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Aviation History
1976
1976 - 0961.PDF
1 FLIGHT International. 5 lune 1976 "" he hears an instruction affecting one of his aircraft, he 'K acknowledges it with the correct formula and communi- i, cates the new heading or flight level, for instance, to the * computer via a conventional keyboard. Should the com- k puter decide that the pilot needs to make a call to a con troller—on passing a reporting point, for instance—it will put the required message on an electronic data display (EDD) in front of the appropriate controller. Once the 4 message has been relayed, the driver informs the com puter, which then removes it from the screen. * Of the nine major ATC simulators ordered from Ferranti i for installation around the world, the unit at Hum's ATCETJ is the biggest. Over 100 aircraft of ten different ^ types can be simulated on a playing area 800 n.m. square. j The secondary-radar sensors, each co-located with a primary radar, can process all 4,096 codes in the six trans- + ponder modes, including height-encoding. There are 500 1 navigation reference points, six ILS runways and ten "blip- drivers" capable of handling up to 40 aircraft each. The 1 Ferranti FM1600C which drives the unit is very compact, ^ much of the computer-room space being taken up with data stores and peripherals. •* In recent years, the advent of the commercial digital , computer has revolutionised ATC simulation, with the result that training, evaluation and validation are now * safe, cost-effective and fully realistic. * Adsel - second-generation SSR T HE USE of secondary radar and its airborne complement, the transponder, is expanding rapidly. First experi-\ mental trials started in the early 1950s, but it was some years before secondary radar was accepted by Icao and W implemented by national authorities. In the UK it is now R,* mandatory in much of the controlled airspace and avail able, to the mutual benefit of controller and pilot, in many I" other areas. ! Basically, the ground-based secondary surveillance radar (SSR) antenna emits a series of coded interrogation 1 pulses at a frequency of 1,030MHz. Any transponder with- i in this rotating beam responds to the interrogation by ( sending down a coded reply to the ground antenna. A different frequency, 1,090MHz, is used for the reply to ; minimise target-obliteration problems. : In civil air traffic, two modes of reply—A and C—are I used. Other modes—1, 2 and 3—are used by the military, 3 being the same as the civilian Mode A; Mode B is defined but is not yet in use. i In Mode A the air-ground link has a 12-pulse code, as well as control pulses which define the start and finish of the reply. Using the binary system, these 12 pulses generate 4,096 (two to the power 12) available codes. The 12 pulses are divided into four groups of three; each : group has a maximum value of 7 (all three pulses present) because the first pulse is given the value 1, the second 2 * and the third 4 (2°, 21, 22 respectively). An octal code i results, and this is why there are never any 8s or 9s in a transponder code. There are thus 4,096 codes to be shared between all participating aircraft. In practice this figure is reduced, however, as certain codes have specific meanings—7700 for general emergency, for instance. When the ATCO asks the pilot to "squawk 4321" ("squawk" is the ATC phrase meaning "operate your 1 transponder, using code . . . ") the pilot sets 4321 in the r window on his transponder. The unit senses the interroga tion pulse and, if the mode selected by the pilot is correct, sends back "4321" in octal code framed between two marking pulses. This is then decoded in the ground com puter for correlation with returns from primary-radar *- transmissions (a fuller description of SSR was given in : Flight, December 4, 1975, page 816). 1 Blocks of codes are assigned to national authorities, f mainly to ensure that no two aircraft flying within the same SSR beam are asked to squawk the same code by •v different controllers. In the more advanced systems the I ground computer can convert the transponder code into 1503 the aircraft's callsign before displaying it to the con troller. Mode C is slightly different in that it replies with air craft height rather than identity. This is usually done automatically, the height code, taken from a special alti meter or digitising device, being additional to the Mode A information. The height code is transmitted immediately after the Mode A reply, again using a 12-pulse train of data. The ground computer can then have this information displayed as a flight level (it is always transmitted in flight-level format), or converted to any other pressure- related datum. In this way the controller can not only monitor the progress of each target, suitably labelled with an identity, but can also keep an eye on its height. Both functions increase flight safety and reduce R/T chatter. In Britain Mode A is mandatory in all controlled air space and Mode C in all controlled airspace above FL100 and south of 52° 30'N; use of the latter is expected to spread in future. Shortfalls being overcome Successful as SSR has been in helping maintain the orderly flow of air traffic, its one or two drawbacks have led to development of a second-generation SSR system. Cossor has now begun trials at its Matching Green, Essex, test site of Adsel (selective address SSR). The main short falls of traditional SSR are a relatively low positional accuracy, and mutual interference resulting in the loss from the display of aircraft with similar range and bearing. Adsel is designed to overcome these problems and to pro vide additional benefits. The company received a British Government develop ment contract for Adsel in 1971. The US is working on a similar second-generation system, Dabs (discreet address beacon system); Dabs is some two years behind in develop ment but uses a similar data format. British flight trials are about to begin, with British Midland carrying the first SSR/Adsel-compatible transponders. These units function exactly as normal transponders as far as the crew and other, non-Adsel, ATC teams are concerned. It is probable, once the newer system has been adopted, that SSR and Adsel will have to work side^by-side for several years. Adsel (and Dabs) provides a unique address for each aircraft. The 12 pulses of ordinary SSR are increased to 24, giving over 16 million possible addresses. It is thus entirely practicable for each aircraft to have its own indi vidual Adsel code for its whole life, and for all ATC computers to have the code/callsign conversion per manently stored. Collision-warning possibilities The use of more refined radar-transmission techniques greatly improves accuracy; mutual interference is also eliminated. Further, the increased data capacity enables messages to be transmitted to the crew, or between air craft. Detailed applications for Adsel have yet to be deter mined—present trials are designed to show the feasibility of the concept—but collision-avoidance is one of the functions being discussed in the US. It is proposed that the three-dimensional position, speed and heading of all co-operating aircraft should be correlated in a computer and relayed in the interrogation pulse-chain to the aircraft. Each aircraft could then determine whether there was any threat of a collision or near-miss, and tell the crew which way to turn to avoid the hazard. Other possible uses for this data link include the continuous broadcast of weather information to the aircraft, and of aircraft instrument readings—heading, airspeed, climb-rate, for instance—to the controller. In the US, Dabs has been undergoing preliminary trials in the experimental facility (Dabsef) at Lincoln Laboratory, Cambridge, Massachusetts. Dabs is still mainly a genera] research tool for the investigation of beacon signalling and processing techniques. The facility is located near the busy Hanscom general-aviation airfield, and collision- avoidance tests have been conducted with Cherokees, Cessna 172s and Beech Bonanzas. The concept tested, inter-
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