[time-nuts] TruePosition GPSDO Holdover Issues
marklgoldberg at gmail.com
Fri May 11 16:45:06 EDT 2018
It definitely works better on a windowsill than under the roof. It also did
not work at all with a patch antenna.
I can compare it to another GPSDO used as a reference and I see ADEV in the
range of 2.5E-10 for low Tau 10-1000s, heading down for larger Tau. I see
phase noise of -102 dBc/Hz at 10 Hz offset, getting better from there.
On Fri, May 11, 2018 at 12:33 PM, <gandalfg8 at aol.com> wrote:
> Hi Mark,
> The antenna is a Micropulse 1373FW/D, described on the label as an L/W
> Survey GPS Antenna, whatever that might mean, Land and Water perhaps?:-)
> I did try a small Motorola magnetic patch, I regularly use these with
> Thunderbolts etc with no problems, but as Mark Sims predicted the
> Trueposition reported an antenna error with that, although I'm not sure why.
> This antenna is on top of the metal case of some test gear, just below
> ceiling height in a single story building and at the moment the unit is
> tracking six sats.
> As commented earlier, we're at 55 degrees north so there's a big empty
> space between northwest and northeast and the number of sats is a bit
> What I'm seeing though is about par for the course with this setup, this
> one is showing six sats at the moment and seven or eight, the max for this
> GPSDO, isn't unusual and this is generally much the same as I'd expect
> anyway, but this is the only one I've seen dropping in and out of holdover.
> I did run a survey when I first set it up and it's fine re position.
> I haven't got a counter on it at the moment but was monitoring it with a
> CNT91 earlier in the week and I couldn't see any change in the output
> either, but having said that if it was handling the holdover properly then
> there shouldn't be any obvious change in the short time scales we're
> talking about anyway.
> I'm happy with the setup, dropping down to just a few sats isn't unusual
> here, but again this is the only one I've seen behaving like this so I'm
> still inclined to think it's doing what it's supposed to.
> I'll try to set up some continuous logging over the weekend and see if I
> can get any more definite correlation between number of sats tracked and
> indicating holdover.
> Nigel, GM8PZR
> -----Original Message-----
> From: Mark Goldberg <marklgoldberg at gmail.com>
> To: gandalfg8 <gandalfg8 at aol.com>; Discussion of precise time and
> frequency measurement <time-nuts at febo.com>
> Sent: Fri, 11 May 2018 19:59
> Subject: Re: [time-nuts] TruePosition GPSDO Holdover Issues
> 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
> 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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