FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1956
1956 - 0541.PDF
FLIGHT, 4 May 1956 541 LONG RANGE NAVAIDS . . . the short, nearly straight sections of its hyperbola: at the base-line. In the event of the receiver being unable to receive both master transmitters, phase comparison then takes place betweenthe transmissions from one master and a stable oscillator of known characteristics carried in the aircraft. Dectra operates in the normal Decca frequency band and canbe completely integrated with standard Decca. This even applies to the ground installation. For example, in the case of the Prest-wick-Gander route the master and "purple" slave of the North British Decca chain would be used to fulfil the additional roleof providing Dectra coverage for the eastern end of the route. This would, however, necessitate the re-siting of the purple slave,so that the master-slave base-line would be bisected by the Prest- wick-Gander Great Circle track. A further pair of Dectrastations would, of course, be required in Newfoundland. Present indications are that an accuracy in tracking to within 5 n.m. shouldbe obtainable over a route length of 1,600 n.m. with a propor- tionate increase at greater ranges. Ranging errors throughout thesignificant coverage should not exceed 5-10 n.m. Although basically a meter-presentation system, Dectrainformation can be displayed by means of the Decca Flight Log. In fact, the same Flight Log and the same receiver may be usedto receive and display standard Decca in addition, thus achieving complete integration of the long- and short-range components.A complete Flight Log chart roll can be made up covering the terminal areas, en route section, and diversion requirements. Forthe Prestwick-Gander route (see Fig. 2) this would consist of an ordinary standard Decca Prestwick approach chart based on theDecca North British chain, then a long en route section based on the two Dectra pairs. This en route chart would be marked witha number of eastbound or westbound tracks spaced at H-lane intervals. (A lane is the area bound by two in-phase hyper-bola). This spacing represents a track separation of 30 n.m. at the centre of the route, reducing to 5 n.m. at 150 n.m. from theterminals. Marked along these tracks (which are numbered Wl, W2, W3 or El, E2, E3 and so on, according to whetherwesterly or easterly) are the miles to go to destination and the miles elapsed. The appropriate point at which to change over tothe distant chain is also shown. In the case under discussion, assuming a westbound flight, this would be the Gander chain.Dectra accuracy reaches a very high order indeed near the trans- mitters, and accordingly the Gander approach would be made ona Dectra approach chart. The scales of the approach charts would approximate to 7 n.m.to one inch, that of the en route Dectra chart being 10 n.m. to the inch. In addition, a 100 n.m. to the inch diversion chart wouldbe included giving immediate diversion information to enable the most suitable alternate to be selected from any point. This chartcould be superimposed in a contrasting colour over the en route chart to provide greater speed of action. Delrac is a projected system that may be regarded as beinga very-long-range version of Decca. Operating in the V.L.F. (very low frequency) band, Delrac is an area coverage aid. Thename, like that of Dectra, is derived from the function—in this case Decca Long Range Area Coverage. The system is designedto provide high-accuracy position-fixing coverage for both air- craft and surface vessels on a global basis. For that matter, allthe Decca systems described in this article are suitable for joint use, although in the case of Dectra there is the obvious qualifica-tion that the air and sea routes should coincide as nearly as possible. It will be readily appreciated that any system that has adual application will be considered more favourably inter- nationally, if only from the cold economical view. The predicted performance of the Delrac system is such thatit is estimated that 21 pairs of stations would provide world coverage, and all of these stations would be able to operate inthe 10 to 14 kc/s band. Each pair consists of a master and slave, and by comparing the phases of the two individual transmissionsfrom the stations of each pair whole families of hyperbolae would result, one family to each pair of transmitters. An importantpoint is the way in which the inherent Decca ambiguity has been eliminated by means of the superimposition on the pattern ofsecondary patterns, coarser by three, nine, and possibly twenty- seven times. Delrac presentation can be by meter and/or FlightLog; the meters would normally be carried to provide Flight Log setting data. Navarho. Although normally the leader in most matters aero-nautical, the United States have lagged some way behind in the development of navigational aids for civil aviation. There is, infact, only one American system officially projected for long range navigation. Known as Navarho, it is developed from an area-coverage azimuth aid called Navaglobe, with the addition of rho, •or distance information. Navarho provides full rho/theta—i.e. distance/bearing—information, and indicates these features to the pilot by means of meters. The maximum range is stated tobe 2,000 n.m. with bearing accuracies of | to 1 deg and range accuracies to within 1 per cent of the distance. Thus, at 2,000 n.m., the range error may be anything up to 20 n.m.Navarho operates in the 90-110 kc/s band section. The principle of operation calls for three aerials set up in theform of an equilateral triangle, the sides of which measure 0.36 of the operating wavelength. The aerials at each end of a side ofthe triangle are excited in turn for a given period and a set pulse rate. Thus, if we consider the three aerials to be lettered X, Y,and Z, we obtain a radiating cycle of XY, YZ, and ZX. Following radiations from each side of the triangle in turn all three aerialstransmit simultaneously. This is a synchronizing transmission and takes place on an additional frequency at the same time. Itis also used in the distance-measuring technique. As each pair of aerials radiate, their transmissions are exactlyalike in phase and amplitude, resulting in a radiation diagram like a figure eight. As the siting triangle is equilateral this will resultin the pattern rotating through three separate positions 60 deg apart. There follows an omni-directional radiation from all threeaerials. The airborne equipment receives these three patterns in sequence and notes the variation in the three amplitudes. Com-parison of these amplitudes electronically is resolved into bearing information.Following the reception of the three directional transmissions comes the "all-stations" dual-frequency pulse. A very highlystable oscillator is incorporated in the airborne receiver and com- parison of its phase against the phases of the two frequenciestransmitted by the ground stations is resolved into distance information. As the system is based on a triangle there is anambiguity of 180 deg in the bearing component, but this is not a serious matter in a long-range system; in any case an ordinaryA.D.F. receiver tuning down to the operating frequency will easily resolve it.As mentioned, basic presentation is by means of meters giving the bearing and distance from the station. Additional equipmentwill allow for pictorial presentation, left-right indication showing divergence from a given track, together with ground speed, andautopilot couplers. The weight of basic Navarho is the same as that of basic Dectra—i.e. 70 lb—but the system cannot beintegrated with a short-range aid. Conclusions. In discussing the various aids little direct com-parison has been attempted. This has been left until now because it is preferable to discuss with complete basic knowledgerather than to debate the pros and cons of each system during description. In the first place, we now have to consider theaids in the light of I.C.A.O. requirements. Quite obviously Consol and Loran fall far short of these requirements, not somuch by virtue of their performance but because of their failure to provide an easily interpreted continuous presentation. Then,again, as a result, unless a track made good by a series of observations is extrapolated, the aids cannot be used for accuratetrack-holding—certainly without the accuracy required for lateral separation. Consol, however, possesses several advantages thatshould lead to its retention as a reserve system. Its ground installation is not critical, and is relatively inexpensive, but byfar the most important factor is the complete lack of special air- borne equipment. The ordinary A.D.F. or communicationsreceiver in the aircraft is all that is required, and these receivers are normal equipment in any case. There is little likelihood ofnon-directional beacons going out of fashion in the foreseeable future, as they are likely to be kept as a secondary short-rangeaid to back up the chosen international standard. Therefore, Consol has no airborne weight at all, and may be integrated witha short-range aid. Loran requires the carriage of fairly weighty airborne equipment which cannot be used for any other purpose,neither can it be integrated with a terminal aid. Dectra is completely integrated with normal Decca, the samereceiver performing both functions. Furthermore, the addition of a Flight Log pictorial presentation is still common to bothsystems, whereas in the case of Navarho it would be necessary to carry additional equipment to cover the terminal stages of aSight. Navarho has a preferable meter presentation in compari- son with the Dectra meters, which have to be referred to chartsin order to be interpreted; but it is scarcely likely that Dectra would be used with meters alone, and with Flight Log the aidrepresents the most highly integrated navigational system of them all. The presentation is clear and easily understood, and apermanent track record from take off to landing is maintained. Decca has already been specified as the United Kingdom stan-dard system for Airways and air traffic control; it can only be hoped that I.C.A.O. will recognize this and call on memberstates to base their A.T.C. systems on Decca. Otherwise, for transatlantic flights at least, this ideal integration will be wasted,as some other terminal aid will have to be carried for use at the far terminal; the U.S.A. is still rho/theta-minded and is far fromkeen on Decca. And last, but not least, the words of the Tech- nical Committee of the International Air Transport Associationmust be considered—that present development of navaids lags five years behind development of transport aircraft. But hereI.A.T.A. are a little unfair. It is not in the development but in the provision of navaids that the five-year lag occurs.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
