On Thu, Aug 9, 2012 at 20:29 UTC, Charles Elliott wrote: > The process shown in the graph repeats indefinitely using my BU-353W. In > other words, the offsets always begin at about 300 ms, rise slowly to about > 360 ms, vary erratically between 260 and 360 ms for about two hours, settle > at 260 ms, and then rise slowly to 360 ms to repeat forever. > > It is not clear to me if the problem is the USB interface or the GPS device.
Drift of that magnitude must be due to the GPS not USB-triggered latency. Moreover, the real problem is you're assuming the GPS is designed to provide the NMEA sentence(s) at a consistent delay relative to the top of the UTC second. Very often, they are not -- the timing of the NMEA sentences wanders by 100 msec or even more. To get better-than-WAN-NTP performance out of such a GPS, you need to be using its PPS signal. Essentially all USB-interfaced GPSes do not wire the PPS signal through to DCD or another suitable input handshaking line on their serial-to-USB chip. Thanks to Eric Raymond's bufferbloat-related efforts, there is hope we will see PPS exposed on more USB GPSes in the future, however. Cheers, Dave Hart _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
