[time-nuts] "Deaf" receivers ?

[shouldbe q931]

On Wed, Oct 23, 2019 at 9:20 AM Martin Burnicki
<martin.burnicki at burnicki.net> wrote:
>
> shouldbe q931 wrote:
> > I recently acquired a pair of Meinberg M600's (with GPS-CON downconverters).
>
> Just a summary of what it's for:
>
> Most Meinberg GPS receivers expect an input signal that has been
> down-converted from the original L1 frequency.
>
> The appropriate Meinberg antennae have a built-in down-converter which
> is DC-powered from the receiver, via the antenna cable.
>
> The GPS-CON is an external down-converter that expects an input signal
> from a standard L1 antenna, and outputs a down-converted signal that is
> appropriate for the receiver.
>
> The down-converter itself is powered by the receiver, just like the
> original antenna. The DC power that can be applied to the GPS-CON is
> only forwarded to the 3rd party antenna, whether a voltage is to be
> applied depends on the antenna.
>
> > From the usual auction site got a dual band antenna (for
> > futureproofing), a 2nd hand mini-circuits ZN4PD1-63W-S+, several SMA
> > BIAS-TEE to prevent DC issues, and an assortment of coax cables to
> > connect everything up.
> >
> > To power the antenna I used a ublox M8Q series without a BIAS-TEE
> > connected to the splitter, and then tested with another ublox (a NEO
> > 7M) via a BIAS-TEE that it could also see satellites and that both
> > could get a 3D fix.
> >
> > When I connected up each of the M600's, they eventually acquired
> > positions and acheived sync, but while the ublox receivers rarely do
> > not show a 3d fix, both of the M600's will show the a similar number
> > of satellites in view as the ublox, and sometimes more than one "Good"
> > satellite, but the "Selected Satellite Set:" never seems to change
> > from "00 00 00 00".
>
> When the receiver is powered up and has a good last known position and
> almanac, it expects to be able to track the satellites that should be in
> view at the given location. If those satellites can't be tracked then
> they are kicked out, and the number of good satellites goes down to 0.
>
> Eventually with a few minutes delay, the receiver should enter "Warm
> Boot" mode when this happens. In warm boot mode it tries to track any of
> the 32 possible GPS satellites, and whenever a satellite has been found,
> the number of "good" satellites is increased.

So if I understand you correctly, the number of "Good" satellits, is
the number of tracked satellites ?

>
> If at least 4 satellites can be received, the receiver position is
> determined / verified, and the receiver changes to "Normal Operation" again.
>
> See also the description here:
> https://kb.meinbergglobal.com/kb/meinberg_gps_receivers
>
> > I presume that as its a timing device, that once it has surveyed its
> > position, it only needs one satellite to solve for time, and the MRS
> > status shows "Signal available, Is master, Is locked, Is accurate"
> > with an offset that is usually within -50 to -120ns but I'm lost as to
> > what the "Selected Satellite Set:" should be displaying.
>
> If more tha 4 satellites can be tracked, it should display the 4 main
> satellites that yield the highest accuracy, depending on the constellation.

So if I understand you correctly, the "Selected" part is only filled
in if there are more than four tracked satellites ?

>
> > At the moment, one of the Meinbergs shows 10 in view and one good, and
> > one of the ublox shows 12 in view and 8 used.
>
> This sounds like the input signal is not quite correct, or the device is
> in "Warm Boot" mode where it has just detected a single satellite, but
> is still searching.
>
> A single satellite is enough to keep the time if the stored position is
> known to be the correct one, so the GPS signal propagation can be
> accurately compensated.
>
> If the real position has changed, e.g. because the device has been
> shipped from the previous owner to you, the compensation of the signal
> propagation delay would be wrong if it is based on the previously saved
> receiver position.
>
> This is why the GPS receiver only claims to be fully synchronized if it
> has at temporarily tracked at least 4 satellites, and has verified or
> determined its current geographic position.

Both devices showed their previous location when initially powered on,
I initialised both with cold boot, and both eventually acquired
position, so I would presume that they both managed to acquire 4
satellites during this process.

>
> > I tried with one of the Meinberg's (via the GPS-CON) connected
> > directly to the antenna with no improvement, and tried with a patch
> > antenna directly connected, again with no improvement, and different
> > ports on the splitter etc.
>
> That won't work if the assumption is correct that the built-in GPS
> receivers are models that expect a down-converted signal.

Apologies, that was badly written by me, I tested both a patch antenna
and the dual band antenna direct to a GPS-CON to a M600, and via the
splitter to the GPS-CON to the M600, and each setup with each GPS-CON
and each M600, there was no obvious difference between any of them

>
> > The antenna location is by no means ideal (west facing window in
> > fairly central London), but none of the ublox receivers (just basic
> > Max8 and Neo7m, no timing ones yet)  have had any trouble getting and
> > keeping a 3D fix.>
> > From my "naive" viewpoint, it looks as if the Meinbergs are a little
> > "deaf", but I'm guessing that I'm missing something.
>
> I have to confirm that some current 3rd party receiver models are more
> sensitive than the Meinberg ones, so they may be able to track signal
> that are just reflected by walls of the surrounding signals.
>
This is what I clumsily meant by "deaf"


> Anyway, the best approach for a reliable, continuous operation is to
> install the antenna in a location with sufficient view to the sky.
>
The fun of being in a ground floor flat, getting the dual band antenna
mounted to the chimney stack is planned

> > Any suggestions gratefully received.
>
> You could also contact techsupport at meinberg.de for support. Those guys
> will even help you, if possible, even if you have acquired your devices
> from someone else, and even if the devices are old.
>
> It would be good to provide for each device at least the serial number,
> or even better the diagnostics file that can be generated via the web
> interface, so the guys know enough details to be able help you.
>
I'll wait untill I either can take them somewhere with a good skyview,
or I can get the dual band antenna mounted, as I think it might just
be the poor antenna position combined with the sensitivity.

> Depending on the device details, there even may be firmware updates
> available that you can install on your devices.

Both devices were initially running firmware that was at least two
releases old, both have already been updated to current (-:

>
> Martin (working at Meinberg)
>

Many thanks for your very informative reply!


Cheers

Arne