Radar System Engineering

638 MOVING TARGET INDICATION [SEC. 16.5
it k also difficult if the stable r-f oscillator is to be locked, because of the
high Q required to make the oscillator stable. Locking a coherent i-f
oscillator is easy since the Q can be smaller by a factor of 100 for the
same allowed rate of frequency change. The scheme of Fig. 16.13b,
which uses i-f locking and i-f adding, is inconvenient because it requiresa
power amplifier.
PERFORMANCE CRITERIA AND CHOICE OF SYSTEM CONSTANTS
The primary objective of moving-target indication is, of course, to
attain a high degree of discrimination in favor of the echoes from moving
targets compared tothose from fixed ones. In order that this beaccomplished
it is necessary that echoes, or clutter, from fixed targets be
eliminated or greatly reduced and that those from moving targets be
retained with optimum sensitivity compared to the residual clutter and
the inherent
noise of the receiver.
Since the recognition of moving targets is based upon changes in the
returning echo from one pulse to the next, any changes present in the
signals from the fixed echoes will interfere with their elimination. Such
undesirable variations are of two sorts: (1) those inherently present in the
returning echoes, principally due to internal motions of the targets
themselves or to the effects of scanning; (2) spurious variations due to
instabilities or other shortcomings of the radar equipment. The methods
used and the constants chosen for the set should be selected in such a
way as to minimize the effects of the inherent fluctuations. The elimination
of spurious variations is largely a matter of careful engineering design.
Another factor to be considered in the choice of system parameters is
that since different radial velocities produce different changes in relative
phase from pulse to pulse, certain velocity intervals are much more
effective than others in producing large uncanceled signals. Indeed, a
phase change of one cyclethat is, a radial motion of one-half wavelength
between pulses—is equivalent to no change at all. Thus there are certain
“blind” velocity intervals in which the mass motion of the target does
not lead to a discernible indication. Since the velocities about which
these intervals are centered are determined by the wavelength and the
pulse repetition rate of the set, it is possible, within limits, to arrange that
they fall as little as possible in the range of velocities to be expected for the
targets of interest.
The various factors dealing with clutter elimination ,and target
visibility on a fixed system will be discussed in detail from both the theoretical
and the practical standpoint in the next few sections. The additional
factors present when the system is moving will be introduced later.
16.5. Stability Requirements. Frequency of the Beating Oscillators.—
The amplitude of the video signal from a stationary target can be written