[time-nuts] Re: Garmin GPS 25 Module

[Andy Talbot]

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
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