[aprssig] gps accuracy?

Scott Miller scott at opentrac.org
Mon Nov 23 11:27:22 EST 2009

Garmin seems to have a history of picking the units that suit them best. 
  Take a look at their receiver specs - the GPS 18X is rated at -185 
dBW.  Of course, just about everyone else on the planet uses dBm.

The same data sheet says < 15 meters, 95% for the accuracy.  I have no 
idea what standard they use for displayed EPE.  Probably 50% CEP or 
whatever gives them the best numbers.


n4lbl wrote:
> I've always presumed that the accuracy reported is at some particular
> confidence interval but I surely don't know that.  If the GPS says the
> accuracy now is 10 feet that sounds like a 100% confidence interval is
> implied but I have trouble believing that.
> On Mon, Nov 23, 2009 at 6:43 AM, Tapio Sokura <oh2kku at iki.fi> wrote:
>> 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.
>>  Tapio
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