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Aviation History
1957
1957 - 0470.PDF
DEVELOPMENTS of air navigation equipment initiatedduring the past decade are likely to bring about a gradualrevolution in techniques. Partly under the stimulus of long- and medium-range missile requirements and partly to satisfymilitary requirements for independence of ground aids and tell- tale radio transmissions, much work has been done with the objectof producing self-contained computers capable of tracing the flight-path of an aircraft and of indicating continuously its positionand relation to a target or diversion airfield. The navigator's basic task has always been to assess his vehicle'sdirection and speed of movement and, by integrating these with time, to find his position. Similar calculations have allowed himto decide the course he must follow to reach a given destination. The equipment and methods traditionally used in air navigationhave been developed largely on an ad hoc basis, each new aid sup- plementing those already in use and gradually replacing thosefound to be redundant. The calculations used in air navigation are relatively straightforward but have always depended for theirexactitude on the accuracy of the various sources of information. In ideal working conditions, and with sufficient knowledge ofthe limitations of the equipment in use, accuracy can be achieved. But increasing speeds and ranges and progressive reduction incockpit space and the number of crew members have meant that the task of navigation must be simplified and speeded up and, ifat all possible, carried out by the pilot himself. Just as integration is the present leitmotif in flight instrumentation and automaticflight control, so has the simplification of navigation aids been a major objective. This is particularly true of radio navigation sys-tems, for which special instrumentation and equipment is con- tinually being developed. Present radio navigation aids are beingused in combination, one supporting and supplementing another, and there have been relatively few instances of a complete andcomprehensive system being based on a single aid. The out- standing example of a complete and single radio aid is the DeccaNavigator, which has achieved wide recognition both for medium- range airliners and for helicopters. Certain missiles such as the Matador and Snark require a singleself-contained navigation device which, in conjunction with either a ground based or programmed guidance system, will take themaccurately to their target. For piloted or unmanned aircraft the four principal newdevelopments in navigation are the highly stable frequency- generator which allows an aircraft to interrogate a ground navi-gation aid without first synchronizing its oscillator with that of the ground station; inertial navigation systems, which will recordprogress over the earth's surface without any form of outside reference; navigation computers, which will accept a number ofinformation inputs about the velocity of an aircraft and calculate from them a present position, a course to steer and almost anyother navigational information which may be required; and the so-railed Doppler navigators, which, by measuring the Dopplerfrequency-shift and directional characteristics in radar signals reflected from the ground, determine ground speed and drift. Tothese might be added automatic star tracking, though this so far has only found application in high-altitude missiles and does not appear likely to find favour for the rather lower altitudes andchangeable visibility conditions of civil aircraft operations. Much of this equipment is subject to security regulations, butit is probably safe to say that at present only the navigation com- puters and Doppler radars have reached operational service in anynumbers, and then only in military aircraft and missiles. Inertial systems are still under intensive development in a wide variety offorms and applications. Here the main problems are technological rather than theoretical. Automatic star tracking has been used inthe Northrop Snark and in certain of the high-altitude research rockets fired in America. Current airline navigation techniques still rely on the methodsand aids proven over the past twenty years, depending almost entirely on radio aids over the more populated areas and support-ing these with celestial observations and dead reckoning in areas not well provided with aids. The various aids in current use can be Broadly divided intoarea-coverage and point-source systems, and subdivided between (a) those using medium and high frequencies and (b) those employ-ing very-high and ultra-high frequencies. Some of these require specialized receivers and indicators while others utilize normalcommunications-type equipment. In the latter case several com- panies have developed special indicators or adaptations of indica-tors to simplify the pilot's task in interpreting the signals. Rapidly taking its place among the accepted group of primary navigationaids is such equipment as airborne weather radar and the radar altimeter. A British example of the latter equipment is that madeby Standard Telephones and Cables, Ltd. Several radio navigation aids depend for their position indica-tions on the synchronization of the frequency-source on the ground with a corresponding source in the aircraft. This synchronizationis usually achieved by interrogating the ground aid to obtain an initial synchronizing reference for the airborne oscillator. Formilitary operations, where radio emanations from an aircraft can betray its presence to a listening enemy, it is obviously desirableto have a ground aid which can be invoked without interrogating it. But this is practicable only if the aircraft can carry an oscillatorwhich will independently maintain itself in exact harmony with that of the ground station; and the accuracies required have provedquite beyond the capabilities of any conventional oscillator avail- able. A major development in this field was recently revealed,namely the atomic clock. Though still rather bulky, this device does provide the necessary accuracy of oscillation. Air navigation has always been bedevilled by the inaccuracy ofthe various sources of information concerning the immediate environment of the aircraft. Neither has wind-finding beenaccurate without specialized equipment. Indicated airspeed must be corrected to provide rectified airspeed, and this in its turncorrected to show true airspeed. Outside air temperatures, pres- sure altitude and other factors used in these corrections mustthemselves be corrected. But this information is required for navi- gation, flight control and many other purposes and, while severalsensing systems have been generally used in an aircraft, such muliplication cannot be continued. Recent requirements have called for a central source of highly
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