Secondary radar

A secondary Radar or SR (Secondary Monitoring RADAR) is an air inspecting device which " interroge" sky. The initials IF , ( I dentification F riend gold F oe), indicate an embarked electronic device developed by the Allies during the Second world war allowing, by radar interrogation, to identify the aircraft " amis" or " ennemis".

This device was improved since, at least for a civil use, and gave rise to the transponders (interrogation by secondary radar in mode has, B or C).
According to the mode used, the operator radar on the ground can identify an apparatus by a code allotted in advance (squawk) and thus obtain his position, his level of flight (altimetric reference 1013hpa): mode C. Its road and its speed are calculated by the secondary radar. The Transpondeur S of the Avion S which are with its range answer him. The answers of the Avion S are captured by the antenna of the secondary Radar, are analyzed and treated electronically and visualized thereafter on the screens of the air-traffic controllers.

Characteristics

This type of Radar makes it possible to determine, like all the radars, the position of the plane using a Gisement and of a distance relating to the antenna. The secondary radar has several advantages which are clean for him. As a Transpondeur is necessary to be detected by a secondary radar, only the relevant studs will be visualized, the obstacles or birds will not appear, thus avoiding a " pollution visuelle". Another advantage is the fact that data can be transmitted to the radar system. The data depend on the mode of the transponder used by the plane, and of the capacities of the antenna radar. The IF amongst other things makes it possible to obtain the altitude of the apparatus by mode of interrogation. The mode of 3/A interrogation is directly connected to the altimeter of the apparatus. From its design, the radar IF is of very reduced size, the way of information is divided by two compared with any radar.

Mode has (Alpha)

The mode has is simplest of the modes of transmission of data between the plane and the ground. Only transmitted information is a code SR, of four digits between 0 and 7 included. The code will be posted on visualization radar of the controller. Each plane is seen allotting a single code, therefore this code makes it possible to establish a relation between a stud and a plane, to identify with certainty that this stud corresponds has this plane. One calls that the Identification radar. With the mode has, the controller thus has the position of the plane, and a means of identification radar.

Mode C (Charlie)

The charlie mode is currently used the most on the French territory. It is acted in fact of a mode improved, by adding information of altitude. This data is measured on the aircraft, is transmitted to the radar, then visualized on the screen of the controller.

Mode S (Sierra)

The mode S is still an evolution of the secondary radar. The number of codes available in mode has and C is limited (4096 codes only) and becomes insufficient for the present needs. The mode S will thus allow a true data link. Instead of a code, the registration or code of the plane could be transmitted. Instead of altitude, any data could be transmitted, as well of the plane towards the ground as of the ground towards the plane. The applications are numerous.

Military modes

The IF carries out in the military aircrafts the same functions as the transponder. He adds a military of identification friend/hostile function. There exist various military modes: 1,2,3,3C, 4 and 5. Mode 3 and 3C are the military equivalents with the modes Alpha and Charlie. Modes 1,2,4 and 5 are exclusively soldiers. Modes 4 and 5 are encrypted and mode 5 allows a true data-processing dialog.

Radar data processing

Once the data transmitted to the antenna, they are sent to the center needing this information. They undergo a treatment which fulfills several functions. First is to increase the precision of information by mixing information coming from several radars. By recutting information, one refines the position of the aircraft. Then a comparison with preceding information is carried out, in order to determine the road plane, and its speed. This information will be then posted on visualization radar.

The fact that a on-board equipment is necessary to be visualized makes the radar secondary unsuitable to a military use. For an exhaustive detection, one will prefer the primary education Radar.

See too

Transponder
Control of the air traffic
aeronautical Alphabet

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