Radar System Engineering

SEC.5.7] RANGE-MEASURING DOPPLER SYSTII.lT 139
plied with direct current or conceivably with alternating current of
frequency above the pass frequency of the audio amplifier.
5.7. Range-measuring Doppler System.—In order to measure the
range of one or more targets, the outgoing wave must be marked, or
modulated, in some way and the time required for the marks on the
wave train to return must be measured. The modulation may be of
either the amplitude or the frequency type, the techniques for the two
methods being quite different.
The most familiar method for measurement involving amplitude
modulation is to emit a pulse of waves and determine the time delay in
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the arrival of the reflected pulse. This modulation gives rise to a large
number of sidebands, and the distance of the target may be regarded
as determining the relative phase shifts of the various sidebands on their
return path.
The fundamental point here is the determination of the relative phase
shifts, not the multiplicity of sidebands. Actually only two frequencies
are needed, as we will now show by considering the simplest possible
amplitude-modulated c-w system capable of measuring range.
The system consists, in principle, of two separate systems like that
just described, the two systems having transmitter frequencies differing
by an amount f,. The transmitted and received spectra then appear
as in Fig. 5.8 with the two receivers receiving ~0 and .fO+ ~, by leakage