Discussion:
TruePosition GPSDO Holdover Issues
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Artek Manuals
2018-05-13 14:57:44 UTC
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Nigel
As I recall there is more to it than just the number of satellites.
involved. Also factored in to all the math is the signal strength of the
sats plus there relative angles to your spot on the earth. If all three
are directly overhead the solution is not that accurate compared to if
they were all at 45 degree elevation at widely divergent compass points
gives a much more accurate solution . Likely the math / firmware has a
quality factor built in depending on all the above factors not just the
number of sats

My memory on this may be faulty  (the older I get the more bytes that
seem to get dropped) And if it is faulty I am sure others will be more
than happy to correct me

-DC
Ok, I give up....
I've been logging this with Lady H and was watching again this afternoon as the sat count dropped but this time there was no dropping into holdover as the number of sats dropped from four to three....damn, it just carried on doing its thing until the count went up again:-)
I'm still seeing the occasional reported random holdover event but am still no nearer to knowing why.
Otherwise it's a nice unit and does handle the holdover well, even a longer event yesterday whilst there was a supposed antenna fault didn't reflect into the frequency plots, but time to call a halt for now.
Nigel, GM8PZR
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gandalfg8--- via time-nuts
2018-05-12 11:08:38 UTC
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Today using Lady Heather I have observed the TruePostion GPSDO dropping into holdover as the number of tracked sats dropped from four to three.
There does seem to be some hysteresis in the system though, the number of tracked sats eventually dropped to two and then the unit came out of holdover as the number increased from two to three.

I can't say that this explains all the holdover events I've seen but it does seem to explain at least some of them.

Nigel, GM8PZR
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gandalfg8--- via time-nuts
2018-05-13 17:13:33 UTC
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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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Bob kb8tq
2018-05-13 18:05:50 UTC
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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
Post by gandalfg8--- via time-nuts
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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Bob kb8tq
2018-05-13 15:08:08 UTC
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Hi

Assuming the device has the “normal stuff” in it, it will look at the location in memory and
compare that to the current solution. If the solution is off by more than the limit, it will
reject it. If it rejects enough data ( = no good solution) it goes into holdover. Some devices
will operate down to a single satellite against a memorized position. That is not true of all
gizmos. Even with the “single sat” devices, there is a multi sat / bad location solution situation
that will send you into holdover.

Bottom line is that it usually is antenna location. Clear view of the sky and an appropriate
elevation mask are what you are after.

Bob
Ok, I give up....
I've been logging this with Lady H and was watching again this afternoon as the sat count dropped but this time there was no dropping into holdover as the number of sats dropped from four to three....damn, it just carried on doing its thing until the count went up again:-)
I'm still seeing the occasional reported random holdover event but am still no nearer to knowing why.
Otherwise it's a nice unit and does handle the holdover well, even a longer event yesterday whilst there was a supposed antenna fault didn't reflect into the frequency plots, but time to call a halt for now.
Nigel, GM8PZR
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Gabs Ricalde
2018-05-15 07:40:37 UTC
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On Sun, May 13, 2018 at 10:57 PM, Artek Manuals
Post by Artek Manuals
Nigel
As I recall there is more to it than just the number of satellites.
involved. Also factored in to all the math is the signal strength of the
sats plus there relative angles to your spot on the earth. If all three are
directly overhead the solution is not that accurate compared to if they were
all at 45 degree elevation at widely divergent compass points gives a much
more accurate solution . Likely the math / firmware has a quality factor
built in depending on all the above factors not just the number of sats
Hi,

For a position/time solution, the dilution of precision (DOP) depends
on the number of satellites and the satellite geometry. When a timing
receiver is in 1D (position hold) mode, the TDOP is just 1/sqrt(# of
sats).
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Mark Goldberg
2018-05-11 20:45:06 UTC
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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.

Regards,

Mark
W7MLG
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
variable.
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.
Regards,
Nigel, GM8PZR
-----Original Message-----
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.
Regards,
Mark
W7MLG
On Fri, May 11, 2018 at 10:56 AM, gandalfg8--- via time-nuts <
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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gandalfg8--- via time-nuts
2018-05-11 17:56:10 UTC
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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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gandalfg8--- via time-nuts
2018-05-15 16:38:26 UTC
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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 <***@n1k.org>
Discussion of precise time and frequency measurement <time-***@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
Post by gandalfg8--- via time-nuts
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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Bob kb8tq
2018-05-15 16:44:01 UTC
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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
Post by gandalfg8--- via time-nuts
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
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
Post by gandalfg8--- via time-nuts
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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gandalfg8--- via time-nuts
2018-05-15 18:24:47 UTC
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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 <***@n1k.org>
To: gandalfg8 <***@aol.com>; Discussion of precise time and frequency measurement <time-***@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
Post by gandalfg8--- via time-nuts
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
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
Post by gandalfg8--- via time-nuts
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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Bob kb8tq
2018-05-15 18:32:31 UTC
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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
Post by gandalfg8--- via time-nuts
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-----
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
Post by gandalfg8--- via time-nuts
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
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
Post by gandalfg8--- via time-nuts
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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Gary E. Miller
2018-05-15 21:22:03 UTC
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Hal!

On Tue, 15 May 2018 12:52:58 -0700
Neat. Thanks. That raises several questions.
How high do satellites get if you are at a pole?
44.6 degrees.

RGDS
GARY
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