[time-nuts] "Deaf" receivers ?

[Martin Burnicki]

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.

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.

> 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.

> 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.

> 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.

Anyway, the best approach for a reliable, continuous operation is to
install the antenna in a location with sufficient view to the sky.

> 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.

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

Martin (working at Meinberg)