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Automatic dependent surveillance-broadcast (ADS-B) is a cooperative surveillance technique for air traffic control and related applications. An ADS-B-equipped aircraft determines its own position using a global navigation satellite system and periodically broadcasts this position and other relevant information to potential ground stations and other aircraft with ADS-B-in equipment. ADS-B can be used over several different data link technologies, including Mode-S Extended Squitter (1090 ES), Universal Access Transceiver (978 MHz UAT), and VHF data link (VDL Mode 4).

ADS-B provides accurate information and frequent updates to airspace users and controllers, and hence supports improved use of airspace, reduced ceiling/visibility restrictions, improved surface surveillance, and enhanced safety, for example through conflict management.

Under ADS-B, a vehicle periodically broadcasts its own state vector and other information without knowing what other vehicles or entities might be receiving it, and without expectation of an acknowledgment 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.

A similar solution is the Automatic Identification System (AIS), a system used by ships and Vessel Traffic Services.

Theory of operation

ADS-B consists of three components:

• A transmitting subsystem that includes message generation and transmission functions at the source, e.g. airplane.
• The transport protocol, e.g. VHF (VDL mode 2 or 4) or 1090ES.
• A receiving subsystem that includes message reception and report assembly functions at the receiving destination, e.g. other airplanes, 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 directly measures the range and bearing of an aircraft from a ground-based antenna. 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 based on the global positioning system (GPS) and distributed via VHF radio or Mode S transponders, meaning integrity of the data is no longer susceptible to the position of the aircraft or the length of time between radar sweeps.

Primary Surveillance Radar does not require any cooperation from the aircraft. It is robust in the sense that surveillance outage failure modes are limited to those associated with the ground radar system. Secondary Surveillance Radar depends on active replies from the aircraft. Its failure modes include the transponder aboard the aircraft. Typical ADS-B aircraft installations use the output of the navigation unit for navigation and for cooperative surveillance, introducing a common failure mode that must be accommodated in air traffic surveillance systems.

Today's ATC systems do not rely on coverage by a single radar. Instead a multiradar picture is presented via the ATC system's display to the controller (ATCO). This improves the quality of the reported position of the airplane, provides a measure of redundancy, and makes it possible to verify the output of the different radars against others. This verification can also use sensor data from other technologies, such as ADS-B and multilateration.

Relationship to ADS-A/ADS-C

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 differs from ADS-A in that ADS-A is based on a negotiated 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 and polar), reports are periodically sent by an aircraft to the controlling air traffic region.

The transmission delay caused by protocol, satellites, etc., is significant enough that significant aircraft separations are required. The cost of using the satellite channel leads to less frequent updates. Another drawback is that no other aircraft can benefit from the transmitted information.

Relationship to other broadcast services

The ADS-B link can be used to provide other broadcast services, such as TIS-B and FIS-B (see below).

Another potential aircraft-based broadcast capability is to transmit aircraft measurements of meteorological data.

Benefits of ADS-B

ADS-B is intended to increase safety and efficiency. Safety benefits include:

• Improved visual acquisition especially for general aviation under visual flight rules (VFR).
• Reduced runway incursions on the ground.

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

However, although ADS-B is suitable for surveillance of remote areas where the siting of radars is difficult, some ATC providers are not yet convinced that it is currently suitable for use in high traffic volume areas, such as in UK and Northern European airspace. Changing from conventional SSR to ADS-B would also require investment in ATC infrastructure, something which many European providers may be unwilling to sanction. Furthermore, ADS-B provides no ground verification of the accuracy of the information provided by aircraft and this could have adverse security implications.

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 TIS-B station transmits surveillance target information on the ADS-B data link for unequipped targets or targets transmitting only on another ADS-B link.

TIS-B uplinks are derived from the best available ground surveillance sources:

• 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

Multilink gateway service

The multilink gateway service is a companion to TIS-B for achieving interoperability in low altitude terminal airspace. In some airspaces, 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 directly share air-to-air ADS-B data. In terminal areas, where both types of ADS-B link are in use, ADS-B/TIS-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.

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 Data Link (VDL) Mode 4.

A comparison of these link solutions was made in Gatwick/Heathrow in year 2002 for the Eurocontrol ADS programme.

1090ES

In 2002, the FAA has announced a dual link decision using 1090 MHz ES and UAT as media for the ADS-B system in the United States, with the 1090 MHz extended squitter ADS-B link for air carrier and private/commercial operators of high performance aircraft, and Universal Access Transceiver (UAT) ADS-B link for the typical general aviation user.

Europe has not officially chosen a physical layer for ADS-B. A number of technologies are in use. However, the influential Eurocontrol CASCADE program uses 1090ES exclusively.

With 1090ES, the existing Mode S transponder (TSO C-112 or a stand alone 1090 MHz transmitter) supports a message type known as the extended squitter (ES) message. It is a periodic message that provides position, velocity, 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 (TSO C-166A) 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 (Mode S) receivers to receive these signals, and would only require enhancements to accept and process the additional Extended Squitter information. 1090ES does not support FIS-B service.

Universal access transceiver

The UAT system is specifically designed for ADS-B operation. Because UAT is more flexible and has greater bandwidth, many Air Traffic management experts think that UAT will become the dominant ADS-B link standard within the next ten years. UAT is also the first link to be certified for "radar-like" ATC services in the U.S. Since 2001, it has been providing 5nm enroute separation (the same as radar) in Alaska. UAT is the only ADS-B link standard that is truly bi-directional: UAT users have access to ground-based aeronautical data and can receive reports from proximate traffic (FIS-B and TIS-B, which provides reports for proximate aircraft through a multilink gateway service that provides ADS-B reports for 1090ES equipped aircraft and non-ADS-B equipped Radar traffic. UAT equipped aircraft can also "see" each other with accuracy and range that exceeds even TCAS. Viable ADS-B UAT networks now exist on the US East Coast, Alaska and Oregon. Since 2006, a US company has been helping the Civil Aviation Flight University of China run the largest ADS-B system in the world - a network that spans more than 1,200 nm across Central China.

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, invented by Swedish Håkan Lans in 1988) that allows it to be self-organizing, meaning no master ground station is required.

In November 2001 this protocol was published by ICAO as a global standard.

This medium is best used for short message transmissions between a large number of users.

VDL Mode 4 systems required by airlines in Europe as a mean for more efficient ATC service .

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 Information (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.

Airborne collision avoidance

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 interrogations 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.

Conflict management

ATS conformance monitoring

Other applications

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

U.S. implementation timetable

The U.S. FAA ADS-B implementation is broken into three segments each with a corresponding time line. Ground segment implementation and deployment is expected to begin in 2009 and be completed by 2013 throughout the National Airspace System. Airborne equipage is user driven and is expected to be completed both voluntarily based on perceived benefits and through regulatory actions (Rulemaking) by the FAA. The cost to equip with ADS-B Out capability is relatively small and would benefit the airspace with surveillance in areas not currently served by radar. The FAA intends to provide similar service within the NAS to what radar is currently providing (5NM en route and 3NM terminal radar standards) as a first step to implementation. However, ADS-B In capability is viewed as the most likely way to improve NAS throughput and enhance capacity.

In December 2008 Acting FAA Administrator Robert Sturgell gave the go-ahead for ADS-B to go live in southern Florida. The south Florida installation, which consists of 11 ground stations and supporting equipment, is the first commissioned in the USA, although developmental systems have been online in Alaska and along the East Coast. The completed system will consist of 794 ground station transceivers. The December 2008 action is in compliance with a late-term Executive Order from George W. Bush which mandated accelerated approval of NextGen.

FAA segment 1 (2006-2009)

ADS-B deployment and voluntary equipage, along with rule making activities. Pockets of development will exploit equipment deployment in the areas that will provide proof of concept for integration to ATC automation systems deployed in the NAS.

FAA segment 2 (2010-2014)

ADS-B ground stations will be deployed throughout the NAS, with an In-Service Decision due in the 2012-13 time frame. Completed deployment will occur in the 2013-2014 time frame. Equipage is expected to begin after the proposed rule is finalized in around 2010:

• Airport Situational Awareness – A combination of detailed airport maps, airport multilateration systems, ADS-B systems and enhanced aircraft displays have the potential to significantly improve Final Approach and Runway Occupancy Awareness (FAROA).
• Oceanic In-trail – ADS-B may provide enhanced situational awareness and safety for Oceanic In-trail maneuvers as additional aircraft become equipped.
• 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 to relay information received from aircraft equipped with ADS-B extended squitters back to the Houston Center to expand and improve surveillance coverage.

FAA segment 3 (2015-2020)

ADS-B In equipage will be based on user perceived benefit, but is expected to be providing increased situational awareness and efficiency benefits within this segment. Those aircraft who choose to equip in advance of any mandate will see benefits associated with preferential routes and specific applications. Limited radar decommissioning will begin in the time frame with an ultimate goal of a 50% reduction in the Secondary Surveillance Radar infrastructure.

Non FAA implementations

• Use of ADS-B and CDTI may allow decreased approach spacing at certain airports to improve capacity during reduced-visibility operations when visual approach operations would normally be terminated (eg. ceilings less than MVA +500).
• United States
  • Cargo Airline Association - Cargo carriers, notably United Parcel Service (UPS), operating at their hub airports operate largely at night. Much of the benefit to these carriers is envisioned through merging and spacing the arriving and departing traffic to a more manageable flow. More environmentally friendly and efficient area navigation (RNAV) descent profiles, combined with CDTI, may allow crews to eventually aid controllers with assisted visual acquisition of traffic and limited cockpit-based separation of aircraft. The benefits to the carrier are fuel and time efficiencies associated with idle descent and shorter traffic patterns than typical radar vectoring allows.
  • Embry-Riddle Aeronautical University - ERAU has equipped their training aircraft at its two main campuses in Florida and Arizona with UAT ADS-B capability as a situational safety enhancement. The University has been doing this since May 2003, making it the first use in general aviation. With the addition of the G1000 to their fleet in 2006, ERAU became the first fleet to combine a glass cockpit with ADS-B.
  • University of North Dakota - UND has received an FAA grant to test ADS-B, and has begun to outfit their Piper Warrior fleet with an ADS-B package.

• China - An American Company, ADS-B Technologies created one of the largest and most successful ADS-B system in the world (an 8 station, 350+ aircraft network that spans more than 1,200 nm across Central China). This was also the first UAT installation outside the U.S.. As of March, 2009, more than 1.2M incident/failure free flight hours have been flown with these ADS-B systems.
• Australia - Upper Airspace Program - A program aimed at providing near-term safety and operational benefits in high level, non-radar airspace. Includes installation of approximately 28 ADS-B ground stations, strategically located across Australia to provide air traffic surveillance above 30,000 feet in continental airspace outside of radar coverage. All sites are expected to be installed and operating by mid 2007.
  • Australian Transition to Satellite Technology - Now in initial planning and development, this is a major, longer term program designed to make ADS-B the primary means of ground to air and air to air surveillance in Australian en route airspace. Includes installation of additional ADS-B ground stations to provide air traffic surveillance in airspace currently covered by en route radar facilities. Intended to lead to the decommissioning of a number of radar sites. Mandatory aircraft ADS-B equipment requirements will apply – funding options to support general aviation operators will be explored.

"Australian Transition to Satellite Technology (ATLAS)". Airservices Australia. August 7, 2007. http://www.airservicesaustralia.com/projectsservices/projects/adsb/. Retrieved on 2007-08-24. 

• Canada - Canada is planning to use ADS-B to provide coverage of its northern airspace around Hudson Bay, most of which currently has no radar coverage. ADS-B will be initially deployed in the Hudson Bay Basin in 2007–2008 and the service is expected to be later extended to cover the rest of the Canadian Arctic and eventually to the rest of Canada. As of November 2008, Nav Canada has installed five ADS-B ground-station receivers around Hudson Bay, which will go into daily use in January.

• Sweden - LFV Group in Sweden has implemented a nationwide ADS-B network with 12 ground stations. Installation commenced during spring 2006, and the network was fully (technically) operational in 2007. An ADS-B supported system is planned for operational usage in Kiruna during spring 2009. 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.

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/MSSR are existing users of the channel. 1090ES ADS-B must not reduce capacity of these existing systems.

The FAA national program office and other International aviation regulators are addressing concerns about ADS-B 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. Additionally, there are some concerns about the integrity of ADS-B transmissions. ADS-B messages can be produced, with simple low cost measures, which spoof the locations of multiple phantom aircraft to disrupt safe air travel. There is no foolproof means to guarantee integrity, but there are means to monitor for this type of activity. This problem is however similar to the usage of ATCRBS/MSSR where false signals also are potentialy dangerous (uncorrelated secondary tracks).

There are some concerns about ADS-B dependence on other systems. This paper from 2001 mentions some of the potential risks. The risks mentioned can be mitigated by using other sources of information, e.g. GLONASS, Galileo or multilateration.

There are some General Aviation concerns that ADS-B removes anonymity of the VFR aircraft operations. The ICAO 24-bit transponder code specifically assigned to each aircraft will allow monitoring of that aircraft when within the service volumes of the Mode-S/ADS-B system. Unlike the Mode A/C transponders, there is no code "1200"/"7000", which offers casual anonymity. Mode-S/ADS-B identifies the aircraft uniquely among all in the world, in a similar fashion as a MAC number for an ethernet card or the IMEI (International Mobile Equipment Identity) of a GSM phone.

Public Access to ADS-B

Currently there are no laws preventing the public from listening to and decoding ADS-B transmissions. Like Cellular Phone however, laws can easily be implemented to make reception a crime, and receivers would then become contraband. The ongoing debate amongst hobbyists is to display real-time activity on personal screens and then delay five minutes on networked displays. Others feel that, like GPS data, it should be freely available.

Two receivers are currently popular, although both are at the expensive end for consumer devices. The first on the market was Kinetic Avionics with their SBS-1. The second was the AirNav Systems RadarBox. Both of these devices are designed to be used for portable operation, although many use them in a fixed base setting. They are both supplied with a short USB cable for interface to a Windows PC, and a short coax to a small omni antenna.

Both products have specialized single-use receivers in them. Unlike most radios there is no Intermediate Frequency (IF). The ADS-B data flows from the supplied antenna through an LNA pre-amp. It is bandwidth filtered, and then the pulses are extracted using a logarithmic amplifier chip. This chip can operate directly at the ADS-B frequency of 1090 MHz. These analog pulses are then applied to a high speed analog to digital (A/D) converter (40 MHz for Kinetic, 8 MHz for AirNav) and then on to the Field-Programmable Gate Array (FPGA), where the Mode-S frames are detected. The detected Mode-S packets are then sent to the PC via a USB interface or, in the case of an SBS-1, through an optional ethernet interface module.

ADS-B technical and regulatory documents

MASPS = Minimum Aviation System Performance Standards
MOPS = Minimum Operational Performance Standards

• DO-289 - Airborne Surveillance Applications (ASA) MOPS
  • High level system architecture and sub-system descriptions.
• DO-242A - ADS-B MASPS
  • Describes system-wide operational use of ADS-B.
• DO-286A - TIS-B MASPS
  • Describes a surveillance service that derives traffic information from ground surveillance sources, broadcasts to ADS-B equipped aircraft or surface vehicles.
• DO-260A - 1090 MOPS for ADS-B and TIS-B
  • Airborne equipment characteristics / requirements for 1090 MHz Mode-S extended squitter.
  • Change 1-DO-260A–PMC consideration review/approval –June 27, 2006
• DO-282A - UAT MOPS
  • Airborne equipment characteristics / requirements utilizing the universal access transceiver.
• DO-XXX - STP MOPS (work in progress)
  • Describes a function that processes information prior to the information being broadcast by the ADS-B transmit function.
• DO-XXX - ASAS MOPS (work in progress)
  • Adds additional subsystems necessary to fully implement AirborneSurveillance Applications:
    • Airborne Surveillance and Separation Assurance Processing (ASSAP)
    • Cockpit Display of Traffic Information (CDTI)
• DO-259 - CDTI Application Description
  • Provides initial CDTI applications descriptions.

See also

• DO-212 Minimal Operational Performance Standards for Airborne Automatic Dependent Surveillance (ADS) Equipment
• NextGen (US)
• Single European Sky ATM Research
• TCAS
• FLARM
• PCAS
• ASDE-X
• Free flight

External links

• Official FAA ADS-B Website
• ADS-B Technologies - ADS-B engineering and consulting
• ADS-B ... Terrorist's Dream, Security's Nightmare
• Commercial Aviation Accidents Before and During the Alaska Capstone Implementation of ADS-B, FIS-B, Terrain Situational Awareness, and Expanded IFR Infrastructure
• Flarm, a simple low-range ADS-B concept implementation
• NUP II Project
• NUP II Plus
• Enhanced General Aviation by ADS-B
• ADS-MEDUP Project
• Eurocontrol CASCADE Programme
• European Organisation for Civil Aviation Equipment
• "Airframes.org". http://www.airframes.org. Retrieved on 2008-04-26. : A publicly accessible database of aircraft registrations, SELCAL codes and ICAO24 address codes. The database is not complete and relies on user feedback to remain up to date.
• ICAO Annex 10 Volume III Chapter 9. Aircraft Addressing System
• Overview of collision avoidance systems
• Joint Planning & Development Office
• Safe Flight 21 ADS-B Projects
• Capstone ADS-B Project
• Airservices Australia ADS-B Programs
• AOPA News Archive
• Kinetic Avionics SBS-1
• AirNav RadarBox
• Capstone Public Interface for Garmin GDL-90
• ADS-B In the Gulf of Mexico, FAA Aviation News
• - RadarSpotters.eu Community Forum for Radarbox & Kinetics SBS & About ADS-B

Commercial implementations of ADS-B

• ADS-B Technologies - ADS-B engineering, manufacturing and consulting
• ACSS' SafeRoute ADS-B Solutions for Approach and Taxi, used by UPS
• AIS-P
• Quadrant ADS-B Sensors

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Published in July 2009.

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