I am surprized no one, including myself was said this using precises and technical terms. In can be stated in two short sentences:
- The purpose of NEMA is to "number the seconds" - The purpose of PPS is to define the start of the UTC second. I tried to say this above in a previous email but when I re-read it I see it is not clear. In the telephone analogy the voice recording "numbers the seconds" and the "beep" defines the start of that second. One could imagine a voice time system that lacks the "beep" and it would be good enough for most people, you'd still get to work on time. A GPS' NMEA output is an ASCII serial message that says the current time and location and by the NMEA standard it is required to be output "during the second to which it applies". (The quote is from the NEMA specification and is very close, but is from memory) This means it can be almost a full second "off". But typically it is not so bad as that. The PPS on the other hand is a simple pulse and the raising edge of the pulse is specified to occur right on the start of the UTC second. Of course there is some error. The amount of error depends on the care you put into selecting parts and your budget. But much care and $100 will buy you an error of only a few tens on nano seconds. NEMA alone will typically get you a few tens on milli seconds. Literally a million times worse. That said, nanosecond level accuracy is wasted on NTP, microseconds are good enough fro driving NTP. But perhaps you have other timing equipment driven by the same GPS reciever? I hope this is now clear. Chris Albertson Redondo Beach, California _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions