Overhead 1090 Explains · Episode 4
What's a squawk code?
Updated October 2026
Every aircraft talking to air traffic control carries a four-digit code. Most of the time it's a routine number for that one flight. A few codes mean something has gone wrong, and a home receiver picks them up straight from the aircraft.
What's said in the video
Every plane carries a four-digit code. Controllers call it a squawk, and most of them are routine. A few are special. 7700 means emergency. 7600: radio trouble. Overhead 1090 flags them the moment your antenna hears one. Next time: who's flying that? Follow for more. It's called Overhead 1090. Search Overhead 1090 on the App Store.
Where the code comes from
Air traffic control gives each flight a four-digit code, and the crew dials it into the aircraft's transponder. Every digit runs from 0 to 7, so there are 4,096 possible codes. The transponder sends the code in its replies, which lets a controller match a target on the screen to a flight plan. Setting the code is called “squawking” it.
Most codes mean nothing on their own: they're assigned for one flight and reused later. A few are fixed. In the US, a light aircraft flying under visual flight rules and not talking to a controller usually squawks 1200.
The three emergency codes
- 7700: general emergency.
- 7600: the crew has lost radio contact.
- 7500: unlawful interference, meaning a hijacking.
A crew can set one of these without waiting to be asked, and controllers' screens highlight it at once. A 7700 sounds dramatic, but it's often precautionary: a technical fault, a sick passenger, or a crew that wants priority to land. Most end with an ordinary landing.
What your own receiver shows
The squawk travels in the aircraft's own transmissions, so a home ADS-B receiver decodes it along with the altitude and call sign. The decoder's map shows it in each plane's details.
Overhead 1090 watches for the emergency codes. Its Sightings flag one the moment your antenna hears it, so you can see which flight it is and follow it on the map. It works the same on iPhone, iPad and Apple TV, from your own receiver. If you run PiAware, the PiAware guide shows how to connect it.
The series
- Ep. 1: What is ADS-B?
- Ep. 2: What does an ADS-B receiver hear?
- Ep. 3: How far can an ADS-B receiver hear?
- Ep. 4: What's a squawk code?
- Ep. 5: Who's flying that?
See every plane over your house on iPhone, iPad and Apple TV: Overhead 1090 on the App Store, one purchase, no subscription.