Secondary Surveillance Radar and Transponder

062-03-04Pablo Asensio Martínez2026-04-012 min

SSR (Secondary Surveillance Radar) is a radar system that relies on the cooperation of airborne equipment called a transponder. Unlike primary radar, which works by passive reflection, SSR uses an active interrogation and reply system.

Aviation Transponder Panel

Operating Principles

  1. Interrogation: The ground station (interrogator) transmits coded pulses on the frequency 1030 MHz.
  2. Reply: The aircraft transponder receives the interrogation and transmits a coded reply on the frequency 1090 MHz.

Advantages over Primary Radar

  • Lower Power Required: Since the reply signal is actively generated by the transponder (it is not an echo), the signal is much stronger and requires less transmission power on the ground to achieve the same range.
  • More Information: Allows transmission of data such as identification, altitude, and other flight parameters.
  • No Clutter: It is not affected by terrain echoes or weather.

Operating Modes

Mode A (Identification)

  • Provides a 4-digit identification code (Squawk Code).
  • The code is octal (digits 0-7), allowing 4096 possible combinations ($8^4$).
  • Interrogation: P1 and P3 pulses separated by 8 µs.

Mode C (Altitude)

  • Provides pressure altitude (referenced to 1013 hPa) in 100-foot increments.
  • Interrogation: P1 and P3 pulses separated by 21 µs.

Mode S (Selective)

  • Selective Addressing: Each aircraft has a unique 24-bit address (over 16 million combinations), allowing interrogation of a specific aircraft.
  • Enhanced Data: Allows data link and transmission of flight parameters (ELS/EHS) such as callsign, speed, heading, etc.
  • Altitude Precision: Reports altitude in 25-foot increments.
  • Compatibility: Fully compatible with Modes A and C.
  • TCAS: Essential for the operation of the TCAS collision avoidance system.

Technical Characteristics

Side Lobe Suppression (SLS)

To prevent the transponder from replying to interrogations coming from the antenna's side lobes (which would give an erroneous position):

  • A control pulse P2 is transmitted 2 µs after P1.
  • If the transponder receives P2 with greater power than P1, it knows it is in a side lobe and does not reply.

IDENT Function (SPI)

When the controller requests "Squawk Ident", the pilot presses the IDENT button.

  • The transponder adds a special pulse (SPI) to its reply for about 20 seconds.
  • This causes the aircraft's echo to blink or highlight on the controller's screen, facilitating positive identification.

Emergency Codes

Although not detailed in the source text, it is standard knowledge associated with Mode A:

  • 7700: General Emergency.
  • 7600: Radio Communication Failure.
  • 7500: Unlawful Interference (Hijacking).