Radar System Engineering

654 MOVING-TARGET INDICATION [SEC. 1610
.
f, = 500 and the wind velocity is 25 mph, as is shown in Table 16.2.
Thus, kj, should be about 1600 for this particular wind speed. In general,
it is easy to obtain the required figure for Aj,, but not for n, witho’ut
sacrifice elsewhere. Scanning fluctuations are the principal difficulty in
MTI design.
The quantity n is proportional to beamwidth and PRF, and inversely
proportional to scanning rate. It has the value 120 for a PRF of 720 with
a 1° beam rotating at 1 rpm. Although this set of constants satisfies the
condition above, the scanning rate is too slow for many purposes. To
attain greater rate without losing subclutter visibility, either the beamwidth
or PRF must be raised; the alternative is to take a loss in subclutter
visibility, which may be the best solution
in some cases. For example, if the
targets are all larger than the clutter, a
subclutter visibility of zero db is sufficient.
In that case the scanning rate can
be raised to 30 rpm. It is, therefore, of
considerable importance to use all possible
methods of decreasing the size of the
clutter relative to the moving targets,
~
o 100
Radial speed in mph
FIG, 16.24,—Distribution of rachal
speeds for random aircraft, assuming
all ground speeds equally likely in
rarige 100 to 400 mph and all directions
equally likely.
such as shaping the beam in elevation,
using short pulses, and choosing the site
carefully.
Blind Speeds.—It has already been
seen that MTI causes an “average”
loss of about 3 db when all radial speeds
are equally probable. 1 This is true no
matter where the blind speeds happen
to fall. We are interested, however, in
distributions
targets such as airplanes and clouds which have non-uniform
of radial speeds, and would like to know the best values of the blind
speeds in these cases.
Figure 16.24 shows the distribution of radial velocities for airplanes
whose ground speed can have any value between 100 and 400 mph with
equal probability and whose direction of flight is random. It is hard to
choose the best blind speeds from this graph. There will probably be
an average loss 1 of about 3 db, as in the case of a uniform distribution,
whether there are many blind speeds in the interval O to 400 mph or
whether the first optimum speed comes at, for example, 400 mph. The
choice of blind speeds is of no particular consequence, per se; but there
may be special cases, such as airport traffic control, where the radial speed
can be regulated. Blind speeds can then be chosen accordingly.
1See footnote on p. 651.