Geoscience Reference
In-Depth Information
5.3
Radar Operating Principles
The basic principles of a radar consist in the transmission, propagation, and
reflection of radio waves. Figure 5.1 shows a functional diagram of a radar system:
the transmitter generates a pulse of electromagnetic energy which is routed through
a transmit-receive switch, called duplexer, to an antenna, which radiates it into
free space. The transmit-receive switch has the function of protecting the sensitive
receiver from the high-power transmitted pulse. When the electromagnetic pulse
encounters objects along its way, it is scattered (reradiated), and part of the scattered
signal returns to the radar where it is collected by the antenna and routed through the
transmit-receive switch to the receiver. The received signal is enhanced, interfering
signals are reduced, and measurements of the object are made by signal processing
and presented on displays.
The purpose of radar instruments is not just to detect objects but mainly to
measure the range between the radar and the object. When a pulse is transmitted,
a clock is started and stopped as soon as a received signal is detected. The range
between the radar and the detected object is expressed by R D c t /2, where R
is the range from the radar to the object in m, c is the speed of light in the
vacuum D 299,792,458 m/s, and t is the elapsed time in s.
As mentioned before, the radar transmits pulsed signals. The characteristics of a
radar waveform are the wavelength of the propagated energy D c / f c in m, where
c is the speed of light in the vacuum and f c is the radar carrier frequency in Hz; the
angular frequency ! D 2 f c in rad/s; the pulse repetition interval (PRI) in s, which
is the time between radar pulses; and its reciprocal the pulse repetition frequency
(PRF) in Hz, which is the number of pulses sent per time interval, with PRF D 1/PRI
(Fig. 5.2 ).
Fig. 5.1
Radar block diagram
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