Automatic Dependent Surveillance-Broadcast (ADS-B) is a function on an aircraft or a surface vehicle that periodically broadcasts its state vector (horizontal and vertical position, horizontal and vertical velocity) and other information. ADS-B supports improved use of airspace, reduced ceiling/visibility restrictions, improved surface surveillance, and enhanced safety such as conflict management. ADS-B should not be confused with the applications it supports.
Under ADS-B, a vehicle periodically broadcasts its own state vector and other information without knowing, a priori, what other vehicles or entities might be receiving it, and without expectation of an acknowledgement or reply. ADS-B is automatic in the sense that no pilot or controller action is required for the information to be issued. It is dependent surveillance in the sense that the surveillance-type information so obtained depends on the suitable navigation and broadcast capability in the source vehicle. 
- 1 Benefits of ADS-B
- 2 Theory of Operation
- 2.1 Relationship to Surveillance Radar
- 2.2 Relationship to Addressed ADS
- 2.3 Relationship to Other Broadcast Services
- 2.4 ADS-B Physical Layer
- 3 ADS-B Supported Applications
- 4 Implementation Timetable
- 5 References
- 6 See also
- 7 External links
- 8 eventually delete this section as it is incorprated in other sections
Benefits of ADS-B
ADS-B is an enabling technology with benefits in safety and efficiency, when fully implemented and when supporting applications are available. Safety benefits include: 
- Improved visual acquisition especially for general aviation under visual fight rules (VFR).
- Reduced runway incursions of the airport surface.
- Provision of graphical weather to general aviation cockpit.
ADS-B enables increased capacity and efficiency by supporting:
- Enhanced visual approaches
- Closely spaced parallel approaches
- Reduced spacing on final approach
- Reduced aircraft separations
- Enhanced operations in high altitude airspace for the incremental evolution of the "free flight" concept
- Surface operations in lower visibility conditions
- Near visual meteorological conditions (VMC) capacities throughout the airspace in most/all weather conditions
- Improved ATC services in non-radar airspace
Theory of Operation
ADS-B consists of three components:
- A transmitting subsystem that includes message generation and transmission functions at the source.
- The propagation medium.
- A receiving subsystem that includes message reception and report assembly functions at the receiving vehicle or ground system.
The source of the state vector and other transmitted information as well as user applications are not considered to be part of the ADS-B system. 
Relationship to Surveillance Radar
|Radar measures the range and bearing of an aircraft. Bearing is measured by the position of the rotating radar antenna when it receives a response to its interrogation from the aircraft, and range is measured by the time it takes for the radar to receive the interrogation response.
The antenna beam becomes wider as the aircraft gets further away, making the position information less accurate. Additionally, detecting changes in aircraft velocity requires several radar sweeps that are spaced several seconds apart. In contrast, a system using ADS-B creates and listens for periodic position and intent reports from aircraft. These reports are generated and distributed using precise instruments, such as the global positioning system (GPS) and Mode S transponders, meaning integrity of the data is no longer susceptible to the range of the aircraft or the length of time between radar sweeps. 
PSR is robust in the sense that surveillance outage failure modes are limited to those associated with the ground radar system. SSR failure modes include the transponder aboard the aircraft. Typical ADS-B aircraft installations use the output of the navigation unit for navigation and cooperative surveillance, introducing a common failure mode that must be accommodated in air traffic surveillance systems. 
Relationship to Addressed ADS
There are two commonly recognized types of ADS for aircraft applications:
- ADS-Addressed (ADS-A), also known as ADS-Contract (ADS-C), and
- ADS-Broadcast (ADS-B).
ADS-B is inherently different from ADS-A, in that ADS-A is based on a negoiated one-to-one peer relationship between an aircraft providing ADS information and a ground facility requiring receipt of ADS messages. For example, ADS-A reports are employed in the Future Air Navigation System (FANS) using the Aircraft Communication Addressing and Reporting System (ACARS) as the communication protocol. During flight over areas without radar coverage (e.g., oceanic, polar), reports are periodically sent by an aircraft to the controlling air traffic region. 
Relationship to Other Broadcast Services
The ADS-B link can be used to provide other broadcast services, such as FIS-B and TIS-B. Another potential aircraft-based broadcast capability is to transmit aircraft measurements of meteorlogical data.
Traffic Information Services-Broadcast (TIS-B)
TIS-B supplements ADS-B air-to-air services to provide complete situational awareness in the cockpit of all traffic known to the ATC system. TIS-B is an important service for an ADS-B link in airspace where not all aircraft are transmitting ADS-B information. The ground ADS-B station transmits surveillance target information on the ADS-B data link for unequippd aircraft or aircraft transmitting only on another ADS-B link.
TIS-B uplinks are derived from the best available ground surveillance source:
- ground radars for primary and secondary targets
- multi-lateration systems for targets on the airport surface
- ADS-B systems for targets equipped with a different ADS-B link
The multilink gateway service is a companion to TIS-B for achieving interoperability in low altitude terminal airspace. Because aircraft that primarily operate in high altitude airspace are equipped with 1090ES, and aircraft operating primarily in low altitude airspace are equipped with UAT, these aircraft cannot share air-to-air ADS-B data. In terminal areas, where both types of ADS-B link are in use, ADS-B ground stations use ground-to-air broadcasts to relay ADS-B reports received on one link to aircraft using the other link. 
Flight Information Services-Broadcast (FIS-B)
FIS-B provides weather text, weather graphics, NOTAMs, ATIS, and similar information. FIS-B is inherently different from ADS-B in that it requires sources of data external to the aircraft or broadcasting unit, and has different performance requirements such as periodicity of broadcast. 
In the US, FIS-B services will be provided over the UAT link in areas that have a ground surveillance infrastructure. (Scardina 2002)
ADS-B Physical Layer
Three link solutions are being proposed as the physical layer for relaying the ADS-B position reports:
- 1090 MHz Mode S Extended Squitter (ES),
- Universal Access Transceiver (UAT) and
- VHF Digital Link (VDL) Mode 4.
The FAA has announced (FAA 2002) its selection of the 1090 MHz ES and UAT as the mediums for the ADS-B system in the United States. 1090 MHz ES will be the primary medium for air carrier and high-performance commercial aircraft operating at high altitudes, while UAT will be the primary medium for general aviation aircraft operating at lower altitudes. (Scardina 2002)
Europe has also chosen 1090ES as the primary physical layer for ADS-B. However, the second medium has not yet been selected between UAT and VDL Mode 4. (Reference?)
With 1090ES, the existing Mode S transponder (or a stand alone 1090 MHz transmitter) supports a message type known as the ES message. It is a periodic message that provides position, velocity, heading, time, and, in the future, intent. The basic ES does not offer intent since current flight management systems do not provide such data – called trajectory change points. To enable an aircraft to send an extended squitter message, the transponder is modified and aircraft position and other status information is routed to the transponder. ATC ground stations and TCAS-equipped aircraft already have the necessary 1090 MHz receivers to receive these signals, and would only require enhancements to accept and process the additional information. 1090ES will not support FIS-B, due to regulatory requirements. (Reference?)
Universal Access Transceiver
The UAT system is specifically designed for ADS-B operation. UAT has lower cost and greater uplink capacity than 1090ES. 978 MHz (in the US) is dedicated for transmission of airborne ADS-B reports and for broadcast of ground-based aeronautical information. UAT users would have access to ground-based aeronautical data and would receive reports from proximate traffic (FIS-B and TIS-B). TIS-B will provide reports for non-ADS-B equipped aircraft and a multilink gateway service will provide ADS-B reports for 1090ES equipped aircraft. (Scardina 2002)
VDL Mode 4
The VDL Mode 4 system could utilize one or more of the existing aeronautical VHF frequencies as the radio frequency physical layer for ADS-B transmissions. VDL Mode 4 uses a protocol (STDMA) that allows it to be self-organizing, meaning no master ground station is required. This medium is best used for short message transmissions from a large number of users. VDL Mode 4 systems are capable of increased range in comparison to 1090ES or UAT systems. (Reference?)
ADS-B Supported Applications
The ADS-B data link supports a number of airborne and ground applications. Each application has its own operational concepts, algorithms, procedures, standards, and user training.
Cockpit Display of Traffic Information
A Cockpit Display of Traffic Informatin (CDTI) is a generic display that provides the flight crew with surveillance information about other aircraft, including their position. Traffic information for a CDTI may be obtained from one or multiple sources, including ADS-B, TCAS, and TIS-B. Direct air-to-air transmission of ADS-B messages supports display of proximate aircraft on a CDTI.
In addition to traffic based on ADS-B reports, a CDTI function might also display current weather conditions, terrain, airspace structure, obstructions, detailed airport maps, and other information relevant to the particular phase of flight.
ADS-B is seen as a valuable technology to enhance ACAS operation. Incorporation of ADS-B can provide benefits such as:
- Decreasing the number of active iterrogations required by ACAS, thus increasing effective range in high density airspace.
- Reducing unnecessary alarm rate by incorporating the ADS-B state vector, aircraft intent, and other information.
- Use of the ACAS display as a CDTI, providing positive identification of traffic.
- Extending collision avoidance below 1000 feet above ground level, and detecting runway incursions.
Eventually, the ACAS function may be provided based solely on ADS-B, without requiring active interrogations of other aircraft transponders.
ATS Conformance Monitoring
Other applications that may benefit from ADS-B include:
- Improved search and rescue
- Enhanced flight following
- Lighting control and operation
- Airport ground vehicle and aircraft rescue and firefighting vehicle operational needs
- Altitude height keeping performance measurements
- General aviation operations control
The timetable for airborne ADS-B equipage will be determined by ground and airborne facility implementation, equipment cost, perceived benefits of equipping and regulatory actions by the Civil Aviation Authorities (CAA). The cost to equip with ADS-B Out capability is relatively small and would benefit the airspace by enabling increased situational awareness. ADS-B In capability can provide additional benefits when ground stations and the critical mass of aircraft are also equipped. This data was taken into consideration when building the following estimated implementation timetable.
Near-term Implementation (2002-2006)
Defined (in the US) as the period prior to deployment of ADS-B national ground infrastructure, as air carriers and general aviation begin to equip. Principle efficiency benefits are expected to be pairwise air-to-air in the terminal environment, except in "pockets" where additional benefits are possible. (Scardina 2002) Some of these include:
- Capstone. In Alaska, the FAA is conducting its Capstone program to improve surveillance in some of the more remote locations of Alaska and as a test bed for implementing elements of ADS-B into the ATC environment. Approximately 190 general aviation users have been equipped with GPS receivers, UAT transceivers and flight deck displays. In addition, 11 ground-based transceivers have been installed for radar-like services, and flight information services data (FIS-B), including weather information, is being uplinked from the ground. Phase II of the program will expand the coverage and add more than 250 additional users.
- Gulf of Mexico – In the Gulf of Mexico, where ATC radar coverage is incomplete, the FAA is locating ADS-B (1090 MHz) receivers on oil rigs and buoys to relay information received from aircraft equipped with ADS-B extended squitters back to the ATC centers to expand and improve surveillance coverage.
- Australia. Australia is implementing ADS-B trials in Queensland to test the feasibility of 1090 MHz ADS-B as an alternative to ground-based radar. ADS-B is expected to be a much more cost-effective method of providing ATC surveillance coverage for remote areas which currently have limited or no surveillance coverage.
- Cargo Airline Association. Cargo carriers operating at their hub airports operate largely at night. Equipage of these aircraft with ADS-B and CDTI displays along with a ground-based transceiver at these hubs will allow better situational awareness at night and in inclement weather and offers the potential for increased airport traffic handling capability.
- Embry Riddle Aeronautical University. Embry Riddle Aeronautical University is equipping the training aircraft at its two main campuses in Florida and Arizona with ADS-B capability as a safety enhancement. The FAA will provide FIS-B and TIS-B uplink capabilities in those areas in support of this equipage.
- Safe Flight 21 East Coast Broadcast Services. The FAA has announced its intention to implement ADS-B coverage for the entire east coast of the U.S. by the end of 2004. Service range will extend inland 150 miles with a goal of providing coverage at altitudes down to 2,000 feet. The medium will be UAT and the implementation will also include TIS-B and FIS-B information.
Mid-term Implementation (2007-2012)
Defined (in the US) as the period during which the ADS-B national ground infrastructure wil be deployed. (Scardina 2002)
Within four to eight years, an increasing number of aircraft with ADS-B Out capability along with the start of ground-based ADS-B infrastructure will begin to make a number of ADS-B applications attractive.
- Benefits of “Pockets of Implementation” will become evident and these areas will be expanded, encouraging more users to equip with ADS-B capability.
- Beginning in 2004 the FAA is expected to deploy ADS-B ground infrastructure based on ASDE-X equipment. This infrastructure will allow the use of ADS-B data for ATC purposes such as surface movement tracking/guidance and airborne surveillance.
- Ground uplinks of TIS-B and FIS-B will commence where the ground infrastructure is deployed.
- Other Civil Aviation Authorities may install ADS-B ground infrastructure and require aircraft to be equipped with ADS-B Out for operation in selected airspace. Australia is expected to be the first country to do so; however, a number of other countries with limited surveillance coverage may find ADS-B an attractive alternative to radar surveillance.
- Significant numbers of users will become equipped with a minimum of ADS-B Out capability. In Europe, 1090 MHz ES will become standard capability for all new Mode S transponder installations after 31 March, 2005. UAT will become increasingly popular in the upper end of the general aviation market.
- Airport Situational Awareness – A combination of detailed airport maps, airport multilateration (ASDE-X) systems, enhanced aircraft displays and ADS-B have the potential to significantly improve Runway situational awareness.
- Oceanic In-trail – ADS-B can provide enhanced situational awareness and safety for Oceanic In-trail maneuvers as additional aircraft become equipped.
- Use of ADS-B and CDTI will allow decreased approach spacing and closely spaced parallel approaches at congested airports with improved safety and capacity during low-/lower-visibility operations.
Long-term Implementations (2012 and beyond)
- Air carriers’ fleets will achieve intended ADS-B benefits in the terminal and en route airspace.
- New Aircraft Separation Assurance applications will take advantage of the increased situational awareness and positional accuracy available in an airspace environment largely equipped with ADS-B capability.
- FIS-B and TIS-B services will encourage general aviation equipage in all market segments.
- ^ a b c d e f g h Minimum Aviation System Performance Standards for Automatic Dependent Surveillance-Broadcast (ADS-B). RTCA, Inc. June 25, 2002. DO-242A.
- ^ a b Scardina, John (June 7, 2002). "Overview of the FAA ADS-B Link Decision" (PDF). Federal Aviation Administration.
- ^ "FAA Announces Automatic Dependent Surveillance-Broadcast Architecture" (Press release). FAA Office of Public Affairs. July 1, 2002. APA 27-02.
- ^ "ADS-B Home Page". Federal Aviation Administration. April 21, 2005. Retrieved December 26, 2005.
- ^ Orlando, Dr. Vincent A (December 3–8, 2001). "Automatic Dependent Surveillance Broadcast (ADS-B) Mode S Extended Squitter" (PDF). FAA Working Group 3: 1090 MHz ES Meeting 8.
- DO-212, Minimum Operational Performance Standards for Airborne Automatic Dependent Surveillance (ADS) Equipment
- DO-267A, Minimum Aviation System Performance Standards for Flight Information Services-Broadcast (FIS-B)
- DO-282, Minimum Operational Performance Standards for Universal Access Transceiver (UAT) Automatic Dependent Surveillance-Broadcast (ADS-B)
- DO-286, Minimum Aviation System Performance Standards for Traffic Information Services-Broadcast (TIS-B)
- DO-260A, Minimum Operational Performance Standards for 1090 Mhz Automatic Dependent Surveillance-Broadcast (ADS-B) and Traffic Information Services-Broadcast (TIS-B)
- Airservices Australia ADS-B info
- EGOA Project
- Safe Flight 21 ADS-B Projects
- Capstone ADS-B Project
- NUP II Project
- NUP II Plus Project
- ADS-MEDUP Project
- Eurocontrol CASCADE Programme
- European Organisation for Civil Aviation Equipment
eventually delete this section as it is incorprated in other sections
The initial use of ADS-B is expected to be by air traffic control and for surveillance purposes and for enhancing pilot situational awareness. ADS-B is lower cost than conventional radar and permits higher quality surveillance of airborne and surface movements. ADS-B is effective in remote areas or in mountainous terrain where there is no radar coverage, or where radar coverage is limited. The outback of Australia is one such area where ADS-B will provide surveillance where previously none existed. ADS-B also enhances surveillance on the airport surface, so it can also be used to monitor traffic on the taxiways and runways of an airport.
Airbus and Boeing are now expected to include ADS-B out (i.e. the transmitter of information) as standard on new-build aircraft from 2005 onwards. This is in part due to the European requirements for Mode S Elementary Surveillance (which uses 1090MHz Mode S transponder which now is normally capable of ADS-B via Extended Squitter), and some common functionality with ADS-B out.
Use of ADS-B for ground-based surveillance requires only ADS-B Out (transmit) capability on the aircraft. With the addition of ADS-B In (receive) capability, the potential for ADS-B applications grows significantly. Some of the equipment and services associated with ADS-B In capability include:
- Cockpit Display of Traffic Information (CDTI), a display of proximate traffic based on ADS-B reports from other aircraft and ground-based facilities.