[time-nuts] GPS message jitter (preliminary results)

Mark Sims holrum at hotmail.com
Mon Jul 18 14:30:08 EDT 2016

I added the ability for Lady Heather to measure a plot the difference of the arrival times of each timing message (actually the time when Lady Heather receives the last byte of the timing message from the operating system).   The end-of-message arrival time is time stamped to nominally nanosecond resolution using either the Windows QueryPerformanceCounter() value or the Linux nanosecond res clock function... (actual resolutions are system depenent).  I measured several different receivers in both NMEA mode and their native binary mode.

A few generalizations:   most receivers get the time messages out within a window less than 50 milliseconds wide with a standard deviation of less than 10 milliseconds.   Timing receivers tend to do a better job of it.  The Trimble Thunderbolt and Resolution-T receivers are particularly good (5 msec window, 1 msec standard deviation).  The Z3801A is also rather good (10 msec window, 1.2 msec standard deviation).

There is usually not much difference the performance of the NMEA messages and the native binary messages.   The Navspark Venus based receivers have the greatest differences..  their NMEA messages are about twice as consistent than the binary messages.  Occasionally you see a minor hicccup in the message time when the operating system goes off and does something else (particularly like when waking up from a screen saver mode).

The number of messages the receiver sends during each timing interval does not seem to make much difference.  I have a Ublox 8M tracking 20+ satellites and sending over a dozen NMEA messages each second.  It gets the timing message out within a 30 msec window, 6.5 msec standard deviation.

Surprisingly, the communications channel type is not that important.   I've seen little difference between hardware serial ports and a USB / RS-232 adapter.   Even running a TCP/IP link between Dallas and Seattle gave surprisingly consistent messages (with a couple of spike to +/- 100 millisecond or so differences.   All of the tested receivers were running at 9600 baud,  the Navspark at 115,200,  the Z3801A at 19,200.

Attached is a plot of the Trimble Thunderbolt running on a 3 GHz, single core Windows XP box.

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