Chapter 6 ______________________________________________________ Processing Algorithms
VAISALA______________________________________________________________________ 191
- It is misplaced in time, that is, the Tx pulse is outside of the window
displayed in the Pb plotting command. In this case, the trigger timing
needs to be changed in order to bring the center of the pulse back to
the center of the window.
- It is mistuned in frequency, that is, the AFC feedback is incorrect and
has caused the burst frequency to fall outside of the passband of the
RVP900 anti-alias filters. In this case the AFC (or DAFC) needs to be
changed so that proper tuning is restored.
The Hunt Mode performs a 2D search in time and frequency to locate the
burst; searching across a ±20 µsec time window, and across the entire AFC
span. If a valid Tx pulse (that is, meeting the minimum power requirement)
can be found anywhere within those intervals then the Burst Pulse Tracker
and AFC loops will be initialized with the time and frequency values that
were discovered. The fine servos then commence running with a good
burst signal starting from those initial points.
Depending on how the hunting process has been configured in the Mb
menu, the whole procedure may take several seconds to complete. The
RVP900 host computer interface remains completely functional during
this time, but any acquired data would certainly be questionable. GPARM
status bits in word #55 indicate when the hunt procedure is running, and
whether it has completed successfully. The BPHUNT (Section 7.26 Hunt
for Burst Pulse (BPHUNT) on page 326) opcode allows the host computer
to initiate Hunt Mode when it knows or can sense that a burst pulse should
be present
6.1.5 Interference Filter
The interference filter is an optional processing step that can be applied to
the raw (I,Q) samples that emerge from the FIR filter chips. The intention
of the filter is to remove strong but sporadic interfering signals that are
occasionally received from nearby man-made sources. The technique
relies on the statistics of such interference being noticeably different from
that of weather.
For each range bin at which (I,Q) data are available, the interference filter
algorithm uses the received power (in deciBels) from the three most recent
pulses:
To Next Page
Loading...
Need help?
General