AP 3302 Pt. 3
Section 1
CHAPTER 5
Basic outline of CW radar
where:
Fd = Doppler shift in c/s
Ft = Transmitted frequency
in c/s
v = Velocity of target in m.p.h.
c = Velocity of radio waves
in m.p.h.
If the transmitted frequency is 1,860 Mc/s and the velocity of a target directly approaching the aerial is 360 m.p.h. then:
Doppler shift Fd = (2 x 360)/(186,000 x 60 x 60) x 1,860 x 106 = 2 Kc/s
This
means that the frequency of the received signal Fr is Ft + Fd = 1,860 Mc/s +
2 kc/s. If the target had been moving away in a direct line at 360 m.p.h. the
frequency of the received signal would have been Fr = Ft - Fd = 1,860 Mc/s -
2 kc/s.
In practice it is the velocity of a target we wish to find, so we work the other way round from the measured value for the Doppler shift and the other known factors. Knowing the relation-ship it is simple to convert any difference in frequency between the received signal and the trans-mitted signal into the relative velocity of the target.
So far we have assumed that the target is moving in a direct line either towards or away from the radar aerial. If the target is not moving along such a path, the difference in frequency which Doppler effect causes is less. From Fig 7 we can see that the important factor is the radial velocity, i.e. that component of the target's speed which is in a direct line with the aerial.
When the target is not moving along a radial line the radial velocity is less than the actual velocity. In fact if the target is moving at right angles across a radial line its radial velocity is zero. It is only the radial velocity which can be measured by the Doppler effect.
