Every aircraft you see on a radar screen is there because it chose to announce itself. The announcement comes from a small box called a transponder, and the four-digit number it broadcasts is the squawk code — the aircraft’s temporary radar identity.
How the transponder system works
When a flight is cleared by air traffic control, the controller assigns a unique squawk code: a number between 0000 and 7777, using only the digits 0–7 (it is an octal number, giving 4,096 possible combinations). The flight crew dials it into the transponder before or shortly after departure.
Ground-based radar antennas send out interrogation pulses. When the pulse hits the transponder, the box replies with the squawk code, which the radar then displays as a labelled blip — usually showing the code, flight callsign, altitude, and speed. Without a working transponder the aircraft is effectively invisible to secondary surveillance radar.
Modern transponders operate in several modes:
| Mode | What it sends |
|---|---|
| Mode A | Squawk code only |
| Mode C | Code + pressure altitude |
| Mode S | Code + altitude + unique 24-bit aircraft address (ICAO) |
| ADS-B | Full position, speed, heading broadcast continuously |
Most commercial flights today use Mode S and ADS-B together, which is why flight-tracking apps can show a plane’s precise location without needing ground radar at all.
The special emergency codes
Three squawk codes are reserved worldwide and carry immediate meaning the moment they appear on a radar screen:
7700 — General emergency. Any declared emergency: engine failure, medical crisis, fuel exhaustion, structural problem. Controllers worldwide are trained to react instantly.
7600 — Radio failure (NORDO). The crew can no longer communicate with ATC. Controllers will clear airspace and watch for the aircraft to follow standard lost-communications procedures.
7500 — Unlawful interference (hijack). The most serious code. Setting 7500 discreetly signals to controllers that someone has taken control of the flight against the crew’s will. Aviation authorities and military are alerted immediately.
A fourth code — 1200 — is used in the United States for Visual Flight Rules (VFR) flights not talking to ATC. It is the default “I’m just flying around” identifier that keeps the skies untangled in uncontrolled airspace.
What you’ll see on the radar
On the FlyOverhead radar, each aircraft is broadcasting its Mode S or ADS-B data, which includes the current squawk code. Under normal conditions you’ll see routine four-digit codes — different for every flight, assigned by each country’s ATC system.
If you ever spot a flight labelled with 7700, 7600, or 7500, that is a real alert: the crew has deliberately set an emergency code. These are uncommon on any given day, but flight trackers pick them up the moment they are set. Routine squawk codes change every flight; the three reserved ones are instantly recognisable because they are the same everywhere in the world.