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Aviation History
1971
1971 - 0812.PDF
FLIGHT International, 20 May 1971 A PRECISION SECONDARY RADAR . . . tion which is significantly in error. The human operator's task of defining the true track is now more difficult even though the label and the primary radar "paint" provides assistance. As air-traffic densities increase, the controller's work-load increases and less individual attention can be given to each aircraft track. The load can be eased by the introduction of com puters and data-handling systems. This often requires automatically pre dicting the aircraft's position and it is at this point that serious errors in angular position resulting from lost replies become unacceptable. A bear ing error which exceeds the limits, in the computer programme, for a straight track is interpreted as the beginning of an aircraft turn, and as a result, the track is lost and must be re-initiated by a time-consuming process. The Royal Radar Establishment has been conducting studies on SSR prob lems such as these for some time and last January, C. Ullyatt of that estab lishment reviewed this work in a lecture given to the Institution of Electronic and Radio Engineers, with specific reference to Adsel (SELective ADdress). Adsel is fully compatible with SSR as defined by ICAO Annex 10, but in addition, employs selective address thus permitting each transponder to be interrogated individually. This facility, if universally applied, would remove all interference problems. Accurate position information can be obtained from range and bearing measurements made on the single interference-free reply resulting from a single directional interrogation. The available bandwidth is more fully exploited by including, in both the ground-to-air and air-to-ground mes sages, data to assist the pilot and ATC. Clearly a selective address system for SSR must use internationally agreed message formats and modula tion methods. The process by which such international agreement is reached is always lengthy, and since it must be followed by the develop ment and production of the equip ment it could take many years before such a system became an inter national standard. However some of the techniques which Adsel employs can be applied to SSR without con travening the Icao recommendations or requiring modifications to the transponder. A theoretical study of the methods which might be used for, and the accuracy which would result from, direction finding on individual radio frequency pulses of a reply (as opposed to finding the centre of a number of video pulses) shows that an overall system error of 16min of arc (r.m.s.) on an aircraft at 160 n.m. may be expected. Since only a single 717 Right, latest SSR 700 installation to be completed at Burrington, Devon. Below, CSD 1000 high-speed graphic display which may be used in con junction with a fibre-optic light pen for the external insertion of infor mation pulse is necessary to obtain this in formation good positional accuracy re sults even when a reply has much of its content corrupted by interference. When a number of pulses of the reply are "good" the accuracy improves still further. The method employed also prevents signals which are received in the side lobes of the aerial from reaching the output of the receiver. Thus much of the interference, which would other wise corrupt the information in a wanted reply, is removed. In the existing environment, a single interrogator using the new sys tem would still experience lost replies, but because the measurement of angle is made on every received pulse this loss will have no serious effect on accuracy. If all interrogators were fitted with these improvements it would be possible to make a consider able reduction in the mean interroga tion rate, and thus reduce the number of lost replies. As few as four inter rogations per beamwidth could give adequate redundancy, a reduction of five times in the current interrogation rate. To incorporate the necassary modi fications in existing SSR stations is a practicable operation. Modifications to the aerial feed arrangements are required to enable the signals (which contain the bearing information) to be extracted and passed to a special dual-channel receiver. The bearing of each reply pulse is automatically cal culated and translated into digital form. The information is assembled together with the range and validated identity and height of the aircraft into a format which is suitable for transmission over a narrow-band link and for presentation on a synthetic random-access bright display. Further extensions are practicable. If some of the potential reduction in interrogation rate is sacrificed and a back-to-back aerial arrangement is adopted an appreciably higher data rate can result. This would enable a more nearly continuous track of the aircraft to be presented to the con troller. An important feature of Adsel may be adopted. A relatively infrequent search routine, at a low interrogation rate, would be used to acquire aircraft entering the surveillance area. Sub sequent to the acquisition the accurate positional information would permit auto-tracking at the radar head, with interrogations made only in the direc tion of the aircraft. Lost replies would be still further reduced as a result, and the accurate positional informa tion from several sensors could be correlated at air-traffic centres to give the controllers highly reliable, accu rate and frequent up-dating of their display. Cossor Electronics are engaged on the development of this system, which they have named Cossor Precision Secondary Radar. It combines im proved accuracy with decreased generation of, and less susceptibility to, interference, and is also compatible with the requirements of an advanced data link such as Adsel. • REFERENCES 1 Sensors for the ATC environment with special reference to SSR. C. Ullyatt, UK Symposium Electronics for Civil Aviation September 1969. 2 Secondary Radar in the era of auto- tracking C. Ullyat, IEE Conference Publication No 28. 3 Report of Department of Transporta tion Air Traffic Advisory Committee. December 1969 (USA).
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