[time-nuts] "Deaf" receivers ?

[Martin Burnicki]

shouldbe q931 wrote:
> On Wed, Oct 23, 2019 at 9:20 AM Martin Burnicki
> <martin.burnicki at burnicki.net> wrote:
>>
>> shouldbe q931 wrote:

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

Yes. More exactly, the number of satellites that can be tracked and
used. Sometimes individual satellites are set to "maintenance mode", in
which case they can be tracked, if they are visible above the horizon,
but should not be used for navigation or timing.

Such satellites are not considered "good".

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

At least 4 satellites are required to solve the navigation equation and
determine the position and time (x, y, z, and t) of the receiver from
the satellite data.

If there are 4 or more "good" satellites, the selected set represents
the 4 satellites that would yield the best accuracy, according to their
constellation relative to the receiver position.

If there are only less than than 4 "good" satellites, a selected set
doesn't really exist. I'm not quite sure if the receiver would then
display a partial selected set with less than 4 satellites, or if the
select set is completely "empty" in this case. I'd have to investigate
and check this.

[...]
> 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.

Yes. Since the GPS satellites are not stationary, it is possible that 4
or more satellites can be tracked at certain time, but only less than 4
at other time, depending on view of the sky from the antenna.

>>> 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

OK, then I misunderstood.

>>> 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"

Understood. ;-)

>> 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

OK, I know this can be a hassle, but having the antenna installed with
clear view to the sky should improve the situation in any case.

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

You can even get firmware updates (if available) if the device is out of
warranty. ;-)

>> Martin (working at Meinberg)
>>
> 
> Many thanks for your very informative reply!

You're welcome. I'm happy to help make things clearer, if I can.


Regards,

Martin