Hi > On Nov 29, 2015, at 11:31 AM, Tom Van Baak <[email protected]> wrote: > > Hal, > > Right. The orbits are nominally circular -- but not exactly. The set of > orbital parameters cover these details. A quick google search suggests the > eccentricity for GPS is around 0.01. Still, that's enough to cause +/- 23 ns > of accumulated phase error per orbit. I'm pretty sure the receivers take care > of this math, since eccentricity is a key part of any orbit model. I wish we > could see the source code to a GPS timing receiver. > > I'm not sure I understand your elevation question. Are you talking about > elevation as in mountain vs. sea level altitude? Or elevation as in satellite > Az/El? > > GPS satellites in view are about 20,000 km (overhead) to about 25,000 km > (horizon) away, so the signal gets to you within about 65 to 85 ms. Whether > you apply the full 4.5e-10 relativistic correction or no correction to the SV > clock at all, it makes only a 1 cm time-of-arrival difference. That's why I > said for trilateral navigation purposes, the relativistic effects are in the > noise. For UTC time-transfer, however, an uncorrected 4.5e-10 frequency error > would continuously accumulate, giving 38 us/day phase error, the number you > often hear. > > About survey grade -- I suspect the post-processing takes into account > anything you can think of, from the shape of the antennas to space weather to > the phase of the moon (literally).
Most survey work is done as a “delta from known references”. It’s much like common view time transfer. That alone takes care of a whole raft of things. If you dig into the gravity stuff, they get into questions like “do we put in a term for the gravitational effects of Pluto? Yes, there are Gravity Nuts…. Bob > > /tvb > > ----- Original Message ----- > From: "Hal Murray" <[email protected]> > To: "Discussion of precise time and frequency measurement" > <[email protected]> > Cc: <[email protected]> > Sent: Saturday, November 28, 2015 2:37 PM > Subject: Re: [time-nuts] Einstein Special on PBS > > >> >> [email protected] said: >>> The GPS spec implies the satellites have a fixed frequency offset to >>> compensate for relativistic effects. But do they actually dynamically and/ >>> or individually adjust the frequency to adjust for orbit variations and >>> eccentricities? >> >> I think the orbits are circular so the frequency won't depend on the orbital >> position. >> >> The next question is does the math in the receiver have to correct for >> changes due to elevation? Does it become relevant if you are trying for >> survey grade results? >> >> -- >> These are my opinions. I hate spam. >> > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
