Radar System Engineering

SEC. 16.13] BEATING DUE TO FINITE. PULSE PACKET 657
The simplest way of employing the noncoherent method is to use an
A-scope presentation and watch for echoes that are fuzzy on top. This
type of echo indicates the presence of a moving target in the clutter
at the range shown on the scope. The scheme is useful only when very
slow scanning can be tolerated. An alternative indication can be
obtained by gating off all but a short portion of the A-sweep and putting
the output through a pair of headphones. A moving target within the
“gate” then produces a musical note in the phones.
A more useful arrangement is to put the video output through the
usual cancellation circuits and display the moving targets on a PPI.
With such equipment in an airplane, roads will show up on the scope as
intersecting lines of dots due to the moving vehicles.
The drawback to the noncoherent method is that a moving target can
be detected only when there is ground clutter at the same range and
azimuth as the target. Thus although the method works well out to the
range where the clutter begins to get patchy, beyond this range moving
targets become lost in the clear places. A plan for overcoming this
difficulty is to have noncoherent operation for short ranges and coherent
operation for long ranges. This can be accomplished by gating off the
coherent reference oscillations for the required number of miles at the
start of each sweep. Since the station is moving, it is necessary, during
the coherent part of the sweep, to compensate for this motion. The
method of doing this has been described in the preceding section.
16.13. Beating Due to Finite Pulse Packet.—In addition to the
fluctuations due to scanning and wind, there is another kind of fluctuation
when the system is moving. This arises from the fact that the clutter
elements illuminated by the beam at a given instant do not, because of the
spread of radial velocities within the finite illuminated area, all have
the same doppler frequency. Signals from clutter will therefore beat
with each other at frequencies up to ~1 = 2 Au/A, where AU is the spread
in radial velocity.
Let v be the velocity of the system and 0 the azimuth angle. Then the
radial velocity is o cos O, and thus we have
AU = –v9 sin 0 for O>> 8,
where e is the beamwidt h. If the beating is represented by a factor cos
(2m~,t),then the fractional change in amplitude from pulse to pulse is of
the order 2Kjl/.f,, which can be written as % Av/(V,) or % AV/VI, Where V1
is the first blind speed as defined in Sec. 16.9. For example, suppose
v = 300 mph,
VI = 800 mph,
EI = 2° = & radian, e = 90°,
Then AU = 10 mph and the fluctuation
is 8 per cent.