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Aviation History
1976
1976 - 0019.PDF
FLIGHT International, w/e 3 January 1976 FIRE POWER Airborne radar Back to first principles GREATER NUMBERS of combat aircraft are now being equipped with more versatile and higher-performance radar for the detection of other aircraft or ground targets. As micro-circuitry techniques advance so radar technology packs more functions and greater flexibility into the con fined nose space of an aircraft. A layman's description of radar might read: "Radio energy is emitted from an aerial, reflects off any object, returns and is collected by the same aerial. By determin ing the time of travel, the radar computer can calculate the distance to the object." The device is, of course, far more complex. Since the early days of radar at the beginning of the Second World War, advances have been continuous until only the basic principle remains the same. The first link in the chain is the creation of a stable radio frequency, of around 9 X 109Hz (9GHz), by an oscil lator. If this were transmitted as a coherent signal there would be no time for reception of the reflected beam, so the oscillator output is usually chopped into pulses. This also enables the radar to determine which inbound pulse is related to which outbound pulse. With a continuous transmission a separate receiving aerial is required. The oscillator-generated pulses are then passed through a switching unit which directs them to the antenna. After transmission and reflection from a target, the pulses are returned to the same antenna before the next pulse is transmitted. The incoming beam is then switched to the receiver and subsequently processed for display. If the radar beam is coherent, returns from stationary ground targets or spurious signals generated by the elec tronics can be separated from the returns of the moving targets. In a coherent radar the pulser periodically inter rupts a continuous signal. Thus the returning wave is phase-related to the outgoing pulse, enabling direct corre lation. Such direct correlation permits the speed of any target to be computed. The phase of the return signal is com pared directly With a reference and the Doppler frequency 21 •i ;.i ... ::. n fluionics can be calculated from the phase shift of a number of suc cessive returns from the same target. The frequency of the returned pulse varies with the speed of the target relative to the radar antenna. The frequency rises with an approaching target and falls with a receding one. When this shift has been detected, targets in any particular speed band can be either rejected or displayed. It must be emphasised that the speed of the target is relative to the (moving) antenna and therefore targets travelling at the same relative speed will show no Doppler frequency shift. However, the computer can allow for the speed of the antenna in its rejection/acceptance computation. With a moving aerial, returns from ground clutter will exhibit a Doppler frequency shift and this also can be allowed for. The big advantage of using Doppler techniques in air borne radar is that an enemy aircraft against a land back ground can be detected by the Doppler suppression of ground clutter returns. The disadvantage is that if the target is travelling tangentially to the antenna then it has no radial velocity component and therefore no Doppler frequency shift to differentiate it from the ground clutter. However, this is extremely improbable with a moving, airborne antenna. Most airborne radars transmit in the X-band between 8,500MHz and 10,680MHz. While the frequencies used for airborne radar transmissions generally have increased over the years, the aerial is the area for compromise; the choice lies between the larger aerials needed by the lower-frequency S-band, and the smaller units for the high-frequency Ku-band (also known as J-band), in which transmissions can be blocked by weather conditions such as precipitation or clouds. The X-band frequency is generated as a continuous sine wave. Until recent years this very high frequency was generated at around 40MHz and multiplied within the electronics. Now, with the development of Gunn diodes, this frequency can be generated directly, which allows simplified circuitry. The pulse-repetition frequency (PRF) is the rate at which The components of a typical airborne attack radar are seen here in this Hughes equipment for the F-15. Behind the fiat-plate antenna a large part of the nose is taken up with the processing system ANALOG PROCESSOR DATA PROCESSOR DIGITAL PROCESSOR 3LC Ji' tf £
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