[time-nuts] Re: Garmin GPS 25 Module

[Bob kb8tq]

Hi

In terms of upgrading the GPS module(s): 

You might want to look at just *what* NMEA messages are being used. 
While the format is “standard” it’s not quite dead nuts in some cases. 
Vendors get to add this and that and it still is NMEA. Converting from 
one family to another is not always easy. 

I would also want to check the GPS antenna at the remote site. They
don’t last forever. Flakey sat signals can drive a module a bit nuts. 

GPS modules have gotten pretty cheap over the years. If this is a long
drive / crazy access sort of thing, redundancy is not as expensive on 
the module (or antenna) side as it once was. 

Bob

> On Feb 9, 2022, at 4:36 AM, Andy Talbot <andy.g4jnt at gmail.com> wrote:
> 
> 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
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