Hi Actually they do know a bit about the light delay. They include that data in the information the stat's broadcast. The data is fairly coarse grained. I posted some links a week or so back that go into all the grubby details.
Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Tom Holmes Sent: Wednesday, September 08, 2010 10:19 AM To: 'Discussion of precise time and frequency measurement' Subject: Re: [time-nuts] What position is measured? Thanks, Jim. I assume that neither the satellite nor the receiver knows what the variation in the light time delay is, so it must be small enough to allow the claimed nanosecond accuracy of the PPS edge. Although one sat is sufficient for time work, would using more improve the PPS accuracy? Seems like having more inputs would help with the light delay and other corrections, but it probably is no different than having multiple Rb's in the lab (the guy with two is never quite sure and all that). Mostly just curious, as my Z3801 is quite good enough for my needs. Tom Holmes, N8ZM Tipp City, OH EM79 > -----Original Message----- > From: [email protected] [mailto:[email protected]] On > Behalf Of jimlux > Sent: Wednesday, September 08, 2010 9:42 AM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] What position is measured? > > Tom Holmes wrote: > > One other delay contributor would appear to be processing delay in the > > receiver, which thus begs the question of how the PPS signal is actually > > synchronized to the GPS system. > > > > The GPS nav messages is synchronized to the seconds, so it's a matter of > making sure the output pulse is synced to the appropriate time in the > GPS signal. The delay in the receiver is (reasonably) constant, so the > mfr essentially calibrates it out. > > It's not done precisely like this, but conceptually, you have a 1pps on > the spacecraft driven by a Cs clock, you receive the signal in your > receiver (some time later than the actual "change of second") and > subtract out the light time delay from satellite to you. (or, more > accurately, delay the signal from the receiver to the "next" second). > > It's controlling for that "light time delay" that's the tricky part, > since it varies depending on the degree of ionization of the ionosphere. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
