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.