Radar System Engineering

642 MOVING-TARGET INDICATION [SEC. 16.6
oscillator is off-tune. Taking A, = 0.04 psec (for r = 1 ~sec) and allowing
a phase change of & cycle, we get Aj = ~ Me/see.
Summary of Requirements. -Table 16.1 summarizes the stability
requirements. It should be mentioned that these figures represent almost
ideal conditions, in which the clutter to be canceled shows no fluctuations
of its own. If for any reason (such as rapid scanning rate) the clutter
should fluctuate by 10 or 15 per cent from pulse to pulse, there would be
no sense in canceling to 3 or 4 per cent. In such a case, some of the requirements
in the table could be relaxed
considerably.
TAZLE 16 1.—STABILITY REQUIREMENTSFOR HIGH-PERFORMANCE SYSTEM WITII
PRF OF 1000 PPS, PULSE LENGTFI 1 ASEC, GROUND CLUITER OUT TO 50 }’lILES
Component
Quantity
Xfaximum allowable value
Stable local oscillator, Frequency drift
Coherent oscillator. Frequency drift
Magnetron . . . . . . . . Frequency drift
Magnetron . . . . . . . . . . . . . . . . Frequency change within
pulse
Modulator and magnetron. Pulselength variation from
pulse to pulse
Coherent oscillator. Detuning from intermediate
frequency
Modulator . . . . . . . . . . . . . . . . . Stability relative to trigger
Delay lines. . . . . . . . . . . . . . . . Relative drift in delay time
20 kc/secl
20 kcjsecx
10 Jfc/sec2
~ L1c/secZ (and probably
much more)
4 y.
~ hfc/sec
& psec
~lrp.sec/hr
16.6. Internal Clutter Fluctuations.-In this section we shall consider
echo fluctuations due to internal motions of the clutter—for example, the
motions of trees in the case of ground
(d (b)
FIG. 16.15.—C omposit e nature of
ground clutter. (a) Illuminated area of
clutter. (b) Contributions to signal: fixed
clutter = R,; moving clutter = Rz; resultant
signal = R; resultant for next pulse =
R’; variation between pulses = r.
clutter. The ground-clutter pattern
may include strong echoes from single
targets, especially in regions
where there are large structures having
simple geometric al shapes.
Echoes from water towers and building
faces may easily equal in intensity
the composite echo obtained
from a mountainside. H o w e v e r,
echoes from structures are generally
found to be fairly steady since the
targets
do not sway much in the wind
and do not present a serious cancellation problem when the antenna is
stationary.
Most ground clutter is composite in the sense that the echo amplitude
at a given instant is the vector sum of many small echoes from the individual
targets scattered over a land area determined by the beamwidth