Guide
What is ADS-B, and how does flight tracking work?
Aircraft broadcast who and where they are, continuously, unencrypted, on a frequency anyone can receive for about thirty pounds. This is what that signal contains, how it reaches a screen, and why some aircraft never appear.
The signal
Every aircraft is telling anyone who listens exactly where it is.
ADS-B stands for Automatic Dependent Surveillance – Broadcast. Automatic because nobody presses anything; dependent because the aircraft works out its own position from GNSS rather than being found by a radar; surveillance because that is what it is for; broadcast because it goes out to everyone rather than to one recipient.
The aircraft transmits roughly twice a second on 1090 MHz. Light aircraft in some countries use 978 MHz instead. There is no encryption and no subscription: the signal is a public broadcast, in the same sense that a radio station is.
Each transmission carries a 24-bit ICAO address, a permanent identifier assigned to the airframe, plus position, altitude, ground speed and vertical rate. Separate messages carry the callsign the crew has entered, which is usually the flight number.
What it does not carry is the aircraft type, the operator, the registration or the route. Those come from separate databases, matched against that 24-bit address after the fact, which is why two flight trackers can disagree about what an aircraft is while agreeing exactly on where it is.
Receiving it
How a broadcast becomes a screen.
An aerial and a receiver
A 1090 MHz aerial and a software-defined radio dongle, together about £30 to try, and setting one up takes an evening. Height matters more than anything else: an aerial in a loft or an upstairs window will typically hear aircraft a hundred kilometres away, because the limit is line of sight rather than power.
Decoding
Software such as dump1090 or readsb turns the radio signal into a list of aircraft with positions. This is the part that is genuinely solved: the decoders are mature, free and open source.
Enrichment
The 24-bit address is looked up to find the registration, type and operator, and the callsign is matched against route data. This is where trackers differ, and where a wrong answer looks exactly like a right one.
If you would rather not put up an aerial, community networks such as adsb.lol aggregate what thousands of volunteers receive. You get wider coverage than any one aerial could manage, at the cost of a few seconds of delay and a dependency on somebody else's server.
The gaps
Why some aircraft never appear.
It is not transmitting
ADS-B is mandatory for most commercial aircraft in controlled airspace, but not for everything. Older light aircraft, gliders, balloons and some military types either have no transponder capable of it or are not required to use one. If an aircraft is not broadcasting, no receiver anywhere will show it.
It is out of range
1090 MHz is line of sight. An aircraft at 38,000 feet can be heard 400 kilometres away with a good aerial; the same aircraft on the ground behind a hill is invisible from a mile.
It is filtered
Some networks withhold particular aircraft, usually military or private, either voluntarily or under a blocking scheme. The aircraft is transmitting and your own receiver would show it; the network chooses not to pass it on.
Nobody knows what it is
Position without identity is common: a valid 24-bit address that no database recognises. An honest display says so. A less honest one guesses, and a plausible guess is worse than a blank, because you have no way to tell which you are looking at.
Questions
The ones we are asked.
- Is receiving ADS-B legal?
- Receiving it is legal in the UK and most countries: it is an unencrypted broadcast intended to be received. What you may do with the data afterwards varies, and republishing it can carry licence conditions. Feeding it to a community network is normal and encouraged.
- Do I need an internet connection?
- Not if you have your own aerial. A receiver, a decoder and a display can run entirely offline, which is why an aerial is worth having even where coverage is good. Without one you are reading somebody else's receivers over the internet.
- How far can I receive?
- Line of sight, so it depends on height at both ends. A modest aerial upstairs commonly reaches 100 km; a good one with a clear horizon can reach 400 km for high-altitude aircraft. Terrain matters more than the equipment.
- What is the difference between ADS-B and radar?
- Radar finds an aircraft by bouncing a signal off it. ADS-B has the aircraft work out its own position and announce it. Radar does not depend on the aircraft cooperating; ADS-B is far cheaper to receive and carries identity as well as position.
- Why do two flight trackers show different information?
- They almost never disagree about position, because that comes straight off the air. They disagree about type, operator and route, because those are database lookups, and the databases are maintained separately and are of varying quality.
- Can I see military aircraft?
- Often yes. Transport and tanker aircraft frequently broadcast normally, and a receiver of your own will show whatever is transmitting. Some networks filter them; your own aerial does not.
The product this comes from
Airborne draws what is overhead, on a screen in your room.
The airline's own livery on that airframe, with the route and the height beside it, redrawn every thirty seconds. Five views on one screen. If it cannot identify something, the frame stays empty rather than guessing.