Primary Surveillance Radar (PSR) : a surveillance radar system which uses reflected radio signals. Source: ICAO Doc 4444 PANS-ATM

Principle of operation: The radar antenna rotates (usually at 5-12 rpm) emits a pulse of radio wave. Upon reaching an aircraft (or other object) the wave is reflected and some of the energy is returned to the antenna. The PSR output data uses polar coordinate system: it provides range and bearing of the targets found in respect of the antenna position. Note that the range is the slant distance from the antenna and not the horizontal distance. The range is determined by the time difference of the emitted and received pulse (the speed of propagation is the speed of light) and the bearing is obtained from the antenna azimuth. The rotation speed of the antenna is usually between 5 and 12 rpm. The antenna radiation patern is a narrow beam when seen from above and, whith some approximation, can be considered as a trapezium if seen from the side.

The advantage of Primary Surveillance Radar (PSR) is that it operates totally independently of the target aircraft (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.

Secondary Surveillance Radar (SSR) requires an airborne transponder which responds to the receipt of a pulse from a ground-based antenna by transmitting a return signal. Because the transponder transmits a much stronger signal than that which is reflected off an aircraft in primary radar systems, greater range and reliability can be achieved with secondary radar and cheaper and more efficient ground equipment can be used. Additionally, information such as altitude and a code can be added to the returned signal from the transponder which is then displayed on the operator’s screen.

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.
Another principal SSR mode currently used 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.