T&T: GPS Error?

[Jonathan Haas]

I use righ resolution GPS receivers as a tool to map archaeological sites in the field.  We have high end Trimble receivers that lock on to about a dozen satellites and with these backpack units we are able to walk along one side of a 2 ft thick wall and then down the other side of that wall and the 2 ft thick wall shows up exactly on our maps.  Then, however, when we get these locational data back from the field we have to "correct"  or "post process" them using a receiver at our base station. This receiver set on the roof of our headquarters - surveyed in with cm accuracy - receives data from even more satellites 24/7.  When post processed, the GPS data taken from the field receivers are shifted over about 6+ ft or so.  The shapes stay exactly the same, but their location on the face of the earth is nailed down with an accuracy of less than a meter (it's actually about 20 cm, but we just call it "sub-meter accuracy").  The only reason we have a base station at the field
 headquarters is because we are in Peru and the only base stations available for post processing are controlled by the military.  Around the U.S. and particularly along the shores, base stations or CORS are more readily available and can be accessed through NOAA's National Geodetic Survey:  http://www.ngs.noaa.gov/CORS/cors-data.html 
 There are also services that provide base station post-processing in other parts of the world (e.g. GPS Control Ltd.)

 As of right now, all of the portable GPS receivers need to be post-processed with data from a base station to achieve the higher levels of accuracy.  They are coming out with more accurate (and very expensive - $7000+ portable units, but they still need to be post-processed.  
 
 So in something like a canal, even if the charts are accurate, you might still see yourself as cruising along the towpath if it is 6-10 ft away, or in a marina you might be in your neighbor's slip.
 
 Hope this might be of some help.
 
 Jonathan Haas
 (on the coast of Peru, but not on a boat ... yet)

Ron Rogers <rcrogers6 at kennett.net> wrote: Not since some point during the Clinton administration. The civilian signal 
is no longer degraded. Rather, there is a separate encrypted military signal 
of incredible accuracy. The civilian signal, plus WAAS, and DGPS stations is 
sufficiently accurate that military bases and naval vessels to include the 
USCG jam and spoof GPS near them.

Unless there is a satellite hardware failure, I'd blame errors on bad 
mapping.

Ron Rogers

----- Original Message ----- 
From: "Rob Dorsey" 
|
| 3. There is a built in offset in the GPS signal for military security
| reasons. That's where the 10m accuracy comes from. In fact the GPS 
position
| CAN be so accurate that you could park in the slip with it. 
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