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

root root at physics.ubc.ca
Fri Jan 25 09:36:14 UTC 2008


"Dennis Hilberg, Jr." <timekeeper at dennishilberg.com.invalid> writes:

>Jason wrote:
>> All,

>I could be wrong here, but it seems to me that your ntpd is having a hard 
>time finding out the actual time, as your only source of time is nearly 2 
>seconds off and varies wildly (GPS time does that, although not usually by 

No GPS NMEA should not do that. The length of the sentence is almost fixed 
length, so the timing on it should vary by perhaps a few msec, as you found, 
certainly not by seconds. It sounds like you have troubles. 

You could try using minicom ( assuming you are on Linux) to look at the output
on the serial port.

>that much), and you have nothing else to go by.  The PPS is probably not any 
>good unless it's within a second as ntpd would have no idea which second the 
>PPS is ticking, which might be why you mention the PPS's reach value will 
>increase when the GPS time offset gets under one second.

The GPS sentence should certainly be able to pull you into the second range
very easily.
Anyway use some serial port terminal program to read what the gps is sending and how often. 
(Remember at 4800Bd, it takes about 2 msec to send each character.


>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.




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