[aprssig] gps accuracy?

Alex Carver kf4lvz at yahoo.com
Tue Nov 24 16:09:04 EST 2009

> Can anyone say what the accuracy of the GPS system is for a
> civillian grade
> CA receiver outside the USA?  With WAAS enabled, my
> garmin "told" me it's
> accuracy was 11 feet (3.35m).  I had occasion to test
> it against three known
> points and it was actually off the mark by 16 to 21 feet
> (4.8 to 6.4m).
> Never the less, it got me thinking about the 1990's when
> selective
> availability was on and I wonder if SA is on outside the
> USA....
> particularly in the middle east.
> -- 
> Wes
> ---

Selective Availability worked by randomly altering the timing of the timestamp beacon that the satellites transmitted on the L1 civilian channel.  All GPS receivers are computing their position based on the known satellite locations (by the Keplerians that the satellites transmit at regular intervals) and the differences in the received timestamps plus phase delays between the signals.  the SA system was an all or nothing sort of thing.  You either randomly altered the timing or you didn't.  There was no way to make SA operate over a specific area.

Part of the decision to eliminate SA was based on the fact that the European Union was going to launch Galileo and Russia had GLONASS both of which didn't use any form of SA so you could always get a receiver for those systems.  The newest US GPS satellites do not even have a provision for SA any longer.  In the years since SA was turned off, the DoD has come up with plenty of methods for degrading GPS service in local areas that can also degrade Galileo and GLONASS.  It became more useful (and less expensive) to just knock out the GPS system in the needed areas than to alter it all across the globe the way SA did.  The hardware required on the satellite to implement SA for civilian signals and keep it clear for the military signals (both groups share L1) was very expensive.


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