ADS-B (Automatic Dependent Surveillance - Broadcast) is a surveillance technology that works by broadcasting GNSS position, velocity and identity to interested parties. It can be used as a supplement to or alternative to radar systems for tracking aircraft. It is a component of the FAA's NextGen ATC improvements, as well as the European SESAR programme.
ADS-B (Automatic Dependent Surveillance - Broadcast) is a cooperative surveillance technology for tracking aircraft. It is:
- Automatic in the sense that it operates without external stimulus (unlike radar)
- Dependent in the sense that the surveillance information is derived from on board systems
- Surveillance in the sense that it's primary intended to provide surveillance information to other parties
- Broadcast because it transmits to all listeners without knowing who does listeners are, there is no two-way communication.
The capability of an aircraft to transmit ADS-B signals is called ADS-B OUT, the capability of an aircraft to receive ADS-B signals is called ADS-B IN.
ADS-B data contains information about the identity, position, altitude and velocity of the aircraft. The position and velocity information is derived from GNNS (GPS).
There are three technologies defined for ADS-B
1090ES is operating on 1090 MHz, the same frequency used for Secondary Surveillance Radar (SSR) and Airborne Collision Avoidance Systems (ACAS/TCAS). Messages are transmitted by the same transponder that also replies to radar interrogations. This will be a Mode-S capable transponder because the data is formatted in a similar way. Since this frequency is the worldwide used standard for aviation, international aviation will use this ADS-B standard.
Universal Access Transceiver (UAT) works on 978 MHz. A special transceiver is required to use this form of ADS-B. The transceiver is able to send and receive data, it will also receive ADS-B transmissions from other aircraft. UAT is only allowed within the USA, the frequency 978 MHz is reserved for other purposes in other countries. Within Europe it is for example used for mobile telephony (GSM). To encourage people to equip with ADS-B voluntarily, the FAA provides additional services on UAT. These are Traffic Information Service - Broadcast (TIS-B) which transmits position of aircraft detected by FAA's SSR's and Flight Information Service - Broadcast (FIS-B) which provides the receiver with weather information and operational data such as airspace restrictions and NOTAMS.
- VHF Datalink Mode 4 (VLDm4) uses a frequency in the 118-136.975 MHz block, which is internationally reserved for aviation. Normally these frequencies are used for voice communications, so a special radio is required to support VDLm4. Just like UAT, VDL mode 4 allows for services like TIS-B and FIS-B. VDLm4 was tried in Sweden and Russia, but has not been able to gather much mass. It is unlikely that it will be used on a large scale.
For more information on ADS-B see the Wikipedia page.