[ntp:questions] First attempt GPSD/PPS ->NTP time server

Dennis Hilberg, Jr. timekeeper at dennishilberg.com.invalid
Fri Jan 25 16:47:46 UTC 2008

David J Taylor wrote:
> Dennis Hilberg, Jr. wrote:
> []
>> The GPS time is not very accurate anyway, and can vary wildly,
>> probably depending on the device, so don't expect perfect offsets. On my 
>> Garmin GPS 18 LVC, I use 0.190 which gets it in the ballpark,
>> but can randomly jump +16ms to -10ms at any time.
> With a good view of the sky, the GPS time should be within a microsecond. 
> The fact that you have a 0.2s offset suggests you are synching to the 
> trailing edge of the PPS signal and not the leading edge.  Is that 
> possible?  Have you checked the polarity, and the flag2 setting?
> Cheers,
> David 

I was actually referring to the time emitted in the NMEA data (GPS time), 
not the PPS signal.  Sorry, I should have specified that.  The PPS is 
accurate to a microsecond, but not the GPS time.  I happen to use a fudge 
factor of 0.190 to get the GPS time close to zero, but the PPS is of course 
very accurate and doesn't require any fudging.

He's not using the GPS_NMEA (type 20) driver, so he has to deal with the PPS 
and GPS time seperately, hence SHM(0) and SHM(1).  The type 20 driver uses 
both the PPS (if the OS supports it) and GPS time, but the offset you see in 
'ntpq -p' is the PPS offset.

Dennis Hilberg, Jr.     \  timekeeper(at)dennishilberg(dot)com
NTP Server Information:  \  http://saturn.dennishilberg.com/ntp.php

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