Radar System Engineering

656 MOVING-TARGET INDICATION [SEC.16.12
depression angles; some other method must be used, such as the noncoherent
method now to be described.
16.12. The Noncoherent Method. -We have already seen in Fig.
1619 that a target moving in the clutter produces a resultant echo that
varies in both amplitude and phase. To detect the amplitude variations,
all that is required is a receiver that does not limit—that is, one with a
linear-logarithmic response. Since no coherent reference oscillator is
used, this is called the noncoherent method of detecting moving targets.
The ground clutter itself acts as the reference signal,
Modulator Transmitter TR
I
r
Mixer
1
Stable local
oscillator
Signals
I
Mixer
Coherent Phase shift Reference
oscillator
unit
signal
(a)
Receiver
t
Reference signal j, f.+ fd ~c+~d_Phase.shifted
Mixer
Filter
from coherent
fc-f,
reference signal
oscillator
f.hDoppler
frequency
oscillator
(b)
FIG. 16.25 .—MTI on a moving system. (a) Block diagram of system showing location
of phase-shift unit. (h) Simplified block diagram of phase-shift unit
In the noncoherent method the local oscillator does not have to be
stabilized since phase changes are not to be detected. Ho~\-everj the
transmitter must have the same stability as in the coherent method
because the overlapping ground clutter beats ~vith itself if the transmitter
frequency varies from pulse to pulse. As shown in Table 16.1 the stability
requirement is not at all severe.
Another great advantage of the noncoherent method is that it works
even when the radar set is carried on a moving station. This is because
the set is sensitive only to amplitude fluctuations, whereas the motion of
the station causes mainly phase changes in the received echoes. An
amplitude effect, due to the finite beamwidth, will be discussed later.