[time-nuts] TruePosition GPSDO Holdover Issues

Mark Goldberg marklgoldberg at gmail.com
Fri May 11 14:59:11 EDT 2018

What antenna are you using and where is it located?  I have the
TruePosition with a Motorola PCTEL 8508851k66 antenna on a windowsill with
a ground plane under the antenna and it generally sees 4-7 sats. I have
seen some holdovers but it has been better after running for a while and
doing a survey. The TruePosition seems to switch between modes 0 and 1 as
new sats come into view. The 10 MHz out does not change that I can see when
this happens.



On Fri, May 11, 2018 at 10:56 AM, gandalfg8--- via time-nuts <
time-nuts at febo.com> wrote:

> 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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