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: http://www.insidegnss.com/pdf/ig0806_gnss-solutions.pdf For deeper technical details, start with these papers: http://spot.colorado.edu/~kristine/gpsrep.pdf http://www.isprs.org/proceedings/XXXVII/congress/4_pdf/162.pdf http://web.gps.caltech.edu/classes/ge167/file/Ragheb2007.pdf And finally, to see the effect on a GPSDO, I have some ADEV plots at: http://leapsecond.com/pages/sidereal/ http://leapsecond.com/pages/sidereal/14years.htm /tvb _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
