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Aviation History
1951
1951 - 1680.PDF
FLIGHT, 31 August 1951 DISTANCE-MEASURING EQUIPMENT... a monitoring system. This monitoring system automatically and continuously checks such characteristics as transmitter power output, receiver sensitivity and timing accuracy. A full installation is planned to consist of two complete beacons, each with a monitor- ing system, together with main and stand-by power supplies. In the event of any falling-offin performance or failure of this beacon, the second beacon will automatically come into operation and replace it. Similarly, a power failure will bring in the stand-by power source. These features make the beacon entirely automatic in operation and suitable for installation at a point distant from an airfield if necessary and unattended apart from routine maintenance visits. Remote-control facilities are provided for switching on, and an indication of failure, should one occur, is given at the SH08T UMJL LONS «N I SEARCHC0KTROI MIX. 8£i£(»S£BtACOM 259 control point. When considering the system for world-wide use, the fact that the complete ground installation (combined with the associated V.O.R., if any) is housed within one building is patently an advantage. There may be difficulties in maintaining and pro- tecting an interdependent chain of ground installations, for the elements of such a chain may be many miles apart in isolated or uncivilized areas. Furthermore, a ground installation of the beacon type is comparatively inexpensive to provide and requires fewer technicians to maintain. The beacon aerial consists of a vertical array mounted on a mast normally 30ft high. The complete array is housed in a Fibreglass dome. Coverage of the aerial can be predicted from a knowledge of the geographical contours in the neighbourhood of the site. The airborne equipment consists of three main assemblies, a transmitter, a receiver and a timing unit. These are housed in pressurized containers which permit the equipment to be installed, if so desired, outside the pressurized portion of the aircraft. Three aerials are provided and these are of the suppressed type, i.e., they are completely enclosed within the skin of the aircraft so as to avoid drag. An omni-directional or "distance" aerial is normally mounted in the lowest portion of the aircraft fuselage and two homing aerials are installed in the nose to give forward cover. A range of non-suppressed aerials is also available for particular applications. A control unit is provided for the pilot, with an additional unit for the navigator if desired. The unit controls channel selection, which follows automatically on the setting of the channel number on two rotary switches. Two distance/homing meters supply the information to pilot and navigator. The meter display adopted is logarithmic, with an open scale at the shorter ranges at which the most precise information is likely to be required. This type of scale enables the full range of 0-200 miles to be presented in one revolution, so that unambiguous readings can be obtained at a. glance. The homing facility gives the pilot, on a centre-zero instrument incorporated in the distance meter, an indication of whether the beacon is to port or starboard of the aircraft, with a rough indication as to the amount of turn necessary to bring the beacon ahead. It has been established, state the makers, that satisfactory homings can be performed in cross-wind conditions. Further developments towards a complete navigational system on the 1,000 mc/s band are a method of aircraft identification in conjunction with surveillance radar, the provision of bearing information, and an off-set computor. It is proposed to cover all these facilities without requiring additional frequency alloca- tions in the 1,000 mc/s band. Trials at London Airport have shown the feasibility of an identification system using the airport ground radar as an interrogator and the D.M.E. transmitter as a responder. Bench trials have also been successfully carried out on a bearing system making use of the existing receiver circuits of the D.M.E., together with an additional bearing discriminator unit. A model of a mechanical computor has been made which gives every indication that this can be made both small and accurate. It has already been stressed that the subject of navigational aids cannot be judged nationally; any realistic and worthwhile examination of the problem must be carried out on an international basis. Turbine-powered aircraft cover great distances and may cross continental frontiers. The fact that their fuel consumption is high at low altitude means that the time taken for the let-down to the destination must be kept to a minimum period by the use of a suitable aid. The appearance of these aircraft on the world's air routes underlines the importance of an internationally accepted system. The three identical main units of the interrogator—transmitter, receiver and timing unit Interrogator Con- trol Unit. Channel selection is by the two rotary switches near the centre; the channel number appears in the windows between. (Left) The beacon aerial,shown on the right with dome re- moved. Provision is made for the fitting of standard obstruction lights.
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