Modes A, C and S
Mode A: the radar asks 'who are you?' and the transponder replies with a 4-digit squawk code. Mode C adds pressure altitude, so controllers see height without radar elevation data. Mode S (Select) is a digital datalink: each aircraft gets a unique 24-bit address, and the radar interrogates one at a time, enabling hundreds of aircraft in the same airspace without garble.
ADS-B: the next step
Automatic Dependent Surveillance-Broadcast removes the interrogation entirely. Aircraft broadcast their GPS position, altitude, velocity and intent once per second, unsolicited. Ground stations and other aircraft receive it. The surveillance is no longer dependent on radar at all — the aircraft tells everyone where it is. Mode S transponders include ADS-B Out as standard now.
Interrogation and reply frequencies
The ground station transmits at 1,030 MHz; aircraft reply at 1,090 MHz. This frequency split means the reply is 70 MHz away from the interrogation, reducing interference. A single Mode S radar can handle 800+ aircraft in its coverage volume. The reply pulses are only 0.5 µs wide, allowing precise time-of-arrival measurement for multilateration backup.
Limitations and spoofing
If the transponder fails, the aircraft vanishes from secondary radar — primary is the backup. If a pilot squawks the wrong code, confusion ensues. And ADS-B is unencrypted; spoofing a fake aircraft into the system is technically trivial. Researchers have demonstrated ghost aircraft injection with cheap software-defined radios. The aviation industry is slowly moving toward authenticated ADS-B.
The Interrogator Sidelobe Suppression System
A critical technical challenge in secondary radar is the issue of sidelobe interrogation. Since the radar antenna's main beam is highly directional, but secondary lobes still radiate energy, an aircraft close to the station might respond to a sidelobe rather than the main beam. This creates 'ghost' targets or 'ring-around,' where the aircraft appears at multiple bearings simultaneously. To solve this, the ISLS (Interrogator Sidelobe Suppression) system uses a second, omnidirectional antenna. The system transmits a control pulse, P2, exactly two microseconds after the initial P1 pulse. If the transponder receives P2 at a higher power level than P1, it recognizes it is being hit by a sidelobe and suppresses its reply for 35 microseconds.
This suppression logic is hardwired into the transponder's receiver circuitry to ensure the radar only processes data from its narrow main-lobe beam. Without ISLS, high-traffic terminal areas would be cluttered with redundant replies, making it impossible for controllers to differentiate between aircraft at the same range but different headings. Modern Monopulse Secondary Surveillance Radar (MSSR) further refines this by using phase comparison across the antenna face, allowing the system to calculate the exact azimuth of a single reply pulse with far greater precision than traditional beam-scanning techniques allowed.
From IFF to Civilian Control
Secondary radar was not originally a civilian tool but emerged from the 'Identification Friend or Foe' (IFF) systems developed during World War II. The British Mark III IFF allowed radar operators to distinguish between returning Allied bombers and Luftwaffe interceptors by looking for a 'blooming' signal on the cathode ray tube. Post-war aviation authorities realized that primary radar's inability to penetrate weather or distinguish individual flights was a bottleneck for the growing commercial sector. This led to the 1950s standardization of the Air Traffic Control Radar Beacon System (ATCRBS), which adapted military interrogation protocols for civilian use, permanently decoupling the surveillance function from the physical reflectivity of an aircraft's fuselage.
Early ATCRBS transponders utilized sliding-scale dials to set the 4,096 possible squawk codes, a numerical limit dictated by the 12-bit binary structure of the reply pulse train. While 7500 (hijack), 7600 (radio failure), and 7700 (emergency) became internationally recognized constants, the transition to Mode S in high-density sectors was required to resolve the 'fruit' and 'garble' interference caused by too many aircraft responding to multiple ground stations simultaneously. Today, the legacy of IFF remains visible in the pulse spacing and frequency standards that have remained remarkably consistent for over seven decades.
The Pulse-Position Modulation Mechanism
In secondary surveillance radar (SSR), data is not transmitted as a continuous stream but via pulse-position modulation (PPM). A standard Mode A or C reply consists of two framing pulses, designated F1 and F2, spaced exactly 20.3 microseconds apart. Between these brackets, twelve information pulses (A, B, C, and D bits) are arranged at specific 1.45-microsecond intervals. This spacing allows for 4,096 discrete combinations—the familiar octal squawk codes used by air traffic control. If an aircraft lacks an altitude encoder, the transponder simply omits the pulses corresponding to the altitude data, sending only the identity framing.
The timing precision required to decode these pulses is significant. Secondary radar systems use a process called 'defruiting' to filter out synchronous interference known as fruit—False Replies Unsynchronized In Time. Because all transponders respond on 1090 MHz, a ground station will inevitably receive replies triggered by other interrogators in the vicinity. By comparing time-of-arrival against its own interrogation triggers, the SSR processor discards any signals that do not align with its internal clock, ensuring that the 'dots' displayed on the controller's screen represent only the aircraft specifically addressed or triggered by that station.
Squawk Codes: Emergency and Special Purpose
While most squawk codes are assigned dynamically by controllers, certain four-digit sequences are reserved globally for specific operational conditions. Code 7700 is the universal signal for a general emergency, which triggers an immediate visual and auditory alert on the controller's console. Code 7600 indicates a complete radio failure, allowing the pilot to fly a standard lost-communications procedure while the controller maintains separation via radar. Perhaps most critical is Code 7500, the discreet signal for an unlawful interference or hijacking, which informs ground authorities of the situation without the pilot needing to speak over the radio.
Beyond emergencies, the code 1200 is standard for Visual Flight Rules (VFR) in North American airspace, while 7000 is used for the same purpose throughout much of Europe. These codes signify that the aircraft is not currently under positive radar control but is still visible to the system. The transponder also features an 'Ident' button, which adds a special Position Identification pulse (SPI) to the reply for approximately 18 seconds. This causes the aircraft's target on the controller's screen to pulse or change color, providing an unambiguous confirmation of the aircraft's identity during high-traffic handoffs.