Re: My Computer Drifts!


David J Taylor
 

On 04/06/2022 08:31, Dirk wrote:
David, talking about GPS:
With NTP alone you can reach a time deviation in the lower millisecond range.
Using only the GPS time information you CAN reach a time deviation also in the
low double-digit millisecond range, if GPS hardware and software can be told to
decode only ONE NMEA sentence.
With GPS alone or with NTP plus GPS you CAN reach a time deviation in the
microsecond range, using certain GPS hardware.
If you e.g. read the Meinberg documentation, the use of the GPS *PPS signal* is
described in detail. With the PPS signal the beginning of each second can be
detected.
Dirk,

I think may Linux users would be disappointed to get low milliseconds from NTP
- and Windows makes it much more difficult (although better with Windows 8 and
later). The problem with GPS alone is that while you may get a low deviation,
that deviation is referenced to other than UTC - certainly having a single NMEA
sentence and high baud rate can help. I've seen differences in excess of a second!

GPS alone is not good enough for microsecond deviation, you must have PPS as
well. A GPS over USB, for example, does not typically have a PPS signal, and
even if it does it subject to the USB polling rate (125 microseconds).

Getting good results can depend on pot luck, to a degree, as different network
cards and drivers influence the results. Just for interest:

Windows stratum-1 PPS-synced PCs:
https://www.satsignal.eu/mrtg/performance_ntp.php#windows-stratum-1

Windows (& Linux) synced over the LAN from stratum-1 servers
https://www.satsignal.eu/mrtg/performance_ntp.php#windows

Cheers,
David
--
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