1-25
rest of the pulse train; no such cross-section fluctuations can affect the measurement. An additional
advantage of monopulse tracking is that no mechanical action is required.
ELECTRONIC SCANNING used in search radar systems was explained in general terms earlier in
this chapter during the discussion of elevation coverage. This type of electronic scanning is often called
FREQUENCY SCANNING. An in-depth explanation of frequency scanning theory can be found in the
fire control technician rate training manuals.
RADAR TRANSMISSION METHODS
Radar systems are normally divided into operational categories based on energy transmission
methods. Up to this point, we have mentioned only the pulse method of transmission to illustrate basic
radar concepts. Although the pulse method is the most common method of transmitting radar energy, two
other methods are sometimes used in special applications. These are the continuous-wave (cw) method
and the frequency modulation (fm) method. All three basic transmission methods are often further
subdivided to designate specific variations or combinations.
CONTINUOUS-WAVE METHOD
When radio-frequency energy transmitted from a fixed point continuously strikes an object that is
either moving toward or away from the source of the energy, the frequency of the reflected energy is
changed. This shift in frequency is known as the DOPPLER EFFECT. The difference in frequency
between the transmitted and reflected energy indicates both the presence and the speed of a moving
target.
Doppler Effect
A common example of the Doppler effect in action is the changing pitch of the whistle of an
approaching train. The whistle appears to change pitch from a high tone, as the train approaches, to a
lower tone as it moves away from the observer. As the train approaches, an apparent increase in
frequency (an increase in pitch) is heard; as the train moves away, an apparent decrease in frequency (a
decrease in pitch) is heard. This pitch variation is known as the Doppler effect.
Let’s examine the reason for this apparent change in pitch. Assume that the transmitter emits an
audio signal at a frequency of 60 hertz and that the transmitter is traveling at a velocity of 360 feet per
second (fps). At the end of 1 second, the transmitter will have moved from point P to point P1 as shown
in view A of figure 1-20. The total distance from point P to the observer is 1,080 feet. The velocity of
sound is 1,080 feet per second; thus, a sound emitted at point P will reach the observer in 1 second. To
find the wavelength of this transmitted signal, you divide the velocity of the signal (1,080 fps) by the
frequency (60 hertz). The result is 18 feet, as shown below:
