Automatic Dependent Surveillance–Broadcast, better known as ADS-B, changes the way an aircraft can be observed by air traffic services and by other appropriately equipped aircraft. Instead of a ground radar station having to illuminate a target and infer its position from the returned signal, an ADS-B Out installation determines aircraft state information using onboard navigation sources and broadcasts a digital surveillance message. The FAA describes ADS-B as combining a positioning source, aircraft avionics and ground infrastructure to create a surveillance system. [1]
In the United States, the FAA states that ADS-B Out broadcasts information including the aircraft’s location, altitude and ground speed once per second. That message can be received by the ground ADS-B network and by aircraft equipped with appropriate ADS-B In capability. The word “broadcast” matters: the aircraft is not waiting for each ground station to interrogate it before sending the surveillance message. [1]
What the four letters actually mean
The FAA breaks the name into four useful concepts. Automatic means the system operates without requiring an external interrogation for every report. Dependent means the surveillance report depends on aircraft navigation and other onboard data. Surveillance reflects its use for tracking and traffic awareness. Broadcast means the information is transmitted for suitably equipped receivers to use. [2]
“Dependent” is particularly important because ADS-B does not independently measure its own geographic position by scanning the ground. A compliant ADS-B Out installation receives position information from an approved navigation source — commonly a suitable GNSS/GPS source — and packages that data into the broadcast surveillance message along with other required parameters. The quality and integrity of the source data are therefore part of ADS-B performance. [2] [3]
ADS-B does not mean a satellite is tracking the aircraft
A common oversimplification is that ADS-B is “satellite tracking”. In a conventional ADS-B Out installation, satellite navigation may be used by the aircraft to determine position, but the aircraft then broadcasts its ADS-B message by radio. Ground stations can receive that radio transmission, and other aircraft with compatible ADS-B In equipment may receive it as well. In some regions, specialised space-based ADS-B services place ADS-B receivers on satellites, but that is a separate receiving architecture and should not be confused with the basic principle of an aircraft obtaining GNSS position and broadcasting it. [2]
This distinction explains why an aircraft’s onboard GPS receiver and its ADS-B transmitter have different jobs. The navigation source estimates position and supplies integrity information. ADS-B Out formats surveillance data and transmits it. The ground or airborne receiver then consumes the message. A failure or degradation in one part of that chain cannot automatically be treated as a failure of every other part. [1]
What ADS-B Out broadcasts
The FAA’s public description identifies position, altitude and ground speed among the information broadcast by ADS-B Out, together with other data used by the surveillance system. Technical compliance is defined more precisely by the applicable equipment standards and operating rule. In U.S. rule airspace, 14 CFR §§91.225 and 91.227 establish where ADS-B Out is required and the performance requirements for the broadcast. FAA Advisory Circular 90-114 provides operational guidance for ADS-B equipment and use. [1] [4]
ADS-B messages also contain identification and status information needed for surveillance processing. The exact data set depends on the link and equipment standard, so a general explanation should not invent a fixed list beyond what the regulatory material specifies. The central concept is that the broadcast is much richer than a simple latitude-and-longitude coordinate and is designed to give surveillance systems a coherent aircraft track. [4]
Why once-per-second position updates matter
The FAA contrasts ADS-B’s frequent digital reporting with traditional rotating-radar surveillance. A once-per-second ADS-B Out report can provide ground automation with frequent position updates when infrastructure and coverage are available. That does not mean every controller display literally redraws every aircraft once per second or that all surveillance decisions are based on ADS-B alone; ATC automation can fuse and process several surveillance sources. [1]
The direct digital message also avoids having the surveillance computer estimate every target’s position solely from a radar return. The aircraft reports a position derived from its onboard navigation solution. Surveillance performance still depends on the accuracy, integrity and continuity required by the approved system, which is why U.S. regulations specify performance requirements rather than treating any GPS coordinate plus any radio transmitter as equivalent to certified ADS-B. [3] [4]
1090 MHz Extended Squitter and 978 MHz UAT
The United States supports two ADS-B data-link choices for different operations. The FAA identifies 1090 MHz Extended Squitter, or 1090ES, and the 978 MHz Universal Access Transceiver, or UAT. For operations at and above Flight Level 180 in U.S. ADS-B rule airspace, the FAA requires 1090ES; UAT is an option for appropriately equipped aircraft operating below that level where permitted. [3]
That U.S. two-link arrangement should not be presented as a universal global rule. 1090ES is the internationally dominant link for high-performance transport aircraft, while 978 UAT is specifically associated with the U.S. system. An airline operating internationally therefore has equipage requirements that cannot be inferred solely from a domestic general-aviation example. [3]
ADS-B Out and ADS-B In are different capabilities
ADS-B Out is the transmitting function required in designated U.S. airspace under the applicable rule. ADS-B In is a receiving capability that can support traffic and other information displays when installed and approved for the intended use. An aircraft can therefore be compliant with an ADS-B Out mandate without necessarily providing its pilots with an ADS-B traffic display. The FAA explicitly separates “Out” and “In” in its equipage guidance. [2]
Likewise, seeing a traffic target on an ADS-B In display does not prove that the target was received directly from that aircraft in every case. In the U.S. architecture, ground services can rebroadcast traffic information to help bridge differences between the two ADS-B links and incorporate radar-derived traffic where applicable. The precise services available depend on coverage, equipment and airspace. Pilots must use the display according to its approved limitations rather than assuming it is a complete collision-avoidance picture. [2]
Why ADS-B and TCAS are not interchangeable
ADS-B can support traffic awareness, but it is not the same system as TCAS II/ACAS II. TCAS has its own surveillance and certified collision-avoidance logic capable of generating Resolution Advisories. ADS-B is principally a surveillance broadcast architecture. Some modern avionics can use ADS-B information to enhance surveillance efficiency or traffic displays, but an ADS-B target on a screen is not automatically equivalent to a TCAS Resolution Advisory. [1] [5]
The distinction is operationally important. Surveillance tells a system where traffic is believed to be. Collision avoidance adds specialised threat prediction, advisory generation and — for ACAS-equipped encounters — coordination. ADS-B can contribute data to modern traffic systems, but it should not be described as if ATC tracking and airborne collision avoidance were one function. [5]
How ATC receives ADS-B
Where ground infrastructure is provided, ADS-B ground stations receive the aircraft’s radio broadcast and feed surveillance information into the air traffic system. The FAA’s nationwide ADS-B network was built as part of the NextGen surveillance architecture and is integrated with ATC automation. The controller generally interacts with a processed surveillance track on an operational display rather than with raw ADS-B messages. [1]
The availability of ADS-B does not mean primary and secondary radar have vanished from every location. Surveillance infrastructure is designed around operational requirements, and different sensors can coexist. Radar may continue to provide independent surveillance or coverage in places where it remains operationally valuable. The FAA describes ADS-B as a major surveillance technology, but a specific facility’s sensor mix must be established from current air-navigation-service information rather than assumed. [6]
Identity and the transponder connection
For 1090ES installations, ADS-B Out is closely associated with the Mode S transponder function. Correct aircraft identification and altitude information are therefore important to surveillance consistency. FAA installation guidance warns operators about configuration and source requirements, including the need for compliant position and altitude data. An incorrect configuration can produce surveillance discrepancies even when the radio transmitter itself is operating. [3]
This is why ADS-B compliance is not established by asking whether a cockpit contains “a GPS”. The aircraft needs an approved combination of position source, ADS-B equipment, antenna installation, configuration and performance that meets the applicable standards. The FAA publishes Technical Standard Order and installation guidance for the relevant equipment. [6] [4]
Position accuracy and integrity are not the same thing
A position can appear numerically precise while the navigation system has insufficient assurance that the number is trustworthy. ADS-B performance therefore includes integrity-related parameters in addition to basic accuracy. The U.S. operating requirements in §91.227 and FAA guidance define required performance metrics for ADS-B Out. The surveillance receiver can use this quality information when deciding how the report may be used. [4]
This is a fundamental difference between certified surveillance and an informal consumer tracking device. A compliant aviation system must communicate enough information about data quality for downstream safety functions to make appropriate decisions. Consequently, a temporary navigation-source problem can affect whether an ADS-B report meets the required surveillance performance even if the transmitter continues to emit radio messages. [4]
Why public flight-tracking websites are not the ATC system
Public flight-tracking services can receive ADS-B data through their own networks of ground receivers and combine it with other information. Their maps are useful for public awareness, but they are not the certified ATC surveillance display and they do not necessarily have identical coverage, latency, filtering or data sources. The existence of a public track therefore should not be used to infer exactly what a controller could see at the same moment. FAA ADS-B infrastructure and operational ATC surveillance are governed by separate approved systems and requirements. [1]
What ADS-B changed
ADS-B shifted part of surveillance from “ground sensor determines where the target is” toward “aircraft determines its own position using an approved navigation source and broadcasts surveillance data”. That supports frequent updates, digital data content and direct air-to-air reception for equipped users. It also creates dependence on correct aircraft configuration, navigation-source performance and radio-link availability. [2] [4]
The result is not a single magic box. ADS-B is a chain: navigation source, avionics processing, transmitter, antenna, radio link, ground or airborne receiver and the automation that uses the data. Understanding that chain explains both the power and the limitations of the technology. The aircraft is not simply “visible because GPS exists”; it is visible to an ADS-B receiver because approved onboard data is packaged into a standard surveillance broadcast that a compatible network can receive and process. [1] [3]
Verified Sources / References
- Federal Aviation Administration — Automatic Dependent Surveillance–Broadcast. FAA overview of ADS-B architecture, broadcast content and update rate.
- FAA — ADS-B In and Out. Explanation of Automatic, Dependent, Surveillance and Broadcast, plus Out/In capability differences.
- FAA — ADS-B Installation. FAA equipage guidance covering position sources, 1090ES and UAT.
- FAA Advisory Circular 90-114C — Automatic Dependent Surveillance–Broadcast Operations. Current FAA operational guidance.
- FAA — Airborne Collision Avoidance System. Used to distinguish TCAS/ACAS collision avoidance from ADS-B surveillance.
- FAA — ADS-B Quick Links and Technical Resources. FAA technical standards and programme resources.
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Editorial Notice: This article was prepared using information considered reliable and publicly available at the time of publication. Every reasonable effort has been made to ensure accuracy; however, aviation requirements, technical standards and operational guidance may change as further information or revised regulation becomes available. This article is for general aviation education and reporting and is not a substitute for approved aircraft manuals, operator procedures, regulatory material or professional training. Cockpit King does not allege fault or responsibility against any person or organisation unless confirmed by an authoritative source. If you believe any material is inaccurate, misleading, improperly attributed or should be reviewed for amendment or removal, please contact us with the article title, the specific passage concerned and supporting evidence. We will assess legitimate requests promptly and, where appropriate, correct, clarify, update or remove the material.