AP 3302 Pt. 3
Section 1
CHAPTER 5
Basic outline of CW radar
Introduction
In an earlier chapter we noted that all pulse-modulated radars have a 'blind' area surrounding the installation within which no targets can be detected. The blind area depends upon the value of pulse duration but even with a pulse of 0.2 us no target within 100 feet of the radar aerial can be detected. Sometimes it is necessary to detect and to measure the distance of targets from the radar aerial down to almost zero feet. Pulsed radar cannot be used for this; frequency-modulated continuous wave radar (f.m.c.w.) can.
The only way to measure the speed of a target in pulsed radar is to try to estimate the distance the echo on the c.r.t. screen moves in a given time. This is a rather indirect and inefficient method. A better method involves the use of unmodulated c.w. radar and the 'Doppler effect'.
We are therefore concerned with two different forms of c.w. radar-f.m.c.w. and c.w. Doppler. In this chapter we shall consider the elementary ideas of both forms, illustrating the application of each with examples.
Frequency-modulated
CW Radar
One application of f.m.c.w. radar is in aircraft altimeters. The normal barometric altimeter is operated by air pressure and has two limitations:
a. If the atmospheric pressure changes while
the aircraft is in flight the altimeter reading will change.
b. The
barometric altimeter indicates height above sea level, or some other pr-set
level. It does not tell the pilot his actual altitude above the ground (Fig
1).
These limitations led to the development of the radar altimeter.
We have seen how the distance between a radar aerial and a reflecting surface can be measured by pulse-modulated radar. If we transmit pulses directly downwards from an aircraft we can measure the actual distance to the ground below.
Altimeters which work on this principle give satisfactory results while the aircraft is at a high altitude. However, since all pulsed radars have a certain blind area, altimeters of this type would be useless when the aircraft is flying near the ground, e.g. when it is landing. For this we need a f.m.c.w. radar altimeter.
