How ADS-B Lets Aircraft Broadcast Their Own Position
ADS-B aircraft work out their position from satellite navigation and broadcast it about once a second. See what is sent, who hears it and what limits it.
In this article
ADS-B stands for Automatic Dependent Surveillance-Broadcast. An aircraft with ADS-B works out where it is, usually from satellite navigation, and broadcasts that position automatically, around once every second. Air traffic control, other aircraft and anyone with a suitable receiver can pick it up. It is a different approach from traditional radar, which finds an aircraft by sending out a signal and waiting for the echo.
What the name means
- Automatic: the broadcast happens without a ground station asking for it.
- Dependent: the data depends on the aircraft's own navigation equipment.
- Surveillance: its purpose is to let others see where aircraft are.
- Broadcast: the signal is sent to anyone listening, not to one named receiver.
Wikipedia's article on ADS-B describes it as a technology in which the aircraft determines its own position, usually from satellite navigation, and broadcasts it so that it can be tracked. SKYbrary, an aviation safety reference, quotes the ICAO definition: a means by which aircraft, aerodrome vehicles and other objects can automatically transmit and receive data.
What an aircraft needs
Two things are required. One is a source of position, normally a GPS receiver or another certified satellite navigation unit. The other is the ADS-B transmitter itself, a datalink that sends the message. Unlike secondary radar, no interrogation from the ground or from another aircraft is needed. The aircraft simply transmits.
What is in a message
According to SKYbrary, the data includes the following:
- Latitude and longitude
- Barometric altitude (the height measured from air pressure)
- Quality indicators, which say how good the position data is
- The aircraft's identification, including its 24-bit address
- Emergency status
- A special position indicator, when the crew selects it
Wikipedia adds that velocity, including ground speed, is part of the broadcast, and that position and velocity are sent about once per second.
Two radio links
There are two main datalinks, and which is used depends on the region and type of flying.
- 1090 MHz extended squitter (1090ES): in effect a modified Mode S transponder. Existing air traffic control receivers and collision-avoidance equipment can already receive these signals. SKYbrary says it is the basis of the most widely shared global use.
- 978 MHz Universal Access Transceiver (UAT): a separate link used in the United States, mainly by general aviation. Wikipedia says it can carry weather and flight information services that 1090ES cannot, and that the US FAA has suggested aircraft staying below 18,000 feet use it to reduce crowding on 1090 MHz.
SKYbrary notes that using different datalinks in different regions limits full uniformity.
ADS-B Out and ADS-B In
ADS-B Out is the sending side. Ground stations or satellite receivers collect it for surveillance. ADS-B In is the receiving side: an aircraft with the right equipment can see nearby traffic with altitude, heading, speed and distance. In the United States, ground services can add traffic reports from other sources and weather information. Wikipedia says ADS-B In is not mandated in US airspace.
How ground systems use it
SKYbrary explains that ground stations pass the data to processing systems, where it is combined with radar or multilateration inputs to build one picture of the traffic. One benefit it lists is lower infrastructure cost than radar. Another is surveillance coverage from gate to gate, including air-to-air coverage.
Where it is required
Rules differ by country. Wikipedia says that ADS-B Out has been required in most controlled airspace in the United States, including Classes A, B and C, since the compliance date of 1 January 2020. In Europe, equipment has been mandatory for some aircraft since 2017, and in Australia it is required for all aircraft flying under instrument rules, and for all aircraft at or above flight level 300. SKYbrary adds that European rules apply to instrument flight aircraft above certain weight or speed thresholds. Check current rules with the aviation authority concerned, as the details change.
Limits
- Aircraft with no transponder, or only older transponder equipment, do not appear on ADS-B displays.
- The system depends on onboard navigation, which can fail.
- Signal reception weakens with distance, and terrain, weather or interference can stop messages being decoded.
- Gliders using FLARM collision-avoidance equipment cannot see ADS-B-only aircraft, and the reverse also holds.
Privacy and security
ADS-B messages are not encrypted, so anyone with a receiver can read them. Wikipedia reports that researchers have shown spoofed messages can create phantom aircraft, and that cross-checking against radar or multilateration is one proposed defence. Because each aircraft has a unique 24-bit address, its movements are easy to follow, and the FAA offers some privacy programmes that do not change the underlying broadcast.
In short, ADS-B moves the job of finding the aircraft from the ground to the aircraft itself, which then shares what it knows with everyone in range.
Sources
Share
Keep reading
Related articles
How Cabin Pressurisation Keeps Airliner Air Breathable
At cruise height the outside air is too thin to breathe. Airliners pump in air and control how fast it leaves. Here is how, and why cabins sit near 6,000 to 8,000 feet.
How Ailerons, Elevators and Rudder Steer an Aeroplane
Three movable surfaces control roll, pitch and yaw. Learn what each one does, why turning needs more than ailerons, and how trim tabs ease the pilot's load.
What Bypass Ratio Means in a Jet Engine
Bypass ratio compares the air going around a jet engine's core with the air going through it. Here is why airliners use high ratios and fighters do not.