Explaining ADS-B's basic components, services and transmission protocols

As part of NextGen, onboard navigation systems will replace radar as the primary position source for ATC.


ADS-B Out functions. Note that there is a pilot input capability. This is a function for added data—ADS-B Out will be a fully automated operation not necessarily requiring any pilot input.

In AC 20-172, FAA defines ADS-B In as "reception of ADS-B direct, traffic information services–broadcast (TIS-B) and automatic dependent surveillance–rebroadcast (ADS-R) messages."

The ADS-B In final rule is due to be issued in 2013. FAA chartered an aviation rulemaking committee (ARC) in 2010 to provide for the aviation community to define a strategy for incorporating ADS-B In technologies into the NAS.

ADS-B In capability, combined with a cockpit display, provides greater situational awareness to both high and low-altitude operators by providing highly accurate traffic. ADS-B In further offers low-altitude users essential flight data such as weather and special activity airspace information.

The ARC is composed of about two dozen representatives from various aviation user groups as well as segments of industry and government.

The ARC's initial recommendations on ADS-B In will be given to FAA leadership. These findings are expected to provide a clearer definition of how the aviation community should proceed with ADS-B In while ensuring compatibility with ADS-B Out avionics standards.

Feedback provided by the aviation community in response to these recommendations will be incorporated into an ARC final report due by Jun 2012 which will detail suggested next steps and, hopefully, a final rule by 2013.

ADS-B In sources and onboard processing are illustrated in the diagram above, which shows a 1090ES protocol system.

With ADS-B direct, 1090ES aircraft "Out" position information will be received directly by other 1090ES aircraft that are "In" equipped without ATC intervention or input. Unlike TCAS, this position information is only an aid to the crew and is not a standalone source to deviate from an assigned heading or altitude.

ADS-B In functions. Shown are data sources to the aircraft's antenna. Note that TCAS will integrate with other ADS-B In sources. However, only TCAS is a standalone source for traffic separation. IFR traffic will receive standard separation from other traffic via ATC using ADS-B position information, as is true today with radar positioning. ADS-B (direct), TIS-B and ADS-R traffic is only an aid to flight and ground safety and does not relieve the crew of the requirements of see-and-avoid under FAR 91.113 (b).

ADS-R is the transmission from ATC ground stations of other protocol positioning information, such as UAT-to-1090ES and vice versa. Like ADS-B direct and TIS-B, traffic information is secondary to TCAS, ATC traffic separation, and visual see-and-avoid procedures.

Like TIS-B, ADS-R will be of most benefit to UAT and 1090ES aircraft that are not TCAS equipped.

TIS-B is the transmission of traffic information from ATC ground stations using best source data. TIS-B will display the position of non-ADS-B aircraft using radar data or other sources.

Text information such as TFRs and Notams will be available to the cockpit via TIS-B. Unlike TCAS, TIS-B information is secondary to ATC or visual separation. TIS-B will transmit to both 1090ES and UAT receivers—however, the intent is to serve the UAT-only fleet that generally does not have TCAS. TCAS equipped aircraft may opt not to install the TIS-B function.

Traffic information, ground and air ADS-B In protocol will provide a cockpit display of ground traffic information when the aircraft is taxiing, as an aircraft acquisition and following aid for ATC assigned visual approaches, and traffic advisories when airborne. All of the functions are aids to the crew and, unlike TCAS, do not constitute a stand­alone traffic separation system.

Flight information system–broadcast (FIS-B) will only transmit to UAT protocol systems. Advisory weather and terrain information will be available to the cockpit via service providers without a subscription fee. As with traffic information, FAA considers this information an advisory aid to the crew and it does not constitute a single data source for decisionmaking.

However, experience in Alaska with the Capstone project showed that this type of advisory information greatly enhanced safety. The FIS-B link is not shown in the diagram on this page since it represents 1090ES protocol.

Today's decision, tomorrow's expense

From the single-pilot owner/ operator to the large corporate flight department or fractional, decisions today about future ADS-B avionics and when to upgrade are challenging.

Decisionmakers should first decide on the mission profile, then equipment level, protocol, and ensure growth potential is built in to new avionics. Any change in magnitude of NextGen and ADS-B promises only one thing—more change.

Bill Gunn is the compliance manager for the State of Texas Aviation Division. He lectures nationally for a private aviation advocacy group and flies for work and pleasure.


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