[Xastir] Question about APRS GPS position precision

[Gerry Creager]

Richard Polivka, N6NKO wrote:
> Jim,
> 
> Most GPS units are good to 4 decimals. Any higher precision requires 
> post-processing or L1/L2 reception (not avail in consumer equipment). 
> When you factor in multipath and all the other variables, 4 decimals is 
> quite good but it takes time and patience - think searching for a 
> geocache in a forest. Plus, at four decimals, on a patch antenna minus 
> ground plane, it is quite unstable.

9 cm more or less should be plenty good enough for most of our users. 
That's 4 decimal-place precision.  That said, an L1 signal (L5 won't be 
available for some time still) position assuming really good geometry 
and a stable antenna platform is likely to be good only to ~6m 
horizontal and ~13.7m vertical... at best.

> Plus, I have a feeling that when Bob B. designed APRS, he was not 
> looking at this being used for what we are doing.
> 
> Until the data output is smoother and better accuracy, five decimals in 
> - broadcast 4 - rewrite the standard, this may be the best for now.

The limitations in precision are in rank order, the spec and the spec. 
For accuracy the limitations are:
User equipment antenna configuration
Ionosphere
Troposphere
Multipath
GPS Signal Specification for L1

When I resolve cm accuracy, or better, I do it using dual-frequency 
(L1/L2) receivers, multiple stable baseline processing on ground-plane 
or choke-ring antennas, at a fixed and measured height about the ground, 
and post-process the data to include a least-squares adjustment of the 
position.  The process is as much statistical as matrix-mathematical in 
accomplishment.

gerry

> Jim Tolbert wrote:
>> Hi, all..........
>>
>> I have been told that the limiting factor in position precision is the 
>> APRS system transmission standard-- that transmissions are limited to 
>> 1/1000th of a degree or approximately +/- 60 feet.   Is this true?  If 
>> so, why?
>>
>> If not, what is the limiting element in the final display of tracker 
>> position on Xastir?
>>
>> For those of you using Xastir for Search & Rescue, do you run on the 
>> APRS frequency or a different (quiet) frequency?   What are the 
>> arguments for each school of thought?   We are going to be running 
>> some field tests in the near future with a variety of equipment 
>> setups....  does anyone have suggestions of things we should test?  We 
>> have a list, but new ideas and comments would be appreciated from 
>> those that have already taken the stumbles<grin>.
>>
>> Many thanx.............. jt
>>
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-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
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