[time-nuts] Re: Garmin GPS 25 Module

[Bob kb8tq]

Hi

I would suggest that killing off anything that’s unused is a really
good idea, especially when switching to a new module. You 
eliminate a lot of weird corner case issues that you *really* don’t
want to find out about …. 

Bob

> On Feb 9, 2022, at 10:57 AM, Andy Talbot <andy.g4jnt at gmail.com> wrote:
> 
> Yes, I had to disable the GSV sentence to get the data flow down to a level
> that could be sent at the lower speed.
> I'm only using  GPRMC, so all the others could be removed as well, but as
> GPRMC comes first in the block every second, there's nothing to be gained
> from killing any of the others sentences.
> 
> I did fall foul once of a changed format.   Originally I wrote for an RMC
> string that just gave integer seconds.   On using my code with a later
> module it failed, and I spotted that it was sending decimal seconds, so had
> to rewrite my PIC code to cope with that.
> Then later, using other module types, GPRMC became GNRMC to cope with
> Glonass etc. so that was another change needed.
> 
> Andy
> www.g4jnt.com
> 
> 
> 
> On Wed, 9 Feb 2022 at 15:49, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
>> 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
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe
>> send an email to time-nuts-leave at lists.febo.com
>>> To unsubscribe, go to and follow the instructions there.
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send
>> an email to time-nuts-leave at lists.febo.com
>> To unsubscribe, go to and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
> To unsubscribe, go to and follow the instructions there.