GPSd/NTPd timesync issue in JSON mode

[Jürgen Hausladen]

Hi,

I have a device that is running with a 1Hz GPS update rate, and NTPd configured in JSON mode. The device has a real PPS pulse towards it that GPSd makes use of.

However, a problem occurs when the GPS update rate is set to 10Hz. Then the time starts to drift away and ends up floating somewhere between 100ms-200ms off to the reference devices (Usually the device is off around 1-2ms and 15ms at max.).

When looking at the respective PPS & TOFF messages in NTPd, one can see a strange jump in the real_nsec field of the TOFF message, when the time starts to jump to the ~100ms offset.

{"class":"PPS","device":"/dev/pps1","real_sec":1660832190,"real_nsec":0,"clock_sec":1660832189,"clock_nsec":999151159,"precision":-20}
{"class":"TOFF","device":"/dev/ttyGNSS","real_sec":1660832190, "real_nsec":497767,"clock_sec":1660832190,"clock_nsec":76266124}
{"class":"PPS","device":"/dev/pps1","real_sec":1660832191,"real_nsec":0,"clock_sec":1660832190,"clock_nsec":999154925,"precision":-20}
{"class":"TOFF","device":"/dev/ttyGNSS","real_sec":1660832191, "real_nsec":498929,"clock_sec":1660832191,"clock_nsec":70566339}
{"class":"PPS","device":"/dev/pps1","real_sec":1660832192,"real_nsec":0,"clock_sec":1660832191,"clock_nsec":999157310,"precision":-20}
{"class":"TOFF","device":"/dev/ttyGNSS","real_sec":1660832192, "real_nsec":500092,"clock_sec":1660832192,"clock_nsec":74100536}
{"class":"PPS","device":"/dev/pps1","real_sec":1660832193,"real_nsec":0,"clock_sec":1660832192,"clock_nsec":999163719,"precision":-20}
{"class":"TOFF","device":"/dev/ttyGNSS","real_sec":1660832193, "real_nsec":99501370,"clock_sec":1660832193,"clock_nsec":179516908}
{"class":"PPS","device":"/dev/pps1","real_sec":1660832194,"real_nsec":0,"clock_sec":1660832193,"clock_nsec":999166266,"precision":-20}
{"class":"TOFF","device":"/dev/ttyGNSS","real_sec":1660832194, "real_nsec":99502533,"clock_sec":1660832194,"clock_nsec":187970582}
{"class":"PPS","device":"/dev/pps1","real_sec":1660832195,"real_nsec":0,"clock_sec":1660832194,"clock_nsec":999169546,"precision":-20}
{"class":"TOFF","device":"/dev/ttyGNSS","real_sec":1660832195, "real_nsec":99503695,"clock_sec":1660832195,"clock_nsec":188442119}

Has anyone experienced this problem so far as well?

Could this be a potential GPSd issue or might this be related to NTPd?

Running GPSd configured in shared memory mode with 10Hz seems to work fine so far but still subject to long-term testing. 

Thanks in advance,

Jürgen