[time-nuts] Furuno GT-87

[Mark Sims]

Bob Darlington shipped me a GT-8736 to test.   It is a small, Motorola M12 format GPS receiver with a claim to fame of +/- 1.7 ns sawtooth error.   I was unable to test it until recently because my M12 to Adafruit adapter board was fab'd with holes drilled too small for the 2x5 pin 0.05" M12 pin header.   I got in some headers and bodged one onto the top of the adapter board and used a cable (from Adafruit) to interconnect the two boards.

Besides the M12 binary format the GT87 can also talk Furuno ESIP format.   Lady Heather now fully supports ESIP data (basically just NMEA with some custom sentences,  similar to Furuno's PFEC protocol (which Heather also supports)... why Furuno  felt they needed Yet Another Protocol is beyond me).  ESIP mode provides better control and monitoring than the Motorola mode.   Oh, and don't believe/do anything you might read about putting the G87 into PFEC mode... it effectively bricks the unit and puts it into some unknown binary language.

The G87 supports GPS and GLONASS (with Galileo supposedly on the way) and SBAS.   I typically track over 22 satellites.    The thing is very sensitive.   I disconnected the antenna from the 3" MMCX to BNC antenna pigtail I was using and could still track some satellites indoors.

Anyway, I connected the 1PPS output to a TAPR TICC and did some measurements.   It's rather impressive.  The span of the 1PPS was around +/- 4 nsec over 24 hours.  ADEV is in the mid E-13's at 10,000 seconds.  It should make for a rather nice GPSDO.

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