GPS Not Correct
12-14-2015 | 08:35 PM
#17
Drifting
Joined: May 2007
Posts: 2,415
Likes: 13
From: Pacific Northwest
GPS receivers have known errors in its calculations for altitude, its well known and thats what its augmented for any kind of usage for aircraft approaches or landings. The geometry of the constellation has quite a lot to do with it, as the receiver does the calculations. The satellites just send timing signals and ionospheric correction via the WAAS, the GPS receiver in the car does the processing and figures out where its located and then uses the CD-rom road map to place the little icon. Plus not all the maps you see use the same datum as GPS, which is WGS-84. There can be quite a difference between positions and altitude calculations when different datums are used. Most older maps and road signs are not based on WGS-84. So there you go.
Its getting signals from friggen SPACE. So your location is the pointy end of a bunch of triangles with very long sides from the satellites to your location. The most error sensitive part of the trigonometry is the altitude computation. So as mentioned, one can use the location and look up the altitude based on a map system of elevations, but those maps need to be of the right coordinate system with the GPS coordinates and then the elevation resolution of that data set isn't accurate down to the foot and updated yearly.
12-14-2015 | 10:39 PM
#18
Track Day
Joined: Dec 2015
Posts: 20
Likes: 0
From: Tempe, AZ
In all seriousness, if you've had your windshield replaced the gps antenna can get damaged and will cause operational blips here and there. Mine eventually had my car in Mexico after a couple of weeks creeping south after a windshield replacement.