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Aviation History
1958
1958 - 0645.PDF
FLIGHT, 16 May 1958 NAVIGATION . . . in advance, rather than by additional boxes in the aircraft. Further,if the aircraft is to operate in any area where Decca/Dectra are not available, Doppler can be used in the pictorial form and thus nomental re-orientation is required of the pilot. The basis of the French Radio Mailles (also known as RadioMesh and Radio Web) is that, if two separate stations transmit signals modulated by adjacent but distinct frequencies the signalsare received in phase along a moving interference pattern, or equi- phase line, hyperbolic in shape. The speed is governed by thechoice of signal frequencies, and the distance between two stations is therefore scanned at a pre-determined rate. The aircraft equipment consists essentially of a counting devicemeasuring the time interval between the passage of two equi-phase lines from which position lines are derived. In a further variationof this system the appropriate air traffic control receives a position signal from the aircraft when an equi-phase line passes through it.In Radio Mailles the stations are normally located at the corners of a square and, although it was initially planned as a short rangesystem, the possibilities of choosing suitable frequencies, increasing the sides of the square and thus converting the system for longrange application, are being considered. Radio Mailles is in a purely experimental stage and no test results are so far available. Doppler radar, finally shorn of the last shreds of security cover,is now preparing to make its debut in civil service. Already one adapted system has flown in scheduled service with PanAm; andthe first genuine civil Doppler is being demonstrated in a Viking. Companies engaged in preparing civil equipment include Britishand Canadian Marconi, Decca Radar, Computing Devices of Canada, General Precision Laboratories, Collins, Bendix andRadio Corporation of America. The last three companies have only recently entered the field. Considerable experience of Doppler radar in operational servicehas now been gained; and probably more in Britain than in America. Some of the results of R.A.F. experience were detailedduring a meeting of the Institute of Navigation last November Flight for November 29). Further details on the use of the Mar-coni AD.2000 were given in a paper presented in March to the Institution of Electrical Engineers convention on radio aids toaeronautical and marine navigation Flight for April 11). These military systems have formed the basis for the designof highly specialized Doppler navigators for civil airliners and the following remarks from Decca Radar, Ltd., show the reasoningwhich led that company to the design of the Doppler element of D.I.A.N. It appears to bear a fairly dose relationship to avery compact Decca military Doppler displayed at R.R.E. Malvern late last month. Doppler information may be derived by a variety of types oftransmitter/receiver, and at this stage in the evolution of Doppler it is not possible to state categorically that one technique is betterthan another. Inevitably the technique which is selected repre- sents that compromise which the particular designer considerswill best meet the end he has in view. Several factors influence the type of system used, the two maintypes being coherent systems and self-coherent pulse systems. The first type may be further sub-divided into the pure continuouswave systems, frequency modulated continuous wave systems, and coherent pulse systems. Each of these has both advantages and disadvantages, the salientfeatures of each being as follows: — Continuous Wave. Unmodulated continuous wave seems at firstsight to offer a simple straightforward system. But this proves to be a false assumption since very great frequency stability isrequired, and reflections from the radome and other structure which may be vibrating cause unwanted frequency modulation ofthe transmitter which, unless transmitter-to-receiver isolation is of a very high order, cause spurious signals and extra noise in theDoppler band. This forces the use of separate transmitting and receiving aerials, and so is wasteful of space in the aircraft. It alsomakes it almost impossible to use an aerial which swings in azimuth for drift measurement. Frequency Modulation. One way of overcoming some of thedifficulties associated with continuous wave systems, is to adopt frequency modulation of the transmitter. When this scheme isused, the Doppler frequency is obtained by mixing the returned signal with a sample of the transmitted signal. This mixing pro-duces a signal at modulation frequency, and harmonics of this frequency with the Doppler frequency as sidebands to these fre-quencies. One property of this scheme is that there is zero signal returned from zero range thus removing difficulties caused byzero-range reflections; and improved discrimination against such effects can be obtained by using a harmonic of the modulationfrequency. The modulation index is adjusted to give the maximum power at the required harmonic. There is, however, some price to be paid for freedom fromtrouble. For example, using the 3rd harmonic introduces a loss of 13 db compared with a C.W. system. The Doppler frequency fromeach aerial beam can be obtained with a good signal-to-noise ratio The amused look of Minneapolis-Honeywell designer Philip LaHue belies the intensive work which went into the design and production of the floated mertiai-quality gyro, which he is here comparing with a child's toy. by using coherent detection. Unfortunately these frequency spectrabecome very broad at high altitude due to unwanted F.M. of the transmitter; and this has to be cancelled out by mixing the spectrafrom forward- and backward-looking beams before tracking. This procedure causes the generation of extra noise and so most of theadvantage of coherent detection is lost. Coherent Pulse. The coherent pulse system may be thought ofas a sampled C.W. system, the zero-range reflections being removed by the receiver gating. The previous remarks aboutcoherent detection and mixing before tracking apply. Principal disadvantages originate in the need for close frequency controland the difficulties associated with obtaining coherent pulses of microwave energy. A further difficulty is "altitude hole" effectby which, at heights which correspond to the echo return time, the return signal is received while the receiver is gated for trans-mission and a reduced signal of lowered accuracy is obtained. But this can easily be overcome by frequency modulation of thePJR.F. at a low frequency. Self-coherent Pulse. A self-coherent system has the merit thatthere is no stringency in respect of transmitter control. The Doppler frequency is obtained by using the energy returned fromone beam as the reference for that returned from another, so that a continuously operating reference oscillator is not needed providedthere is time-coincidence between the returns from the two beams. Neither is there any trouble from transmitter noise because thereceiver is gated off during transmission. But there are "altitude hole" difficulties.The coherent systems mentioned here all have the advantage that no microwave local oscillator is needed for the receiver, but allrequire carefully regulated power supplies for the transmitting tube, which is normally a klystron. Furthermore, the transmittingtube has an efficiency of only about 3 per cent so D.C. power requirements per transmitted Watt are high. The question of comparing the realizable efficiency of the foursystems is quite complicated, but to a first approximation the C.W. is the most efficient, the coherent pulse probably next inorder, and the frequency modulated and self-coherent roughly similar at perhaps 15 db below the C.W. performance.This comparison refers to the efficiency with which Doppler power is obtained from transmitted power, but when the com-parison is made to include the efficiency of the transmitting valve, the self-coherent system is substantially the more efficient becausethe transmitter valve, being a magnetron, has an efficiency of between 20 and 30 per cent, whereas in the coherent systems thetransmitting valve is a klystron with an efficiency of about 3 per cent. Furthermore, if it is necessary for any reason to increaseperformance, this can be done with a self-coherent system simply by increasing transmitter power. If the same must be done witha C.W. system, or an F.M. C.W. system, it is necessary not only to increase the transmitted power but also to increase the degreeof isolation between transmitting and receiving aerials. If it is hoped to use a duplex system, the degree of performance to beexpected from a C.W. system may be limited by the performance of the duplexer. In the case of the self-coherent system, duplexer operation wouldcertainly be used, which offers the practical advantage that crystals may be located in the T.R. unit remote from the aerial. In thecase of C.W. and F.M. C.W. systems, either a duplexer with limited operational capabilities can be used with crystals mountedin the T.R. system, or separate aerials can be used in which case either the crystals may be mounted on the aerial or a second wave-guide must be installed. When all the factors above-mentioned had been taken into con-
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