From:
http://www.airwaysmuseum.com/Surveillance.htmThe first ATC radars used in Australia were wartime air defence units which were used experimentally. These radars were of the type that later became known as '
Primary' radars. That is, they worked on the well-known 'Battle of Britain' principle in which the radar transmitter sends out a pulse of radio energy, of which a very small proportion is reflected from the surface or structure of the target aircraft back to the radar receiver.
The azimuth orientation of the radar antenna provides the bearing of the aircraft from the ground station, and the time taken for the pulse to reach the target and return provides a measure of the distance of the target from the ground station. The bearing and distance of the target can then be converted into a ground position for display to the Air Traffic Controller. Target elevation (altitude) is not normally measured by ATC primary radars.
The advantage of
Primary Surveillance Radar (PSR) is that it operates totally independently of the target aircraft - that is, no action from the aircraft is required for it to provide a radar return.
The
disadvantages of PSR are that, firstly, enormous amounts of power must be radiated to ensure returns from the target. This is especially true if long range is desired. Secondly, because of the small amount of energy returned at the receiver, returns may be easily disrupted due to such factors as changes of target attitude or signal attenuation due to heavy rain. This may cause the displayed target to 'fade'. Thirdly, correlation of a particular radar return with a particular aircraft requires an identification process. When PSR was the only type of radar available, this was typically achieved by the Controller instructing an aircraft to turn and observing same on their display, or by correlating a DME distance report by the aircraft with the position of a particular return along a known track.
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Secondary Radar
The disadvantages of PSR outlined above led to the employment of another aspect of wartime radar development. This was the Identification Friend or Foe (IFF) system, which had been developed as a means of positively identifying friendly aircraft from enemy. The system which became known in civil use as Secondary Surveillance Radar (SSR), or in the USA as the Air Traffic Control Radar Beacon System, relies on a piece of equipment aboard the aircraft known as a 'transponder'.
The transponder is a radio receiver and transmitter operating on the radar frequency. The target aircraft's transponder responds to interrogation by the ground station by transmitting a coded reply signal. The great advantages of SSR are three: firstly, because the reply signal is transmitted from the aircraft it is much stronger when received at the ground station, thus giving the possibility of much greater range and reducing the problems of signal attenuation; similarly, the transmitting power required of the ground station for a given range is much reduced, thus providing considerable economy; and thirdly, because the signals in each direction are electronically coded the possibility is offered to transmit additional information between the two stations.
The disadvantage of SSR is that it requires a target aircraft to carry an operating transponder. Thus SSR is a 'dependant' surveillance system. For this reason, PSR will operate in conjunction with SSR in certain areas for the foreseeable future so that 'non-cooperating' targets, such as some light aircraft, can be detected.
SSR has several modes of operation, the basic civil mode being Mode A. In this mode the aircraft's transponder provides positive aircraft identification by transmitting a four-digit code to the ground station. The code system is octal; that is, each of the code digits may be any of the numbers 0-7. There are thus 4096 possible four-digit codes (e.g. 3472).
Another principal SSR mode currently used in Australia is Mode C. In this mode the aircraft's altitude, derived from on-board instruments, is transmitted to the ground station in addition to the identity. The use of Mode C was introduced in Australia in the late 1980s with the acquisition of ground systems, such as ATCARDS, capable of processing the information.
A further mode, Mode S (or 'Mode Select'), is also used. Aircraft equipped with transponders supporting this mode are assigned a permanent identification which can be selectively addressed by the ground radar. This reduces problems of garbling between SSR returns from aircraft in close proximity. Mode S also offers a wider range of data to be transmitted, including potentially an uplink of data from the ground station to the aircraft although this capability is presently not used in Australia.
Additional SSR Modes are used by military aircraft.
Incidentally, the phraseologies associated with the use of SSR link back to the early days of IFF when the equipment was code-named 'Parrot'. Thus an instruction to turn off the IFF eqiupment was to "strangle your parrot" and, conversely, to transmit the identification signal was to "squawk" - a phrase still in use today.
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