[time-nuts] TruePosition GPSDO Holdover Issues

Bob kb8tq kb8tq at n1k.org
Tue May 15 14:32:31 EDT 2018


Hi

Most GPS antennas will do fine as close to the horizon as you would ever want to get. Tilting will 
simply make multi path worse. 

One thing to consider - if you get far enough north, the “hole” closes and you can get sat tracks from the
other side of the north pole. Yes they are a *long* ways away. It’s no worse than a lot of tracks that GPS
thinks are perfectly fine to use. For timing they aren’t going to do much good. They will improve a navigation
fix.

Bob

> On May 15, 2018, at 2:24 PM, gandalfg8--- via time-nuts <time-nuts at febo.com> wrote:
> 
> 
> Yes, and we're quite a bit north of Hadrian's wall:-)
> 
> I've always been aware of the hole, it's just that I've always pointed GPS antennas straight up without stopping before to consider whether or not that's necessarilly the best option, but now it occurs to me that it might not be for this location.
> 
> Whatever the optimum might be there's obviously going to be a limit but I do think it might worth pursuing.
> 
> Web sites such as "In The Sky.org" can generate useful plots for any specified location and time but I don't know if there's anything that will allow building a projected map over a period, and I don't suppose there's going to be anything anyway that allows experimatation with antenna angle etc, so that really leaves using lady H to generate the plots in real time, which she does do really well but for something like this might be akin to watching grass grow:-)
> 
> Nigel, GM8PZR
> 
> 
> 
> 
> 
> 
> 
> 
> 
> -----Original Message-----
> From: Bob kb8tq <kb8tq at n1k.org>
> To: gandalfg8 <gandalfg8 at aol.com>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Sent: Tue, 15 May 2018 17:44
> Subject: Re: [time-nuts] TruePosition GPSDO Holdover Issues
> 
> Hi
> 
> If you are in the northern hemisphere and looking at GPS satellite tracks, there will always be a “hole”
> in the track plots to the north. The orbits do not cross either of the poles. More or less they make it about
> to Hadrian’s Wall and that’s it.
> 
> Bob
> 
>> On May 15, 2018, at 12:38 PM, gandalfg8--- via time-nuts <time-nuts at febo.com> wrote:
>> 
>> 
>> 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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>> 
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