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Aviation History
1976
1976 - 0083.PDF
FLIGHT International. 17 January 1976 W EATHER RADAR — specifically designed to give the crew a long-range view of the weather ahead—has been familiar to airline pilots for some years, but so far it has not been widely adopted in business aircraft. Now advances in microcircuitry, digi tal data-processing techniques and antenna technology are bringing unit size and cost within the scope of that type of aircraft. Weather radar may still appear a luxury item—particu larly in Britain, where cross-country routes are short and the weather is fairly predictable. But on longer routes, and particularly in areas where violent meteorological conditions are frequent, such an installation can soon pay for itself in terms of both time and fuel saved by avoiding long detours around cloud build-ups. More important than this, however, is increased safety, and weather radar helps the pilot to avoid inadvertent flight into potentially hazardous storm cells. The detection of rain itself—viewed as an obstacle across his intended fiightpath—is not of great importance 113 of the range and on maximum versa tility at the other. Bendix has perhaps the most com prehensive range of weather radars on the market and produces eight ver sions — from the $8,500 RDR-150, tailored for the light twin, to the RDR-1400, at $24,000, which is more at home in an offshore helicopter busi ness jet. In the UK Bendix radars are distributed by FieldTech at Heathrow Airport. The basic principles of weather radar exactly follow those of the attack radars described in Flight, January 3, page 21, except that not so many high-technology innovations are needed to give optimum performance in this far less demanding role. X-band (around 9,300MHz) is used for all modern weather radars, giving optimum weather-identification per formance after limitations in antenna size and output power have been taken into consideration. No use is made of Doppler tech niques, since target velocity is unim portant, but the radar is pulsed (a coherent beam is chopped at regular II Auionics Avoid the storms: Weather radar for business aircraft The CONTOUR facility offered by the Bendix weather radars allows the detection of areas of heavy turbulence. Above, with CONTOUR off the storms appear as one shade; below, with CONTOUR switched on the active centres appear black. Presentation quality is improved by the digital processing techniques used to the pilot, but the rate of change of rainfall in the horizontal plane is a reliable indicator of turbulence inten sity. A rapid change in precipitation density over a short distance indicates heavy turbulence. Weather radar must be able to detect and map these con tours with fidelity if it is to be of any use. Areas of high rainfall density are conventionally displayed black, shading to paler tones (usually of green) for lighter rain. The market for weather radar is widening as business aircraft become more common. Several manufacturers, including King, RCA and Bendix, are competing in this developing field, the accent being on cheapness at one end This Bendix flat-plate antenna is installed in the nose of a Learjet • i HCT Ufcfo, ' intervals before transmission) to aid range determination and processing. A low, 100Hz, pulse-repetition fre quency (PRF) with a pulse width of 3-5X 10-6 sec is typical. The recently developed flat-plate antenna is now being applied to weather radars. This type, with its slotted waveguide, offers better beam focusing and, therefore, better target resolution than the older parabolic aerials. A typical weather radar has a beam angle of 3°. Latest addition to the bottom of the Bendix range is the RDR-150. With a list price of under $8,500 (£4,200), this is one of the cheapest radars avail able; a claimed weather-identification range of 160 n.m. is nevertheless offered. (It should be noted that the US list prices, while subject to import duty and local taxes such as VAT, may often be significantly discounted by the dealer.) The RDR-150 has a flat-plate antenna of either lOin or 12in diameter, making it suitable for the very smallest of twin-engined air craft. The cockpit display unit has been kept small—just 4inX614in of panel space is neeeded—so that it should not displace the standard nav/ com equipment. Radar information is presented in a bright-image format: it is easily distinguished in even the brightest daylight conditioms, while being dimmable for use at might. The maximum display range is 160 n.m.; this might seem rather excessive for applications in which cruise speed is not likely to be much above 160kt, but the displayed range can be varied in steps down to 5 n.m. Range rings are provided on the screen, varying in radius with the scale of the display, and targets as close as 1,800ft away can be discerned. In azimuth, marks at 30° either side of centre are displayed and the antenna scans through ±45° at 16 "looks" per minute. Transmissions are made on 9,375MHz, with a power of 8kW. Noteworthy in the RDR-150 is its weather-alert facility, for which two modes, CONTOUR and NORMAL, are alter nated four times per antenna scan. The active part of a storm, identified by the contour mode, is then seen as a flashing display. Although the RDR-150 antenna scans in azimuth it is not stabilised in pitch relative to the aircraft. Any climb or descent manoeuvres tend
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