Radar is a detection device that emits radio waves and detects the presence and characteristics of reflected echoes from targets to obtain target information.

　　The distance to the target can be measured based on the time delay between the radar emitted signal and the echo; the target's relative speed to the radar can be obtained through continuous measurement of the target's distance; the angular position of the target can be determined by measuring the angle of arrival of the echo wavefront at the radar.

　　Radars used in air traffic management mainly include primary surveillance radar (PSR) and secondary surveillance radar (SSR). A primary radar emits radio waves and receives the target's reflected echoes to obtain the target's distance and azimuth information.

　　If the echo is a radiated radio wave retransmitted from a transponder on the target, it is called secondary radar.

　　Primary radars are classified according to the management area used: en-route (path) surveillance radar, airport surveillance radar, and precision approach radar.

　　Advantages of primary radar: it can provide the aircraft's azimuth and distance on the radar screen using light spots;

　　Disadvantages: it cannot identify the aircraft's code and altitude, and the reflected echo is weak, easily affected by fixed targets. Secondary radar was developed to overcome the shortcomings of primary radar.

　　1. Primary Surveillance Radar

　　Primary surveillance radar can be divided into two main categories: ground radar and airborne radar.

　　Ground radar is mainly used in air traffic control systems, such as primary surveillance radar for monitoring en-route or terminal control area flights, precision approach radar for guiding aircraft takeoff and landing, surface surveillance radar for monitoring airport surfaces, and ground meteorological radar for detecting meteorological conditions within the controlled airspace.

　　Airborne radar is mainly used for on-board detection, such as airborne weather radar, altimeters used to indicate aircraft altitude, and Doppler navigation radar used for navigation.

　　2. Secondary Surveillance Radar

　　Secondary Surveillance Radar (SSR) works with ground secondary radar (interrogator) and airborne transponders using a question-and-answer method to monitor active reflection targets in the controlled airspace. Secondary radar is relative to primary radar.

　　The interrogator can determine the azimuth and distance of the responding target based on the propagation time delay and antenna pointing. After receiving this reply signal, the ground radar processes the signal and displays the aircraft code, altitude, azimuth, and distance on the display.

　　3. S-Mode Secondary Surveillance Radar

　　Traditional A/C mode secondary surveillance radar produces synchronous crosstalk when aircraft are in overlapping areas of two adjacent radar stations; the traditional A/C mode secondary surveillance radar uses 12-bit binary numbers for encoding, with only 212 codes, resulting in limited information exchange.

　　The main data information output by the radar in A/C mode includes altitude information, identification code, azimuth information, and distance information. It is easily affected by interference and crosstalk. For airports with a daily traffic flow of over 1000 flights, its monitoring capacity has reached its limit.

　　S-mode is a new air traffic surveillance technology developed in recent years. Compared with traditional secondary surveillance radar, it uses selective interrogation, expands the data link, increases system capacity, reduces internal system interference, and the radar outputs much richer data information than the traditional A/C mode.

　　The 'S' in S-mode stands for 'Selective'. It does not send the same interrogation format to all aircraft within its interrogation beam, but rather interrogates selectively (by name) according to the different addresses of each aircraft, each aircraft's address being unique.

　　By adding various interrogation modes, the S-mode secondary surveillance radar can effectively integrate aircraft equipped with conventional mode transponders and aircraft equipped with S-mode transponders. The S-mode secondary radar and airborne transponder use 24-bit binary numbers to represent the aircraft code, solving the problem of aircraft code shortage.

　　The data information output by the radar in S-mode includes altitude information, identification code, aircraft identification information (flight number), 24-bit aircraft address information, signal strength information, azimuth information, and time information, making it easier for air traffic controllers to understand the detailed status of aircraft.