[email protected] said: > During my Arecibo Observatory days we used NIST's TMAS service to keep our > H-maser-based station clock synced with UTC. Our user community (mainly VLBI > and pulsar timing people) seemed pretty satisfied with +/- 100ns accuracy, so > I tried to do better by keeping things well within +/- 50 ns during my reign. > IIRC, NIST was claiming that TMAS could produce results mostly within about > +/- 20 ns.
I think the VLBI guys use the time from the clock at the receiver as a starting point. What they need is a constant frequency. They can work out the time offset. Here is the example I heard about. Suppose you have N antennas. You need to know thir locations very accurately. You can work out the location of one antenna if you point them all at a good point source and use N-1 antennas to work out where that point source is in the sky and the time/angle of the Earth's rotation. Then you can solve for the position of the Nth antenna that gives the best fit. I assume there are iterative approaches that can be used to refine the positions of multiple antennas. Position is 3D. Time is 1D so I assume the search is reasonably quick. There are interesting similarities/dualities between GPS and VLBI. With GPS, you have N transmitters and one receiver. With VLBI, you have one transmitter and N receivers. With VLBI, you can't work out the distance to the transmitter. With GPS, you can't work out the length of the antenna cable. -- These are my opinions. I hate spam. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
