William Unruh <un...@invalid.ca> wrote: > On 2015-02-07, David Woolley <david@ex.djwhome.demon.invalid> wrote: >> On 07/02/15 08:24, Rob wrote: >>>> >And a side question: Is it the GPS module that calculates when the PPS >>>> >goes active? Is this signal compensated for the time it takes the signal >>>> >from the sats in the module, or on the SV? >>> Yes, the module calculates the position and time fix from the signals of >>> at least 4 SVs, and then outputs the PPS related to the calculated time. >> >> Which is the key to how GPS does the P. GPS receivers need to know >> where the satellites were when the signal was launched, and the orbits >> are specified in terms of the satellite times. It then has to work out, >> from the relative delay times, from at least four satellites, the >> current time and three spatial coordinates (four unknowns). I'm not >> sure it actually needs the time part of the solution (it probably does >> for detailed corrections) but that is going to drop out of solving for >> the spatial position. > > Some receivers allow you to feed in the current location, and then the > receiver can use only one sattelite to determine the time
That is actually just because they are single-channel receivers. I do have a GPSDO like that. It initially determines the position using 4 SV mode and after half an hour or so it switches down to 1 SV mode for just time and frequency. However, it does that because it has one of those ancient receiver modules that have only one physical receiver that is multiplexed over at most 8 SV. Modern receivers do not need to restrict themselves to 1 SV because they can receive multiple SV in parallel. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
