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Aviation History
1979
1979 - 1115.PDF
FLIGHT International, 7 April 1979 terrain. To cope with low-level targets, modern radars use pulse-Doppler techniques. Moving targets display Doppler shift, the exact frequency of the radar echo being slightly modified due to target velocity. With these basics in mind, the column headings in our data tables may now be considered and the significance of some parameters grasped. Designation In many cases, European radars have a distinct name such as Seaspray, Cyrano or Agave. US equipment usually has an alpha-numerical designation such as AN/APG-63. This is not pure Pentagon Yuckspeak but a logical system which can be simply explained. AN simply indicates that the equipment designation conforms to the standard code and has been omitted from our tables. The remaining three letters specify platform, nature of equipment and purpose. Most designations for airborne radars begin with the letter A (manned aircraft) but a few open with U (utility). The second character can be P (radar), L (countermeasures), S (special) or W (armament). The final letter can be G (fire control), N (navigation), Q (multi-purpose), S (detection and/or range and bearing measurement) or Y (surveillance and control). If Nato nations manufacture US equipment under licence, the original alpha-numerical designation is usually retained. Sweden and Israel use their own alpha-numerical system. Although the Soviet designations for many items of defence equipment become known in the West, they rarely appear in print. Probably first discovered by sensitive intelligence-gathering operations, they are promptly classi fied at a high level to protect the source and are then never de-classified due to the inertia which infests all such security systems. Soviet radar designations are alpha- numerical and used in our tables wherever possible, but most equipment is listed under the relevant Nato reporting name, normally a two-word designation such as Skip Spin, or Fox Fire. Manufacturer All company names appear in shortened form in our tables. A complete list appears at the end of this feature. Operating frequency Whenever possible this is quoted in GHz but often only the band is known. E 2-3GHz F 3-4GHz I 8-10GHz J 10-20GHz K 20-40GHz Antenna Whenever possible, dimensions are given in inches. Paraboloid antennas are the well known "dish" or "orange-peel segment" shapes used to focus the micro wave energy physically, but the newer planar arrays com prise a flat plate of individual radiating elements. Peak output This column contains one or two surprises. The small APG-153 set of the F-5E apparently produces 80kW, while the AWG-9 from the F-14 generates only lOkW. The secret lies in the word "peak". A relatively simple radar can generate impressive peak powers but very little average power. A set which generates one- microsecond-long lOOkW pulses is not working very hard. Even if 1,000 pulses are transmitted every second, the set is only "on the air" for one thousandth of the actual operating time, bringing the average power down to a much less impressive 100W. Advanced radars use much more sophisticated pulse techniques and are worked harder. Our hypothetical radar generated lOOkW peak power but only 100W average power. By contrast, Israel's Elta EL/M-2021 generates an unimpressive 2kW peak power but has an average output of 200W. PRF Since a simple radar must listen for echoes between its own pulses, the pulse-repetition frequency must be kept low for long-range targets but can be increased to improve the average power output and thus echo strength 1069 Egyptian pilots pose in front of a Sukhoi Su-7 Fitter. The intake centrebody contains a radar known to Nato as High Fix. Soviet designation of this equipment is understood to be SRD-5M at shorter ranges. In practice, the latest radars can use high PRFs of up to 300kHz rather than low PRFs of up to 1,000Hz. This puts much more energy into the beam, but since several pulses will have been emitted before the echo from the first is received, simple range-measuring techniques cannot be used. Range must be calculated indirectly from target speed, which is measured by Doppler techniques. Faced with the choice of using low PRFs to obtain direct range data, without speed measurement or high PRFs to obtain speed data but no direct range information, some design teams have opted for medium PRFs of around 12kHz, obtaining both range and velocity by indirect computation. This is a relatively new innovation in airborne radar and presents the designer with formid able parameter-definition and signal-processing problems. Medium PRFs give better range resolution than high and are better suited to rear-hemisphere look-down attacks. For this reason they are widely used in pulse-Doppler radars. High rates are best kept for long-range and head- on targets. Pulsewidth As PRF is increased, pulsewidth is reduced, improving the range resolution of the set and decreasing the minimum range at which it can operate. Even travel ling at the speed of light, a one microsecond pulse is around 1,000ft long, limiting the minimum range and range resolution to this value. Maximum range depends on target size, so this has been given wherever possible. Figures given without target size are of limited value. Although the data tables give the main parameters for each radar, brief descriptions of selected equipment are also given in the text. Abbreviations used CRT Cathode-ray tube CW Continuous wave ECCM Electronic counter-countermeasures Flir Forward-looking infra-red GHz GigaHertz kHz kiloHertz kW kilowatt LD Look-down LRU Line-replaceable unit LU Look-up MTI Moving-target indicator PPI plan & position indicator PRF Pulse-repetition frequency (pulses/sec)
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