Secondary Surveillance Radar (SSR) and Related Air Traffic Control Terminology

What is Secondary Surveillance Radar (SSR)?

Secondary Surveillance Radar (SSR) is an advanced, cooperative radar system fundamental to modern air traffic control (ATC). Unlike primary radar, which passively detects aircraft by analyzing reflected radio signals, SSR operates on the principle of active electronic cooperation between ground-based interrogators and aircraft-mounted transponders. This approach allows for precise, real-time acquisition of aircraft identity, position, and altitude, as well as additional flight data in advanced modes.

SSR dramatically improves situational awareness for controllers by assigning each aircraft a unique squawk code, correlating radar returns with flight plans. With Mode S, SSR enables selective interrogation using a unique 24-bit ICAO address, supporting high-density operations and advanced safety systems like TCAS and ADS-B. SSR is the backbone of controlled airspace surveillance worldwide, ensuring safety, capacity, and efficiency in increasingly crowded skies.

Key Components of SSR

Transponder

A transponder is an essential electronic device installed in an aircraft, enabling SSR’s cooperative surveillance. On receiving 1030 MHz interrogations from ground stations, it automatically transmits coded replies on 1090 MHz. Pilots enter squawk codes and select functions (e.g., IDENT, ALT, STBY) via a cockpit panel. Modern Mode S transponders transmit unique aircraft addresses, flight identification, and other status data, ensuring global interoperability and supporting safety systems like TCAS and ADS-B.

SSR Interrogator (Ground Station)

The SSR interrogator is the ground-based system that sends out coded interrogations via a rotating directional antenna, typically co-located with primary radar. It receives transponder replies, processes them for aircraft identification, altitude, and other parameters, and integrates this data with ATC automation systems. Advanced interrogators use digital signal processing and monopulse techniques for increased accuracy and reliability, even in congested or overlapping airspace.

SSR Modes: A, C, and S

• Mode A: Responds with the four-digit squawk code for identification.
• Mode C: Adds pressure altitude to the Mode A reply, encoded in 100-foot increments.
• Mode S: Enables selective, targeted interrogation using unique addresses, reducing reply overlap and supporting advanced data services.

Squawk Codes

A squawk code is a four-digit octal number (0000-7777) assigned by ATC to each aircraft. It is critical for correlating radar returns with flight plans. Special emergency codes include:

Squawk codes are managed dynamically as aircraft transition between ATC sectors, ensuring unambiguous identification across busy airspace.

SSR Frequencies

SSR operates on two internationally standardized UHF frequencies:

• 1030 MHz: Ground-to-air interrogations.
• 1090 MHz: Air-to-ground transponder replies.

These channels are protected and coordinated globally to ensure interference-free operation, supporting additional systems like ADS-B and TCAS.

Technical Features and Challenges

Fruiting

Fruiting occurs when a ground station receives valid transponder replies from interrogations it did not transmit, often due to overlapping SSR coverage. This can lead to false or ghost targets on radar screens. Techniques like time-based filtering, reply suppression, and Mode S selective interrogation are used to minimize fruiting.

Garbling

Garbling is caused by simultaneous or nearly simultaneous SSR replies from multiple aircraft, resulting in overlapping signals at the ground receiver. This can degrade radar accuracy. Mitigations include monopulse processing, staggered interrogation timing, and selective addressing in Mode S.

Mode S Selective Interrogation & 24-bit Address

Mode S introduces selective interrogation: the interrogator addresses individual aircraft via a unique, ICAO-assigned 24-bit address. This reduces reply overlap and supports transmission of additional surveillance and intent data. The address structure ensures global uniqueness, supporting seamless flight tracking and advanced safety functions.

Pulse Position Modulation (PPM)

SSR signals use Pulse Position Modulation (PPM), encoding information in the precise timing of RF pulses within a reply. Each reply contains a standard sequence of pulses, with specific arrangements representing squawk codes, altitude, and, for Mode S, additional data and error detection (parity).

Altitude Reporting

SSR altitude reporting is based on pressure altitude, derived from the aircraft’s barometric altimeter set to the international standard (1013.25 hPa). This altitude is encoded in Mode C and S replies, supporting accurate vertical separation and alerting in high-density airspace.

SSR Technical Specifications

SSR systems are designed for continuous, high-reliability operation, with built-in redundancy and remote diagnostics to ensure safety and uptime.

Monopulse SSR

Monopulse SSR employs simultaneous reception in multiple antenna beams to determine precise aircraft bearing in a single sweep, significantly improving angular accuracy and reducing errors from multipath or overlapping replies. This technology is standard in modern SSR installations.

Traffic Collision Avoidance System (TCAS)

TCAS is an airborne safety system using SSR (especially Mode S) to monitor nearby aircraft and prevent midair collisions. By actively interrogating surrounding transponders and analyzing replies, TCAS provides pilots with real-time resolution advisories, directing them to climb or descend as needed.

SSR in Controlled Airspace

SSR is mandatory for most operations in controlled airspace, supporting all core ATC tasks: sequencing, conflict detection, handoff, and integration with automated flight data systems. Mode S requirements are increasingly standard in busy regions, reflecting technology’s vital role in capacity and safety.

SSR Data Link Capabilities

Mode S transponders support data link, enabling exchange of supplementary information (flight identification, airspeed, vertical rate, intent data) and digital communication for Controller-Pilot Data Link Communications (CPDLC). These capabilities are central to next-generation airspace management concepts.

SSR Redundancy and Maintainability

Modern SSR installations feature multiple layers of redundancy (duplicate transmitters, receivers, processors) and remote monitoring. Built-in test equipment and modular design allow for rapid fault detection and repair, ensuring uninterrupted ATC surveillance.

Regulatory and Standards Framework

SSR is governed by a robust regulatory and standardization framework:

• ICAO Annex 10, Vol IV: Technical standards for SSR.
• ICAO Doc 4444: Operational procedures for ATC and SSR code assignment.
• FAA Order JO 7110.65: U.S. ATC procedures.
• EASA CS-ACNS: European technical and operational standards.
• ITU Radio Regulations: International frequency allocations.

These standards are regularly updated to match evolving technology and operational needs.

SSR and Primary Radar Integration

SSR is typically co-located and integrated with primary radar (PSR), combining the strengths of both systems: PSR detects all targets (including non-cooperative ones), while SSR provides precise identification and altitude for equipped aircraft. This integration supports high-integrity surveillance and safety net functions.

Summary

Secondary Surveillance Radar (SSR) revolutionized air traffic control, providing accurate, real-time surveillance, identification, and altitude data through cooperative transponder technology. With advanced modes like Mode S, SSR meets the demands of modern, high-density airspace, seamlessly supporting ATC, safety nets, and next-generation digital communications. Its robust regulatory foundation, technical sophistication, and ongoing evolution ensure SSR remains essential for safe, efficient, and scalable airspace management worldwide.