In message <blu125-w29155884599c7e38cb3a21ce...@phx.gbl>, Mark Sims writes:
>But, the key word is most. Some were off over 50 feet lat/lon >and 100 meters altitude. Unless you have a known position to compare >against you may never know for sure. [...] Actually, you will, if you monitor the per-sat time residuals from the GPS. A couple of years ago, I got to play around with 10 M12M's during a burn-in test, and managed to get some hacked up code to improve the pos-hold coords, while staying in pos-hold mode. The basic trick is to project the per-sat time residuals onto their hemispherical coords (alt+azi) and determine if there is a net E/W or N/S imbalance. The E/W direction worked great simply by comparing the eastern to the western hemisphere and moving the pos-hold longitude accordingly. It is easy to see that if your pos-hold longitude is too far east, sats west of you will have a slightly longer signal path making their signals arrive a little too late, and vice versa. To test my hacked up code, I would intentionally give it a bogus pos-hold to start with, and over a month it would edge onto the right longitude. It can probably be tuned to be much faster. I never got the N/S direction working to my satisfaction, because at 55°N, I have no usable sats north of my antenna, and I never found a good way to do the N/S imbalance with only data for southern half the plane. And then I had to deliver the NTP servers and never got around to play with it again... It should be possible however, because the residuals vary strongly with altitude: almost no effect on a sat right overhead, big effect on sats near horizon (think: basic triangle geometry). Some details at: http://phk.freebsd.dk/raga/sneak/ The above observation gave me another idea which I didn't get to play with, so I don't know if it will work: If your pos-hold is not correct, your time solution will jump whenever a sat is added/deleted from the solution. It may be possible to detect the sign of these jumps, by monitoring the per-sat residuals, and use it to twist the pos-hold coords without the tedious detour over mapping and balancing hemispheres. It is important to not be fooled by near-horizon artifacts, so a high mask-angle is probably required for this to work. Somebody[tm] should really pick up this idea... Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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