Radar System Engineering

650 MOVING-TARGET INDICATION [SEC. 16.9
This expression is zero when fd is a multiple of the PRF and, since
j, = 2v/~, the first “blind” speed is given by
f,T = 2~T = 1,
or
“=27=5’
A
Af,
(15)
where j, is the PRF. In other words, the target appears to be at rest
when it travels a distance x/2 (or a multiple of h/2) between pulses,
Numerically,
First blind speed = Xj,/89 (A in cm, speed in mph) (16)
Going back to Eq. (13) we see that the average canceled signal, after
rectification, is given by
M = : ~olsin (~.fdT)l (17)
We have to compare this amplitude with that of noise after cancellation.
The noise amplitude in the delayed channel is completely independent of
that in the undelayed channel. We must therefore add the noise powers
in the two channels, obtaining an increase of @ in noise amplitude after
cancellation. Hence the change in signal-t~noise ratio caused by the
addition of MT1 is represented by
! !;~~ ‘he fact~~lsin.Tl 1~
Radialveloc,ty of target
FIG. 16.21.—Responsecurve for target
in the clear. The first blind speed has the The numerical factor is about 0.9.
value kj, /89 (X in cm, speed in mph).
Thus MT1 causes scarcely any 10s8
for a target moving at the optimum speed but does cause a loss at other
speeds. Figure 16.21 shows the voltage response for MT1 relative to the
res~onse for a normal radar set.
‘It will be observed that the response at small speeds is proportional
to the speed v. It can be written as follows:
Response
Llax. response
z ~jdT = = Z,
VI
where VI is the first blind speed. As an example, consider a set with
A = 9.2 cm and~, = 2000 pps. Then from Eq. (16) we have v, = 206 mph.
For a storm cloud traveling radially at 30 mph, the ratio in Eq. (2o)
becomes 1/2.2 or – 7 db, whereas Table 16.2 shows that the fluctuation due
to internal motion of the cloud is only – 17 db. From the MTI point of
(20)