Automatic Dependent Surveillance–Broadcast (ADS–B) is a surveillance technology in which an aircraft determines its position via satellite navigation or other sensors and periodically broadcasts it, enabling it to be tracked. The information can be received by air traffic control ground stations as a replacement for secondary surveillance radar, as no interrogation signal is needed from the ground. It can also be received by other aircraft to provide situational awareness and allow self-separation. ADS–B is “automatic” in that it requires no pilot or external input. It is “dependent” in that it depends on data from the aircraft’s navigation system.

The system involves an aircraft with ADS-B determining its position using GPS. A suitable transmitter then broadcasts that position at rapid intervals, along with identity, altitude, velocity and other data. Dedicated ADS-B grounds stations receive the broadcasts and relay the information to air traffic control for precise tracking of the aircraft.

• Automatic – Requires no pilot input or external interrogation.
• Dependant – Depends on accurate position and velocity data from the aircraft’s navigation system (eg. GPS).
• Surveillance – Provides aircraft position, altitude, velocity, and other surveillance data to facilities that require the information.
• Broadcast – Information is continually broadcast for monitoring by appropriately equipped ground stations or aircraft.

ADS-B data is broadcast every half-second on a 1090MHz, digital data link.

Broadcasts may include:

Flight Identification (flight number callsign or call sign)
ICAO 24-bit Aircraft Address (globally unique airframe code)
Position (latitude/longitude)
Position integrity/accuracy (GPS horizontal protection limit)
Barometric and Geometric Altitudes
Vertical Rate (rate of climb/descent)
Track Angle and Ground Speed (velocity)
Emergency indication (when emergency code selected)
Special position identification (when IDENT selected)

The ability of a ground station to receive a signal depends on altitude, distance from the site and obstructing terrain. The maximum range of each ground station can exceed 250 nautical miles. In airspace immediately surrounding each ground station, surveillance coverage extends to near the surface.

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