1-27
The Doppler method is the best means of detecting fast-moving objects that do not require range
resolution. As a moving object approaches the transmitter, it encounters and reflects more waves per unit
of time. The amount of frequency shift produced is very small in relation to the carrier frequency. This is
because the velocity of propagation of the signal is very high compared to the speed of the target.
However, because the carrier frequencies used in radar are high, larger frequency shifts (in the audio-
frequency range) are produced. The amount of shift is proportional to the speed of the reflecting object.
One-quarter cycle shift at 10,000 megahertz will provide speed measurements accurate to a fraction of a
percent.
If an object is moving, its velocity, relative to the radar, can be detected by comparing the transmitter
frequency with the echo frequency (which differs because of the Doppler shift). The DIFFERENCE or
BEAT FREQUENCY, sometimes called the DOPPLER FREQUENCY (fd), is related to object velocity.
The separation of the background and the radar contact is based on the Doppler frequency that is
caused by the reflection of the signal from a moving object. Disadvantages of the Doppler system are that
it does not determine the range of the object, nor is it able to differentiate between objects when they lie
in the same direction and are traveling at the same speed. Moreover, it does not "see" stationary or slow-
moving objects, which a pulse radar system can detect.
To track an object with cw Doppler, you must determine the radar range. Since the Doppler
frequency is not directly related to range, another method is needed to determine object range. By using
two separate transmitters that operate at two different frequencies (f1 and f
2), you can determine range by
measuring the relative phase difference between the two Doppler frequencies. In such a system, a mixer is
used to combine the two transmitted frequencies and to separate the two received frequencies. This
permits the use of one transmitting and receiving antenna.
Instead of using two transmitter frequencies, you can find the range by sweeping the transmitter
frequency uniformly in time to cover the frequency range from f1 to f2. The beat, or difference, frequency
between the transmitted and received signals is then a function of range. In this type of radar, the velocity
as well as range is measured.
Q24. The Doppler effect causes a change in what aspect of rf energy that strikes a moving object?
Q25. The Doppler variation is directly proportional to what radar contact characteristic?
Q26. The Doppler method of object detection is best for what type objects?
Q27. The beat frequency in a swept-frequency transmitter provides what contact information?
FREQUENCY-MODULATION METHOD
In the frequency-modulation method, the transmitter radiates radio-frequency waves. The frequency
of these rf waves is continually increasing and decreasing from a fixed reference frequency. At any
instant, the frequency of the returned signal differs from the frequency of the radiated signal. The amount
of the difference frequency is determined by the time it took the signal to travel the distance from the
transmitter to the object.
An example of a frequency-modulated signal, plotted against time, is shown in figure 1-21. As
shown, the 420-megahertz frequency increases linearly to 460 megahertz and then quickly drops to 420
megahertz again. When the frequency drops to 420 megahertz the frequency cycle starts over again.
