[time-nuts] TruePosition GPSDO Holdover Issues

gandalfg8 at aol.com gandalfg8 at aol.com
Tue May 15 12:38:26 EDT 2018


 Thanks Bob, and others, for comments on this.

>From my observations with this running in position hold mode following a self survey, and based on previous experience in this location, my general impression is still that the TruePosition GPSDO does seem more prone to dropping into holdover than others I've used here under similar circumstances.

Having said that though, now running on a better sited antenna giving consistently higher signal strengths and with always at least 5 sats indicated, not counting PRN120:-), it hasn't dropped into holdover in the past 40 hours or so, so it is only under more marginal conditions that it would be evident.

As a bonus, the slightly tilted antenna is so far looking to be a reasonable success, with the hole to the north noticeably reduced and tending more to a closed circle and signal levels generally higher all round too, partially at least perhaps due to a change in antenna gain but either way another can of worms opened and begging further investigation:-)

Nigel, GM8PZR




From: Bob kb8tq <kb8tq at n1k.org>
Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Sun, 13 May 2018 19:05
Subject: Re: [time-nuts] TruePosition GPSDO Holdover Issues

Hi

When you are looking at timing, the SBAS / WAAS sat’s really don’t count as part of the total 
of 4 that you need for a basic fix. There also iis the subtle distinction of “tracking” vs “locked to”
on some devices. Tracking means we might get adequate data soon and locked means it is 
good enough to use on those devices.In that case, only the “locked” sats count towards the 
minimum of 4 that you must have. 

Past the minimum of 4 rule, most GPSDO’s also want to see that set of devices for some period
of time before they come out of holdover. You will drop in very quickly ( a second or two), but come
out slowly ( many minutes). Local noise can in some cases be enough to put you in holdover.

Bob

> On May 13, 2018, at 1:13 PM, gandalfg8--- via time-nuts <time-nuts at febo.com> wrote:
> 
> The location at 55N, 5W, isn't ideal, there's quite a large hole to the north but this isn't something I've seen here before with any other GPS module or GPSDO.
> 
> When first noticed I'm sure it was whilst tracking six or seven sats, it was certainly five or more, which is why I commented in the first place, it was only later I thought there might be some correlation with it tracking low numbers.
> 
> There doesn't seem to be any adjustment for elevation mask on these, at least not via Lady H, but I've switched now from a flat survey antenna to a Symmetricom pod on a stub mast, so I can cheat a bit and angle it south slightly:-)
> 
> It'll take some time to build up a picture of the effect of that but it's tracking 8 sats at the moment.
> Inmarsat-3, PRN120, seems to have joined in the mix now and keeps popping on and off the bottom of the list but I'm not sure whether or not that could contribute anything useful anyway.
> 
> Nigel, GM8PZR
> 
> 
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