Dear Said,
[email protected] wrote:
Hi Guys,
changing the AMU thresholds to 2.0 as suggested by Warren fixed the
problem, the unit is now working properly, thanks much for the hint. And no, this
was not obvious to me.
It is always great to get constructive help rather than the "you should
know better" type of comments.
I certainly understand that one.
In the end, it looks like this turned out to be a configuration issue
rather than a receiver sensitivity issue. But this also begs the question: how
much performance degradation will I get when using 2.0 as the AMU threshold
rather than the factory default. There must have been a reason why Trimble
set the default so high, they mention something about multipath rejection
in the manual.
I kindly disagree... sorry for the long rambling motivation.
While I have not been able to conclude exactly what the AMU measure
really is (Amplitude Measurement Unit), but it is similar at least to
C/N measures. When the receiver has a bad C/N measure from a certain
source, it may be wise not to rely on it. The signal level from each sat
can be configured to be delivered in either AMU or in dB-Hz where the
later allows comparision among other GPSes. The best description of AMU
I have is from the Lassen iQ manual where it states:
Note – The signal level provided in this packet is a linear measurement
of the signal strength after correlation or de-spreading. Units, either
AMU or dBHz, are controlled by Packet 0x35.
Thus, you can request the 20*log10(AMU)+Const that converts it into
dB/sqrt(Hz) measure.
The Lassen iQ AMU limit is default at 2 and for extended sensitivity
1.2. The Thunderbolt AMU values may not need to have the same
relationship to dB/sqrt(Hz) as the Lassen iQ, but there may be more to
it. The Lassen iQ is tailored towards navigation gear (hence the garage
startup) where as the Thunderbolt is aimed at timing.
Regardless, AMU or C/N measures is an indication of correlation strength
and thus a measure of signal quality. It should rightfully contribute to
the TDOP measure. The AMU limit is a pre-T-RAIM cleanup protection value.
If you needed to lower your AMU limit from 4 to 2, you are feeding the
Thunderbolt a signal level below the intended levels. A 6 dB raise and
you would not have needed to shift the limit for the same tracking
result. Regardless of where the limit is, the lower AMU values will
indicate a higher noise level and thus higher uncertainty in time than
otherwise obtained.
So, the 10 dB loss in the 1-to-4 splitter is significant here. It's just
unfortunate that the Thunderbolt is not clearer about it.
Why do I want to get this unit up and running: this unit has been tested
to have very good phase noise performance. I'll use anyone's low noise
source anyday.
Which is also why I like to see mine operational, alongside my Z3801,
GPS-4 and others.
Cheers,
Magnus
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