>> The only way to know is to compare to another reference assumed to be >> correct. Pool NTP servers would be accurate enough for that. The >> GPSes (Motorola Oncore) I use have a related "problem" in that they >> allow the pulse to be adjust so that it happens before the UTC second >> or any time during the second. So I actually have a choice. But how >> to set it and know it is right? > > ??? That would make the pps totally useless. If it does not occur on the > UTC second, it is pointless.
No, if it is not _processed" right at the UTC second it is pointless. The Motorola GPS allows you to adjust the timing of the pulse to account for delay in the antenna feed line and serial line. A GPS might have 50 feet of antenna cable and that would cause about a 50 ns delay in the signal. And then in my case the PPS, after it leaves the GPS goes through one 74F04 gate (5 ns delay) followed my a MAX232 which has a bit more delay. The Motorola UT+ and newer, have pulse accuracy that is far better than the cable speed of light delay so if the unit did not have the adjustment the cable would be the major source of error. So even if the GPS is perfect if the pulse has to go down a cable the pulse comming out the end of the cable will be late. Altogether it makes sense to advance the pulse about 80 nS. >> >> What you'd do is adjust the timing until it was a best match to the >> other reference clocks you have or lacking that to a set of pool NTP >> servers > Since you are using the pool as your time source, just use them. This > device adds nothing in that case. > I am assuming, as with the GPS18. that the unit emits a PPS pulse > exactly on the seconds transition of UTC time. How do you know the GPS18 has the pulse right on the second? Certainly there is some delay however small. I bet that if you had a second source of UTC seconds it would not match, they never do Only a person who owns one clock thinks he knows what time it is, anyone who owns two or more is never really sure. You'd think setting the UTC PPS phase to match the pool servers would make your local reference clock only as good as a pool server but that would be true if you sync'd on only one pulse. If you averaged many thousands of pulses you get much better. The local GPS even if it has a small offset to UTC will have much lass jitter than the pool server -- ===== Chris Albertson Redondo Beach, California _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
