How does a flight radar work?

In short: the signal’s journey from aircraft to map

ADS-B – the aircraft reports its own position

ADS-B (Automatic Dependent Surveillance – Broadcast) is a system in which an aircraft determines its position by GPS and broadcasts it several times a second on 1090 MHz. Each message contains, among other things:

• the 24-bit ICAO address – the transponder’s permanent “serial number” (shown on the map as “ICAO 24-bit”, e.g. 48AE23),
• position and altitude,
• ground speed, track and vertical rate,
• the flight’s call sign, e.g. LOT281,
• the transponder code (squawk) assigned by air traffic control.

The signal is not encrypted and anyone with a suitable antenna can receive it. In Europe, ADS-B is mandatory for aircraft with a maximum take-off mass above 5.7 tonnes or a cruising speed above 250 knots, which is why practically every passenger and cargo aircraft appears on the map.

How does the data reach the map?

The ADS-B signal travels in a straight line, so a receiver can only see aircraft up to the horizon – for an aircraft at cruising altitude that is usually 250–400 km. Covering a whole country takes many stations. Most of them are home receivers run by aviation enthusiasts: an antenna on the roof, a cheap SDR receiver and a small computer.

Aircraft positions over Poland and the surrounding area come from the OpenSky Network – an open community network of such receivers run by a non-profit organisation. Every 15 seconds or so our server fetches them in a single request and shares them with all users at once. Between updates we move the icons along their track at their speed – and aircraft that are turning along an arc – so movement on the map is smooth. Aircraft from the rest of the world, ticket flight numbers and passenger flight schedules come from AirLabs.

Routes (departure and arrival airport) come from AirLabs schedules and, if missing, from the adsbdb.com database, and photos from planespotters.net, always with the photographer credited. The aircraft does not transmit its route – the route is linked to the call sign in the database, so for charter, cargo and changed flights it can be out of date. That is why we check it against the departure point and the direction of flight: if they don’t match, we show the departure airport or “route unknown” with an explanation.

How to read the map

How to find your flight

1. Type the flight number from your ticket into the search box at the top, e.g. LO281, FR1001 or W61301. The search converts it to the call sign used on the radio.
2. Not on the list? The aircraft may still be on the apron or not broadcasting – try again after take-off. You can also enter the registration (SP-LWA) or an airport (Kraków, WAW).
3. Click “Share” to send a link via Facebook, X, WhatsApp or email that opens the radar straight on this flight. “Notify me when it lands” lets you know when the aircraft begins its approach and when it lands.

Looking for all flights of a particular airline or aircraft type? Type e.g. ryanair or 787 – only matching aircraft will be highlighted on the map.

Why can’t some aircraft be seen?

• No ADS-B transmitter – small sport aircraft, gliders, balloons and some helicopters.
• Military aviation – often flies with ADS-B switched off or only broadcasts it in certain situations.
• No coverage – low to the ground, in the mountains and far out at sea the signal may not reach any receiver. The aircraft then “disappears” just before landing.
• Blocked data – some owners of private aircraft ask for their identifying data to be hidden.

MLAT – when the aircraft doesn’t report its position

Older transponders (Mode S without ADS-B) send messages without coordinates. The position of such an aircraft can be calculated by multilateration (MLAT): if at least four time-synchronised stations receive the same signal, the differences in its arrival time make it possible to work out where it was sent from. MLAT positions are less accurate and refreshed less often – the flight panel shows this in the “Position source” field.

Altitude and speed

The aircraft reports its pressure altitude, measured from the standard pressure of 1013.25 hPa. At cruising altitude all aircraft use the same reference, so separation between them remains safe even when the weather changes. The actual height above sea level can differ by a few hundred metres. Flight level FL350 is 35,000 feet, or about 10,670 m.

The speed shown is ground speed. At cruising altitude there is a strong westerly wind (the jet stream), so an aircraft flying east can exceed 1,000 km/h, while the same type flying west is 150–200 km/h slower.

Squawk codes: 7500, 7600, 7700

A squawk is a four-digit transponder code assigned by air traffic control. A few codes have a special meaning:

We highlight aircraft squawking 7500, 7600 and 7700 in the flight panel, in the “Emergency” filter of the flight list and in the “Interesting now” section on the home page.

Flight number, call sign and registration

Your ticket shows the IATA flight number, e.g. LO281. On the radio the aircraft uses its ICAO call sign, usually with a three-letter airline code, e.g. LOT281. The numbers don’t always match – Ryanair and Wizz Air use call signs with letters (e.g. RYR5CL) to avoid confusion on the radio. The search accepts both formats.

The registration is the aircraft’s “number plate”. The prefix shows the country: SP – Poland, D – Germany, PH – Netherlands, UR – Ukraine, G – United Kingdom, EI – Ireland, 9H – Malta (where part of the Ryanair and Wizz Air fleets is registered).

“What is flying over me?”

On the What is flying over me? page you share your location (or enter a place name), and we show the aircraft within the chosen radius, a sky view with the points of the compass and which aircraft will pass closest to you next. The angle above the horizon helps you find the aircraft in the sky: 90° means directly overhead, and a clenched fist at arm’s length covers about 10°. Your location stays in your browser.

Your own ADS-B receiver

An ADS-B station costs little: a USB receiver of the RTL-SDR type, an antenna tuned to 1090 MHz, a small computer (e.g. a Raspberry Pi) and free software such as readsb. The antenna should see as much of the horizon as possible. In a good spot such a set-up picks up aircraft 200–400 km away. You can feed the data to open networks such as the OpenSky Network – our map over Poland uses its data, so you improve coverage in your area for everyone.

Glossary