[aprssig] gps accuracy?

Tapio Sokura oh2kku at iki.fi
Mon Nov 23 08:43:08 EST 2009

Wes Johnston, AI4PX wrote:
> Can anyone say what the accuracy of the GPS system is for a civillian grade
> CA receiver outside the USA?

For non-differential use, it's the same as inside the USA. If some kind 
of differential corrections are used, the accuracy naturally also 
depends on the source of the corrections.

> 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).

That's typical of Garmins, they usually overestimate the accuracy with 
SBAS solutions. And I have a feeling they usually overestimate without 
SBAS as well.

> 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.

SA is a system-wide (satellite-wide) degradation of the signal, it 
really can't be turned on or off for certain geographical areas. There 
are other ways to degrade GPS performance for specific geographical 
areas I've been told, but I don't really know how they work, if you 
disregard direct RF interference.


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