Radar System Engineering

132 C-W RADAR S’YSTEMS [SEC.56
biguous range, and it will appear later to be true also of frequencymodulation
systems.
5.6. Utility of C-w Systems.—In spite of the fact that c-w systems
are limited in their rate of information transmission they are of value for a
number of reasons. First, there are instances in which a rapid rate of
transmission is of no advantage. For example, in the case of an altimeter,
there is one target, the earth. Its general direction is known and fresh
information as to its distance is hardly needed more than a few times a
second. Observation 100 times a second is actually employed in radar
altimeters, to make possible the reduction of certain errors by averaging,
but even so the rate of information transmission is small.
Second, there are the situations in which, though a little information
may be obtained with ease, a lot is impossible to obtain. Such a situation
arises in the presence of very severe clutter, where pulse systems, even
with MTI equipment, may fail to give any information. Certain doppler
systems, on the other hand, will provide useful and even adequate
information.
Third, the price of rapid transmission of data is a certain degree of
apparatus complexity. In some cases, the gain is not worth the price
and the simpler c-w system is adequate.
Finally, there are some things which pulse-type systems simply
cannot dfiwork down to zero range, for example.
In the descriptions of c-w systems to follow, they will be presented
roughly in the order of their complexity as to objects, conception, and
apparatus. Most of the systems described have seen some use, although
a few that are included have been tried only briefly.
SPECIFIC
SYSTEMS
5.6. Simple Doppler System.—We describe first a system capable of
detecting one or more moving objects in the presence of large amounts of
ground clutter. In detail, the specification called for the detection of an
airplane 50 ft above the ground at 10 miles range and aircraft at higher
altitudes at 15 miles range. Also, the system later proved most useful
for measuring velocities of projectiles.
It will be observed that, in principle, the simple device shown in
Fig. 5.1 can do all that is required. Practically, however, two important
modifications must be made.
First, steps must be taken to keep as much transmitter power as
possible out of the rectifier. There are two reasons for this: (1) the
only practical rectifiers at microwave frequencies are crystal detectors
and these would burn out if connected, as in Fig. 5.1, to even a low-power
transmitter; (2) if the rectifier did survive, it would respond to amplitude
modulation of the transmitter and, since it can be found from Eq. (2.4u)