[ntp:questions] best gps receiver for time synchronization
martin.burnicki at meinberg.de
Wed Mar 11 09:06:27 UTC 2009
> Martin Burnicki <martin.burnicki at meinberg.de> writes:
>>David Woolley wrote:
>>> Unruh wrote:
>>>> No. That is what GPS does for you. It determines your postion, then
>>>> uses that to determine the time delay from the sattelite to your
>>> Actually, it is the other way round. It uses the differences in the
>>> time delays to solve for the position (and the actual time delays).
> Actaully they solve for the position and the time delay all at once.
> But if the receiver knows its position, it only needs one sattelite to
> determine the time, and it uses the position to determine the delay.
>>Right. However, it computes the position of the *antenna*. So if you
>>receiver is connected via a long antenna cable you have to compensate the
>>signal delay on the cable. This can not be done automatically by the GPS
> Agreed. On the other hand, 300m is 1usec and most computers are completely
> incapable of discipling the clock to that accuracy ( interrupt latencies,
> etc) If you actually have a system which could do better than that, then
> antenna delay is important. As may proper cable termination so the signal
> bouncing back and forth along the cable does not mess up the timing.
Agreed, too. However, if we are discussing about Garmins with 50 ns accuracy
then 1000 ns delay is magnitudes worse.
Also, PPS pulses must not necessarily be used only as PPS signals for
computers. We have also applications where just a piece of hardware is
synchronized by a PPS signal, in which case 1 microsecond *is* relevant.
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