Chapter 6 ______________________________________________________ Processing Algorithms
VAISALA______________________________________________________________________ 235
Intersecting this measured noise level with the green straight line gives:
From which we see that the input power at the point of intersection is,
indeed, I
o
.
I
o
is the received signal level that will produce 0dB SNR, that is, signal
power equal to noise power. This should not be confused with the
minimum detectable power P
MDS
which typically will be several dB
lower, depending on processor settings. In the above example, a 1.2dB
LOG detection threshold is shown (horizontal blue line) for the received
signal. If the RVP900 is applying sufficient range and time averaging so
that thermal noise alone produces very few false alarms above 1.2dB, then
P
MDS
will be a full 5dB lower than I
o
. We would expect a detection rate
of roughly 50% for echoes arriving at this "minimum detectable" level.
Typically a CW test signal is used to generate the test curve shown in
Figure 49 on page 232. Follow the instructions provided by the radar
manufacturer for injecting a test signal. During calibration, the radar
should be fully operational, so that all sources of noise are present. Ideally
the transmitter should be turned on during calibration.
To perform the calibration, insert signals at steps of 5 or 10 dB over the
entire range of the system. Draw the plot shown in Figure 49 on page 232.
You can utilize fine resolution steps at the ends of the scale to observer the
details of the roll off. Be sure to raise the antenna up a few degrees to avoid
ground thermal noise. Also tune the frequency of the signal generator using
the setup command pr, and displaying the received signal spectrum. Be
sure to check the tuning at the end of the calibration to make sure the signal
generator and IFDR have not drifted apart.
Each time that a new signal level is injected, the measured power values
are obtained by first invoking the SNOISE command and then reading-
back the results using the GPARM command. The Log of Measured Noise
Level (Word 6) from GPARM should be used. This procedure averages
many samples together. For IRIS users, this is all handled by the zauto
utility.
NOTE
Verify with the radar manufacturer that no damage will occur to the signal
generator if the transmitter is running during the calibration.
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