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Aviation History
1959
1959 - 0533.PDF
262 FLIGHT, 20 February 1959 FLYI NG, Al DS . . . ~J « ; The Decca flight Log, Decameters and Dectra range meter on the central panel of the M.o.S. Comet 2E cockpit NAVIGATION Airborne and Ground-Based Systems THERE is virtually no danger of an airliner becoming lost inthis day and age. Conventional plotting and observation,combined with information obtainable from any of the tradi- tional radio equipment such as A.D.F., Loran and probably V.H.F. and H.F. can provide sufficiently accurate knowledge of position to make arrival at destination a foregone conclusion. But intensive development of navigational aids is still necessary because the problem is now not so much how to get there but exactly what track should be made good and what stage along a given track has been reached at any time. The fact that the traditional navigating skills are fast disappearing from the flight deck is an additional factor, at least on the shorter range opera- tions; and the speed of aircraft is increasing to an extent where time becomes critical. A revival of the art of astro-navigation has taken place in bomber flying where independence of ground-based aids is considered essential and cruising heights are high enough to ensure a view of stars unobscured by cloud. But already the independent auto- matic systems such as initial navigation, automatic star tracking (and combinations of these two) and Doppler/inertial are spread- ing from the exigent but relatively short-term missile field and entering what is in some ways the more difficult field of conven- tional fixed-wing operations. A notable example is the Doppler/ inertial hybrid AN/ASQ-42 bombing and navigation system made by S perry for the Convair B-58 Hustler. The stable platform element of an inertial system is inherently suitable as a source of attitude information for flight control and such additional use has been made of it in ASQ-42. The possibili- ties have been carried a good way further in the inertial system supplied by Sperry for the North American X-15 research aircraft. In the absence of atmospheric sources of dynamic flight data at extreme altitudes a complete flight-instrument system is here supplied from inertial data. This is described in greater detail in a later section of this account. The frequency reference of a stability so far only rarely attained even in laboratory conditions could, if produced in a form for installation in aircraft, allow precise navigation of high-speed air- craft with relatively simple ground-based apparatus. It could additionally afford discrete communication, aircraft identification A Vortac ground-beacon, in this case made by International Telephone and Telegraph Corp. The Tacan aerial is the smaller unit above the ^rr. ^;, ,.-...-J cone housing the VOR aerial and position reporting as part of a single integrated system. One such device was developed by the National Company in America, using the resonant frequency of the cesium atom as a reference. First details were published during 1956 and at that time an air- borne version weighing about 60 lb was forecast. Since then virtually nothing has been heard of this equipment. The earth satellite for navigation has particular advantages since its radiations would penetrate cloud cover which obscures stars and because its high speed would make it possible to provide world-wide cover with one satellite. Both the actual path and the time along the path have been very accurately maintained by satellites so far launched. Either direction-finding or Doppler techniques could be applied to assessing position. At 200 miles above the earth the horizon distance would be 1,100 miles and very high frequencies above 100 Mc/s offer good possibilities of avoiding path distortion of the signals when passing through the ionosphere. A navigation satellite is obviously a costly project which would either become a military prerogative or have to be supported internationally. Within the sphere of influence of I.C.A.O., navigation aids are to some extent internationally standardized, the present short- range system being the American V.H.F. omni-directional range, VOR. A very considerable number of VOR beacons has been set up in the U.S., but the number installed in other areas is not large enough to be able to claim that the I.C.A.O. standardization requirements have yet been fully implemented. This can be ascribed in part to the fact that a very large number are required to provide the coverage envisaged. Compared with other aids at present in use, VOR is very popular among pilots, particularly because it is simple to use, but it has certain fundamental limitations which have become apparent even before complete implementation is accomplished. VOR is not exceptionally accurate, an average estimate of total errors in the system being of the order of five degrees in bearing. Some estimates put the total VOR errors as high as 10 deg. Though this might be acceptable for purely navigational purposes at moderate altitudes, it will not suffice in dealing with high-density traffic on crowded airways. Even the addition of the distance- measuring facility now afforded by the Vortac system is not likely materially to improve this situation. VOR operates at a fixed frequency in the V.H.F. band between 112 and 118 Mc/s and has a working range of about 130 miles at 12,000ft. Since it transmits along a line of sight, its range and coverage at lower altitudes is reduced and subject to terrain con- ditions. The beacon transmits two modulations on a V.H.F. carrier, one being a fixed-phase reference signal and the other having a phase delay in relation to it which corresponds to the bearing of the receiver in the aircraft. The bearing is displayed on a meter or, alternatively, the heading error with respect to a chosen radial set on a selector can be shown by a left /right pointer. The latter feature is particularly important since it makes pos- sible the use of a selected VOR radial bearing as a reference for steering information in an integrated instrument display. All the principal manufacturers of flight instrument and radio navigation equipment, both in America and Britain, have produced integrated displays using this principle. Guidance for I.L.S. localizer and glide-path flying have also been incorporated and the variations in instrument presentation and performance are many. For such purposes VOR has proved to be extremely flex- ible and has led, during recent years, to something of a revolution in semi-automatic flight. [Continued cm p. 263
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