On 2011-09-22, Terje Mathisen <"terje.mathisen at tmsw.no"> wrote: > unruh wrote: >> On 2011-09-22, David Woolley<[email protected]> wrote: >>> The orbit plane is fixed relative to the fixed stars, so, for the orbit >>> to cover the same ground each time, it has to have a period that exactly >>> divides the sidereal day. The period is between successive maximum >>> North points. >> >> It is the sattelite you see, not the orbital plane. The sattelite goes >> around its orbit in a much shorter time period than one siderial day. >> > David is correct, the GPS sats are in fact using orbits that have a > period of almost exactly half sidereal day, or about 11 hours 58 minutes. > > This is not a requirement though, afaik the upcoming Galileo sats will > have significantly higher orbits/longer periods, but still a rational > factor of the sidereal day, so that they will return to the same point > after a number of days.
You and he are right, I am wrong. Although there is no particular reason why that needs to be true. The GPS system would also work if the sattelites were not in a siderial day orbit. > > Terje > _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
