[time-nuts] Using GPS for space-based instrument

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Nov 10 17:52:39 EST 2008

Strauss, Karl F wrote:
> I've been tasked (or was it I was volunteered?) to do some basic design & definition work on an ultra-stable master frequency system for a proposed instrument that is currently planned to be in an Earth-trailing orbit.   Given the first order accuracy requirement of 1 part in 1E-10, my first thought was to grab the GPS timing signal.
> Sorry for the newbie questions here:    a) Do all satellites in the constellation broadcast a signal into space  (as opposed to, say,  bouncing off some ionospheric boundary layer); and b) is there some website/technical paper describing expected signal strength for these space-radiated signals?
> Again, apologies to all for the Newbie Q's.  Hopefully this will be a fun topic of discussion
> Thanks
> Karl

The GPS SVs are well above the ionosphere.
However they use circularly polarised directional antennas that point at
the Earth.
Signal strength in orbits closer to the Earth than the GPS SVs will be
comparable with that on the surface of the Earth.

When the orbit id further out than those of the GPS SVs the signal
strength will be somewhat lower than expected from the inverse square
law, as the orbit will only be illuminated by weak sidelobes from
favourably positioned GPS SVs.


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