Secondary Surveillance Radar

In the aviation sector, flight safety and air traffic management are among today’s highest priority areas. In this context, secondary surveillance radars are critical systems used for monitoring aircraft and effectively controlling airspace. Secondary surveillance radars assist in determining aircraft position, altitude, speed, and identification information by relying on data transmitted from transponders onboard the aircraft. These data, collected via secondary surveillance radars, are utilized at air traffic control centers to support the provision of air traffic control. Secondary surveillance radars are an essential system for ensuring the safe management of flights under today’s complex air traffic conditions. With technological advancements, the effectiveness and coverage of these radar systems continue to increase, enabling them to better meet the needs of the aviation industry.

Secondary surveillance radars differ from primary surveillance radars not only in their operating principle but also in other aspects. While primary surveillance radars detect only the presence and location of aircraft by analyzing reflected radio waves, secondary surveillance radars can provide air traffic controllers with information such as aircraft identity, altitude, and position through transponders mounted on the aircraft. This enables air traffic controllers to monitor and manage aircraft more effectively. Additionally, these systems can generate a more efficient air traffic picture while consuming less energy, thereby enhancing air traffic safety.

Historical Development

The fundamental principles of secondary surveillance radars were developed in the 1950s. The earliest secondary radar systems were based on transponder technology that enabled aircraft to respond to radar signals. When aircraft received radar signals, they automatically responded, allowing pilots and air traffic controllers to obtain additional information about the aircraft. In the 1960s, the use of secondary radar technology became widespread, and many countries integrated it into their air traffic management systems. During this period, secondary surveillance radars became the standard for identifying and tracking aircraft in airspace. In particular, Mode A and Mode C were introduced as the first transponder modes. In the late 1980s and early 1990s, secondary surveillance radars were further enhanced with the introduction of Mode S. Mode S offered greater information capacity, enabling simultaneous communication with multiple aircraft and the transmission of more detailed data. During this period, air traffic control systems were able to achieve improved visibility in airspace by using data from secondary radars.

In the early 2000s, with the global expansion of aviation and increasing air traffic, secondary surveillance radars underwent further modernization. The Automatic Dependent Surveillance–Broadcast (ADS-B) technology was integrated into secondary radar systems, allowing aircraft to periodically broadcast their own position information to secondary radar stations and other aircraft. This system enabled more real-time monitoring of aircraft and provided additional data flow to air traffic control. Today, secondary surveillance radars remain a vital component of air traffic control and safety, and transponder systems that enable data transmission to these systems are mandatory on aircraft in accordance with aviation standards.

Operating Principle

The operating principle of the secondary surveillance radar system is based on interaction between the transponder onboard the aircraft and the ground station. This system, which operates via radio waves, involves a process in which the ground station transmits a signal at a specific frequency, and the aircraft’s transponder responds to that signal. The ground radar station periodically emits short radio pulses. When the aircraft receives these pulses, the transponder activates and transmits a reply signal at a designated frequency. This return signal contains information used by air traffic control, such as the aircraft’s identity, altitude, and position. The interrogation signal transmitted by the ground station operates at 1030 MHz (Megahertz); the reply signal transmitted from the aircraft’s transponder to the ground station operates at 1090 MHz (Megahertz).

Operating Principle of Secondary Surveillance Radar (MDPI)

One of the most important advantages of the secondary surveillance system is its ability to utilize the additional data provided by the transponder. The aircraft, upon receiving signals from the ground radar station, transmits its position and altitude information. The radar station receives these signals and continuously monitors the real-time positions and flight data of aircraft. This process enables air traffic control to track aircraft movements more efficiently and ensures the safe and orderly management of air traffic. Since transponders continuously receive and transmit radar signals during flight, air traffic control centers receive real-time information that minimizes the risk of potential collisions between aircraft.

Importance of the System

Secondary surveillance radar systems play a critical role in modern air traffic management and are essential for ensuring aviation safety. By providing aircraft identity, position, and altitude information, air traffic control centers can monitor aircraft movements in real time. This enables more effective management of airspace and significantly reduces the risk of collisions. In areas with high air traffic density, these systems provide effective monitoring and routing, which is crucial for air traffic safety. Secondary surveillance radar systems also offer significant benefits in emergency management. When aircraft need to transmit emergency signals, the information received via transponders enables air traffic controllers to respond rapidly. This helps prevent accidents or effectively manage emergency situations. In particular, in regions with high and complex air traffic, the information provided by the system can be life-saving for both pilots and air traffic controllers.

Transponder

The transponder is an avionics device onboard aircraft that automatically responds to queries from secondary surveillance radars. The term “transponder” is derived from the combination of the English words “transmitter” and “receiver.” These devices transmit information in response to specific query types requested by secondary surveillance radars. They are critical for air traffic management and safety and must be installed on aircraft and operate in compliance with aviation standards.

Secondary Surveillance Radar Modes

Secondary surveillance radars communicate with transponders onboard aircraft using a query-response principle to collect data used in air traffic management, including aircraft identity, position, and altitude. Different modes have been developed based on the type of information requested by the secondary surveillance radar. The most commonly used modes in secondary surveillance systems are Mode A, Mode C, Mode S, and ADS-B.

Mode A: A radio signal is transmitted from the secondary surveillance radar to the aircraft requesting its identity information. The transponder onboard the aircraft responds by transmitting the aircraft’s identity code back to the radar.

Mode C: A radio signal is transmitted from the secondary surveillance radar to the aircraft requesting its altitude information. The transponder onboard the aircraft responds by transmitting the aircraft’s altitude to the radar.

Mode S: This mode was developed to extract more information from the aircraft. The air traffic control center sends a radio signal to the aircraft via the secondary surveillance radar, requesting specific data. The transponder onboard the aircraft responds by transmitting the requested information. Mode S includes several internal formats.

ADS-B (Automatic Dependent Surveillance–Broadcast / Otomatik Bağımlı Gözetim-Yayın): This mode enables transponders onboard aircraft to periodically broadcast their own navigation data, such as position, speed, and altitude, without requiring a query. This allows other aircraft and radar systems to use the broadcast information to contribute to air traffic management.

Benefits of the System

Secondary surveillance systems make significant contributions to air traffic control and aviation safety.

Enhanced Identification Capability: The use of transponders allows for more accurate identification of aircraft identity and position, enabling air traffic control to monitor aircraft more effectively.

Altitude Information Provision: By transmitting aircraft altitude information to air traffic control centers, the system ensures safe distance management between aircraft.

Airspace Management: It assists in the orderly and safe routing of aircraft within airspace, reducing collision risks and improving the efficiency of air traffic management.

Reduced Reliance on Primary Radars: Secondary systems require less energy and emit fewer radar waves compared to primary radars.

Enhanced Communication Capacity: They provide additional capacity for data exchange between aircraft and control centers. For example, Mode S transponders can transmit not only identity but also additional data.

Improved Emergency Management: They enable aircraft to transmit emergency signals more quickly and effectively, accelerating emergency response times.

Data Storage Capacity: By centralizing flight data, aircraft positions, and other critical information, the system enables comprehensive monitoring and analysis.

International Standards Compliance: The system complies with standards established by ICAO (International Civil Aviation Organization), facilitating air traffic coordination between different countries.