How (ADS-B R) Magic Happens

The “Hockey Puck” behind the scenes!

The draft ROTOR Act legislation in the USA mandates ADS-B IN for all airlines operating in USA controlled airspace. However, the USA operates two incompatible ADS-B technologies, one for airlines and one for general aviation. For ADS-B IN to work, the overall system must integrate both technologies. The following article explains how the magic happens.

Datalink Dichotomy: Two Inherently Incompatible Systems

Within the United States National Airspace System (NAS), the FAA established a dual-datalink architecture for ADS-B. This decision, while pragmatic, resulted in two technically incompatible systems operating simultaneously, necessitating a complex and sophisticated ground-based infrastructure to ensure a unified and safe surveillance environment for all airspace users. Airline ADS-B uses 1090 MHz Extended Squitter (1090ES) while general aviation ADS-B uses 978 MHz Universal Access Transceiver (UAT), two systems which are inherently incompatible.

The FAA mandated 1090ES for all aircraft operating in Class A airspace—at and above Flight Level 180 (approximately 18,000 feet Mean Sea Level). This regulation effectively makes 1090ES the default and required system for the entire commercial airline fleet and most high-performance corporate and private aircraft that operate at FL180 and above.

In stark contrast to the evolutionary 1090ES, the Universal Access Transceiver (UAT) is a purpose-built datalink, designed from the ground up specifically for ADS-B services within the United States. It operates on 978 MHz and is operationally restricted to airspace below 18,000 feet. Further, its use is prohibited in Class A airspace. This limitation aligns with the typical operating profile of the majority of piston-engine aircraft. Crucially, because UAT is not recognized by international aviation authorities, it is not a compliant ADS-B solution for any flight operations outside of the United States, a significant consideration for pilots who may fly to neighboring countries.

The Bridge That Unites

The solution to the datalink incompatibility problem is not found in the air but on the ground. The FAA engineered and deployed a comprehensive terrestrial network, known as the Surveillance and Broadcast Services (SBS) system, to serve as the technological bridge between the 1090ES and UAT worlds. This infrastructure is the linchpin of the U.S. ADS-B implementation, performing the critical functions of surveillance, data processing, and targeted information broadcasting. The architecture of this network, and the specific operational logic it employs, are designed not only to solve the technical problem of incompatibility but also to manage spectrum efficiently and incentivize participation in the NextGen ecosystem.

The Ground Based Transceiver (GBT) Network

The physical backbone of the SBS system is a nationwide network of more than 700 Ground Based Transceivers (GBTs). These unmanned radio stations are strategically located across the country, including in remote and mountainous terrain where traditional radar coverage is limited or non-existent, to provide seamless coverage throughout the NAS. Collectively, these GBTs form the Surveillance and Broadcast Services Subsystem (SBSS), the ground segment of the overall ADS-B architecture.

Each GBT is a sophisticated piece of infrastructure equipped with both receivers and transmitters capable of operating on both the 1090 MHz and 978 MHz frequencies. This dual-frequency capability allows them to perform two critical and distinct functions simultaneously. First, they serve a surveillance function. The GBTs continuously listen for and receive the ADS-B OUTbroadcasts from all equipped aircraft—both 1090ES and UAT—within their reception area. This raw position data is then relayed to FAA air traffic control facilities, where it is used to display aircraft positions on controller screens. This provides ATC with a surveillance picture that is more accurate and updates far more frequently (approximately once per second) than legacy radar systems, which typically have a sweep interval of 5 to 12 seconds.

Second, the GBTs perform a broadcast function. Based on the surveillance data they collect and process, the GBTs transmit a suite of uplink data services back to aircraft that are equipped with ADS-B IN receivers. These services are the key to creating a complete traffic and information picture in the cockpit and include Automatic Dependent Surveillance-Rebroadcast (ADS-R), Traffic Information Service-Broadcast (TIS-B), and Flight Information Service-Broadcast (FIS-B). It is this re-broadcast function that provides the mechanism for an aircraft using one datalink to “see” traffic on the other.

The “Hockey Puck” Concept

The traffic-related uplink services, ADS-R and TIS-B, are not broadcast indiscriminately across the NAS. Instead, the SBS system employs a highly efficient “client-based” service model. To receive these tailored traffic broadcasts, an aircraft must first be recognized by the ground system as a “client.” This status is achieved automatically when an aircraft transmits valid ADS-B OUT messages. This requirement is a critical prerequisite; an aircraft that is only equipped with a passive ADS-B IN receiver is not considered a client and is therefore not eligible to activate the full suite of ground-based traffic services.

Once the ground system identifies an aircraft as a client, it establishes a customized, dynamic surveillance area around that aircraft. This service volume is commonly referred to in technical literature and by pilots as the “hockey puck.” This is a three-dimensional volume of airspace that typically extends for a radius of 15 nautical miles horizontally and extends 3,500 feet above and below the client aircraft’s current altitude. The SBS system then populates this personalized volume with relevant traffic information and uplinks it specifically for that client. Any other aircraft, whether ADS-B equipped or not, that penetrates this hockey puck volume will be included in the traffic data package sent to the client aircraft.

This architecture is far more than a technical solution; it is a policy tool artfully disguised as a system design. The FAA’s primary objective for NextGen was to achieve near-universal ADS-B OUT equipage to unlock the system’s full potential for safety and efficiency improvements. While the 2020 mandate provided the regulatory “stick,” the client-based service model provided a compelling “carrot.” The most valuable benefit of ADS-B IN for a pilot is enhanced situational awareness, which is contingent on having a complete and reliable traffic picture. By making the most comprehensive traffic services—ADS-R and TIS-B—available only to those aircraft actively participating by transmitting ADS-B OUT, the FAA created a functional incentive to equip. An operator with a passive, receive-only (”IN-only”) system is not a client and can only “eavesdrop” on the traffic picture being generated for a nearby client. This results in an intermittent, incomplete, and fundamentally unreliable view of the surrounding traffic, creating a tangible safety and utility gap that strongly encourages full compliance with the ADS-B OUT mandate.

From an engineering perspective, the “hockey puck” model is a critical solution for managing finite resources and ensuring system scalability. The 1090 MHz frequency is an exceptionally congested resource. A naive approach of broadcasting all detected traffic in a given region to all aircraft in that region would quickly saturate the datalinks. In dense terminal areas like Los Angeles or New York, this would lead to a cascade of message collisions, data loss, and ultimately, system failure. The “hockey puck” architecture elegantly solves this scalability problem by functioning as a highly efficient data filter. The ground system performs the computationally intensive task of tracking and correlating all targets within a large geographic area, but it only uplinks a small, relevant subset of that data—the traffic within the puck—to each individual aircraft. This personalization of the data stream dramatically reduces the bandwidth required for each uplink transmission, ensuring that the system remains robust, reliable, and effective even under the highest traffic loads.

The Core Translation Service: Automatic Dependent Surveillance-Rebroadcast (ADS-R)

At the very core of the FAA’s solution to datalink incompatibility is the Automatic Dependent Surveillance-Rebroadcast (ADS-R) service. This is the primary mechanism that directly answers the question of how a 1090ES-equipped aircraft can integrate signals from a UAT transponder. ADS-R is not an airborne system but a ground-based service, orchestrated entirely by the SBS infrastructure, that functions as an essential “translator” to bridge the communication gap between the two disparate ADS-B communities.

Principles of Operation

The operational principle of ADS-R is straightforward in concept but complex in execution. It is a service that receives ADS-B messages broadcast on one frequency, processes and reformats the data, and then rebroadcasts it on the other frequency for reception by aircraft equipped for the opposite link. In essence, it acts as the “interpreter” that allows the 1090ES and UAT systems to “talk” to each other, even though they cannot do so directly.

The ADS-R service for any given aircraft is not always active. It is initiated only when a GBT receives a valid ADS-B OUT transmission from a “client” aircraft, as defined by the client service model. When the SBS system receives this transmission, its software notes the datalink being used by the client (e.g., 1090ES for an airliner). This triggers the system to begin actively scanning for traffic on the other datalink (in this case, 978 UAT) that is currently operating within, or is projected to enter, the client’s personalized “hockey puck” service volume. When such a target is detected, the ADS-R translation and rebroadcast process is activated for that specific target relative to that specific client.

The Dual-Link Exception: A Mark of System Intelligence

The FAA’s technical specifications for the SBS system include a crucial and intelligent exception to the ADS-R service. The system is designed such that aircraft equipped with dual-link ADS-B IN receivers—that is, avionics capable of simultaneously receiving signals on both 1090 MHz and 978 MHz—are not provided with ADS-R services.

This exclusion is not a flaw or a limitation but a sophisticated design feature intended to ensure data integrity, prevent pilot confusion, and optimize the use of the radio spectrum. An aircraft with a dual-link receiver already has the native capability to receive the UAT aircraft’s 978 MHz transmission directly, air-to-air, without any ground system involvement. If the ground system were to also provide an ADS-R uplink of that same UAT aircraft on the 1090 MHz link, a dangerous situation could arise. The receiving aircraft’s avionics would simultaneously process two distinct position reports for the exact same target, one received directly from the aircraft on 978 MHz and another received from the ground on 1090 MHz.

This data redundancy could cause the display system’s track-keeping algorithms to fail or, worse, lead to the presentation of a “ghost” or duplicate target on the cockpit display of traffic information (CDTI). Such an artifact would create dangerous clutter on the traffic display, potentially masking other real threats and fundamentally eroding pilot trust in the reliability of the system. By intelligently detecting an aircraft’s dual-link capability (which is declared in its own ADS-B OUT broadcast parameters) and actively suppressing the now-redundant ADS-R service for that client, the SBS system demonstrates a high degree of sophistication. This logic conserves ground processing resources, saves valuable uplink bandwidth on the congested 1090 MHz frequency, and, most importantly, guarantees a clean, unambiguous, and reliable traffic picture for the operators who have invested in the most capable avionics.

Integrating Non-ADS-B Targets into the Datalink Environment

TIS-B is a service designed to bridge the technological gap between the NextGen ADS-B environment and the legacy radar-based surveillance system. Its function is to uplink surveillance data for aircraft that are not broadcasting ADS-B OUT but are equipped with a traditional Mode A, Mode C, or Mode S transponder and are being actively tracked by the FAA’s network of ground-based secondary surveillance radars.

The mechanism for TIS-B involves a larger integration of national surveillance assets. The SBS system ingests real-time track data from the national radar network. When a radar target is identified as being within the “hockey puck” of an ADS-B OUT client, the SBS system synthesizes a TIS-B message containing the radar-derived position, altitude, and track of that target. This message is then broadcast as a TIS-B target to the ADS-B IN client. This service is available on both the 1090 MHz and 978 MHz frequencies, ensuring that all ADS-B IN users, regardless of their datalink, can benefit. In essence, TIS-B “fills in the blanks” in the traffic picture, allowing ADS-B-equipped aircraft to see nearby traffic that would otherwise be invisible on their displays.

The Integrated Traffic Picture for Airline Flight Crews

In summary, the flight crew of a typical commercial airliner operating in the U.S. NAS, equipped with a standard 1090ES ADS-B IN system, benefits from a comprehensive traffic picture that is a composite of data from three distinct sources. They receive real-time position information directly, air-to-air, from other 1090ES-equipped aircraft. They see nearby UAT-equipped aircraft thanks to the ground-uplinked ADS-R translation service. And they are made aware of nearby legacy transponder-equipped aircraft through the ground-uplinked TIS-B service, which provides radar-derived data. This fused display provides an unprecedented level of situational awareness.

Emerging Trends in Equipage and the Future of the 978 UAT Spectrum

While the dual-link system was designed to accommodate different user groups, recent data from the FAA on fleet equipage reveals a clear and accelerating trend away from UAT and towards 1090ES, even within the General Aviation community that UAT was designed to serve. This market-driven shift appears to be influenced by several practical factors. As older Mode C transponders reach the end of their service life, many aircraft owners are finding it simpler and more cost-effective to replace them with an all-in-one 1090ES transponder that satisfies both the transponder and ADS-B OUT requirements in a single box, often with a less complex installation than a separate UAT unit. Additionally, the desire for international compatibility, even for occasional flights to Canada, the Bahamas, or Mexico, makes 1090ES a more future-proof investment.

This organic, market-driven consolidation towards the single, global 1090ES standard is an unintended consequence of the initial policy, and it could significantly reshape the future use of the NAS. The FAA’s dual-link system was a well-intentioned compromise to facilitate initial adoption. However, long-term market dynamics—driven by equipment lifecycles, global standardization, and installation simplicity—are now favoring a single standard. As UAT usage declines, the 978 MHz spectrum, originally chosen for its high bandwidth and low congestion, becomes progressively underutilized.

This emerging “spectrum opportunity” was not part of the original NextGen plan. This high-quality, underutilized frequency band is now being seriously considered by regulators, both in the U.S. and abroad, as a potential solution for accommodating the next generation of airspace entrants, most notably Unmanned Aircraft Systems (UAS). These new users require a robust datalink for electronic conspicuity (the ability to see and be seen) that does not further congest the already saturated 1090 MHz band. The declining use of 978 MHz by manned aircraft may, therefore, inadvertently provide the perfect technological home for the safe integration of UAS into the NAS. Thus, the market’s gradual pivot away from the FAA’s initial dual-link strategy may ultimately provide an elegant solution to a future challenge that was not fully envisioned when the system was first designed.

Conclusion:

The integration of 978 MHz UAT transponder signals into an airline’s 1090ES ADS-B IN system is a complex yet elegant process that is not achieved through direct airborne communication. It is, instead, a sophisticated synthesis of data wholly dependent upon and orchestrated by the FAA’s terrestrial Surveillance and Broadcast Services infrastructure. Through the core translation service of Automatic Dependent Surveillance-Rebroadcast (ADS-R), complemented by the supplemental radar data from the Traffic Information Service-Broadcast (TIS-B), the ground network constructs a customized, unified traffic picture for each participating aircraft and uplinks it on the appropriate frequency.

Who knew?

A “Hockey Puck” solves one of the biggest avionic challenges in aviation.

PFM … Pure Flippin Magic!