Navigational Aids
Decca
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Major system characteristics |
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System type |
Hyperbolic navigation system |
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Frequency band |
70 - 130 Kkc/s |
The text that follows in this section is taken from "The Services Textbook of Radio, Volume 7, Radiolocation Techniques" by Brig. J. D. Haigh, O.B.E., M.A., M.I.E.E., Edited by the staff of "Wireless World", H.M.S.O., London, 1960 pages 252 - 254. This volume was known to the British armed forces as Admiralty B.R.600(7), War Office 10224(7) and Air Ministry A.P.3214(7).
C.W. Systems - Decca
This system, which was primarily developed as a long distance marine navigational aid, is basically a 'range difference' system, but as c.w. is used the range differences have to be interpreted in the form of phase differences. Lines of constant phase difference will once again take the form of a family of hyperbolae with their foci at the transmitters. A fix is obtained by making use of two or more families of constant phase hyperbolae established by a master and two or more slave transmitters. A difficulty inherent in the use of c.w. is at once apparent ; if all transmitters were on the same frequency it would be impossible to sort out their phase relationships. To overcome this difficulty all the transmitters work on different, but related, frequencies. The frequencies used lie in the band 70 - 130 kc /s.
In order that phase comparison may be easily effected the frequencies of the Master station and the slave stations (normally known as the Red, Green and Purple slaves to correspond with the colours in which the associated families of hyperbolae are printed on charts) must be harmonically related. This relationship is established, for example, by choosing a generating frequency f kc/s and allotting frequencies 6f, 8f, 9f and 5f to the Master and Red, Green and Purple slaves respectively. Any other integral multiples of f could be used. At the receiver there are four radio-frequency amplifiers, fed from a common aerial, each tuned to one of the four transmitters. Phase comparison has to be carried out at a common frequency so the received Master signal is frequency divided to f and then frequency multiplied to 8f , 9f and 5f for phase comparison with the signals from the three slaves.
The Master and Slave stations are usually spaced many wavelengths apart so that the family of hyperbolae will contain many that correspond to zero (or 2np) phase difference and there will therefore be a multiple ambiguity. The area between two adjacent lines of zero phase difference is known as a lane and phase comparisons carried out at the receiving point will merely show the position within a lane. To get complete information it is necessary that the phase comparator be of such a kind that it integrates the total phase change. Provided that the lane in which the point of departure lies is known, and continuous reception of the signals is maintained, the phase comparator will indicate not only position within a lane but also the number of lane boundaries that have been crossed. The latest type of indicator, having had the 'Decca co-ordinates' of the point of departure set in, shows both the lane and the position within the lane. There are of course three such indicators to show the positions in the Red, Green and Purple lanes.
For very long range navigation such as is involved in crossing the North Atlantic the Decca Navigator Company have proposed a system to which they have given the name Dectra.
The Dectra system is designed to give navigational assistance over a particular route. To do this a master and slave pair of stations is located at each end of the route in such a way that the route perpendicularly bisects the line joining the slave to the master of each pair. The sets of hyperbolae due to each pair of stations make a pattern such as is shown in Fig. 17-10.
The method of establishing the hyperbolae, and of measuring phase differences in the air, differs from the normal Decca system in that the master and slave stations do not transmit simultaneously but alternately. Thus the master transmits for a period and then, by momentarily changing frequency, signals to the slave to start transmitting. The slave, which has been using the master station's transmissions to control the phase of a stable oscillator, radiates in phase coherence with the master by using this stable oscillator to drive its transmitter. After a period the slave shuts down and the cycle repeats.
In the air a somewhat similar process is followed. The transmissions from the master station are used to phase lock an oscillator with very good short-term stability; then when the slave station radiates, the phase of its transmission, at the aircraft, is compared with that of the oscillator.
Different frequencies are used for the master-slave pair at either end of the route. in order to provide range measuring facilities the two frequencies F1 and F2 are made to differ by a frequency f which is also an integral sub-multiple of both F1 and F2 A typical set of frequencies might be F1 = 85.100 kc /s, F2 = 84.915 kc/s and f = 185 c/s.
Now if the stable oscillator is tuned to, and is being phase locked at F1 its frequency is subdivided until the frequency f is produced. A second r.f. amplifier is tuned to F2 and its output mixed with that of the other r.f. amplifier tuned to F1. This also gives rise to signals at frequency f. We now have two signals at frequency f. One is derived directly from the F1 signals and its phase is directly related to that of the F1 signals. The other is derived from both the F1 and F2 signals and its phase will be related to the phases of both these signals. If we compare the phases of the two signals at frequency f, the phase determining effects of the F1 signal cancel out and we are left with a phase dependent only on the F2 signal and a phase meter will plot the intersections of the course with a series of hyperbolae with the two master stations as foci and with a lane spacing determined by F2. By tuning the stable oscillator to F2 (when approaching that end of the route) a second set of hyperbolae, with lane spacing F1 can be established.
A block diagram of the system is given in Fig. 17 11.
In case, at either end of the route, the signals from the distant master stations arc not audible, an alternative ranging system is provided. This consists of a high stability, crystal-controlled, oscillator which is set up, before departure, to be in phase with the transmissions of the master station at that end of the route. Thereafter it is free running and by keeping it in an accurately controlled oven its frequency stability is assured to about one part in 108 for a period of an hour. After take-off comparison of the phase of the master station's transmission with that of the crystal-controlled oscillator gives a direct measure of the distance travelled.
In practice the crystal controlled oscillator works at a frequency NF1 or NF2, and is divided down to F1 or F2 for phase comparison.
