[time-nuts] GPS position survey

Stewart Cobb stewart.cobb at gmail.com
Mon May 6 03:34:57 EDT 2013

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.


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