[time-nuts] TruePosition GPSDO Holdover Issues

gandalfg8 at aol.com gandalfg8 at aol.com
Fri May 11 13:56:10 EDT 2018

I've had one of the cheap TruePosition GPSDOs running here for the past week, this is the earlier version with the Bliley oscillator, and as Mark reported have been seeing what I consider to be excessive holdover reports from Lady Heather.

At 55 degrees north the number of Sats visible can be a bit variable, and with this running on an indoor antenna probably even more so, but I've run plenty of other GPSDOs here in similar fashion and have not seen this before.

I've not been running Lady H in logging mode, just running in the background when this PC is on so all a bit hit and miss, but I have been seeing holdover reports every day and with the time building up as the day goes by. The time never exceeds 5 or 6 minutes though so if it was just being used as a reference without monitoring the status this could easily go unnoticed.

The LMU300 Location Measurement Unit that this comes from will need to track four Sats for positioning purposes, and the manual for that does state that the LMU300 will indicate a holdover alarm when "The LMU cannot lock on to the minimum number (4) of GPS satellites and the LMU GPS receiver board has been in holdover for over 15 minutes", and I had started to wonder if the firmware in the GPSDO might also be treating less than four Sats tracked as a holdover situation.

Earlier this afternoon I happened to catch it with between three or four Sats being tracked and it did seem to be dropping in and out of holdover as the number varied, once the number of tracked Sats increased again and stayed there, it's been six for some time now, it settled down again and hasn't returned to holdover since.

Hardly the stuff of scientific observation I know, and I need to start logging what's happening, but it does seem more likely now that these units could be reporting a holdover event when the number of tracked Sats drops below four.

Nigel, GM8PZR

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