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.

By Bill Gunn
ATP/CFII. Citation, Sabreliner, Westwind

Mandated in Europe for large aircraft by Feb 2015 and in the US by Jan 2020, ADS-B represents the next great leap forward in air traffic control.

Automatic dependent surveillance–broadcast (ADS-B) is one component of FAA's NextGen overhaul of the US air traffic control (ATC) system.

A simple explanation of ADS-B is that onboard precise navigation systems will replace radar as the primary position source for ATC. However, ADS-B will add far more capability than is available today with radar.

The explanation for the acronym is telling as to the process. It transmits information automatically without requiring pilot input. It is dependent on approved systems (currently WAAS enabled GPS) for aircraft position.

A surveillance function is provided for aircraft and ground vehicles. And broadcast is automatic to any aircraft, vehicle or ground station equipped to receive. The breakout—type of service, equip­ment protocol and value of the information provided—is complex but logical.

FAA published its ADS-B Out final rule on May 21, 2010, mandating ADS-B operation in the National Airspace System (NAS) after Jan 1, 2020. ADS-B Out capability will be required in all areas that today mandate Mode C/A or Mode S transponders.

How FAA will implement ADS-B in stages and what strategy flight departments and aircraft owners will have to assume by Jan 2020 looms large as the implementation date draws closer.

An overall view of the Outs and Ins of ADS-B

Concept for ADS-B in the US. Although 1090ES and UAT are shown as separate protocols, any aircraft may equip with both. An example would be 1090ES for "Out" and "In" positioning and UAT "In" to receive FIS-B weather and terrain information.

In the US there are 2 protocols, or means of transmission—1090 extended squitter (1090ES) and universal access transceiver (UAT). Either of these protocols may provide 2 overall functions—out from the aircraft and in to the aircraft.

The ICAO standard protocol is 1090ES. "1090" is the transmit frequency (transponder frequency range) of 1090 MHz, while "ES" simply means that the aircraft unit sends out an extended range signal without being interrogated. Much of the logic for 1090ES ADS-B is based on existing Mode S technology. In the US 1090ES is required above FL180 and may be used optionally below that altitude.

The other protocol—UAT—operates on a frequency of 978 MHz, just below the 1090ES frequency. UAT may only be used below FL180 and in the US. It has its beginnings in FAA's Flight 2000/Capstone project dating back to 1997.

By 1999 FAA had equipped 200 GA and commuter aircraft in Alaska with early-version UAT "Out" and "In" capability. Operating in the southwest area of the state, this tested the concept of ADS-B. The success of this project and lessons learned became the basis for the 2-tiered protocols (1090ES and UAT) unique to the US.


"Out" refers to the capability of an individual aircraft to transmit ADS-B position data to ATC, other aircraft or any ground receiver using similar protocol. The May 2010 final rule only mandates the ADS-B Out function after Jan 1, 2020.

FAA defines ADS-B Out as "aircraft broadcasting own-ship information" in AC 20-165. Although only the "Out" feature is required by Jan 1, 2020, aircraft may opt for full or limited capability today. The Gulf of Mexico is ADS-B operational and selected areas of the US are providing ADS-B In on test.

The illustration on the right shows possible aircraft "Out" source inputs. While FAA does not mandate WAAS enabled GPS as the positioning source, this is the only solution to date that meets the stringent ADS-B positioning accuracy and reliability standards.

Position/velocity/heading will be GPS derived. Baro altitude must be the same source as Mode C or S transponder readout to avoid conflict. RVSM will be retained with 1090­ES protocol. Only TCAS II traffic avoidance will be compatible with ADS-B. Although the diagram shows dual external antennas, a single bottom-mounted antenna may be sufficient for ADS-B Out function.

ADS-B has been operational and controlling traffic in the Gulf of Mexico since Dec 2009. Offshore oil operator PHI is fully invested in the ADS-B Out function in daily operations in the Gulf. (See Pro Pilot, Sep 2011, pp 38–43.)

ADS-B Out provides the following—ID, position, velocity and altitude from the aircraft to any like-equipped receiver. The primary function is to provide data so that ATC can separate traffic, as is done with radar today.

Mode C/A or S transponders remain required beyond Jan 1, 2020 for all aircraft operating in today's transponder mandated airspace. FAA will retain at least single radar coverage for existing service volumes and use primary and secondary radar to complete the traffic picture, or as an alternative means to separate traffic if ADS-B is not available.


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