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How ADS-B Works: A Look at the Foundation of NextGen

By

ADS-B

Implementation of ADS-B Ground Stations, 2013

Photo © FAA

Overview:

ADS-B is the foundation of the FAA's Next Generation Transportation System (NextGen). It was developed to help transform the nation's airspace system into a more efficient one. The air traffic system will undergo a much-needed modernization plan through the implementation of NextGen, and ADS-B is the primary component.

The main role of ADS-B is to provide precise aircraft location information to air traffic controllers. It's a step above RADAR, which has been in use for years.

ADS-B stands for Automatic Dependent Surveillance-Broadcast. It uses GPS satellite signals to broadcast aircraft information continuously to air traffic controllers and other participating aircraft. ADS-B is the most accurate surveillance system the aviation industry ever seen. It will enable aircraft to fly more direct routes, ease congestion, decrease carbon emissions and save aircraft operators time and money.

Components:

  • GNSS Satellite Constellation: ADS-B is a satellite-based system. Data is continuously sent from the set of satellites to the aircraft's on board GPS devices, where it's interpreted and then sent to ADS-B ground stations.

  • Ground Stations: There will be at least 700 ground stations in the United States that receive satellite data and transmit the data to air traffic control stations.

  • IFR Certified, WAAS-enabled GPS receiver: Aircraft must be equipped with a compatible GPS receiver for ADS-B to work.

  • A 1090 MHz Extended Squitter link with a Mode S transponder OR a 978 MHz Universal Access Transceiver (UAS) for use with an existing transponder: The latter option is available for aircraft flying below 18,000 feet in the United States.

 

How it Works:

ADS-B works by using satellites signals and aircraft avionics systems to interpret aircraft data and broadcast it to air traffic controllers on a continuous basis and in almost real-time. Satellite signals are interpreted by an aircraft GPS receiver. ADS-B technology takes the satellite data and additional data from aircraft avionics to create a very accurate picture of the aircraft's location, speed, altitude and over 40 other parameters. This data is transmitted to a ground station and then to air traffic controllers. Other properly equipped aircraft in the area will also receive the data, increasing situational awareness for pilots.

There are two different functions of ADS-B: ADS-B In and ADS-B Out.

  • ADS-B Out is the first and main function that the FAA has addressed. An aircraft that is capable of ADS-B Out has the capability of broadcasting its position, speed and altitude to air traffic controllers and other ADS-B equipped airplanes. According to an FAA mandate, all aircraft that want to fly in airspace that currently requires a transponder must be equipped with ADS-B Out capabilities before January 1, 2020.

     

  • ADS-B In remains an optional capability - at least for now. The ADS-B In capability will allow aircraft to receive traffic and weather information in real-time on the aircraft cockpit display. The ADS-B In function goes above and beyond today's traffic systems (such as TCAS) as it offer more precise data and more data parameters than current TCAS systems do. For example, TCAS can display vertical distance from aircraft but not lateral. ADS-B In will display the speed, locations, altitude and vectors of other participating aircraft, along with many other pieces of data.

 

Errors and Limitations:

Currently, the biggest limitation for ADS-B is the cost of installing the necessary equipment on virtually every aircraft in the country. While the program makes flying safer and more efficient, most flight departments and general aviation pilots are having a difficult time justifying the costs.

ADS-B has very few system errors; in contrast, it is known for its reliability. No human-made system is fool-proof, though, and some experts claim that ADS-B (and GPS in general) is vulnerable to system infrastructure attacks such as hackers or GPS jamming. Additionally, since ADS-B is reliant on the GNSS system, normal satellite errors such as timing errors and satellite weather errors can affect ADS-B.

 

Current Status:

According to the FAA, as of August 2013 there were 547 operational ground radio stations. Complete deployment of all 700 ground stations is expected to occur in 2014. These stations offer weather services and traffic information to ADS-B equipped aircraft across 28 TRACON facilities. The FAA stands by its mandate that all aircraft operating in designated airspace must be ADS-B Out equipped by January 1st, 2020.

 

Practical Use:

There is uncertainty centered on the specific types of equipment needed for different aircraft and operators. The equipment installation varies depending on the type of flying and currently installed equipment.

A 978 MHz UAS link, for example, will suffice for an aircraft with a WAAS-enabled, IFR certified GPS unit and a Mode C transponder already installed, unless the operator would like to fly outside of the United States or above 18,000 feet, in which case a 1090 MHz ES link would be necessary. But a 1090 MHz ES link isn't compatible with TIS-B or FIS-B, which means an operator would have to find another way to get traffic information (such as TCAS).

And an operator that doesn't already have a WAAS-enabled GPS unit in his aircraft will have to purchase a new GPS unit along with a 978 MHz UAS or 1090 MHz ES link and potentially a Mode C or Mode S transponder.

Once in use, ADS-B is a valuable tool, providing the most accurate data to air traffic controllers and pilots that we've ever seen. When implemented nationwide the benefits are positive.

There's no arguing, though, that ADS-B is quite expensive and complicated. The FAA is hopeful that the long-term benefits will outweigh the cost, but the project leaves aircraft owners in a difficult position.

 

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