ADS-B Out System Components and Airframe Requirements

Automatic Dependent Surveillance–Broadcast (ADS-B) Out is the cornerstone of the FAA's NextGen airspace modernization program. Unlike traditional radar, which depends on ground stations actively interrogating aircraft transponders, ADS-B Out works by having each equipped aircraft continuously broadcast its own position, altitude, velocity, and identification to ground stations and, in some configurations, to nearby aircraft. For an Aviation Maintenance Technician (AMT) working on airframes, understanding what hardware is required, how it must be installed, and what regulations govern that installation is not just exam knowledge — it is the foundation of maintaining a legal, airworthy aircraft in today's National Airspace System (NAS).

The FAA mandate under 14 CFR Part 91.225 requires ADS-B Out capability in most controlled airspace, and 14 CFR 91.227 specifies the precise performance standards the avionics and installation must meet. An AMT who understands these rules can verify compliance, troubleshoot system anomalies, and advise owners on upgrade paths.

How ADS-B Out Works

ADS-B Out is described as dependent because it depends on the aircraft's own onboard systems — primarily a GPS or GNSS position source — rather than on external interrogation. It is automatic because it broadcasts continuously without crew input. The aircraft transmits a data package roughly once per second that includes the aircraft's precise three-dimensional position, ground speed, track, geometric altitude, barometric altitude, ICAO 24-bit address, flight identification, and several integrity and accuracy metrics.

Ground-based receivers called ADS-B ground stations receive these broadcasts and forward the data to FAA air traffic control facilities. The data then appears on controller displays with much better update rates and position accuracy than traditional secondary surveillance radar. Because each aircraft self-reports, coverage can extend into areas where radar returns are poor or absent, such as mountainous terrain or low altitudes.

Required System Components

A compliant ADS-B Out system consists of three tightly integrated elements. The AMT must understand what each does and how they interconnect, because a fault in any one element can make the entire system non-compliant.

1. GNSS Position Source

The heart of the system is a WAAS-enabled GPS receiver (or equivalent GNSS source) meeting the accuracy and integrity requirements of 14 CFR 91.227. The position source must deliver Navigation Accuracy Category for Position (NACp) of at least 8 (meaning a horizontal accuracy better than 0.05 nautical miles, or about 92.6 meters, 95% of the time) and a Source Integrity Level (SIL) and Source Accuracy Level (SAL) that indicate the probability of providing misleading position information is extremely low. Standard IFR-certified GPS units generally meet these requirements, but older units without WAAS augmentation typically do not, making them non-compliant position sources. The AMT must verify that the installed GPS is approved for ADS-B use and that its antenna is properly located, oriented, and free of obstructions that could degrade signal reception.

2. ADS-B Transponder or Transmitter

The second element is the broadcast transmitter. There are two frequency-based implementations used in the United States:

1090ES (1090 MHz Extended Squitter): This system uses an upgraded Mode S transponder that, in addition to responding to radar interrogations, automatically transmits ADS-B Out messages on the same 1090 MHz frequency used by traditional transponders. It is the only compliant technology for aircraft operating above 18,000 feet MSL (Class A airspace) and is universally accepted at all altitudes. The "Extended Squitter" refers to the longer data message appended to the standard transponder reply.
UAT (Universal Access Transceiver) on 978 MHz: The UAT transmits ADS-B Out on a separate dedicated frequency of 978 MHz. It is permitted only for aircraft operating below 18,000 feet MSL and offers the additional benefit of receiving ADS-B In services, including FIS-B (Flight Information Services–Broadcast, which provides weather and NOTAMs) and TIS-B (Traffic Information Services–Broadcast). UAT-equipped aircraft cannot fly in Class A airspace and must be aware that 1090ES-equipped traffic is only visible to them if the ground infrastructure rebroadcasts that traffic via TIS-B.

The transponder or UAT unit must be a TSO-approved device. The relevant Technical Standard Orders are TSO-C166b for 1090ES equipment and TSO-C154c for UAT equipment. These TSO standards define minimum performance requirements for the transmitters, including message format, transmit power, and data content.

3. Altitude Source

ADS-B Out must broadcast barometric pressure altitude (the same altitude read on the altimeter with a standard setting of 29.92 in. Hg) in addition to GPS-derived geometric altitude. This barometric altitude is encoded and transmitted at 25-foot resolution — a finer granularity than the 100-foot encoding used by older Mode C transponders. The encoding device is typically an encoding altimeter or a blind encoder connected to the pitot-static system and wired to the transponder. The AMT must ensure this altitude reporting system is inspected and tested per 14 CFR 91.411, which requires a transponder and altitude reporting system check every 24 calendar months.

Airframe Installation Requirements

Because ADS-B Out involves both avionics and structural/airframe considerations, AMTs must attend to several installation details:

Antenna placement: The ADS-B transmit/receive antennas must have an unobstructed view of the sky. For 1090ES, the existing Mode S transponder antenna is often used or a dedicated antenna is added. For UAT, a separate 978 MHz antenna is required. Antennas are typically mounted on the belly of the aircraft for best ground-station visibility. Separation from other antennas (VHF comm, GPS) must meet interference-avoidance guidelines. GPS antenna placement is especially critical — it must have a clear view of the sky hemisphere with minimal shadowing from the airframe or other structures.
Wiring and data buses: The GPS position source must be electrically and data-connected to the ADS-B transmitter. Many modern installations use a digital data bus such as ARINC 429 or RS-232 for this link. The installer must ensure the wiring meets acceptable practices (AC 43.13-1B for aircraft alterations and repairs) and that connectors are properly secured and bonded.
Supplemental Type Certificate (STC) or Field Approval: Installing ADS-B equipment in a certificated aircraft is an alteration that typically requires an FAA-approved data source. Most installations are accomplished via an existing STC issued by the avionics manufacturer or a dedicated avionics installation STC. For aircraft without a matching STC, a Form 337 Field Approval process may be required. The AMT must document the installation in the aircraft maintenance records and ensure the equipment list and Weight and Balance are updated.
Functional testing: After installation, the system must be verified using an FAA-authorized ADS-B test and verification tool or by checking the FAA's ADS-B Performance Monitor website, which provides a report on the quality of ADS-B Out transmissions received from a given aircraft. Avionics shops often use a ramp tester to confirm correct message content before the aircraft flies into monitored airspace.

Why It Matters

Non-compliant ADS-B Out is not a minor paperwork issue. Aircraft operating in the airspace defined by 14 CFR 91.225 — which includes Class A, Class B, Class C, above the ceiling of Class B and C airspace, and within 30 nautical miles of a Class B primary airport below 10,000 feet MSL — without a compliant system are operating illegally. An AMT who returns an aircraft to service with a faulty or non-TSO'd ADS-B installation may be placing the operator in violation and may face certificate action. Beyond the regulatory angle, ADS-B Out directly enhances safety by providing controllers and other pilots with accurate, timely traffic data, reducing the risk of midair collisions.

Key Numbers and Rules

14 CFR 91.225: Defines where ADS-B Out is required (Class A, B, C, and specific surface areas).
14 CFR 91.227: Defines performance standards — accuracy, integrity, message content, and update rates.
1090ES frequency: 1090 MHz — required above 18,000 feet MSL; permitted at all altitudes.
UAT frequency: 978 MHz — permitted below 18,000 feet MSL only.
TSO-C166b: Governs 1090ES equipment certification.
TSO-C154c: Governs UAT equipment certification.
Altitude resolution: 25-foot encoding required (vs. 100-foot for older Mode C).
NACp ≥ 8: Minimum GPS horizontal accuracy requirement (~92.6 m, 95% confidence).
14 CFR 91.411: Altitude reporting system check required every 24 calendar months.
AC 43.13-1B: Acceptable methods for airframe wiring and structural alterations.

Common Test Traps

Confusing ADS-B In with ADS-B Out: ADS-B Out is the transmit function required by regulation. ADS-B In (receiving traffic and weather data) is not mandated and is a separate, additional capability. An aircraft can be legally compliant with only ADS-B Out installed.
Assuming any GPS qualifies: A standard VFR GPS or older IFR GPS without WAAS does not meet the NACp and integrity requirements of 91.227. Only WAAS-enabled or equivalent GNSS sources are acceptable position inputs.
UAT above 18,000 feet: UAT (978 MHz) is prohibited as the sole ADS-B Out system in Class A airspace. A UAT-only installation is non-compliant for any flight above 17,999 feet MSL.
Skipping the 24-month altimeter/transponder check: Even after installing new ADS-B equipment, the altitude reporting system and transponder must still receive the required 14 CFR 91.411/91.413 check before the aircraft is used in IFR conditions or ADS-B-required airspace that depends on accurate altitude reporting.
Forgetting to update records: An ADS-B avionics installation is an alteration. Failure to document it on a Form 337 (if required), update the equipment list, and revise the Weight and Balance is an airworthiness violation regardless of how well the avionics itself performs.