Yeah but in the real world, it still puts me off the road or runway.
---------------------------------------------------------------------------- Andrew Rich VK4TEC [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]> http://www.tech-software.net -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] 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 >> > _______________________________________________ > Xastir mailing list > Xastir@xastir.org > http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir -- Gerry Creager -- [EMAIL PROTECTED] 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 _______________________________________________ Xastir mailing list Xastir@xastir.org http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir _______________________________________________ Xastir mailing list Xastir@xastir.org http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir
