Strange RTCM GPS behavior

Hello all,
Last night I noticed some very strange GPS behavior for the first time in the 5+ years of my RTCM voter system’s operation.

1.Two of the sites’ RTCMs kept losing GPS lock, with the “GPS Data time period elapsed” message at the console. Upon looking at some GPS debug data, I noticed that the GPS would promptly reestablish connection with 9-10+ satellites after this message. Nothing has changed at either of these sites RF wise. It is also interesting to note that these sites are less than a 1/4 mile apart from each other.

Note that I am using the Garmin 18X LVC pucks at 4800 baud, where the ASL wiki says I should use them at 9600 if I get this error message. What’s interesting though, is they’ve been 4800 for years and I’ve never noticed this issue before until last night. Looking at the debug console again today, I am not seeing any of “GPS Data time period elapsed” at either site at all. It’s peculiar that two of my three identical sites would exhibit this problem at the same time out of the blue, so I thought I was worth sharing and seeing if anyone had any ideas.

  1. Looking at my third site’s RTCM GPS debug, I was seeing no “GPS Data time period elapsed” whatsoever, but I did notice something very bizarre.

GPS-DEBUG: mon: 2, gps_time: 953153551, ctime: Wed Mar 15 20:52:31 2000

Restarting the RTCM, turning off and on the GPS would not fix it. Do we suspect a bad puck? This site is identical to the other two, same Garmin puck in same configuration.

Any ideas on either issue would be appreciated. I do plan to change all three pucks to 9600 baud, but again after 5+ years I’ve never had the “GPS Data time period elapsed” error until last night.


Looks like I may have found an answer to the second issue I was seeing with incorrect date on my third RTCM, and it even might even have something to do with the first problem I mentioned. It looks like the Garmin’s firmware wasn’t coded to rollover during the “GPS Rollover” event on April 26, 2019.

I noted that the Hours/Minutes/Seconds was being read exactly correctly at the same on all three RTCMs’ consoles, but the week and year was obviously off on the third RTCM (953192179 vs the 1572507379 on the other two working properly). Looks like my third one’s firmware couldn’t handle the GPS rollover it but the other two survived).

Per Garmin’s website…

This explains the “GPS rollover”:

This explains that the model 18X receiver needs to be updated with new firmware:

I’m sure there are plenty of people out there who are using the 18x that would like to know about this. I’d be happy to contribute a blurb to the wiki.

Any thoughts on this?

Thanks for posting this.

Yes, I experienced the same problem about a month ago. My RTCM disconnected based on GPS issues and I noticed that while it had many satellites in view and that the time was correct, that the date was not. I as write this, I am remotely logged into my RTCM and the GPS18 is reporting a date of 03/19/2000.

It looks like a swap GPS receivers is in order, or an update to the GPS18 software.

Thanks again,

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