4.1 Range Measurement Techniques
59
Fig. 4.4 Principle of
frequency range
measurement method
and transmitted oscillations. The above said results to frequency range measurement
method requiring a frequency modulation (FM) of oscillations.
At frequency range measurement method (Fig. 4.4), a frequency modulation with
linear law of frequency change (shift) is most commonly used:
f rad = f 0 + kt
(4.2)
Herewith, a frequency of received oscillations (input frequency) equals to
frequency of sounding oscillations sent earlier at a delay time t D :
f rcv = f 0 + k(t − t D ).
(4.3)
Equation (4.3) is correct at measurements on a steady target or a target moving
in direction perpendicular to radial direction. In other words, to use formula (4.3)
in order to determine a distance to a target, it is necessary that its radial velocity
equals to zero. If not, an additional Doppler frequency F d will appear in a frequency
of received oscillations. There are special methods permitting to detect which part
of frequency change in received oscillation is stipulated by a delay time (range) and
which—by Doppler effect (radial velocity).
At the output of mixer where oscillations continuously coming from frequency
transmitter f rad and from frequency receiving antenna f rcv , a signal on difference
frequency (beat frequency) is formed:
F D = f rad − f rcv = kt D = k
2D
c
,
(4.4)
giving a possibility, using a frequency analyzer, to determine a desired distance to a
target D = k 1 F D . Functional diagram of frequency range measurement method is
performed in Fig. 4.5.