2020-07-03 The GPS ticks! 1 month ago
I remembered the junkbox contains an active GPS antenna which I bought together with the gpskit gps unit in 2003(!). And some other bits and pieces included a SMA to BNC adapter so I put the little GPS antenna outside and connected it to the GPS/RTC Hat. Before I was back behind a computer it was showing a location and within a few minutes it had a PPS pulse. I was used to cold start taking at least 15 minutes with the gpskit! So I tested with ntpd talking to gpsd via shared memory. This gave an interesting offset between local gps time and a nearby ntp server.$ ntpq -p remote refid st t when poll reach delay offset jitter ============================================================================== *SHM(2) .PPS. 0 l 6 64 377 0.000 -0.149 1.672 xSHM(0) .GPS. 0 l 5 64 377 0.000 -104.51 1.943 +ntpritchie.idef 22.214.171.124 3 u 101 64 376 2.774 0.950 13.948 +metronoom.dmz.c 126.96.36.199 2 u 99 64 376 10.482 -0.844 10.638 $ ntpdc -c kern pll offset: -0.000136461 s pll frequency: -11.054 ppm maximum error: 1.3748e-05 s estimated error: 1.7071e-05 s status: 2001 pll nano pll time constant: 6 precision: 1e-09 s frequency tolerance: 500 ppmI'm not too happy about the fact that the GPS NMEA messages are seen as wrong, so I'm going to stop using gpsd and go for a setup optimized for timing.