DELRAC


INTRODUCTION

The Decca Company will long be remembered for its efforts to get the Decca Navigator adopted as a standard aviation navaid, culminating in a hard fought battle at the 1958 International Civil Aviation Organization meeting in Montreal, Quebec which it lost. Many Decca employees believed in Navigator as much as anyone, but in retrospect, the outcome in favour of VOR/DME was the right decision. Decca's susceptibility to static interference and tendency for ambiguity (false lane identification) were the two major reasons for it being rejected for use in aviation. It didn't matter much for ships who could afford to wait a bit for the next fix if a thunderstorm arrived and average out a number of lane identification cycles. Although Decca was a very successful marine navaid it had limited success in aviation. Besides the Navigator system, Decca had a relatively unpublicized effort in several other low frequency, hyperbolic navaids. One such example was Delrac. (DEcca Long Range Area  Coverage).

HISTORY

As early as 1945, Decca carried out a study of the feasibility of a long range navaid using very low frequencies and as a result came out with a proposal in February 1946 for a VLF Decca chain to cover the North Atlantic. The chain was to have two stations in western Ireland generating a hyperbolic pattern, the right bisector of which was to coincide with the Great Circle track from Shannon to Gander, and a third to a ranging station in Bermuda. It never went ahead partly because the Irish Post Office were worried about it jamming the Irish telephone system. In addition, the interest in such a such a system was very low at the time as all trans-Atlantic aircraft were still flying via Iceland and Greenland.

Delrac was never implemented, but theoretical work continued, and in 1951, a paper was issued describing a fully developed system that provided a wide area coverage and not limited to specific tracks. After Decca had acquired the POPI (Post Office Position Indicator) patent rights, the proposal was resurrected and details were circulated widely in both the UK and United States in 1954. Dectra was proposed to the International Civil Aviation Organization to meet an RTCA requirement for an accurate long range navaid. Briefings given in the USA included one at the Pentagon, where it was discussed in relation to an American system called Radux that was then being developed under military classification.

To obtain worldwide coverage, Delrac would have required some 28 transmitters. Decca surmised that designing for comparatively limited range and shorter baselines but using more transmitters would provide a better system than attempting very long range cover with only a few transmitters. Otherwise, Delrac had many similarities to Omega, which had not yet appeared. Delrac had lane identification using frequencies of f, f+1/3f, f+1/9f, and f+1/27f, providing ambiguity resolution in steps of 3: 1. It had been Decca's experience with their Navigator system that 3: 1 was the biggest step that could safely be used. A table of possible radio frequencies was published in the system description. One set consisted of the sequence: 10.2, 10.578, 11.333 and 13.6 kHz, the frequencies eventually used for Omega. Claimed accuracy was better than 10 miles at the 95 percent level at up to 2000 miles range. Although Delrac received UK Government support it did not proceed, being superseded by Dectra. However, it was claimed by Decca that the Omega design team had used the main features of Delrac and this led to legal action in 1973.

GENERAL DESIGN SUMMARY
(From DELRAC Issue 5 document)

The proposed Delrac system would use VLF frequencies to ensure consistent signals at all distances. It was proposed that the most ambiguous, and therefore most stable pattern used for actual fixing would be generated by transmissions from pairs of stations in a frequency range 10 kc/s to 12 kc/s. In that way, phase anomalies at the region of signal equality of ground wave and first hop sky wave nodes would be avoided and continuous fixing cover assured from short to long ranges. The ambiguities of this pattern would be resolved in steps by the use of coarse patterns generated by additional transmitted frequencies.

The whole system was to be time multiplexed as well as frequency multiplexed and the synchronization of airborne and ground switching was to be achieved either by coded guard period lengths or simple clock mechanism, thus avoiding the necessity of a special triggering signal liable to failure in circumstances of noise or for other causes. The transmitting station, with very high Q aerials , would have switchable aerial tuning since only a single frequency was to be transmitted from any station at any one time.

Delrac would have used pairs of ground transmitting stations to generate hyperbolic position lines. Reception from two such pairs would provide a fix.  The position line information would be displayed in the aircraft or ship by means of Decometer indicators used in conjunction with overprinted charts. Alternatively, the fix could be plotted automatically and displayed pictorially by the Flight Log. About twelve pairs of ground stations would have given the necessary world coverage with a fix accuracy of better than 10 miles at the 95% probability level.

The system would give an unambiguous fix by the use of a continuous Lane Identification system which resolved the ambiguity in stages sufficiently gradual to ensure reliability under all operating conditions.

LATER DEVELOPMENTS

Proposals for Delrac had been forthcoming on a regular basis since 1947. Decca kept low level background work going and in 1951 came out with a much revised version of Delrac designed specifically to take advantage of the properties of VLF. They had also by then acquired the patents taken out by the British Post Office on a VLF system known as POPI (Post Office Position Indicator). One of the salient features of POPI was to be the use of very stable oscillators locked to the off air signals and make the phase comparisons between these oscillators. In this way, any gaps in the inherently and rather noisy off air signals could be overridden. Details were circulated widely to both the British and American Governments in 1954 and significantly, as it turned out, a full presentation was made to U.S. Armed Forces Chiefs in the Pentagon on 30th April. At the time, before inertial navigation really became a practical proposition, navigation of long range bombers was a major problem and various types of low frequency radio aids were being pursued. The U.S already had a VLF. system known as Radux on trial but it was not proving very successful and it was seen that Delrac might solve some of the existing problems. The document describing Delrac contained a list of proposed frequencies, one set of which was 10.2, 10.578, 11.33 and 13.6 kHz and explained how these frequencies would be switched between transmitters. However, no formal agreement was signed with Decca and Delrac lapsed.

REFERENCES:

1) Walter Blanchard article July 1998; Past President, Royal Institute of Navigation.

2) Delrac Proposal Issue 5; dated May 1954. The Decca Navigator Company.

3) The Journal Of Navigation - Chapter 4.W.F. Blanchard, Royal Institute of
    Navigation; Vol 44, No. 3; Sept 1991. Used with permission.
 

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May 31/08