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Aviation History
1976
1976 - 0020.PDF
22-23 FLIGHT Internationa/, w/e 3 January 1976 The Saab AJ37 (above) has an Ericsson UAP 1011 radar, Saab computer and Marconi-Elliott head-up display. Ericsson has developed the radar into the UAP 1023 pulse-Doppler system (right) for the JA37 Viggen FIRE POWER-AVIONICS the coherent signal is chopped by the pulser. A low PRF is about 1kHz, a high one about 300kHz. A high PRF gives greater range to the beam by putting more energy into each pulse but, because several pulses will have been emitted before one has returned from a long-distance target, range information cannot be directly obtained; it can, however, be computed from Doppler information. A lower-PRF radar has lower range, but range information can be obtained directly as a pulse will return before the next pulse is transmitted. However, Doppler frequency shifts cannot be measured directly because the relatively large interval between pulses means that speed information cannot be obtained. Use of a medium PRF (around 12kHz) does not solve these problems as neither speed nor range information can be computed directly. However, modern techniques permit the manual selection or automatic programming of a wide range of pulse-repetition, frequencies, according to the clutter conditions present. For instance, for a look-up mode (scanning above the horizon) a low PRF will be used, while medium PRF will be selected for look-down to reject ground clutter, and high PRF for a long-range re quirement. Radar beamwidth is directly dependent on antenna size when using the planar array mounted in aircraft. With a typical antenna 3ft in diameter the beamwidth is about 2X2° in X-band. This seems narrow, but the beam is nearly half a mile wide at ten miles range—far too wide for the ground mapping of air-ground targets. Doppler tech niques again help the azimuth resolution. The Doppler frequency shift from a radar beam looking at the ground will vary across the beam. The technique, known as Doppler beam sharpening, is used to give very fine azimuth resolution. The nose of the strike aircraft limits the size of the antenna and therefore the beamwidth. An advanced technique, synthetic-array radar, is used to compare effectively the return from one point on the ground from several successive pulses. This gives the effect of one long antenna, greatly improving discrimination. The trade-off between discrimination and effective beam power due to varying pulse length (not the same as PRF) is felt in air- ground target seeking. With a pulse length of one micro- sec the leading edge has travelled 1,000ft before the trailing-edge has left the antenna. This gives a discrimina tion of 500ft (i.e. targets less than 500ft long will not be detected), which is not practical for locating tanks or air craft on the ground. The pulse could be shortened to 10 nanosec (10~H sec), giving a resolution of 5ft, which would be excellent for battlefield use; but the emitted power would be so small as to make the range unpractically short. The physical properties of the beam seem to rule out an operational requirement, and again an advanced tech nical solution has been found. A short pulse is generated and fed through a dispersion filter before transmission and similarly compressed on reception. Alternatively, the pulse can be split into two halves, each with a different frequency. On reception the first is held in a delay gate until the second half is re ceived, when the two combine to give a signal twice as powerful and half as wide. Pulsercompression factors of ten are possible in practice, effectively reducing a one microsec pulse to ten nanosec and giving adequate target discriminatiorL The physical properties of radar aerials prevent the concentration of radar energy into a single beam. Spurious emissions, known as sidalobes, can be predicted and mini mised by precision manufacturing of the antenna but never totally eliminated. They are a problem not only be cause of thei waste of energy, but also because if they are strong enough they may induce a false paint on the dis play. The computer has no way of knowing that the return is not from the main beam. Development of the planar array has greatly reduced the sidelobe problem. This is a flat plate with radar energy fed down waveguides running across the plate. The energy escapes through rectangular slots and, with careful placing of the slots, the beam can be focused to maximise the energy in the main beam. Another method of reducing sidelobes is to employ a second (guard) antenna. This emits a fat main lobe coupled to the same computer as the main antenna. If the reflected energy from a target sensed by the guard antenna is stronger than that sensed by the main antenna then the target is in a main-antenna sidelobe and the reply is supressed. To a limited extent the use of a minimum-range filter can also help suppress sidelobes, which will normally have a much shorter length than the main beam. So airborne radar has developed a long way since the continuous-wave one-mode system of 20 or 30 years ago. Pulse radar, with varying pulse length and pulse-repeti tion frequency, is employed to optimise performance according to operational demands. Target speed can be determined by using Doppler frequency-shift computations, and stationary targets eliminated by Doppler filtering. Advanced electronic techniques improve the distinguishing of small ground targets, and accurate construction of the antenna reduces unwanted sidelobes to a minimum. Coupled to all this technology in the radar itself are the rapidly developing electronics of the computer; more in formation can be displayed to the aircrew or fed directly to the automatic weapon-aiming system. Adapted from an article in Vectors, trie Hughes Aircraft Co magazine.
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