[time-nuts] "GPS once a day issues" ?

Tom Van Baak tvb at LeapSecond.com
Mon Oct 20 09:49:20 EDT 2014

The GPS satellites are at an altitude that gives them an orbit of 12* hours. But during that time the earth has made half a rotation. Thus it takes -two- SV orbits and -one- earth rotation to get back to the same geometry. It is this 24* hour ground-track repeat time that is of interest in high-precision work.

That's why you often see GPS time-transfer data based on days*, rather than just a few thousand seconds or 12 hours. This is not likely to affect any of you working on home GPSDO projects. But it is a concern for the folks that do positioning at mm levels.

* Fun facts:
1) Right, it's not actually 24 hours (solar day); instead it's closer to 23h 56m (sidereal day).
2) However, if you look closely you find it's not precisely a sidereal day (86164 s) either; instead the repeat time is closer to 86155 s, due to gravitational effects (inclined orbits, non-spherical earth).
3) If you look even closer you find each SV has its own repeat time; 86155 is merely the constellation average.
4) Also the per-SV repeat times are not constant; they slowly drift by about 10 seconds a year. As the orbit decays and the repeat time gets out of spec, an orbital maneuver puts the SV back.

For a nice description of this effect, here's a short 2-page summary:

For deeper technical details, start with these papers:

And finally, to see the effect on a GPSDO, I have some ADEV plots at:


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