Automatic Dependent Surveillance–Broadcast

Automatic Dependent Surveillance-Broadcast (also called ADS-B) is a system by which airplanes constantly broadcast their current position and altitude, category of aircraft, airspeed, identification, and whether the aircraft is turning, climbing or descending over a dedicated radio datalink. This functionality is known as "ADS-B out" and is the basic level of ADS-B functionality.

The current ADS-B system was developed in the 1990s though its lineage dates back to the 1960s. It relies on data from the Global Positioning System, or any navigation system that provides an equivalent or better service. The maximum range of the system is line-of-sight, typically less than 200 nautical miles (370 km). Research has also been performed on ADS over satcom, which covers much of the world. ADS over satcom is referred to as ADS-C, the 'C' is for contract, and is used by airliners over oceanic routes.

The ADS-B transmissions are received by air traffic control stations, and all other ADS-B equipped aircraft within reception range. Reception by aircraft of ADS-B data is known as "ADS-B in".

Usage

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.

ADS-B equipped aircraft may also have a display unit in the cockpit picturing surrounding air traffic from ADS-B data (ADS-B in) and TIS-B (Traffic Information Service-Broadcast) data derived from air traffic radar. Both Pilots and air traffic controllers will then be able to "see" the positions of air traffic in the vicinity of the aircraft, and this may be used to provide an ASAS (Airborne Separation Assurance System).

Airborne Collision Avoidance Systems may in the future also make use of "ADS-B in", supplementing the existing TCAS collision avoidance system by what is called 'hybrid surveillance'.

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 1090 MHz Mode S transponder which now is normally capable of ADS-B via Extended Squitter), and some common functionality with ADS-B out. (A 'squitter' is a burst of data sent without being prompted by an interrogating radar.)

Addressed and Broadcast ADS

The Radio Technical Commission for Aeronautics’ (RTCA) Free Flight Selection Committee defines surveillance as “detection, tracking, characterization and observation of aircraft, other vehicles and weather phenomena for the purpose of conducting flight operations in a safe and efficient manner."

Automatic Dependent Surveillance (ADS) is described as the process of creating and sending a message including the sender’s current position and other surveillance information, such as velocity, intent and flight identification. This information supports aircraft separation management by improving surveillance information at increased ranges, situational awareness and decision making. ADS data can be used in cooperation with data from current radar beacon systems, such as Air Traffic Control Radar Beacon System (ATCRBS), Mode S, Traffic Collision Avoidance System (TCAS) and primary Air Traffic Control (ATC) radar, and may also be used as a sole means of surveillance.

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-A provides a surveillance data report that is sent to a specific addressee. 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 transoceanic flight, reports are periodically sent by an aircraft to the controlling air traffic region.

When ADS-B is used, aircraft and other vehicles continuously broadcast a message including position, heading, velocity and intent. Other uses may include obstacles transmitting a position message. Aircraft, ground-based stations, and other users monitoring the channels can receive the information and use it in a wide variety of applications. Because of this potential for broad utilization, a system using ADS-B is most often discussed as a replacement for or an augmentation to current methods of monitoring aeronautical traffic.

To understand the full capability of ADS-B, consider how the current Air Traffic Control system creates information. The 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 get farther 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. The enhanced accuracy of the information will be used to improve safety, support a wide variety of applications and increase airport and airspace capacity.

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.
Traffic Information Services-Broadcast (TIS-B), a ground-based uplink report of proximate traffic that is under surveillance by ATC but is not ADS-B-equipped. This service would be available even with limited ADS-B implementation.
Flight Information Services-Broadcast (FIS-B), a ground-based uplink of flight information services and weather data.

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.

Mode S

The FAA has announced 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 while UAT will be the primary medium for general aviation aircraft.

Europe has also chosen 1090 MHz as the primary physical layer for ADS-B. This link will be shared by commercial aircraft and GA traffic. This decision avoids the architectural complexity and additional costs associated with cross-link re-broadcasting (ADS-R).

With 1090 ES, 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. 1090 ES will not support FIS-B (Flight Information Services - Broadcast), due to regulatory requirements.

Ground surveillance radar sites and other ground station interrogators work on 1030 MHz to trigger the aircraft transponders sending their Mode-S packets on 1090 MHz.

An aircraft transponder identifies itself with a unique 24bit aircraft address, which may be stated in octal (FAA) or hex (ICAO convention). Additionally the aircraft crew can set a seven-character identification string (flightnumber / callsign). Blocks of these 24bit aircraft addresses are allocated to states, which assign individual addresses to aircraft in the states' registries.

Examples: ICAO24 address 3C4823(hex) belongs to a 737-86J (B738) of Air Berlin with registration D-ABAC. ICAO24 address A7E579(hex) belongs to a MD-11F (MD11) of FedEx with registration N608FE.

The mode-S protocol specification and ICAO24 bit addressing (including a list of state allocation blocks) is defined in Annex 10 to the Convention of International Civil Aviation.

Universal Access Transceiver

The UAT system, developed by the MITRE Corporation, is specifically designed for ADS-B operation. A 1 MHz channel at 978 MHz is dedicated for transmission of airborne ADS-B reports and for ground-based broadcast of traffic, weather and other non-control aeronautical information. UAT users will have access to these additional ground-based aeronautical data (FIS-B and TIS-B) in addition to receiving reports air-to-air from proximate ADS-B traffic.

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 L Band Mode S (1090 MHz) or UAT systems.

Implementation Timetable

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 (2006-2008)

The next three years will see a continuation of ADS-B trials and some implementation in “pockets” where limited aircraft equipage can bring operational benefits. Some of these include:

Sweden. LFV Group in Sweden is implementing a nationwide ADS-B network with 12 ground stations. Installation will commence during spring 2006, and the network will become fully operational in 2007. Based on the VDL Mode 4 standards, the network of ground stations can support services for ADS-B, TIS-B, FIS-B, GNS-B (DGNSS augmentation) and Point-to-Point communication, allowing aircraft equipped with VDL 4-compliant transceivers to lower fuel consumption and reduce flight times.
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 has completed 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. Australia currently have 5 stations operational across Western Australia, South Australia, New South Wales and Queensland, and plan to implement full coverage of the continent via ADS-B within the next 2 years.
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 (2008-2012)

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. VDL Mode 4 is being implemented more extensively in order to cope with future demands within ATM both in terms of safetey, efficiancy and capacity.
Airport Situational Awareness – A combination of detailed airport maps, airport multilateration (ASDE-X) systems, VDL Mode 4/ ADS-B systems and enhanced aircraft displays 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 full ADS-B capability.
FIS-B and TIS-B services will encourage general aviation equipage in all market segments.
Point-to-Point communication via two way datalink technology such as VDL mode 4 is anticipated to be more widely used.

System Design Considerations of ADS-B

A concern for any ADS-B protocol is the capacity for carrying ADS-B messages from aircraft, as well as allowing the radio channel to continue to support any legacy services. For 1090ES, each ADS-B message is composed of a pair of data packets. The greater the number of packets transmitted from one aircraft, the lesser the number of aircraft that can participate in the system, due to the fixed and limited channel data bandwidth.

System capacity is defined by establishing a criterion for what the worst environment is likely to be, then making that a minimum requirement for system capacity. For 1090ES, both TCAS and ATCRBS are existing users of the channel. 1090ES ADS-B must be designed to not reduce capacity of these existing systems.

There are some concerns about the non-secure nature of ADS-B transmissions. ADS-B messages can be used to know the location of an aircraft, and there is no means to guarantee that this information is not used inappropriately.

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