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different types of objects (aircraft, buildings, vehicles etc.) to be distinguished. Most
surface movement radars use X band or Ku Band.
The time taken for the radar pulse to reach the target and return to the receiver provides a
measurement of the slant range (distance) of the radar antenna from the target. The
angular determination of the target is determined by the directivity of the antenna. The
operating principle of pulse radar involves the transmission of a short pulse of microwave
energy at regular time intervals. This transmission involves amplitude modulation of a
micro-wave signal with the pulse signal. The duration of this pulse is known as the pulse
width or pulse length. The receiver is listening between the successive transmitted
pulses. Returns from an individual target arrive in a fixed interval (round trip time) after the
transmitted pulse.
The pulse width determines the range resolution capability of the radar. This is the ability
of the system to distinguish between closely spaced targets on the same bearing. The
range resolution is governed by the operational requirement; notably the required aircraft
separation standard. The pulse width determines the necessary bandwidth of the radar,
with higher resolutions requiring a bigger bandwidth.
The time period is composed of one transmitted pulse and one listening period and
repeats at a fixed rate; this is called the PRF (Pulse Repetition Frequency) and is
typically of the order of 1 kHz ( long range radars have a low PRF and short range radars
a much higher PRF). The ratio of the pulse width to the pulse repetition interval is called
the duty cycle. Typical values for the pulse width and pulse repetition interval for
aeronautical ground primary radar are 1 µ s and 1 ms respectively. It will be appreciated
that depending on the PRF, antenna turning speed and beamwidth, several returns or
“hits” will be received on each scan (one rotation) of the antenna. A large number of “hits”
increases the signal to noise ratio.
During transmission, the peak of the transmitted pulse defines the peak power. This is
typically in the range 150 kilowatts to 10 megawatts for ATC radars. The mean power of
the transmission is defined by the peak power multiplied by the duty cycle. The mean
power is an important value since it is a measure of the total amount of microwave energy
to be transmitted by the system. The EIRP of ATC primary radar systems is in the range
75 to 95 dBW.
Due to the Doppler effect, there is a frequency change in the received pulse relative to the
transmitted pulse. This depends upon the radial speed of the target and can be used to
discriminate moving objects from fixed ones.
The Radar Cross Section (RCS) or target size is a representation of the magnitude of the
echo received from the target. ATC radars are usually specified against a target size of 1
to 3 sq metres. Radar coverage diagrams indicate the reference RCS for the coverage
indicated. The RCS varies significantly depending on the type and size of target and the
aspect of the illumination.
In order to overcome target size fluctuations, many radars use two or more different
illumination frequencies. This is known as frequency diversity and provides a significant
gain in radar performance. Frequency diversity can be achieved in two main ways. In a
dual channel system, both transmitters can transmit simultaneously on two different
frequencies, thereby providing frequency diversity. Alternatively, a single transmitter with
the required bandwidth can transmit pulses at two different frequencies in sequence.
3.2.2.2 The Radar Equation
Radar performance is governed by the radar equation which defines the maximum range
of target detection relative to radar parameters. The basic form is:
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