From Radar to Revolution: The Story of ADS-B

Look up on any clear day and there's a good chance the aircraft you see above are being tracked by an incredible system: radar-based air traffic control. For decades, this technology has helped guide hundreds of millions of flights to their destinations.

Today, that proven foundation is being enhanced with a complementary technology called ADS-B, which adds new capabilities to the system including the ability for aircraft to broadcast their own precise position, altitude, speed, and identity once per second—or more.

Welcome to the ADS-B transformation.

Known as Automatic Dependent Surveillance-Broadcast, ADS-B technology represents the most significant advancement in aviation surveillance since radar revolutionized air traffic control after World War II. And if you've ever tracked a flight on FlightAware^®—whether on our popular FlightAware app or as one of the nearly 23 million monthly website visitors—you've seen ADS-B in action.

The name itself explains the technology:

• Automatic means the system requires no pilot input as it continuously operates on its own.
• Dependent because it relies on data from the aircraft's onboard navigation systems, primarily GPS.
• Surveillance is its function: determining aircraft positions.
• And Broadcast is how it shares information by transmitting autonomously once per second to anyone within range, without waiting to be interrogated like a traditional transponder.

Building on a Strong Foundation

To appreciate what ADS-B brings to aviation, it helps to understand the remarkable system it builds upon.

Radar surveillance transformed aviation safety when it was adapted for civilian air traffic control in the decades following World War II, and with the widespread installation of cockpit transponders in commercial and private aircraft. Primary radar sends out radar pulses that reflect off aircraft, while Secondary Surveillance Radar (SSR) interrogates transponders aboard aircraft to obtain altitude and identification codes. This infrastructure enabled the managed, efficient air traffic control system that made modern commercial aviation possible.

Radar excels in many environments, particularly in the busy terminal areas around major airports where ground stations provide reliable coverage. The technology has been continuously refined over decades, and the expertise of the controllers who use it has helped make air travel safe.

ADS-B represents the next step in this evolution and provides enhancements that add new capabilities. While radar requires line-of-sight from ground stations, ADS-B relies on satellite navigation to let aircraft broadcast their own precise positions. Through space-based ADS-B, we can now track aircraft over oceans and remote areas where installing ground-based radar infrastructure is physically or economically impractical.

How it Works

An ADS-B-equipped aircraft contains a GNSS receiver (typically GPS, though other satellite navigation systems qualify) that determines the aircraft's position generally within 10 meters or better. This position data, along with altitude from the aircraft's air data system, is fed to the ADS-B transponder.

The transponder then encodes this information into a digital message and broadcasts it on a designated radio frequency. Each transmission includes the aircraft's latitude, longitude, altitude, ground speed, heading, vertical rate, identification (the Mode S address and call-sign), and additional status information.

In the United States, ADS-B operates on two frequencies:

1. 1090 MHz Extended Squitter (1090ES): This is the international standard used worldwide and is required for operations at or above 18,000 feet. The transponder "squitters"-out position reports autonomously.
2. 978 MHz Universal Access Transceiver (UAT): This is a U.S.-only solution designed primarily for general aviation operating below 18,000 feet. The UAT frequency allows for Flight Information Service-Broadcast (FIS-B), delivering weather radar imagery, METARs, and NOTAMs directly to cockpits.

Ground stations receive these broadcasts and forward them to air traffic control. Additionally, aircraft equipped with "ADS-B In" can receive these broadcasts directly from other planes, creating better air-to-air traffic awareness for pilots.

Because the two frequencies cannot communicate directly, the FAA utilizes ADS-R (Automatic Dependent Surveillance-Rebroadcast) to relay traffic information between 1090ES and 978 UAT aircraft. It also provides TIS-B (Traffic Information Service-Broadcast), which "uplinks" radar-detected aircraft that aren't ADS-B equipped so they still appear on cockpit displays.

MLAT: Tracking Aircraft Without ADS-B

Not every aircraft transmits ADS-B, but that doesn't mean they're untrackable. FlightAware uses a complementary technology called multilateration—MLAT for short—to track aircraft equipped with older Mode S transponders that don't broadcast their GPS position.

MLAT works on a principle called Time Difference of Arrival (TDOA). When an aircraft's transponder transmits a signal, that signal travels outward at the speed of light. Multiple ground receivers pick up the same transmission, but because they're located at different distances from the aircraft, each receiver detects the signal at a slightly different time.

By precisely measuring these tiny time differences—in nanoseconds—and using highly synchronized clocks across our receiver network, sophisticated algorithms can calculate where the aircraft must be located for those specific timing differences to occur. It's essentially triangulation using time rather than angles.

For MLAT to work, ideally four or more receivers must detect the same transmission. This is why receiver density matters. FlightAware's network of over 40,000 receivers—including every PiAware and FlightAware FlightFeeder^®—contributes to tracking these non-ADS-B aircraft, providing comprehensive coverage of the airspace.

MLAT achieves position accuracy comparable to ADS-B. This allows FlightAware to track additional aircraft than would be possible with ADS-B alone, providing more comprehensive coverage of the airspace.

Satellite-Based ADS-B: Closing the Coverage Gap

ADS-B receivers can be impractical to deploy over oceanic and remote areas. A partnership between FlightAware and space-based ADS-B provider Aireon^® addresses remote coverage limitations. Aireon’s satellite-based ADS-B receiver network captures 1090 MHz broadcasts from aircraft anywhere on earth. When combined with FlightAware's terrestrial network, this creates truly global surveillance coverage.

Oceanic separation standards that previously required up to 50-100 nautical mile separation are reduced to surveillance-based standards comparable to domestic operations. Routes can be optimized based on winds and traffic, rather than following rigid tracking systems designed around the limitations of high-frequency (HF) pilot position reporting, allowing controllers to see real-time traffic over the North Atlantic, Pacific, and polar regions.

In our upcoming blog, we’ll take a deeper dive into the FlightAware FlightFeeder network and the role it plays in powering global ADS-B coverage. In the meantime, you can explore the ADS-B network and real-time coverage by visiting the FlightAware ADS-B Statistics page, which offers detailed insights into the network’s scale—including active sites, participants, countries represented, and positions reported per hour.

Interested in hosting a FlightFeeder or learning more about how ADS-B works and how you can contribute? Visit our FlightFeeder page to explore resources, requirements, and opportunities to participate in the ADS-B community.

Visit our website to learn more about FlightAware's tracking capabilities and aviaiton data services.