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MODULE A- INTRODUCTION TO RADAR The CW transmitter generates continuously unmodulated RF waves of constant frequency which pass the antenna and travel through the space until they are reflected by an object. The isolator shall prevent any direct leakage of the transmitter energy into the receiver and thus avoid the saturation or desensitisation of the receiver which must amplify the small signals received by the antenna. The CW radar can only detect the presence of a reflected object and its direction , but it cannot extract range for there are no convenient time marks in which to measure the time interval. Therefore this radar is used mainly to extract the speed of moving objects. The principle used is the Doppler effect. The Doppler principle will be explained in detail in following chapters. 4.7 FM-CW Radars The inability of a simple CW radar to measure range is related to the relatively narrow spectrum (bandwidth) of its transmitted waveform. Some sort of timing mark must be applied to the CW carrier if range is to be measured. The timing mark permits the time of transmission and the time of return to be recognised. The sharper or more distinct the mark, the more accurate is the measurement of the transit time. But the more distinct the timing mark, the broader will be the transmitted spectrum. Therefore a certain spectrum width must be transmitted if transit time or range is to be measured. The spectrum of a CW transmission can be broadened by the application of modulation, either by modulating the amplitude, the frequency, or the phase. An example of the amplitude modulation is the pulse radar. 4.8. Moving Target Indication (MTI) Radars The purpose of MTI radar is to reject signals from fixed unwanted signals, sky and/or ground clutter, and retain for detection the signals from moving targets such as aircraft or rain. There are two basic types of MTI namely coherent and non-coherent MTI. The former utilises the Doppler shift imparted on the reflected signal by a moving target to distinguish moving targets from fixed targets, and the latter detects moving targets by the relative motion between the target and an extended clutter background and consequently by the corresponding amplitude changes from pulse to pulse or from one antenna scan to the next. By coherent it is meant that the phase of the transmitted wave must be preserved for use by the receiver if the Doppler shift in frequency is to be detected, whereas in non-coherent systems it is not necessary. TURKEY RADAR TRAINING 1.0 / ALANYA 2005 37
MODULE A- INTRODUCTION TO RADAR 4.9. Pulse Radars Pulse radar is a primary radar unit which transmits a high-frequency impulsive signal of high power. After this a longer break in which the echoes can be received follows before a new transmitted signal is sent out. Direction, distance and sometimes if necessary the altitude of the target can be determined from the measured antenna position and propagation time of the pulsesignal. Weather radars are pulse radars. 4.10. Doppler Radars Conventional radars use MTI in order to remove clutter as explained above. This processing system is used almost entirely to eliminate unwanted clutter from the background, selecting as targets only those objects which move with some minimum velocity relative to the radar or to the fixed background. A more advanced type of system is the pulse Doppler radar, defined as a pulsed radar system which utilises the Doppler effect for obtaining information about the target, such as the target's velocity and amplitude, and not to use it for clutter rejection purposes only. In practice most pulsed Doppler radars have evolved into forms which are quite distinct from the conventional pulse radars. Much higher PRF rates are used in order to eliminate or reduce the number of blind speeds. A blind speed exists when the PRF of the radar equals the Doppler shift frequency, therefore, the higher the PRF, the higher the first blind speed. A Pulse Doppler Radar is characterised by one or more of the following: • A relative high PRF which results in ambiguous range and blind range problems. Unambiguous Doppler can be extracted up to the PRF, otherwise the Doppler frequencies are ambiguous. • A driven transmitter using a klystron or travelling wave tube as a RF power amplifier is used rather than a magnetron oscillator in order to obtain better frequency stability and hence phase stability. However, with the improvement of modern high-frequency magnetrons some pulse Doppler radars are using magnetrons with suitable lock pulse arrangements. • A series of range gates or a movable range gate and a bank of clutter rejection filters 38 TURKEY RADAR TRAINING 1.0 / ALANYA 2005
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