[Xastir] Question about APRS GPS position precision

[Gerry Creager]

Which tells us nothing.  You don't tell me the origins of your map 
sources, the datum, the satellite constellation, antenna occlusions, 
etc.  All these affect how good your accuracy is for GPS positioning.

Or, was this simply flame-bait?
gerry

Andrew Rich wrote:
> Yeah but in the real world, it still puts me off the road or runway.
> 
> 
> ----------------------------------------------------------------------------
> Andrew Rich VK4TEC
> vk4tec at people.net.au <mailto:vk4tec at people.net.au>
> http://www.tech-software.net
> 
> 
> 
> 
> -----Original Message-----
> From: xastir-bounces at xastir.org [mailto:xastir-bounces at xastir.org]On
> Behalf Of Gerry Creager
> Sent: Monday, 8 October 2007 6:54 AM
> To: Richard Polivka, N6NKO
> Cc: Jim Tolbert; XASTIR
> Subject: Re: [Xastir] Question about APRS GPS position precision
> 
> 
> 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
> 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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> 

-- 
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