Radar System Engineering

SEC. 1621] SPECIAL TEST EQUIPMENT 679
Subclutter Visibility Measurement.—A block diagram of a subclutter
visibility meter suitable for measuring the internal performance of most
MTI systems is shown in Fig. 16.37. An i-f pulse originating either in
the locking-pulse mixer or in a separate generator is modulated, delayed
and inject ed into the receiver channel. Thk same pulse locks the
coherent oscillator. The delayed pulse is modulated in phase and in
m /
From locking puls~ meter
7
r
90” phase
shift
I
~
50# sac
delay line
I
Pulsed
Coherent
30 t4c/sec
oscillator
mclllator
J I L I
FIG. 16.37.—Subclutter visibility meter.
-P
Attenuator
7Matching
network
To receiver
amplitude by a controlled amount at a controlled doppler frequency, so
that it imitates a moving target in clutter. The minimum percentage
modulation for threshold signal visibility is a measure of subclutter
visibility. Although the simple modulation scheme shown does not
provide an exact duplication of a moving-target echo, it is believed to be
equivalent for test purposes. The meter does not measure the loss of
subclutter visibility due to fluctuations in the clutter produced by scanning
or the wind. It measures only the quantity K, in Eq. (28).