Commercial, Embedded Avionics

ADS-B Equipage

By By Dr. Ingrid D. Knox | July 1, 2011
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It has been said that those of us who fly within the United States enjoy what is certainly the safest and most accessible airspace system in the world. At the same time, it can be said that the U.S. National Airspace System (NAS) is one of the most congested and jam-packed systems in the world. The solution is the Next Generation Air Transportation System (NextGen).

A critical component of NextGen is Automatic Dependent Surveillance-Broadcast (ADS-B), which combines an aircraft’s positioning source, aircraft avionics and a ground infrastructure to create an accurate surveillance interface between aircraft and air traffic control. The satellite-derived aircraft location system will enhance air traffic controllers’ ability to identify and guide aircraft and can provide coverage in areas where radar is not possible, such as Alaska and the Gulf of Mexico. One of the main advantages of ADS-B is pilots can benefit from weather and enhanced traffic information the technology brings to the cockpit. It is expected that by 2013, ADS-B coverage will stretch nationwide.

Evidence has shown that ADS-B can be more timely and accurate when compared with now-available, conventional surveillance radar systems, which are limited by line-of-sight geometry problems, which stem from mountains and other large obstacles. The more accurate information for controllers and pilots is due to GPS usage as an ADS-B position source for the aircraft.

In ADS-B Out, ADS-B-equipped aircraft are able to broadcast velocity, altitude, identification and position to other aircraft and to the air traffic control. When an aircraft or vehicle receives ADS-B data, the process is known as ADS-B In.

By 2020, all aircraft operating within designated ADS-B airspace will be required to comply with the equipment performance requirement of ADS-B Out, as defined in Title 14 of the Code of Federal Regulations (14 CFR) section 91.225 (14 CFR 91.225).

Advisory Circular (AC) 20-165, which was issued in May 2010, provides guidance for the installation and airworthiness approval of the ADS-B Out system in aircraft. An AC is not mandatory and is not a regulation. AC 20-165 “describes an acceptable means, but not the only means, to install ADS-B Out equipment. However, if you use the means described in this AC, you must follow it entirely,” according to the circular.

The AC covers installation of ADS-B equipment, updates to the flight manual, updates to instructions for continued airworthiness, guidance for interfacing systems, ground test and flight test.

ADS-B equipment must meet the requirements specified in Technical Standard Order (TSO) C166b, for 1090 MHz Extended Squitter transponder, or TSO-154c, for 978 MHz Universal Access Transceiver (UAT). System components for ADS-B Out functionality are described as the ADS-B equipment, a position source, a barometric altitude source, an air-ground status source, a TCAS II source if the aircraft is so equipped, an optional heading source and associated antennas and displays.

Also, it describes the approval process, necessary documentation, ADS-B system installation guidance, and test and evaluation. Also, it provides message element descriptions, identifying ADS-B position sources, latency analysis, airplane flight manual supplement (AFMS) examples and related documents.

System Approach

The federal regulations and advisory circulars outline specific operational requirements for ADS-B, but there are other questions an airline must consider before equipping their fleet.

The installation of an ADS-B system should be rule complaint. The approval requires a “system approach.” A supplemental type certificate (STC) should identify each interfacing system such as ADS-B equipment, antenna, heading, position velocity, baro altitude, pilot input, traffic collision and avoidance system (TCAS) status and air/ground status. The ground and flight tests can be required and flight data can be reused in applicable follow-on installation, which could be stated in the AFM/AFMS. The ADS-B Out system should be shown to meet the equipment requirements of 14 CFR 91.225. All aircraft are required to automatically determine air ground status. Flight test will be required. The flight test profile is outlined in AC 20-165.

The primary purpose of flight in National Airspace Authority ADS-B service volume is to ensure compatibility and validity. Post flight data analysis will need to be accomplished. A federal government authority may provide flight data recorded by ground system upon request. AC 20-165 provides guidance on requesting flight data from the approving authority. The applicant should ensure data is consistent with the actual flight.

ADS-B is not yet a replacement for transponder or ground-based radar. Common knowledge is that numerous radar sites are slated for de-commissioning, such as primary radars. Several of the secondary radars may remain functional as a backup due to a possible GPS outage.

Rules to remember while complying with the requirements depends on circumstances you will be encountering, such as what, how, and where you fly, budgetary constraints, and other factors.

Traffic Information Services-Broadcast (TIS-B), the service provided when ADS-B ground radio stations broadcast traffic information obtained from ATC radar, will not relieve pilots of the responsibility to obtain an official preflight briefing, to see and avoid other aircraft, and to gain complete information for intended flight. TIS-B provides a more complete traffic picture in situations when not all aircraft are equipped with ADS-B for pilots flying aircraft equipped to receive and display ADS-B data.

ADS-B Out probably will not allow instrument approach minima at smaller airports to lower because these minima are more dependent on obstacle and terrain clearance rather than surveillance coverage. However, ADS-B could give controllers additional flexibility meaning clearing aircraft from instrument approach at non-towered airports, which could identify outbound Instrument Flight Rules (IFR) aircraft at lower altitudes possibly all the way to the ground.

Currently there is no fee to receive the ADS-B In services. Aircraft owners and operators will need equipment to visually display the data, such as the multifunction display and the moving-map GPS receiver. The operators must meet prescribed performance requirements to operate in the ADS-B designated airspace. At present, position sources such as GPS or wide area augmentation system (WAAS) are acceptable and meet the performance requirements of ADS-B Out.

New technologies and avionics will be available to meet the ADS-B performance requirements by the 2020 mandate. As ground infrastructure moves forward by 2013, ADS-B is forecast to be available across the NAS everywhere there is radar coverage today. It’s not too early to start understanding the technology and thinking about how operators can benefit from its capabilities.

ADS-B Equipment

The exchange of ADS-B information within the United States is done through a vehicle of ADS-B equipped aircraft generally on one of two frequencies –– 1090 or 978 MHz.

The 978 Mhz/UAT link within the United States is a regional link, primarily used for Flight Information System-Broadcast (FIS-B) services, which transmits graphical weather data and other flight information for UAT-equipped aircraft. ADS-B extends the message element of Mode S with additional information about the aircraft and its position, a process known as extended squitter (ES). 1090MHz is linked to Mode A/C and S transponders and TCAS equipment. The 1090MHz is used by TCAS equipment by Mode A/C and S transponders (Mode A aircraft identify-code; Mode C altitude reporting; Mode S identify information assigned by a federal government authority).

The ADS-B transceiver operating on either aforementioned link will do essentially the same function as the standard transponder but on an exponentially wider span of coverage. The differences are such that while a Mode C transponder provides ATC with position as detected by radar and pressure altitude, the ADS‑B transceiver emits the same information plus elements like aircraft’s type, velocity and geometric altitude, which can be used to develop more accurate depiction and indication of position.

One of the primary advantages is that the ADS-B information will be available to any aircraft equipped to receive the information, so aircraft on like frequencies can see each other in terms of visibility on compatible cockpit displays.

Translation is required to ensure the two links operate simultaneously. ADS-B ground-based radio stations process the messages received on each frequency, then send the messages back out again on the opposite frequency, which is known as the ADS-Rebroadcast (ADS-R). This process is how 1090ES and UAT users of the system can identify one another on traffic displays.

Because some business jets, turboprops and most commercial airlines are required to have Mode S and TCAS installed, the federal government agency responsible for aviation industry oversight will expect these aircraft to choose to equip with the 1090ES link for ADS-B. Also, some general aviation aircraft currently have digital transponders that can be upgraded to 1090ES. Future projections show general aviation aircraft typically characterized as smaller piston airplanes and light twins not required to have TCAS might determine a need for the equipment with the UAT avionics.

With additional input from Mary June Bruner.

Dr. Ingrid D. Knox is an adjunct assistant professor at Embry Riddle Aeronautical University.

Disclaimer: All the article information should be directed to your federal government agency responsible for aviation industry oversight counterpart that makes the final determination and approval decisions. The information is only as good as the information available during the time of the print. The latest requirements and regulations take precedence over the information provided. This information does not interpret requirements and is to be used as information only. It is not meant as a means of compliance or direction to show or find compliance to the regulatory requirements, orders, policies or procedures. The information is public knowledge and does not constitute directions to approval of the systems mentioned.

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