training course on weather radar systems - RTC, Regional Training ...
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MODULE A- INTRODUCTION TO RADAR<br />
4.9. Pulse Radars<br />
Pulse <strong>radar</strong> is a primary <strong>radar</strong> unit which transmits a high-frequency impulsive signal of high<br />
power. After this a l<strong>on</strong>ger break in which the echoes can be received follows before a new<br />
transmitted signal is sent out. Directi<strong>on</strong>, distance and sometimes if necessary the altitude of the<br />
target can be determined from the measured antenna positi<strong>on</strong> and propagati<strong>on</strong> time of the pulsesignal.<br />
Weather <strong>radar</strong>s are pulse <strong>radar</strong>s.<br />
4.10. Doppler Radars<br />
C<strong>on</strong>venti<strong>on</strong>al <strong>radar</strong>s use MTI in order to remove clutter as explained above. This processing<br />
system is used almost entirely to eliminate unwanted clutter from the background, selecting as<br />
targets <strong>on</strong>ly those objects which move with some minimum velocity relative to the <strong>radar</strong> or to the<br />
fixed background. A more advanced type of system is the pulse Doppler <strong>radar</strong>, defined as a<br />
pulsed <strong>radar</strong> system which utilises the Doppler effect for obtaining informati<strong>on</strong> about the target,<br />
such as the target's velocity and amplitude, and not to use it for clutter rejecti<strong>on</strong> purposes <strong>on</strong>ly.<br />
In practice most pulsed Doppler <strong>radar</strong>s have evolved into forms which are quite distinct from the<br />
c<strong>on</strong>venti<strong>on</strong>al pulse <strong>radar</strong>s. Much higher PRF rates are used in order to eliminate or reduce the<br />
number of blind speeds. A blind speed exists when the PRF of the <strong>radar</strong> equals the Doppler shift<br />
frequency, therefore, the higher the PRF, the higher the first blind speed.<br />
A Pulse Doppler Radar is characterised by <strong>on</strong>e or more of the following:<br />
• A relative high PRF which results in ambiguous range and blind range problems.<br />
Unambiguous Doppler can be extracted up to the PRF, otherwise the Doppler<br />
frequencies<br />
are ambiguous.<br />
• A driven transmitter using a klystr<strong>on</strong> or travelling wave tube as a RF power amplifier is<br />
used rather than a magnetr<strong>on</strong> oscillator in order to obtain better frequency stability and<br />
hence phase stability. However, with the improvement of modern high-frequency<br />
magnetr<strong>on</strong>s some pulse Doppler <strong>radar</strong>s are using magnetr<strong>on</strong>s with suitable lock pulse<br />
arrangements.<br />
• A series of range gates or a movable range gate and a bank of clutter rejecti<strong>on</strong> filters<br />
38<br />
TURKEY RADAR TRAINING 1.0 / ALANYA 2005