The problem that they tried to solve when they designed the GPS orbits was to have even coverage world wide such that the number of satellites in view has about the same in any location world wide and that this remain true most of the time. And to do this at minimum cost.
If you just start thinking of random orbits you will have the problem that something bad happens, like one a month all of then are on one side of the Earth or something dumb like that. Quickly you find the orbits must be an integer fraction or multiple of the Earth's rotation period. a factor of "1" is is very high and takes a lot of energy to get there 1/8 is so low that it's footprint is to small and you'd need way to many satellites. Turns out 1/2 works well. There was talk some time ago about a pair of molniyan orbit (12 hours also see here: http://en.wikipedia.org/wiki/Molniya_orbit ) GPS sats for Tokyo so they would have coverage between tall buildings. Circular orbits are not required. On Thu, Sep 22, 2011 at 3:13 PM, unruh <[email protected]> wrote: > 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 > -- Chris Albertson Redondo Beach, California _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
