Open any flight tracking app with a map, find your own location, and look for the moving dot that matches the plane’s direction. Nearly every airliner above you is announcing its identity, position, altitude and speed by radio, about once a second. Your phone is reading a copy of that.
The system is called ADS-B. A plane works out where it is from satellites and broadcasts it, unprompted, to anyone listening. Air traffic control listens. So do ground receivers run by companies and hobbyists, who pass what they hear to the tracking services. In the US, aircraft have needed it since Jan 1 2020 in most of the airspace airliners use, including everything above 18,000 feet.
No camera recognises the plane. An app knows your location and which way your phone faces, then draws a label where the broadcast says the plane should be.
Finding the right dot
The dot straight above you is rarely the plane you’re looking at. A jet at 35,000 feet is 6.6 miles up, so most of what you see is out to the side and a long way off.
| Height above the horizon | Distance to a jet at 35,000 feet |
|---|---|
| Straight up | 6.6 miles |
| 45 degrees | 9.4 miles |
| 30 degrees | 13 miles |
| 10 degrees | 38 miles |
Those are my own numbers, from the angle and a flat Earth, which is close enough at this range. A fist at arm’s length is about 10 degrees, so a plane one fist above the horizon may be over the next county.
To pick between two dots, use altitude. A contrail means high, usually above 30,000 feet. A plane with visible flaps and a growing shape is low, probably within a few miles of an airport. Then check the direction of travel on the map.
The sound won’t help. From 35,000 feet straight up, noise takes about half a minute to reach you, and in that time the plane has moved about 5 miles. It always sounds behind where it is.
If you’re pointing a phone’s camera at the sky, expect the label to drift a little. The compass in a phone is thrown off by a metal case, a car or a railing. Move away from them, turn slowly and give it a second.
A jet at cruise is smaller than it looks. A 737-800 at 35,000 feet spans about 0.2 degrees, less than half the width of the Moon.
What won’t be on the map
Some aircraft are overhead and invisible, and that’s normal.
Military flights are the main group. The rule lets government operators switch ADS-B off on sensitive missions, and plenty of military traffic doesn’t broadcast at all. If you hear a loud jet and can’t find it, that’s usually why.
Small aircraft flying low may not carry it. Gliders and balloons are exempt in much of the airspace, and a light plane below 10,000 feet away from busy airports doesn’t need it. Helicopters at low level can be patchy for a different reason: receivers need line of sight, and buildings and hills get in the way.
Some aircraft appear with the name held back. Owners can ask the FAA to limit what’s shared of their flights. They can also use a temporary address that isn’t linked to the registry, so a receiver sees a plane without seeing whose it is. Those show up as a blank or a private entry, and the position is still there.
And some planes show with no route. The broadcast gives a call sign and a position. It says nothing about origin or destination, so a tracker has to match the call sign to a schedule or a flight plan. A cargo flight or charter may have no match.
Coverage matters too. A map can be thin where few receivers sit: open ocean, deserts and some rural areas. The plane is still broadcasting, and nobody nearby is listening.
How flight trackers work covers how a broadcast becomes a dot, and why it lags. With two trails overhead and only one lingering, the air decides. If someone’s about to land, here’s when to leave to pick them up.
With a plane going over, open Overhead in Skycap and hold up your phone. Names sit beside the planes on the camera, or you can read a board of what’s above you, with each one’s route and altitude. What Overhead shows