[aprssig] APRS resolution

Gerry Creager N5JXS gerry.creager at tamu.edu
Sun Apr 16 13:12:42 CDT 2006

Couple of questions.

1.  What kind of GPS are you using?
2.  Do you have WAAS or DGPS enabled?
3.  What are your GDOP and PDOP?

In general, a civil GPS doing autonomous positioning without 
augmentation is good for 6-10m accuracy.  Lots of things can make it 
worse, luck can make it better.  Garmin's EPE, while it's starting to 
look promising is neither confirmable nor published in method.  There 
are several reverse-engineering attempts that have failed to discern the 
exact formula used.

If you have WAAS or DGPS enabled, it's possible to see real accuracy in 
the meter, or even sub-meter range, with the antenna and multipath 
tending to be the limiting factors.

In general terms, the autocorrelated autonomous position tends to wander 
slowly about the real correct position.  Decorrelating the readings by 
decimating the output to ~30 sec or longer and then doing a long-term 
average (8 hrs of 30-sec readings should do) will provide a reasonable 
estimate of the real position.

Anything you get in the submeter range that's not augmented, well, is a 
combination of luck and an artifact of the receiver's output.


Andrew Rich wrote:
> Just wondering if this was intentional ?
> When I use UI-VIEW i can set my position to 6 figures
> eg 27.34.44
> When I look at my GPS i can see 7 figures
> eg
> When I look at the GPS stream i can see 8 figures
> eg 27.34.5218
> What is the 'best' resolution aprs can return ?
> I got about 50 cm's difference just walking around with the GPS
> -----------------------------------------
> Andrew Rich
> Amateur radio callsign VK4TEC
> email: vk4tec at tech-software.net <mailto:vk4tec at tech-software.net>
> web: http://www.tech-software.net
> Brisbane AUSTRALIA
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Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.862.3983
Office: 1700 Research Parkway Ste 160, TAMU, College Station, TX 77843

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