Primary radar is the primary means of surveillance in air traffic control environments: it transmits electromagnetic waves that illuminate a large portion of space and receives back the reflected echoes. Because the wave travels in a straight path until an aircraft interrupts the radar beam, the system operates independently of the target and can determine range, bearing and altitude within its coverage. Secondary radar interrogates an on-board transponder, receives the coded reply and thereby adds identity, flight data and more precise altitude to the range and azimuth already measured.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a Primary Surveillance Radar (PSR) in aviation?
Primary Surveillance Radar (PSR) is a ground-based, non-cooperative ATC radar system that detects and tracks aircraft by emitting high-energy radio pulses and analyzing the echoes reflected from targets. It is the only surveillance sensor used in civil aviation that does not require any on-board equipment to locate aircraft, allowing it to see aircraft without transponders and to detect non-cooperative targets like light aircraft, drones, birds, and weather phenomena. Operating in the S-Band (2700 to 2900 MHz) with a 60 nautical mile range and a peak effective power of 25 kW, the radar antenna rotates continuously at 5-12 rpm and is mounted on a tower. The system determines range by measuring the time between pulse transmission and echo reception, while bearing is obtained from the antenna's azimuth position. Because it operates independently of the target aircraft, PSR provides surveillance and interfaces with both legacy and digital automation systems, presenting real-time, high-resolution radar data to controllers.
What is a Secondary Surveillance Radar (SSR) in aviation?
Secondary Surveillance Radar (SSR) is a surveillance radar system which uses transmitters/receivers (interrogators) and relies on targets equipped with a radar transponder. It is used in air traffic control (ATC).
Secondary Surveillance Radar (SSR) is a radar system used in air traffic control that provides precise aircraft identification, altitude, and tracking by interrogating aircraft transponders. SSR is a cooperative system that requires a transponder on all aircraft; the ground station transmits interrogation pulses on 1030 MHz, and the aircraft transponder replies on 1090 MHz with its unique identifier and altitude information. This two-frequency operation makes SSR resistant to clutter and allows the directional antenna, collocated with primary radar and rotating at 5-12 rpm, to determine azimuth and range from the time of transmission-return.
The SSR system automatically provides the Flight Level (pressure altitude) of an aircraft and improves the ability to detect and identify aircraft while receiving additional data like identification codes. Operating continuously in Modes A and C and selectively in Mode S, SSR bridges the gap between communications systems and classic radar systems, offering better accuracy in tracking positions. Mode S, a data packet protocol, can interface with older ATCRBS transponders and enables the direct exchange of data between aircraft for collision avoidance, making SSR a cornerstone of contemporary air traffic management.
What is the difference between primary and secondary radar in aviation?
The differences between primary and secondary radar in aviation are given in the table below.
| Primary Radar | Secondary Radar |
|---|---|
| Measures bearing and distance using detected reflections of radio signals | Measures bearing, distance, identification, altitude, and flight status using transponder replies |
| Relies on a signal being reflected or bounced back from an object | Relies on transponders installed on aircraft to transmit return signals |
| Operates independently of the target aircraft | Works in conjunction with primary radar to enhance surveillance capabilities |
| Requires no action from the aircraft | Requires the aircraft to be equipped with a transponder |
| Can be affected by ground reflections and weather conditions | Results in clearer radar images |
| Uses enormous amounts of power to ensure returns from the target | Uses less power as it relies on transponder responses |
| Does not normally measure altitude | Provides information such as identification, altitude, and flight status |
| Sends a wave and the same wave is reflected back | Transmits EM signal energy towards a target, and the transponder responds by transmitting a coded reply signal |
Primary radar transmits high-frequency signals from the antenna site, then measures the time-of-flight of the echoes it receives after those pulses are reflected by the aircraft's surface. Because it relies on a signal being bounced back, it operates independently of the aircraft itself and requires no action from the flight crew. The same reflected signals give the controller bearing and distance, yet they carry no identification code, altitude, or flight status. Target elevation is not normally measured by ATC primary radars, and the picture degrades due to ground reflections and weather conditions. To guarantee detectable returns, enormous amounts of power must be radiated, so primary radar's main disadvantages are high energy demand and limited data content.
Secondary radar works in conjunction with primary radar to enhance surveillance capabilities. A ground transmitter sends pulses that the transponder aboard the aircraft picks up. The transponder responds by transmitting a coded reply signal back. Because secondary radar relies on targets equipped with a radar transponder, it focuses solely on those active replies rather than on passive reflections. The reply signal contains information including identification codes, pressure altitude, and, when requested, flight status. The result is a clearer radar image displayed on the controller's screen, free from ground-clutter interference and achievable with far lower transmitted power. Gap-filler radar systems apply the same secondary-radar-only principle to give lower-altitude radar coverage between two larger radar systems, assuring seamless surveillance without additional heavy primary installations.
