FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1956
1956 - 0540.PDF
540 ' FLIGHT, 4 May 1956 LONG-RANGE NAVAIDS Part 2: Dectra, Delrac and Navarho Systems By J. H. GROVER, M.I.N: 60*W A50*K ALTHOUGH still to be evaluated or even tried, two Britishsystems haye, as previously mentioned, been selected byI.C.A.O. for development. These are Dectra and Delrac, and at present discussions are in progress between the DeccaNavigator Company and the various official bodies concerned, aimed at installing an experimental Dectra layout covering thePrestwick-Gander route. Trials are expected to begin shortly, and the aid has reached an advanced stage of development.Delrac, however, remains a paper project for the time being. Both Dectra and Delrac operate on hyperbolic principles, basedon already well-established Decca techniques. This is of con- siderable importance, because it means that much of the evalua-tion necessary has already been accomplished. Let us in the first instance consider the way in which these Decca techniqueshave been applied to Dectra, and the way in which Prestwick- Gander coverage would be accomplished. All Decca aids are hyperbolic, operating (like Loran) on thedifference between master and slave transmission. However, unlike Loran, Decca measures the difference in phase betweentwo transmissions in order to form a hyperbolic family of position- lines. Dectra is merely an adaptation of Decca to cover a specificroute, whereas Decca covers an area. To mark a route, or track, Dectra employs two pairs of trans-mitters, each pair straddling the track at either end. A pah- consists of a master and a slave, and these are situated some 80miles apart; the line joining the transmitters is called base-line. For various reasons the perpendicular bisector of the two base-lines should aim at coinciding as nearly as possible with the track. Perhaps the most important of these is the fact that the centralmember of a hyperbolic family is a straight line. The master transmits a continuous signal with periodic inter-ruptions. When these occur the slave radiates on the same fre- quency, having received from the master a short triggering pulsewhich corresponds to the lane-identification signal in standard Decca. Forming part of the airborne receiver is a highly stableoscillator, and a "memory" circuit effects comparison between the phases of the two transmissions. This enables the hyperbolic Fig. 1. The Dectra plan for the Prestwick to Gander route. The five- and ten-mile-accuracy contours are indicated, together with the tracking and ranging patterns. Approximately every ten lanes of tracking pattern and 100 lanes of ranging pattern are shown. SINCE the first part of this article appeared, in last week's issue, it hasbeen announced that the Decca company has received a contract from the British Government for the installation, with the co-operation of theCanadian Government, of Dectra transmitting stations in England and Newfoundland for their evaluation as a north Atlantic navaid. The pro-posed arrangement is discussed in this second and final instalment. position-line appropriate to the position of the receiver to becomputed and indicated. It will be appreciated that as a given track is being marked the main function of Dectra is a track-holding one, and therefore the reason for aligning the straight members of the two hyperbolic families with the track becomesapparent; normally only the hyperbolae at the centre of the family are employed in this track-holding role, although the more curvedmembers will afford excellent assistance off the required track. One pair of transmitters covers one half of the route, while theother pair covers the remainder. There is, of course, an area of overlap at the centre, and in this area the receiver frequency ischanged to cover the second half of the route. It will be seen, therefore, that the technique involves the propa-gation of two families of hyperbolae, the centre members of which are similarly orientated within 180 deg. Now, in addition totracking, Dectra also provides accurate ranging. This is accom- plished in either of two ways. The first effects comparison ofthe phases of the transmissions between the two masters at a common comparison frequency. This results in the formationof a hyperbolic pattern lying across the tracking pattern; and by using the familiar comparative process the receiver is thus ableto indicate not only the horizontal tracking hyperbolae but also the vertical ranging hyperbolae. In fact, if the line joining thetwo masters be considered as being similar to the base-lines men- tioned earlier it will be seen that the ranging pattern aims at using Fig. 2. An actual draft Dectra Flight Log chart. In this case a very small scale is used and the chart is drawn for the indirect Prestwick to Gander route via Reykjavik. This necessitates using the outer cover of the Dectra installation on the fringe of the 10 n.m. accuracy contour (see Fig. 1). Three tracks with a 20 n.m. separation are shown; operational charts would include latitude and longitude and diversion information.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
