>> With inexpensive >> GPS devices, which I have, time does appear to vary about ±40 ms over the >> day as satellites come into and go out of view. But with stratum 1 network >> time sources over the WAN, time accuracy appears to vary wildly in the >> interval ±100 ms or more.
I think you bough the wrong inexpensive GPS. I paid $18 for a GPS receicer on eBay. (There are always many available. I did not find any kind of rare deal.) My $18 Moterola "UT" receiver runs with nanosecond level error and NTP runs with microsecond level error. I'd be a little shocked and disapointed to see even a handfull of milliseconds or error in my setup and with cables and other parts I might have only $50 invested in that one system I think your problem is a lack of a hardwired PPS. Without that a basic NMEA interfaced GPS is only about as good as using a Internet pool server and can be much worse. NMEA can be as much as a full second off and with some Garmin units I've seen even more than a second. One of my very old GPSes was continually "voted off the island" as a false ticker. It was useless. The only GPS receivers worth considering should be those with PPS that is spec's to have 1 sigma error better than 0.5 uS. It is easy to find these for under $20. Chris Albertson Redondo Beach, California _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
