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MODULE A- INTRODUCTION TO RADAR (Doppler filter bank to cover the Doppler spectrum) are used rather than a simple MTI system described above, in order to cancel clutter and to extract the Doppler frequency and amplitude components of the received signal. By extracting the Doppler characteristics of signals it is possible to achieve much better clutter cancellation than in conventional radars, and the target’s radial velocity component can be calculated once the Doppler frequency is measured, in addition it is possible by range gating to measure the Doppler and the amplitude of the returned signal in each radar cell. The location of the radar cell by measuring the return time, the position of the antenna (azimuth and elevation) at the time the signal in the radar cell was received. All this processing is done digitally. A simple block shows the essential components of the Pulse Doppler Radar that is used for weather observation. 4.11. Weather Radars Although these names refer to the application of radar, there is a significant difference in the type of radar that is used which is worth to be illustrated. In general radars measure the location of a target that is range, azimuth and height. The major distinction between meteorological radar and other kinds of radars lies in the nature of the targets. Meteorological targets are distributed in space and occupy a large fraction of the spatial resolution cells observed by the radar. Weather radars are pulsed radars with Doppler capability. So we can call them as Pulsed Doppler Weather Radars. Weather radars can operate in different frequency bands. So a classification can be made based on the frequency band as follows: L band radars Those radars operate on a wavelength of 15-30 cm and a frequency of 1-2 GHz. L band radars are mostly used for clear air turbulence studies. S band radars Those radars operate on a wavelength of 8-15 cm and a frequency of 2-4 GHz. Because of the wavelength and frequency, S band radars are not easily attenuated. This makes them useful for near and far range weather observation. It requires a large antenna dish and a large motor to power it. It is not uncommon for an S band dish to exceed 25 feet in size. TURKEY RADAR TRAINING 1.0 / ALANYA 2005 39
MODULE A- INTRODUCTION TO RADAR C band radars Those radars operate on a wavelength of 4-8 cm and a frequency of 4-8 GHz. Because of the wavelength and frequency, the dish size does not need to be very large. This makes C band radars affordable for TV stations. The signal is more easily attenuated, so this type of radar is best used for short range weather observation. Also, due to the small size of the radar, it can therefore be portable. The frequency allows C band radars to create a smaller beam width using a smaller dish. C band radars also do not require as much power as an S band radar. X band radars Those radars operate on a wavelength of 2.5-4 cm and a frequency of 8-12 GHz. Because of the smaller wavelength, the X band radar is more sensitive and can detect smaller particles. These radars are used for studies on cloud development because they can detect the tiny water particles and also used to detect light precipitation such as snow.. X band radars also attenuate very easily, so they are used for only very short range weather observation. Most major airplanes are equipped with an X band radar to pick up turbulence and other weather phenomenon. This band is also shared with some police speed radars and some space radars. K band radars Those radars operate on a wavelength of .75-1.2 cm or 1.7-2.5 cm and a corresponding frequency of 27-40 GHz and 12-18 GHz. This band is split down the middle due to a strong absorption line in water vapour. This band is similar to the X band but is just more sensitive. This band also shares space with police radars. 4.12. Polarimetric Radars Polarimetric Radars are Doppler weather radars with additional transmitting and processing functionality to allow to further compute additional information on the directionality of the reflected electromagnetic energy received. Most weather radars, transmit and receive radio waves with a single, horizontal polarization. That is, the direction of the electric field wave crest is aligned along the horizontal axis. Polarimetric radars, on the other hand, transmit and receive both horizontal and vertical 40 TURKEY RADAR TRAINING 1.0 / ALANYA 2005
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