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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Texas Mesonet -- AATLT, Texas A&M University
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