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Aviation History
1976
1976 - 0050.PDF
62 FLIGHT International, wje 10 lanuary 1976 -RADAR DISPLAY CRT UNIT/ DISPLAY DRIVE UNIT CONTROL PANEL- HAND CONTROLLER The avionics layout in Sea Harrier is con ventional, with the Ferranti Blue Fox radar in the nose, which has been enlarged to accommodate it pier-velocity data, so full gyro- compassing is not possible. However, should those two inputs both fail the platform can still give acceptable navigation information, The computer programming is more complex than that of MRCA, having to cope with calculations of velocities, distances and times to moving targets (ships in this instance). The known velocity of a ship can be fed into the computer, which then displays the above data. Inertial-platform alignment aboard aircraft carriers can be difficult, one reason that this type of equipment has not been chosen for Sea Harrier. On a deck moving about all three axes, accurate orientation is not pos sible without taking information from a shipboard calibrated platform. Such an installation is quite practicable but rather expensive. The choice of a Doppler-radar velocity sensor could also cause problems in manoeuvres likely to occur frequently during Sea Harrier operations. Rapid changes of pitch or (more especially) bank will produce large changes in Doppler data. In the cockpit the pilot's control and display unit features fibre-optic displays to give the necessary high- brightness symbology. Blue Fox, the airborne radar in Sea Harrier, draws heavily on Ferranti's successful Seaspray radar (in the Westland Lynx) and more generally on the company's experience with the Lightning and the Buccaneer. Blue Fox will have to meet two requirements—air intercept (AI) and air-to-surface search and strike (ASV) —and thus a fairly sophisticated radar is needed. It operates in three modes: search, with single or multiple scanning; attack, both lead-pursuit and chase; and weapon-aiming via the HUD and boresight, using radar rang ing for opportunity attacks. The radar can also operate in conjunction with transponders for the identification of friendly aircraft. Like the majority of airborne radars, Blue Fox operates in the X- band (8-12 X 109Hz), which offers the best compromise between target- detection requirements and resistance to weather interference (see Flight last week, page 21). The Sea Harrier is also frequency-agile, transmitting on a randomly varying frequency. This makes it very difficult to jam with electronic countermeasures; the frequency band is very wide and the time spent transmitting on any particular frequency very short. Beamwidth, and therefore target dis- scrimination, is directly related to aerial size (a larger aerial gives a narrower beamwidth and therefore better discrimination); the electronics used in Blue Fox optimise the per formance of the aerial, the size of which is limited by the dimensions of the Sea Harrier nose. A slotted planar aerial transmits a two-plane mono- pluse beam, two radar beams trans mitting in azimuth and elevation and overlapping at the centre. In this area of overlap, along the radar boresight, target definition is greatly enhanced. Use of a slotted planar array improves transmitter efficiency. Servicing of advanced avionics equipment in an operational environ ment is always a problem. In Blue Fox the difficulties are minimised by splitting the system into line-replace able units like the gyro-platform system, in which faults can be in dividually diagnosed using the automatic built-in test equipment (BITE). Malfunctioning parts are then replaced at first-line servicing level. At second-line and third-line servicing level repair is aided by the modular design of the electronics. Radar information is presented on both a head-down display and on the HUD, which is being produced by Smiths Industries. It is claimed to be the first production-system in the world to combine both cursive (writing) and raster (TV-scan) techniques in an integrated HUD weapon-aiming computer. The Smiths Hudwac for the Sea Harrier consists of three units: a com puter to generate the display sym bology; the pilot's display unit (PDU); and the pilot's control panel (PCP). The PCP is mounted on the right- hand coaming and is used to select the various modes and displays (see photographs). The top knob is used for air-to-air weapon-aiming, relating the target's wingspan to its range and angle. Airspeed or Mach number, baro metric or radar height, and UHF- homing information can all be dis played. There is provision on the PCP for the MEL Madge tactical landing aid. The general weapon-aiming dis play selector permits missile-launch, air-to-air, toss-bombing or reversionary (non-gyro) information to be shown, and the standby gunsight can be pre ferred to the normal gyro-derived sight. Finally, at the bottom of the PCP, automatic or manual weapon- release is available—the automatic release occurs when the predicted bomb trajectory and impact point coincide with the HUD target symbol. A high-speed digital processor in the weapon-aiming computer has a 20,000- word store, of which 16,000 words are reprogrammable. Supplementing the normal HUD symbol generator, a video synthesiser produces raster-type displays under the control of a central processor. The PDU, mounted conventionally in the pilot's direct line of vision, includes a 4]2in lens and an integral standby sight. Provision for raster- type (e.g. radar) information is included. The Sea Harrier avionics package will endow the airframe with great versatility and accuracy in weapon delivery and navigation. The airframe and engine are virtually proven and the avionics, although new, are based on established designs. This approach, for all it was mainly dictated by Ministry of Defence financial and tech nical caution, has nevertheless pro duced a cost-effective solution with plenty of development potential.
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