training course on weather radar systems - RTC, Regional Training ...

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