ADS-B explained: how every flight's path is recorded

Most flight trackers, from the apps on your phone to the replay on our free flight map, start with the same thing: a short radio message that the aircraft itself sends out, about once a second, for the whole flight. That message is ADS-B.

What ADS-B is

ADS-B stands for Automatic Dependent Surveillance-Broadcast. The FAA explains each word:

• Automatic: it transmits on its own, without the pilot doing anything.
• Dependent: the position comes from the aircraft's own satellite navigation (GPS), not from a radar on the ground.
• Surveillance: it tells others where the aircraft is, in three dimensions, and who it is.
• Broadcast: anyone with the right receiver can hear it.

Radar sweeps find an aircraft every 5 to 12 seconds. ADS-B reports its position once a second, and the FAA now treats it as its preferred way of tracking aircraft. In the US, aircraft have needed ADS-B to fly in most controlled airspace since 1 January 2020. It is sent on 1090 MHz, the frequency transponders already use; in the US, aircraft flying below 18,000 ft can use a second system on 978 MHz instead.

What a plane broadcasts

US rules list what every ADS-B message set must contain. The parts that matter for anyone living near an airport:

ADS-B does not carry the route, the airline's schedule or the engine settings. OpenSky, for example, works out origin and destination from where each track starts and ends; schedules and flight numbers have to come from other sources.

How volunteers record it

ADS-B is received, not requested, so the equipment is simple: a small antenna, a USB radio receiver that costs little, and a small computer such as a Raspberry Pi. Thousands of people run one at home and share what they receive with online networks, which combine the signals into a single picture.

Some aircraft send their altitude but not their position. Networks can still place them by multilateration: several receivers note the exact moment the same signal arrives, and the differences in timing fix where it came from. ADSB.lol and ADS-B Exchange do this; OpenSky currently does not.

Where coverage runs out

The signal travels in straight lines. A plane at cruising height can be heard by receivers far away, but one on the runway or a few hundred feet up can only be heard by receivers nearby, and hills and buildings block the view. OpenSky puts it simply: if an aircraft is missing, it may be too low, or no receiver is close enough.

For a busy airport with receivers around it, the gaps are small. At Heathrow, our recordings capture about 1,280 flights a day against the airport's official average of about 1,310, a little lower because a small share of flights are not captured. Around smaller airports, and in valleys, coverage near the ground can be patchier.

A few aircraft are missing for other reasons. Aircraft that only fly outside the airspace where ADS-B is required may not carry it, and military aircraft can switch their transponders off. OpenSky also notes that some aircraft send wrong positions, which networks try to filter out.

Privacy: LADD, PIA and unfiltered trackers

Because anyone can receive ADS-B, private owners have pushed for ways to stay out of view. The FAA offers two:

• LADD (Limiting Aircraft Data Displayed). An owner can ask the FAA to keep their aircraft's data out of the flight data it shares. Tracking companies that take the FAA's feed must hide those aircraft from public display. LADD does not change what the aircraft broadcasts.
• PIA (Privacy ICAO Address). Eligible US-registered aircraft can use a temporary 24-bit address that is not linked to the owner in the public register, together with a call sign from a third-party provider. It only works in US-managed airspace. The FAA says the aircraft still broadcasts and off-the-shelf receivers still pick it up; it is just harder to identify.

Networks that show everything they receive, such as ADSB.lol and ADS-B Exchange, still show these aircraft. These programmes are aimed at private and business aircraft. For noise, what matters is the track and the height, not who owns the plane.

How trackers and noise reports use the data

Live trackers show where aircraft are now. The airport's own tracker, and replay tools like our flight map, show where they were on a past day. Either way, you see the real track over the ground and the height at each point, which is what decides how loud a plane is at a given house. Our guide to free ways to check aircraft noise compares the options.

Noise modelling needs exactly this kind of record. ECAC Doc 29, the method airports use for official noise maps, ranks measured flight paths above standard procedures as a source of track and height data; ADS-B gives a measured path for every flight. See how aircraft noise is calculated for the rest of the method.

A Flight Noise report uses ADS-B data from ADSB.lol for every recorded landing and take-off of the last twelve months, applies Doc 29 to each one at the address you choose, and counts how many were louder than 65 dB outdoors, by day and at night. It is an independent estimate, not an official noise map.