Automatic Dependent Surveillance-Broadcast (ADS-B)

ADS-B is a technique whereby: (1) aircraft position is derived by an onboard GNSS receiver; and (2) aircraft identity, altitude, speed, and position are broadcast directly (without using satellites) to the ground and nearby aircraft. The intent of an aircraft may also be coded and broadcast. ADS-B allows for provision of common situational awareness to all appropriately equipped users of the system.

ADS-B is a technique whereby: (1) aircraft position is derived by an onboard GNSS receiver; and (2) aircraft identity, altitude, and position are broadcast directly (without using satellites) to the ground and nearby aircraft. In addition, other valuable surveillance information could also be relayed to ATM facilities (e.g., ATCRBS code, velocity, heading, maneuver intentions, and in situ weather parameters). The onboard ADS-B avionics will broadcast (squitter) on the international transponder reply frequency, 1090 MHz. With current Mode S/TCAS equipment capabilities, an ADS-B equipped aircraft is expected to provide surveillance information to other in-flight aircraft at ranges up to 40 nmi (without changing the sensitivity of aircraft receivers) and to ground stations at distances up to 95 nmi (with six-sector antennas or low-noise receivers).

Squittered ADS-B messages received by nearby aircraft will provide Cockpit Display of Traffic Information (CDTI) and collision avoidance capabilities. Thus the ADS-B transponder avionics will perform three functions: (1) broadcast of aircraft identification, altitude, position, and other information to the ground and other aircraft; (2) exchange of collision avoidance related information with other aircraft; and (3) transponder turn-around of secondary radar interrogations, for use with TCAS I/II and SSRs during the transition phase. ADS-B will provide the principal ATM surveillance capability in the en route, terminal, and airport surface domains in the post-2015 timeframe.

There are no differences in ADS-B operation from domain to domain except for the update rates, which are adapted based on operating domain. Relatively high rates will be used in the terminal and surface domains (l- to 5-second spacing). Rates will be lower in the en route domain (5- to 12-second spacing).

The preferred ADS-B ground station configuration has triple coverage, to allow redundancy in reception of aircraft messages and to support an integrity monitoring and backup surveillance system employing ground-based passive multilateration. Multilateration will operate with ADS-B squitters, SSR Mode C/S replies and squitters, TCAS II replies and squitters, and TCAS IV squitters. Multilateration can be implemented in the near term without any change in aircraft equipment. Individual sensor sites would not have redundant equipment; in effect, the redundant equipment that would be necessary with single site coverage will be deployed at additional sites, providing better airspace coverage, greater robustness, and multilateration capability at little additional cost.

For a minimum surveillance altitude of 6,000 feet, triple coverage requires approximately 300 en route sensor sites (most using multisectored antennas). Between 5 and 11 sensor sites (using omni and multi- sector directional antennas) are needed for each of the approximately 240 TRACONs.

Up to 240 of the largest airports will use ATIDS (ASTA Target Identification System) to receive aircraft messages on 3 to 7 sensors and provide controllers with target location and identity information. ATIDS will perform multilateration on Mode A/C/S transponder replies/squitters as well as on ADS-B squitters. An additional 200 towered airports with lower traffic density will receive one ADS-B sensor site (which could also be a terminal sensor), allowing surface aircraft ADS-B messages to be collected but not providing backup multilateration capability.

During the transition period, surveillance target reports will be available from four sensor types: ADS-B, multilateration, primary radar, and secondary radar. Moreover, there will usually be redundant reports from sensors of the same type. The Multisensor Interface Processor (MIP) will fuse multiple/redundant target reports into a single report for use by aircraft trackers, thereby ensuring smooth integration of ADS-B into the existing surveillance infrastructure. Approximately 22 MIP installations will be used for the en route domain, and approximately 240 MIPS (one per TRACON) will fuse target reports from terminal and surface sensors.

Users on the surface of the approximately 8,000 non-towered airports will rely on GNSS navigation services and aircraft-to-aircraft surveillance.

Preferred ADS-B Backup Mode of Operation: Passive Multilateration ADS-B involves three basic system elements: GNSS signals, avionics (GNSS receiver and ADS-B transponder), and ground station electronics. A backup mode of operation must be provided that prevents loss of surveillance capability if there is a malfunction in any one of these elements - i.e., no single point of failure. Failure of GNSS satellite signals, including unintentional or intentional radio frequency interference, would have the most impact, since it would cause loss of both surveillance and navigation capabilities for all aircraft in a region such as a metropolitan area.

The preferred method for protecting against GNSS failures is passive multilateration on the ground. Three ground stations measure the time-of-arrival (TOA) of a common aircraft message, either squittered or in reply to an interrogation. The message may or may not contain a GPS time tag. If it does, joint processing of the TOAs at a common ground sites can estimate aircraft three-dimensional coordinates, thereby also providing some protection against altimeter failure. If a GPS time tag is not included, differential (hyperbolic) processing of the TOAs can determine the aircraft horizontal position.

If the ground stations are arranged in an equilateral triangle (a pattern that provides both efficient coverage and advantageous multilateration geometry), the latitude/longitude errors will be approximately 70 ft, 2drms for either technique. When the message contains a time tag (ADS-B squitter), vertical position can be estimated. However, errors will be 10 to 17 times larger due to poor altitude measurement geometry. During normal ADS-B operations, multilateration provides an independent integrity check on the GNSS-derived data in the ADS-B messages.

Three-station passive multilateration has several advantages over other possible backup systems. Multilateration based on air-to-ground signals can be implemented without changing aircraft equipment, and thus serve as a transition path to ADS-B. Horizontal position estimates with three stations are significantly more accurate than for two-station configurations. Because the ground stations do not radiate, there are fewer restrictions on their siting (for example, they could be placed on telephone poles in metropolitan areas). With passive multilateration, aircraft position information derived on the ground can be transmitted to the aircraft via data link, thereby enabling “automatic dependent navigation” to be used to back up aircraft navigation.

Excepting use of primary radars, protection against aircraft equipment failures can only be achieved through additional avionics - either redundant or dissimilar to the principal equipment. It is expected that many aircraft would be equipped with redundant ADS-B transponders. However, a separate data link (e.g., using VHF or a different frequency band) could also be used to back up the squitter link at 1090 MHz. Although many aircraft will carry redundant GNSS receivers, this equipment would not be necessary with the recommended multilateration technique. Primary radars will be retained around the U.S. perimeter and in terminal areas, providing an additional level of protection against transponder failures and unequipped aircraft.

Redundant ground sites serve as the backup for individual ground stations.

[Surveillance Vision Plan, Revision 2 (AND-440), Federal Aviation Administration. 1 July 1996]

Technology Overview [FAA Safe Flight 21 site]

The following sections describe the ADS-B and Broadcast Service functional capabilities currently being deployed in the National Airspace System (NAS).

System Functional Architecture

Automatic Dependent Surveillance-Broadcast (ADS-B)

ADS-B is an aircraft-based surveillance service being deployed in selected areas of the NAS. This underlying technology broadcasts a radio transmission approximately once-per-second from the aircraft containing its position, velocity, identification, and other pertinent information. ADS-B can also receive reports from other suitably equipped aircraft within reception range. No ground infrastructure is necessary for ADS-B equipped aircraft to detect each other. These reports can be transmitted to ground based transceivers (GBTs) and used to provide air traffic surveillance services and fleet operator monitoring of aircraft.

Traffic Information Service - Broadcast (TIS-B) Universal Access Transceiver (UAT) Only

The TIS-B service is intended to improve the pilot’s ability to visually see other traffic in the air and on the airport surface so that pilots can more effectively apply traditional “see-and-avoid” techniques.

TIS-B is NOT intended to be used as a collision avoidance system and does NOT relieve the pilot of responsibility to "see-and-avoid" other aircraft. (See Aeronautical Information Manual, Paragraph 5-5-8, “See and Avoid”). TIS-B shall not be used for avoidance maneuvers during times when there is no visual contact with the intruding aircraft. No avoidance maneuvers are provided for, nor authorized, as a direct result of a TIS-B target being displayed in the cockpit.

TIS-B is a ground-based broadcast service that provides secondary surveillance radar (SSR)-derived traffic data. Messages are uplinked over the ADS-B data link system to the aircraft. This is a different technology than is used for Traffic Information Service (TIS), which relies solely on Mode S terminal radars. The TIS-B service is intended to provide ADS-B equipped aircraft with a more complete traffic picture in situations where all other nearby aircraft are not equipped with ADS-B.

Only conventional Mode C and Mode S transponder-equipped targets reporting their barometric altitude will be included in a TIS-B broadcast. This means most Mode C and Mode S transponders with altitude encoders that are within sight of both nearby radars and GBTs will be shown on the receiving aircraft’s Cockpit Display of Traffic Information (CDTI). Aircraft that have Mode-A transponders (i.e., transponders without altitude reporting capability) and aircraft without transponders will not be displayed on the receiving aircraft’s CDTI.

Receiving aircraft must be in both radar coverage and GBT coverage in a given area to receive TIS-B service in that area. To reduce display clutter, the TIS-B service is only providing traffic information below 18,000 feet.

Updates of TIS-B traffic will occur less often than ADS-B updates because the update rates for SSRs are approximately once every five to twelve seconds, compared to the once-per-second update rate available with ADS-B.

There may be, on occasion, “shadowing” or “ghosting” observed of either one’s own ship symbol or another depicted target aircraft on the CDTI. This may be caused by the ADS-B-to-SSR target association process within the ground system. This can result in duplicate targets for equipped or receiving aircraft. This phenomenon may be minimized by standardized filtering software implemented into the aircraft avionics. Some pilots are making productive use of this capability to confirm that their encoder/transponder combination is transmitting their correct altitude.

Flight Information Services-Broadcast (FIS-B) (UAT Only)

FIS-B is the ground-to-air broadcast service of weather and other non-control, aeronautical information that allows pilots to operate more safely and efficiently. FIS-B products can be textually or graphically depicted.

Flight Monitoring Functionality

ADS-B data can be used for flight monitoring, flight management, surface traffic airline asset management, and other safety and efficiency purposes contingent upon appropriate FAA authorization and approvals. Please review the FAA's Phase I policy guidance (pdf) on Flight Monitoring.

Avionics Overview

There are two basic aircraft equipage data link configurations that provide ADS-B capabilities:

1) Transmit-only systems

2) Combined transmit and receive systems.

Users that desire only to be “seen” by other aircraft or ground stations may elect to equip with the transmit-only capability system, a less expensive solution. A transmit-only configuration does not have provisions for a CDTI or the ability to receive and display up linked broadcast services. For a transmit-only ADS-B system, two main avionics components are required:

• 1090 Extended Squitter (1090ES) or UAT data link transmitter, and

• A GPS receiver or another suitable position source able to provide the aircraft’s ADS-B transmit system with “own-ship” position with suitable integrity and other Message Set Elements (MSEs) for transmission over the selected data link transmitter.

In a combined ADS-B transmit and receive system, there are three main avionics components required:

• 1090ES or UAT data link transceiver

• A Global Positioning System receiver (or other position source) that provides one’s “own-ship” position with suitable data integrity, plus other MSEs, and

• A single or multifunction display and processing system for managing and displaying the broadcast data. CDTIs can be installed in the instrument panel or be portable, depending upon the specific aircraft equipage implementation and/or aircraft certification basis.

For the purpose of FAA operational approval authorization, aircraft that do not have an installed CDTI or that do not have a data link modem certified to either TSO C-154 (UAT) or TSO C-166 (1090ES) do not fall under the umbrella of those aircraft equipped with surveillance systems that require operational approval. First generation 1090ES systems that were certified under TSO C-112A may not be used for any ground-based surveillance function except the flight monitoring function, unless the equipment has been shown to function satisfactorily in the NAS.

ADS-B avionics systems need to be properly installed and maintained. This is called continued airworthiness. Follow the manufacturer’s recommendations to ensure initial and continued airworthiness. Care should be given to ensure that all components are specifically approved for installation in the particular make / model aircraft. Maintenance programs should identify inspection items, establish in-service intervals for maintenance and inspections, and include any calibration procedures necessary to ensure continued airworthiness. [full continued airworthiness guidance document]

Quick Links

• FAA ADS-B Services Delivery Page
• Eurocontrol 1090 MHz Extended Squitter Assessment Report
• Eurocontrol Acquisition Squitter Distribution paper
• Nav Canada/Sensis ADS-B Press Release February 2007