Automatic Dependent Surveillance-Broadcast
Introduction to ADS-B
Automatic Dependent Surveillance-Broadcast (ADS-B) provides advisory services to pilots and is used by ATC for air traffic surveillance and separation.
- Automatic: It periodically transmits information with no pilot or operator involvement required.
- Dependent: Position and velocity vectors are derived from the GPS or FMS.
- Surveillance: It provides a method of determining 3D position and identification of aircraft.
- Broadcast: It transmits the information available to anyone with the appropriate receiving equipment.
Operating Modes
The transmission of ADS-B information from an aircraft is known as ADS-B Out. The receipt of ADS-B information by an aircraft is known as ADS-B In.
ADS-B Airspace
14 CFR 91.225 specifies the airspace in which ADS-B Out equipment is required.
System Architecture
The ADS-B system is composed of aircraft avionics and ground stations.
Onboard avionics determine the aircraft's position and transmit the required information to ground stations and other ADS-B In equipped aircraft.
The ground system includes transmitters and receivers spaced throughout the world that are connected to:
- Control stations (sometimes co-located with the antennas).
- Radar sensors for ATC use and Traffic Information Service-Broadcast (TIS-B).
- Service providers for the Flight Information Service-Broadcast (FIS-B).
- The FAA for aeronautical (nonweather) data.
Frequencies
In the U.S., ADS-B equipped aircraft exchange information on one of two frequencies: 978 or 1090 MHz. The purpose of two frequencies is to reduce congestion on the 1090 MHz frequency, which is used by transponders and the traffic collision avoidance system (TCAS).
1090 MHz: The 1090 MHz frequency is associated with transponder operations. Mode S transponders meet the requirements of ADS-B if they can broadcast the additional information required. This additional information is an extended squitter (ES) message referred to as 1090ES.
978 MHz: ADS-B equipment operating on 978 MHz is a Universal Access Transceiver (UAT). UAT equipment is not approved for international operations or flight at 18,000 feet and above.
Equipment Certification
Certified: ADS-B Out requires panel-mounted avionics. The equipment may be able to both transmit and receive or transmit only.
Noncertified: ADS-B In can be achieved with portable devices. No portable device can be certified for ADS-B Out.
Technical Standard Orders
Technical Standard Orders (TSOs) define the minimum performance standards for ADS-B equipment. ADS-B equipment must meet TSO requirements to operate in airspace where ADS-B is required (specified in 14 CFR 91.225).
- TSO-C166b: For ADS-B equipment operating on 1090 MHz.
- TSO-C154c: For ADS-B equipment operating on 978 MHz (UAT).
ADS-B Out
ADS-B Out transmissions are received by ground stations for use by ATC and other ADS-B services. The broadcast can also be received directly by other nearby aircraft equipped with ADS-B In that are operating on the same datalink (UAT or 1090ES).
ADS-B Out equipped aircraft transmit their position and velocity at least once per second while airborne or while moving on the airport surface, and at least once every 5 seconds while stationary on the airport surface.
The aircraft's position is derived from a GPS. GPS meets the basic requirements of ADS-B. A more precise system is needed to operate internationally or in Class A airspace, such as the Wide Area Augmentation System (WAAS).
Aircraft Identification
Each ADS-B Out equipped aircraft is assigned a unique ICAO address, also known as a 24-bit address. The address is programmed during installation, or the pilot can enter it before flight. The ADS-B Out avionics transmits the ICAO address and the aircraft's Flight Identification (FLT ID). The FLT ID for GA aircraft is the aircraft's registration number.
Anonymous Operations
An aircraft equipped with ADS-B avionics on the UAT datalink may have a feature that allows it to broadcast an anonymous ICAO address and not broadcast the FLT ID. In the anonymity mode, the aircraft's transponder code must set to 1200. The feature may only be used when the operator has not filed an IFR flight plan and is not requesting ATC services.
ADS-B In
There is no regulatory requirement for an aircraft to receive (ADS-B In). All information made available to the pilot is considered advisory.
Aircraft equipped with ADS-B In using the 1090ES datalink:
- Can detect other aircraft on the same datalink (air-to-air).
- Can receive traffic on the other datalink via ADS-Rebroadcast (air-to-ground then ground-to-air).
- Can receive the TIS-B via ground uplinks.
- Cannot receive the FIS-B.
Aircraft equipped with ADS-B In using the UAT datalink:
- Can detect other aircraft on the same datalink (air-to-air).
- Can receive traffic on the other datalink via ADS-Rebroadcast (air-to-ground then ground-to-air).
- Can receive TIS-B via ground uplinks.
- Can receive FIS-B.
Limitations
ADS-B traffic displays (all types):
- Are not intended to be used as a collision-avoidance system and do not relieve the pilot's responsibility to "see and avoid" other aircraft.
- Cannot be used for avoidance maneuvers when there is no visual contact with the intruder aircraft.
ADS-B Air-to-Air Traffic
ADS-B air-to-air traffic is the broadcast of traffic information to ADS-B In equipped aircraft. Position updates occur approximately once every second. Aircraft that are only equipped with Mode C transponders are not included.
Range
ADS-B air-to-air traffic is not dependent on ground stations. The aircraft's equipment and traffic display settings limit range and altitude coverage.
Requirements
To receive ADS-B air-to-air traffic, the aircraft must:
- Be equipped with ADS-B In and have a cockpit display with traffic information.
- Be operating on the same datalink (UAT or 1090ES) as the other aircraft.
ADS-Rebroadcast
ADS-Rebroadcast (ADS-R) allows an ADS-B In-equipped aircraft to receive traffic data from nearby ADS-B Out equipped aircraft that are using the other datalink (UAT to 1090ES and vice-versa). ADS-B ground stations act as an intermediary between the aircraft. Position updates occur approximately once every second.
An aircraft receiving ADS-R service is known as a client. An aircraft on the opposite link that has its messages translated and transmitted by the ground system is known as a target.
Range ("Hockey Puck")
Aircraft receiving ADS-R traffic data are provided with the identification, altitude, ground track, speed, and position of other aircraft within a 15 NM radius and up to 5,000' above or below the aircraft's position.
Requirements
To receive tailored ADS-R traffic, an aircraft (client) must:
- Be equipped with ADS-B Out and In and have a cockpit display with traffic information.
- Be at FL240 or below.
- Fly within the coverage volume of an ADS-B ground station.
Signal Sharing ("Clients" and "Parasites")
ADS-R data sent from ground stations are tailored to each client aircraft. However, if an ADS-B In only (no ADS-B Out) aircraft is within the "hockey puck" of a client aircraft, that aircraft can receive the same traffic data. This aircraft is known as a "parasite" because it relies on a host.
If the parasite aircraft is on the edge of the client aircraft's "hockey puck," it may not be aware of nearby traffic.
Traffic Information Service-Broadcast
Traffic Information Service-Broadcast (TIS-B) is the broadcast of ATC-derived traffic information to ADS-B In equipped aircraft. The source of this traffic information is ground-based radar.
Range ("Hockey Puck")
Aircraft receiving TIS-B traffic data are provided with the altitude, ground track, speed, and distance of transponder-equipped aircraft within a 15 NM radius and up to 3,500' above or below the aircraft's position.
Requirements
To receive tailored TIS-B service, an aircraft must:
- Be equipped with ADS-B Out and In and have a cockpit display with traffic information.
- Be at FL240 or below.
- Fly within the coverage volume of a compatible ground station (not all stations support TIS-B).
System Limitations
- Due to the delay in ground-based radar systems, position updates may take 3–13 seconds.
- Only transponder-equipped targets are transmitted through the ATC ground system architecture.
- If there is no radar coverage in a given area, there is no TIS-B coverage in that area.
- Pilots may receive intermittent TIS-B targets of their aircraft due to radar being slower than ADS-B.
- Difficulties in the radar-to-ADS-B transfer process may cause pilots may see duplicate traffic symbols ("TIS-B shadows").
- There is no indication provided when the aircraft is not operating in a TIS-B service area.
Flight Information Service-Broadcast
The Flight Information Service-Broadcast (FIS-B) is a continuous broadcast of graphical weather and text-based aeronautical information through the ADS-B network via the 978 MHz (UAT) datalink. FIS-B does not replace a thorough preflight briefing.
Requirements
To receive FIS-B products, an aircraft must:
- Be equipped with ADS-B In and have a cockpit display with weather information.
- Be at FL240 or below.
- Fly within the coverage volume of a FIS-B ground station.
FIS-B Products
| Product | Update Interval1 | Transmission Interval2 | Look-Ahead Range3 |
| AIRMETs | As Available | 5 Minutes | 100–500 NM |
| SIGMETs | As Available | 5 Minutes | 100–500 NM |
| Convective SIGMETs | As Available | 5 Minutes | 100–500 NM |
| METARs | Hourly | 5 Minutes | 100–500 NM |
| TAFs | 8 Hours | 10 Minutes | 100–500 NM |
| CONUS NEXRAD | 5 Minutes | 15 Minutes | Contiguous U.S. |
| Regional NEXRAD | 5 Minutes | 2.5 Minutes | 100–250 NM |
| Product | Update Interval1 | Transmission Interval2 | Look-Ahead Range3 |
| NOTAMs & TFRs4 | As Available | 10 Minutes | 100 NM |
| PIREPs | As Available | 10 Minutes | 250–500 NM |
| SUA Status | As Available | 10 Minutes | 250–500 NM |
| Temperature Aloft | 6 Hours | 10 Minutes | 500–1,000 NM |
| Winds Aloft | 6 Hours | 10 Minutes | 500–1,000 NM |
Notes:
- The update interval is the rate at which the product data is available from the source.
- The Transmission Interval is the amount of time within which a new or updated product transmission must be completed and the rate or repetition interval at which the product is rebroadcast.
- The look-ahead range varies depending on the aircraft's altitude, and the type of ground station received.
- FIS-B NOTAM information is limited to the preceding 30 days.
ADS-B Equipment for Light-Sport and Experimental Aircraft
Owners of amateur-built aircraft, light-sport aircraft with an experimental airworthiness certificate, and special light-sport aircraft, can use uncertified ADS-B Out equipment to meet the ADS-B mandate. For special light-sport aircraft (SLSA), the installation of any ADS-B Out equipment must be performed in accordance with an applicable consensus standard and authorized by the aircraft's manufacturer.
Even if the equipment is uncertified, it must still meet the TSO requirements for 1090 or 978 MHz equipment. Otherwise, the aircraft is not compliant for operations in ADS-B airspace.
ADS‐B Performance Reports
The Flight Standards branch of the FAA provides an automated, online tool for aircraft owners and avionics shops to test the validation of ADS-B Out equipment. Public ADS-B Performance Reports (PAPR) may be requested after the aircraft operates in an ADS-B coverage area.
Deviation Authorization Request Tool
Pilots of aircraft that do not meet ADS-B Out requirements may request an authorization to deviate from the rule to access ADS-B Out rule airspace. The ADS-B Deviation Authorization Preflight Tool (ADAPT) manages these requests.
To be considered for an ADS-B deviation authorization with ADAPT, pilots must:
- Have an operational transponder and operational altitude encoder (e.g., Mode C).
- Submit the request no more than 24 hours before and no less than 1 hour before the flight. Inflight authorizations are not provided.
Service Availability Prediction Tool
For flight in ADS-B airspace, pilots must perform an ADS-B Out preflight prediction for the intended flight route (route and time). This does not apply to aircraft equipped with a Satellite-Based Augmentation System (SBAS) such as WAAS.
The FAA's Service Availability Prediction Tool (SAPT) predicts the ability of an aircraft's avionics to meet performance requirements of 14 CFR 91.227 along a given route of flight. Predictions should be conducted within 24 hours of departure and as close to departure time as feasible.