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Aviation History
1956
1956 - 1451.PDF
FLIGHT, 12 October 1956 DECCA MARK 10 Latest Development in the Decca Navigator System THE latest development in the range of Decca Navigatortq£i$r^nlils the Mark 10> which was &st shown at theS.B.A C. Show last month. A development of the Decca Navigator System, it is primarily intended to meet current andtuture air navigational needs in the most effective manner For these applications it represents a significant advance over theexisting system in terms of improved night-time coverage, simpli- fied operation and minimum residual ambiguity of position fixingllie additional transmissions required from the ground stations are compatible with the present system and will in no way affectoperation of existing receivers. The improved performance of the Mark 10 system largelyderives from a modified transmission method requiring a short burst ol all four normal Decca frequencies, which are harmonicallyrelated in the ratio 5f, 6f, 8f and 9f from each station in turn, together with a new frequency, 8.2f, for zone identification pur-Pi°SeL « n?rmal fine Patterns are produced by transmission of the 51, 61, 8f and 9f signals from Purple, Master, Red andGreen stations during the remainder of the operating cycle. Though the present simultaneous transmissions of two signalsfrom each of master and slave stations for identification of the lanes of the pattern on the base-line between them will beretained on the present basis, the separate transmission of a burst of four frequencies from each station enables a new system oflane identification employing a synthesised pulse to be imple- mented with considerable advantage. It can be shown that individual variations in phase of thefour component frequencies forming the pulse will, within normal operating limits, give rise to amplitude variation in the resultantpulse signal, with the result that a pulse amplitude gate may be employed to discriminate between valid and false lane identifica-tion conditions; and the permitted ratio of reflected skywave to groundwave signals for safe lane identification has been increased.As a result of this increased skywave tolerance, the operating range for lane identification should extend at night to a distanceequal to that for effective normal pattern operation, while the use of the pulse-limiter technique enables automatic lane identi-fication to be employed. False indicators are rejected, the Deco- meter pointers adjusting themselves to the correct settings uponreception of the first lane identification sequences and being checked thereafter by succeeding sequences. The high accuracy inherent in the Decca Navigator System isobtained by accepting multiple ambiguities in the fine pattern position fix, and resolving them by lane identification to reducethe residual ambiguity to an average (dependent on base line length) of some 10 to 18 deg of position-line azimuth. Theintroduction of an additional 8.2f signal in the Decca transmis- sion plan, producing, in conjunction with the 8f transmission, a0.2f pattern, now provides a further five times reduction, giving a final ambiguity averaging 50 to 90 deg of position-lineazimuth. By the use of the superheterodyne principle in the receiver,operation on up to 21 Deccp chain frequency groups is facilitated while still retaining the simple chain switching that is a featureof current Decca receivers. A second advantage of the super- heterodyne method lies in the fact that this increased frequencycoverage can be provided with approximately one-quarter the number of quartz crystal filters that are required for nine-chainoperation in the current type of receiver: this reduction in space occupied by the crystals permits the use of a thermostatically-controlled housing and the improved phase stability thus obtained makes it unnecessary to reference or adjust the phase-characteris-tics of individual receiver channels during receiver operation. To improve lane identification at long ranges, Mark 10 Deccaemploys the synthesised pulse technique mentioned above. Harmonics of the pulse recurrence frequency, widely separatedin frequency, are separately received and then combined to pro- duce a pulse stated to have characteristics much better than canbe obtained by conventional methods within die maximum pass- band practicable at the transmitted frequencies.Essentially, the Mark 10 identification method is similar to that of the existing system, a coarse If pattern being effectivelysuperimposed on each of the three fine patterns in turn, embracing 24, 18 and 30 lanes in the Red, Green and Purple patternsrespectively. The If pattern is, however, generated by succes- sive transmissions of a burst of all four frequencies 5f, 6f, 8f and9f from each station in turn, transmissions from the other three stations being suppressed at these times. The information pro-vided by each transmission is effectively stored in the receiver to permit phase comparison. The additional frequency of 8.2f is ofno consequence here, but is necessary for zone identification. The four lane identification frequencies are carefully phased at the transmitter and the resultant waveform appears as shown inthe diagram. It will be seen diat the required If sub-harmonic is very clearly defined by the large peaks at this recurrence fre-quency. In the ideal case, if the four signals are received via separate A.G.C. controlled channels, any amplitude variationsin the individual components will be corrected and this waveform may be reconstituted in the receiver. Furthermore, considerablephase deviation of the individual signals can be allowed before any other false peak of comparable amplitude will appear in thereceiver waveform. While the amplitude of the wanted peak will fall, no false peak can rise to an equal amplitude level underany conditions with skywave/groundwave ratios of up to 44 per cent. An amplitude gate can therefore be set to accept the greatersignal only and will pass either the wanted If component in the form of a short pulse or no signal at all. Not only does the relaxation of skywave/groundwave ratiolimit extend the expected range of lane identification to that at which skywave effects tend to make the fine patterns unacceptablefor normal applications (the approved night-time range of the system is 240 nautical miles), but the use of the pulse-limitertechnique also provides continuous discrimination against fake lane identification within that range. This factor is veryimportant from the user's point of view, as it permits the inclusion 9f The Mark 10 Decca lane identification pulse. of circuits in the receiver to correct automatically the Decometcrreadings to the right position within a zone, providing the reduc- tion in residual ambiguity given by the existing system withoutrecourse to any manual adjustment. With the inclusion of an 8.2f component in the lane identifica-tion signals, when a 0.2f pattern is generated togemer with the 8ft component, a further five times reduction is, of course, madein the residual ambiguity of the system. As this pattern is gener- ated by two frequencies only, die tolerance to skywave effects islower than that of the lane identification system. Zone identifica- tion is therefore not made fully automatic, the indications beingbrought up in succession for Red, Green and Purple patterns to permit checking and, if necessary, manual correction. The continuously operating zone indicator at the base of meDecometer dial is coupled to a sector-shaped pointer mounted concentrically with the lane pointer and performing one revolu-tion per five zones. The zone identification indication is given by the lane pointer, the drive mechanism being automaticallyswitched to perform this secondary function upon reception of the appropriate lane/zone identification signal. Correct zoneindication is shown by the lane pointer falling within the zone pointer sector during identification; and an error in the zoneindicator setting results in a displacement of the sector-shaped pointer from the lane pointer position by 72 deg or a multiple of72 dsg. Correction can be made for such errors by means of the resetting button on the Decometer, which rotates the zone indica-tor and pointer in one-zone steps. This is the only operating adjustment required apart from initial setting-up of the FlightLog, when the latter forms part of the Mark 10 installation. Zone identification provides unambiguous position-fixingwithin a group of five zones. Physically, this area will vary with the length of baseline (i.e. the number of zones in the pattern)and the bearing with respect to that baseline. But an average on entering the Decca coverage is of the order of 60 to 90 degof position-line azimuth referred to the centre-point of that baseline, the higher figure applying, for example, to the existingEnglish Decca Chain.
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