ADS-B Out and In: What Pilots Need to Know About Equipment Requirements

Automatic Dependent Surveillance–Broadcast (ADS-B) has become a cornerstone of modern air traffic management, fundamentally changing how pilots and controllers share information. Unlike older radar systems that sweep the sky periodically, ADS-B broadcasts precise position, velocity, and identification data continuously. This technology not only meets a regulatory mandate but also provides a wealth of real-time data that enhances flight safety, efficiency, and situational awareness. For pilots transitioning from legacy transponders or considering an upgrade, understanding the differences between ADS-B Out and ADS-B In—along with the specific equipment requirements—is essential to staying compliant and getting the most out of the system.

The Core Difference: Out vs. In

At its simplest, ADS-B Out is a transmission function: your aircraft broadcasts its GPS-derived position, altitude, ground speed, and other data to ground stations and nearby aircraft. ADS-B In is a reception function that allows your cockpit to receive broadcasts from other aircraft and from ground-based services. While ADS-B Out is mandatory in many airspace classes, ADS-B In remains voluntary but offers substantial operational benefits. Understanding both sides of the equation helps pilots make informed equipment decisions, whether they are flying a certificated light sport aircraft or a turbine-powered business jet.

What is ADS-B Out?

ADS-B Out is the transmission component of the system. An aircraft equipped with ADS-B Out automatically sends a constant stream of data at 978 MHz (UAT) or 1090 MHz (Mode S Extended Squitter), depending on the airspace and equipment type. The data includes:

  • GPS-latitude, longitude, and altitude (with high integrity)
  • Ground track and ground speed
  • Aircraft identification (Mode S code or call sign)
  • Emergency status (if selected)
  • Velocity vector and geometric altitude

This information is refreshed several times per second, providing air traffic controllers and other ADS-B-equipped aircraft with a much more accurate and timely picture than radar can offer. Radar updates typically every 4–12 seconds, whereas ADS-B Out updates every second or less in the UAT environment. The improvement in update rate is particularly valuable in high-density terminal areas and during final approach, where small positional errors can have significant safety implications.

Because ADS-B Out relies on GPS for positioning, the aircraft must have a compliant GPS source that meets FAA Technical Standard Orders (TSO) C129, C145, C146, or C199. The GPS receiver must also meet performance requirements for accuracy, integrity, continuity, and availability in the intended operational environment. For aircraft operating above 18,000 feet (Class A airspace), the 1090 MHz Extended Squitter (ES) with a Mode S transponder is mandatory. For operations below 18,000 feet in controlled airspace, either 1090 ES or 978 UAT is acceptable, though the latter is limited to altitudes below 18,000 feet and to aircraft not requiring international compatibility.

Performance Standards for ADS-B Out

The FAA specifies minimum performance standards in FAA Advisory Circular 20-165A. Key requirements for a compliant installation include:

  • Position accuracy: Horizontal position must have a Navigation Accuracy Category for Position (NACp) of at least 7 (which equates to <0.1 nautical mile error 95% of the time) for most airspace, and NACp of 8 or 9 for operations under certain conditions such as RVSM airspace.
  • Integrity: The system must broadcast a Navigation Integrity Category (NIC) that bounds the position error with a specified probability. For most operations NIC ≥ 7 is required.
  • Latency: Total system latency from GPS position measurement to broadcast must not exceed 2 seconds for the 1090 ES link and 1.5 seconds for UAT.
  • Pressure altitude: For aircraft operating in positive control airspace, the barometric altitude must be sourced from an encoding altimeter or equivalent, and the system must cross-check the altitude to ensure consistency.

Failure to meet these standards can result in a non-compliant installation, leaving the aircraft subject to enforcement actions or denied access to controlled airspace. Pilots should verify that their chosen equipment is on the FAA’s approved list and that the installation is performed by a certified repair station with a field approval or STC where required.

What is ADS-B In?

ADS-B In is the receiving side of the equation. With appropriate equipment—typically a compatible transponder, a UAT receiver, or a combined unit—pilots can see traffic and weather information displayed on a cockpit screen (such as a multifunction display, tablet, or portable device). This in-cockpit information dramatically improves situational awareness, especially in visual meteorological conditions (VMC) where “see and avoid” remains the primary collision avoidance method. The data available through ADS-B In includes:

  • Traffic Information Service–Broadcast (TIS-B): A service that rebroadcasts radar-derived traffic data for aircraft not equipped with ADS-B Out. This ensures that even traditional transponder-equipped aircraft can be seen by ADS-B In receivers.
  • Flight Information Service–Broadcast (FIS-B): Available only on the 978 UAT link. FIS-B delivers free graphical weather products (NEXRAD, METARs, TAFs, winds aloft, icing, turbulence, etc.) and text-based notices (NOTAMs, PIREPs, SUA status) directly to the cockpit.
  • ADS-B Rebroadcast (ADS-R): A ground-based service that converts 1090 ES data to UAT or vice versa, so that aircraft on different data links can still see each other.
  • Surface indications: At large airports with ASDE-X or ASSC, ADS-B In can show the location of ground vehicles and other aircraft on the airport surface, reducing the risk of runway incursions.

While ADS-B In is optional for most general aviation operators, its benefits are compelling for both VFR and IFR flying. For example, a pilot flying cross-country can see convective storm cells displayed on an iPad using a portable ADS-B receiver (such as the Stratus or Sentry) without needing a fully integrated panel. These portable units, often using the 978 UAT link, provide FIS-B weather and TIS-B traffic even though the aircraft itself may not have an installed ADS-B Out solution. However, to receive all available traffic in terminal areas with both 1090 and UAT traffic, a dual-link receiver or an integrated transponder with both frequencies is recommended.

Regulatory Requirements: What is Mandatory?

The FAA’s ADS-B Out mandate took effect on January 1, 2020, under 14 CFR §91.225 and §91.227. All aircraft operating in designated airspace must broadcast ADS-B Out data compliant with those rules. The airspace requiring ADS-B Out includes:

  • Class A airspace (all operations above 18,000 feet MSL)
  • Class B and Class C airspace (including the airspace up to 10,000 feet MSL within the lateral limits of the Class B) — note that within Class B airspace, ADS-B Out is required even if you are transiting VFR without a flight plan.
  • Class E airspace at and above 10,000 feet MSL (except airspace below 2,500 feet AGL)
  • Class E airspace above the ceiling of a Class B or Class C airspace up to 10,000 feet MSL
  • Class E airspace over the Gulf of Mexico from the coastline to the edge of the Flight Information Region (within 12 NM of the US coast) above 3,000 feet MSL
  • Any airspace specifically designated by an FAA Order for special uses (e.g., some MOAs, restricted areas) where ADS-B is required

It is important to note that aircraft originally certificated without an electrical system (e.g., certain ultralights, gliders, and balloons) may be exempt from the mandate, as are those operating below the airspace floor of Class A, B, or C, or operating in Class G airspace. However, voluntary compliance is still encouraged because ADS-B In can provide significant safety benefits.

For international operations, pilots must be aware that other countries (such as those in Europe and Australia) have their own ADS-B mandates, often based on ICAO standards and different timelines. For example, the European Union requires ADS-B Out for aircraft weighing more than 5,700 kg or flying above FL 100 in certain areas, with a full mandate by 2023 in many airspace sectors. Pilots flying internationally should check the specific requirements of each country they will operate in.

Choosing the Right Equipment

Selecting the correct ADS-B equipment involves considering your aircraft type, typical operations, budget, and future plans. The two main data link standards are:

  • 1090 MHz Extended Squitter (1090 ES): Used internationally and in Class A airspace. If you fly above 18,000 feet or plan to fly abroad, you need a 1090 ES transponder. Many Mode S transponders can be upgraded to 1090 ES with a software or hardware modification. Available units include the Garmin GTX 335/345, L-3 Lynx NGT-1000, and the Avidyne AXP340.
  • 978 MHz Universal Access Transceiver (UAT): Used only in the US below 18,000 feet. UAT can provide ADS-B In (FIS-B and TIS-B) without an additional receiver. It is generally less expensive than 1090 ES and is popular among general aviation aircraft. Examples include the Garmin GDL 82, uAvionix skyBeacon, and the FreeFlight Systems RANGR.
  • Dual-link solutions: Some transceivers (e.g., Garmin GTX 345D, L-3 NGT-2000) support both 1090 ES and 978 MHz, allowing you to transmit on 1090 (for airspace compliance) but also receive on 978 for weather and traffic. This is the most complete solution but also the most expensive.

Portable ADS-B In receivers (like the Stratux or Stratus 3) allow pilots to receive traffic and weather on an iPad or other mobile device without any panel installation. These are excellent for aircraft that are not required to have ADS-B Out, or for pilots who want a backup display. However, they do not satisfy the Out mandate and cannot transmit your position to ATC or other aircraft.

Installation and Cost Considerations

The cost of equipping an aircraft with ADS-B Out varies widely. For a simple light aircraft that already has a Mode C transponder and a GPS navigator, a low-cost solution like the uAvionix skyBeacon (a unit that fits onto the wingtip) can be installed for under $2,000, provided the aircraft is approved for such an installation. For a complex turbine aircraft requiring dual 1090 ES transponders, a new GPS, and integration with the flight management system, costs can exceed $30,000. Mid-range installations for panels with existing Mode S transponders upgraded to 1090 ES plus a new GPS typically run $4,000–$10,000.

Installation must be performed by an FAA-certified repair station with appropriate authorizations. For experimental aircraft, a simpler process exists under the FAA’s “toolbox” guidance, but compliance still requires using approved equipment. Always request an FAA Form 337 major repair and alteration for any required ADS-B Out installation to ensure documentation is correct.

It is also worth noting that the FAA’s Rebate Program (which existed prior to 2020) is long expired, so pilots must budget the full cost themselves. Some insurance companies offer modest discounts for aircraft equipped with ADS-B Out and In, recognizing the safety benefits.

Benefits Beyond Compliance

While many pilots equip for regulatory reasons, the operational benefits of ADS-B extend far beyond meeting a requirement. The traffic and weather information provided by ADS-B In can improve flight planning, reduce the risk of mid-air collisions, and increase pilot confidence in marginal weather conditions. Traffic alerts from TIS-B often show targets that are not painted by legacy flight following services, particularly in areas with limited radar coverage. In terminal areas, ADS-B provides a more complete picture of surrounding traffic, which is especially helpful in congested Class B and C airspace.

Furthermore, ADS-B data is used for enhanced services such as Automated Terminal Information Service (ATIS) in digital form, and some airports now use ADS-B for surface surveillance to issue runway occupancy advisory. The FAA is also developing a Data Communications (Data Comm) system that uses ADS-B to deliver digital clearances, reducing the need for voice exchanges. This technology is already operational at many major airports and is being expanded.

For pilots flying instrument approaches, ADS-B in combination with WAAS GPS can provide precision approach capability without ground-based navaids, even at airports without ILS. The integration of ADS-B with avionics like the Garmin G1000 NXi or the Avidyne Entegra makes traffic and weather information available on the primary flight display, reducing pilot workload.

Common Pitfalls and Compliance Tips

Many early adopters of ADS-B discovered that their equipment was not fully compliant after the mandate took effect. Common issues include:

  • Incorrect GPS source: Using a non-TSO’d GPS, or one that does not meet the performance requirements (e.g., a portable GPS).
  • Missing or incorrect configuration: The ICAO aircraft address (Mode S code) must be programmed correctly; an incorrect address can cause the system to broadcast invalid data or interfere with other aircraft.
  • Firmware updates: Some early units lacked software to handle altitude encoding correctly under all conditions. Keeping firmware updated is essential.
  • Antenna issues: Poor antenna placement or damaged coaxial cable can degrade performance, leading to intermittent broadcasts or dropped data.
  • Certification mismatch: Installing equipment not listed on the FAA’s approved list or without proper STC for the aircraft model can result in non-compliance even if the equipment technically works.

To avoid these problems, pilots should consult an experienced avionics shop and request a FAA Compliance Check using a portable ADS-B test set or by flying through a known ADS-B ground station coverage area and submitting a request to the FAA for a report. The FAA offers an online tool called the ADS-B Performance Monitor that can analyze flight data after a flight and verify compliance.

Future of ADS-B

ADS-B technology continues to evolve. The FAA is transitioning from a ground-based network to the Space-Based ADS-B (SB-ADS-B) using the Iridium NEXT constellation, which will provide global coverage, including over oceans and remote land areas. This will eventually allow for remote oceanic air traffic control without requiring separation based on time or procedural routes. Additionally, the NextGen Air Transportation System integrates ADS-B data with other surveillance sources to improve traffic flow and reduce flight delays.

For pilots, staying informed about updates to standards (such as the potential introduction of ADS-B In mandates for certain operations) and new avionics products is wise. The basic architecture of ADS-B Out is mature and unlikely to change dramatically, but the services available through ADS-B In will expand, including possible delivery of digital NOTAMs, weather avoidance plans, and even trajectory-based operations instructions.

In summary, understanding ADS-B Out and In requirements involves recognizing the regulatory mandate, selecting equipment suited to your aircraft and typical mission, ensuring proper installation and configuration, and leveraging the safety and efficiency benefits that this technology provides. By making informed choices today, pilots can enhance their flying experience and maintain compliance in an increasingly surveillance-rich airspace environment.