites bouncing radar and/or lasers off the surface are used to measure the vertical movement of ground on things like volcanoes swelling up before they might blow.

This is how they made very accurate maps (https://www.google.com/mars/) of the Martian surface also. (The blue blob is my favorite crater, Hellas Basin, as wide as the 48 states and about 10,000 ft deep (https://en.wikipedia.org/wiki/Hellas_Planitia))

The calculations that go on in the software are horrendous. In the "old days" we would spend all day just working out the Doppler shift.