FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1961
1961 - 0861.PDF
FLIGHT, 22 June 1961 873 The first of these new ground interrogator-responsors, engineered to a Ministry of Aviation specification incorporating the ICAO requirements, has recently been installed at London Airport and it is to be expected that before long there will be a requirement for certain classes of aircraft flying over the UK to carry transponders. These moves have a threefold importance. First, the adoption of the system anywhere and to any extent is a contribution to the general safety of air travel and is to be welcomed as such; secondly, the sooner the system is in widespread use—and the actions so far taken can only encourage such use—the more rapidly will data become available on which procedural development (which is embryonic at present) can be based; thirdly, the large-scale instal- lation of capital equipment may check the headlong rush of some enthusiasts in the direction of greater and greater system elabora- tion. Taking the highest common factor of the equipment most recently developed by those manufacturers who make airborne transponders, the aircraft installation (which comprises a transponder, a control unit and an aerial) has, among others, the following characteristics:— (a) The ability to reply to two interrogation "modes" (the name givento the ground-air coding process, the term "code" being restricted to the air-ground transmissions). One of these modes (Mode A) is thesame as one of the modes (Mode 3) of the military IFF MklO secondary radar system, which operates on the same interrogation and replyfrequencies: the other (Mode B) is peculiar to civil aviation. Selection of the mode of interrogation to which the transponder will reply is bymeans of a switch on the control unit. If all civil transponders in an area are set to reply on Mode A, there-fore, and all military transponders are set to reply on Mode 3, a Mode A interrogation will cause all transponders in the area to reply, so thatresponses from all transponder-equipped aircraft can be displayed at once. Alternatively, by causing the civil transponders to reply on ModeB and the military transponders on Mode 3, civil or military aircraft can be interrogated at will. By using a technique known as "modeinterlace," whereby interrogations on one mode are alternated with interrogations on another (the modes are characterized by the timeinterval between a pair of pulses in the interrogation—8 and 17 micro- seconds for A and B respectively) the ground equipment can interro-gate all transponder-equipped aircraft in the area and display responses from civil and military aircraft on the same display or separately,according to the control requirements. (b) The ability to transmit a coded response to an interrogation oneither mode. The code is manually selected by a switch on the control unit and there is a choice of 64 codes, which are represented physicallyby the presence or absence of pulses in six possible positions between a pair of "framing" pulses. The framing pulses, which are alwayspresent in a response and always 20.3 microseconds apart, not only define the positions of the "information" pulses but also help thedecoding equipment on the ground to distinguish between an aircraft response and a set of random pulses from some other source. (c) The ability to transmit, in addition to the pulses forming the codedresponse and outside the code frame, a special identification pulse. The process of radiating this pulse is initiated by pressing a button onthe control box, and the pulse is radiated with every response for a period of about a quarter of a minute thereafter—the long period beingnecessary to ensure that the period of rotation of the ground interro- gator aerial is comfortably exceeded. (d) The ability, upon interrogation by an appropriately designed inter-rogator, to distinguish between the main lobe of the interrogation radiation pattern and its sidelobes and to reply only to the main lobeinterrogation ("sidelobe suppression"). Since no further mention of sidelobe suppression is made it may here be briefly noted (ref 1) thatthe advantages of this technique include a reduction in the transponder workload, the avoidance of confusion on ground displays which canresult from the presence of incorrectly positioned responses to sidelobe interrogations, and a reduction in the total number of radiated responsesin any area (with consequent reduction in the possibilities of inter- ference with one ground equipment by responses from aircraft inter-rogated by another ground equipment). These four characteristics, given suitable ground equipment and an aircraft fitment programme resulting in the installation of transponders in all aircraft in one or two classes (such as turbojet or turboprop aircraft, which can be distinguished from others by their cruising altitudes), provide an extension of existing radar facilities sufficient to justify immediate implementation. After the system has been in use in this simple form for long enough for the basic procedures to have been sorted out and internationally agreed, some of the possible extensions and elaborations can be considered for adoption. By then there will be more widespread knowledge of the system—practical experience of which is at present virtually confined to small groups of people in the UK and the USA—and there will be a better chance of its development in a generally acceptable direction. At the same time, it is most desirable that those who are about to commit themselves to the purchase of equipment—particularly ground equipment, as the basic aircraft installation is already inter- nationally standardized—should take note of the probable direc- tions of system extension and make allowance for the possibility of ;heir being introduced at a later date. Otherwise there is the danger of a situation—not unknown in the field of aviation electronics— m which a desirable development is balked by the existence of a widespread and heavy investment in a form of installation which is incompatible with the development in question. Some of the most important of the possible extensions are listed below, together with some comments on their present status:— (1) Additional Interrogation Modes: Two further modes (C and D),for civil use only, are envisaged; provision for them is required in all current specifications. It is likely that Mode C will be used for obtainingaltitude information from aircraft but the status of Mode D is as yet uncertain. (2) Additional Reply Codes: By adding further information pulsesbetween the framing pulses of the response the number of available codes can be substantially increased. For reasons of system compatibilityduring a transition phase, the only reasonably possible method of increasing the coding capability is by putting the additional pulsesmidway between the existing pulses—giving a total of 8.192 codes. It is extremely unlikely that so large a number will ever be used for pro-cedural purposes; indeed, the present 64 codes appear to be more than adequate. On the other hand, 8,192 is too small a number for an air-frame identification system—though it might be possible to devise a flight identification system within these limits. The most probable useappears to be for altitude telemetering purposes; although some experi- mental work is being done in this field using only 64 codes, there is aschool of thought which advocates the use of a much larger number. Servo rock of ground equipment withdrawn to indicate serviceability, and (right) rear view of servo units, showing wired-in components A disadvantage of a substantial extension of the number of availablereply codes by the addition of information pulses is the increase in the total number of pulses likely to be radiated in a given area during agiven period. Such an increase must lead to an increased probability of false coding and other interference problems. This difficulty can,however, be overcome by accepting the lower coding capability—which is yet much greater than the 64 codes of existing transponders—resultingfrom using all the additional pulse positions but restricting the maximum possible number of information pulses to some low figure. Maximumprotection against false coding can be achieved by using a constant number of information pulses (ref 2). (3) Automatic Selective Reply (ASR): As already described, theinterrogation mode to which the airborne transponder will reply is at present selected manually. Automatic Selective Reply is the namegiven to a technique whereby the transponder itself decides which interrogation mode is being used and replies accordingly; the idea isthat the airborne installation will be set up to reply with a different kind of information on each mode, so that the ground controller cancall off the information he requires by appropriately selecting the interrogation mode. The principal application of ASR currently envisaged is for altitudetelemetering. It is proposed that Modes A and B shall continue to be manually selected (ASR on Mode A is in any case undesirable, sinceit could result in civil transponders always replying to military interro- gators operating on Mode 3) but that an interrogation on Mode Cshall elicit a reply containing altitude information from an appropriately equipped aircraft (and no reply at all from any other aircraft) withoutthe intervention of the pilot. (4) Altitude Telemetering: The main technical features of thisproposed development have already been described. In physical terms it is proposed that the facility shall be provided by the addition of a
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
