[time-nuts] Four hour cycle in GPS NMEA jitter

Mark Sims holrum at hotmail.com
Mon Mar 20 21:54:10 EDT 2017

I did a lot of work in Lady Heather to add timing message arrival time analysis capability.  Heather has a "set the system clock to receiver time" function that is intended to be used on systems without access to something like NTP.  By knowing the arrival time of the last byte of the timing message, Heather can set the system clock more accurately.  You can specify the message arrival time offset of the message time code or Heather uses a pre-computed typical value for the receiver.

Heather has a mode that can analyze and calculate the message timing offset for any receiver / cpu combination.  During the analysis it plots the message arrival time offset in milliseconds,  the jitter in the message arrival time,  and ADEV/HDEV/MDEV and TDEV of those values.  When you exit message analysis mode it analyzes a histogram of the arrival time data to calculate an "optimum" arrival time offset to use for the system + receiver.

Most timing receivers that can operate in binary mode have rather consistent arrival times.  Some NMEA receivers can be quite good and others are rather problematic.  Some receivers have rather consistent time for long periods and then jump to a new "stable" value.  Also,  doing other things on the system during the analysis can cause spikes in the data.

Attached is a table of the arrival time offsets and standard deviations of several receivers.

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