Most planes have a device called a "transponder". Said device sends out the position of the plane as well as a 4 digit number that the pilot enters into the transponder.
The controller's radar screen shows the positions of all planes and this 4 digit number next to it. The 4 digit number is called a "squawk code".
Usually the controller tells the pilot which squawk to use. And there are some special squawk codes, e.g. 7700 means "emergency" which is what the pilot intended to do. 7500 means "plane was hijacked".
1
u/Willingness_Mammoth 26d ago edited 26d ago
What's a squawk code