A subtle point: The GPS satellite orbits are controlled so that their (nominal) ground tracks precisely repeat every day. To make this work, their "day" is not a standard 24-hour day but a "sidereal day" lasting 23 hours, 56 minutes, and about 4 seconds. The satellites actually go around the earth twice in that time, so their orbits take 11 hours and 58+ minutes, but the earth is only halfway around after the first orbit, so the ground tracks only repeat every other orbit.
The ground tracks repeat so that multipath errors can be averaged out by taking data in multiples of one sidereal day. If you want to get the most accurate position by averaging satellite data, do your average in multiples of 86164 seconds (sidereal day) rather than 86400 seconds (24 hours). It also helps to take data during a time when the weather is relatively constant. In particular, avoid precipitation. Cheers! --Stu _______________________________________________ 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.
