[time-nuts] Garmin GPS 25 Module

[Andy Talbot]

I run a set of microwave beacons on a remote site that transmit data modes
whose Tx timing is controlled from GPS.   They were installed some 20 years
ago and used a Garmin GPS25-LVS module to deliver NMA and 1 PPS signals to
all five individual controllers.

After a power outage that lasted a couple of days, the beacons fired up but
it was clear, after a couple of days timing was corrupted.  Each beacon has
slightly different controller firmware, and by monitoring the resulting
corrupted modulation it could be determined what was wrong.    No PPS
signal was present - which killed modulation on two of them. NMEA timing
data was present but the time being reported was out by several seconds,
rendering the data signal transmitted undecodable by most people.

I went up to the remote site to recover the old hardware and am in the
process of replacing the timing source using a Ublox NEO-6.   Since all the
beacons were originally designed based on 4800 baud NMEA, the Ublox was set
for this for backwards compatibility with the Garmin and the configuration
saved in NV ram.

Now the query I have.   I had assumed it was a 1024 week rollover that
killed the Garmin, but on testing teh module when I returned home, after a
longer than normal initialisation period it DID start outputting the
correct date and time, with PPS present.   So the initial reboot failed,
even after a couple of days attempt, but after a power cycle it worked.

I'm puzzled by that behaviour.  Is anyone here familar with the GPS25
family, dating from the turn of the millennium?

As an aside, a Jupiter TU60 GPS module was also in use on site, delivering
a GPS locked 10kHz signal.   This I used to lock a 10MHz reference in
"Probably the Simplest GPSDO Possible"  http://g4jnt.com/SimpleGPSDO.pdf
That module, which is not a lot newer than the Garmin, did appear to have
survived the reboot as the resulting frequencies of the beacons were
spot-on when they returned with their modulation faults.  However, in the
spirit of doing the job properly, it too is being replaced with a
Leo-Bodnar mini GPSDO.
All beacon details at scrbg.org


Andy
www.g4jnt.com