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Aviation History
1958
1958 - 0644.PDF
660 NAVIGATION . . . FLIGHT, 16 May 1958 Like its forerunner, standard Loran, Loran-C is a pulse hyper-bolic system consisting of a master and two or three slaves; Loran-C, however, operates in the 90-110 kc/s band. Master andslave transmissions are synchronized so that the time difference between the receipt of master and slave pulse groups can be plottedas hyperbola:. The major difference between Loran-C and standard Loran, apart from the frequencies used, is that these time-differencemeasurements are made by utilizing both the pulse envelope and the phase of the carrier within the envelope. Loran-C uses at least the entire available band-space for long-range aids, i.e., the 20 kc/s between 90 and 110 kc/s. Very high power is required to achieve the ranges estimated—about 2,000 n.m.over sea and 1,200 n.m. over land—and experimental stations have radiated 60 Kw. Even higher powers—up to one megawatt—havebeen postulated for operational installations. The claimed accuracies of the system are high. Fixed monitorstations in the U.S. report average night and day errors of ± 150ft at 400 n.m. to + 1,000ft at 1,400 n.m., using ground wave propaga-tion. No information on airborne evaluation results are available as yet. The third U.S. system is Omega. This is alleged to be adevelopment of Radux but is almost identical to the British Delrac System, the announcement of which preceded it by some four years. Omega uses frequencies in the 10-14 kc/s or V.L.F. band, andis a C.W. phase-comparison hyperbolic system. From three to six synchronized stations each operate on a basic and secondary fre-quency differing by 500 c/s. Transmissions are made sequentially in a fixed schedule and position lines are obtained by determiningthe phase-differences in the arrival of the basic frequency from any two of the stations: the secondary frequency is similarlycompared to give ambiguity resolution. The range of the system is assessed at 5,000-6,000 miles with apredicted accuracy of one mile, on a 95 per cent probability basis within the coverage. To obtain these ranges it would appear thatvery high powers (100 kW), and correspondingly complex antennas would be required. Full-scale trials of Omega are not expectedbefore 1960. Omega, as stated above, is similar in concept to Delrac, whichwas first announced about four years ago. In Delrac a series of master/slave combinations, also operating in the V.L.F. band, ona 1,000-mile base line, transmit phase-related signals on a time- frequency-multiplex basis to provide position-line information.Fixes are obtained by using position-lines from any two pairs and relating them to an overprinted lattice or by displaying thempictorially on the Decca Flight Log. The ranges predicted for Delrac are of the order of 3,000 mileswith positional accuracy not worse than ± 10 n.m. within coyer- age. Since both Delrac and Omega are C.W. phase comparisonaids, the difference in both range and, particularly, accuracy claimed by their proponents are quite striking. Of the two, it would seemthat the more conservative Delrac claims are those most probably attainable at these frequencies by such techniques. The major difference between the two systems is that, whereasOmega seeks to resolve ambiguity in one "coarse" step, Delrac plays safer and does it in three or more, by using additionalfrequencies. Although Delrac has not yet been evaluated, many of the tech-niques involved have been used and proved in other systems. Of the long-range systems considered here Dectra is furthestahead in evaluation; a trial ground installation has been in opera- tion over the North Atlantic for the past twelve months. Dectra, which operates on approximately 70 kc/s, consists ofa master/slave pair positioned at one end of a given route on a base line approximately 60 to 100 miles long. Phase comparisontechniques lay down a hyperbolic pattern between these stations which extends over the required route and forms the trackinglattice. A further slave is located at the end remote from the tracking pair and similar techniques between it and the remotemaster provide the ranging pattern across the track. The trial installation now operating has master and slave located in New-foundland, with the ranging slave near Stirling in Scotland. The initial evaluation of Dectra was done in an M.o.S. Valiantwhich has so far established tracking accuracies of the order of ±2 n.m. standard deviation at ranges of up to 1,500 n.m. Whilstranging errors have not yet been fully assessed it seems probable that approximately the same order of accuracy is being obtained. The significance of these results is, of course, that they providea means of reducing the separation standards on the North Atlantic and, for that matter, on any similar route where traffic congestionis building up. As is well known, operators on the North Atlantic are often penalized by separations of 120 n.m. lateral and 30 minlongitudinal between aircraft on the same altitude, which fre- quently force them to accept altitudes below the optimum or toendure considerable delay. It is for this reason that a number of civil airlines are nowevaluating Dectra themselves. These include B.O.A.C, PanAm, The Decca Navigator, with Flight Log pictorial presentation (shown in detail below) is suitable for fixed- or rotating-wing aircraft. Above, it is in a U.S. Army H-34 helicopter in Germany. S.A.S., Swissair and T.C.A., with others—amongst them El Al—extremely interested. The airborne installation includes both Dectra and Decca.Presentation is made on the Flight Log. Dectra is set up accurately from a Decca reference and thereafter the aircraft track is recordedcontinuously throughout the trip. The position of the aircraft is immediately known, at any time, by the location of the pen:together with the appropriate Dectra lattice the chart is over- printed with a latitude/longitude grid and carries distance togo/distance gone information plus headings to diversions. Posi- tion reporting is thereby greatly simplified as is the detection ofwind changes and the calculation of ground speed and E.T.A.s. As already reported in Flight, the Decca Company are proposingto combine Doppler with their Decca/Dectra in what they term the D.I.A.N. (Decca Integrated Air Navigation) System. Thereasoning behind this D.I.A.N. concept is that such an integration will enable D.I.A.N.-equipped aircraft to fly accurately in a com-plex traffic area using Decca/Dectra whilst, at the same time, checking and removing Doppler errors—particularly those arisingfrom the compass—by reference to these more accurate systems. In the event of Doppler alone being required it can therefore beused from a datum of high accuracy. Perhaps more important is the fact that Doppler informationwill also be presented pictorially on the Flight Log. This will not only save cockpit instrumentation space and weight but will alsoenable complex computation—the provision of latitude/longitude or rho/theta information and compensation fc-r compass varia-tion and convergence, for example—to be provided on the charts
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