Every second your airliner is in the air, it is quietly telling the world exactly where it is. That is ADS-B — Automatic Dependent Surveillance–Broadcast — the satellite-based technology that has replaced radar as the backbone of modern air traffic surveillance. If you fly, work around aircraft, or simply love watching flights track across a map, ADS-B is the reason it all works. Here is how it actually functions, why the FAA made it mandatory, and what the "Out" and "In" halves really do.
What Does ADS-B Actually Mean?
The name is an acronym that describes the technology precisely, and the FAA breaks it down word by word:
- Automatic — it transmits information continuously, with no pilot or operator action required.
- Dependent — the position and velocity data are derived from an onboard navigation source, primarily the Global Positioning System (GPS) or a flight management system.
- Surveillance — it provides a three-dimensional position and identification for the aircraft.
- Broadcast — that information is transmitted openly to anyone with suitable receiving equipment, whether that is an air traffic control facility or another aircraft nearby.
The key conceptual shift is that ADS-B replaces ground radar's interrogation model with satellite positioning. Traditional radar bounces radio signals off an aircraft and sweeps for a return every 5 to 12 seconds. ADS-B flips that around: the aircraft works out its own GPS position and broadcasts it once per second. The result is faster, far more precise tracking — position updates that are roughly an order of magnitude more frequent than an old radar sweep.
ADS-B Out: Broadcasting Your Position
ADS-B "Out" is the transmitting half, and it is the part that regulators mandate. An ADS-B Out system broadcasts the aircraft's GPS-derived position, pressure altitude, ground speed, a unique identity code, and other status data to ground stations and to other suitably equipped aircraft — again, once per second. Controllers see a cleaner, more current picture than radar ever provided, and because the position comes from GPS rather than a rotating antenna, coverage no longer depends on where you can physically build a radar tower.
That last point matters more than it sounds. Radar signals travel by line of sight; they cannot bend around mountains or reach far out over open water. ADS-B ground stations are small, cheap, and can be sited almost anywhere, which is how the United States finally got reliable surveillance over remote terrain, much of Alaska, and the Gulf — places radar could never economically cover.
Two Data Links: 1090ES vs 978 UAT
In the United States there are two approved ADS-B data links, and understanding the split explains a lot about how the system behaves. The first is 1090 MHz Extended Squitter (1090ES), built on the existing Mode S transponder frequency and certified under TSO-C166c. The second is the 978 MHz Universal Access Transceiver (UAT), certified under TSO-C154d and intended specifically for general aviation aircraft operating below 18,000 feet.
Airliners and anything flying in the high-altitude structure use 1090ES, because it is the international standard and works above 18,000 feet. Many light general aviation aircraft choose 978 UAT instead, partly because it is less congested and partly because — as we will see — it carries free weather that the 1090 link does not. Both links satisfy the mandate; they simply serve different segments of the fleet. The two-link design is unique to the U.S.; most of the rest of the world standardized on 1090ES alone. If you are studying how these systems interact, the free reference material on our aviation tools page is a useful companion.
ADS-B In: Traffic and Weather in the Cockpit
If "Out" is about broadcasting, ADS-B "In" is about receiving — and unlike Out, it is optional. An ADS-B In receiver pulls in traffic and weather information and displays it directly in the cockpit, turning a tablet or a panel-mounted display into a live situational-awareness tool. There are three distinct services worth knowing, and the differences trip up a lot of pilots.
TIS-B — Traffic Information Service–Broadcast
TIS-B fills in the traffic that ADS-B alone would miss. It is a client-based service that gives an ADS-B Out/In equipped aircraft surveillance data about nearby aircraft that are not ADS-B equipped but are within radar coverage and running a transponder. To receive it, your aircraft must broadcast valid ADS-B Out and receive ADS-B In. The ground system then builds a virtual "hockey puck" of protected airspace around your aircraft — 30 nautical miles in diameter and 3,500 feet thick — and relays any qualifying traffic inside that volume to you.
FIS-B — Flight Information Service–Broadcast
FIS-B is the free-weather service, and here is the catch every general aviation pilot should memorize: it is broadcast only on the 978 MHz UAT link, not on 1090 MHz, because of bandwidth limits. FIS-B pushes meteorological and aeronautical data continuously into the airspace — no request needed. Available products include METARs, TAFs, CONUS and regional NEXRAD radar imagery, AIRMETs, SIGMETs and Convective SIGMETs, PIREPs, winds and temperatures aloft, NOTAMs, and special-use airspace status. It is a genuinely powerful safety tool, and it is one of the main reasons a light-aircraft owner might pick UAT over 1090ES. You can cross-check the same weather products with the free decoders on our aviation tools page.
ADS-R — Automatic Dependent Surveillance–Rebroadcast
Because the U.S. runs two links, aircraft on different frequencies could not otherwise "see" each other. ADS-R solves that. It is a ground-based relay: an aircraft broadcasting on 1090 MHz has its position rebroadcast to nearby aircraft receiving on 978 MHz, and vice versa. So a UAT-equipped Cessna and a 1090ES-equipped jet end up on the same shared traffic picture, stitched together by the ground network.
The Ground Network Behind It All
None of the "In" services would exist without the ground infrastructure. The FAA completed baseline nationwide deployment of its ADS-B ground network in April 2014, with roughly 634 radio stations distributed across more than 300 service volumes. Coverage now spans the continental United States, large parts of Alaska, Hawaii, and Puerto Rico. That network is what receives every aircraft's ADS-B Out broadcast, feeds it to controllers, and generates the TIS-B, FIS-B, and ADS-R services flowing back up to the cockpit.
The Mandate: Where and When You Need ADS-B Out
ADS-B Out stopped being optional in U.S.-controlled airspace on January 1, 2020. The requirement lives in two parts of the federal regulations: 14 CFR 91.225 defines the airspace where equipage is required, and 14 CFR 91.227 sets the performance standards the equipment must meet. Broadly, you need ADS-B Out to fly in Class A, B, and C airspace; in Class E airspace at or above 10,000 feet MSL (excluding the layer at and below 2,500 feet above the ground); within the 30-nautical-mile Mode C veil around the busiest Class B airports; above the ceilings of Class B and C airspace; and over the Gulf at and above 3,000 feet within 12 nautical miles of the U.S. coast. In plain terms, if you are flying anywhere busy or high, you must be equipped and transmitting.
Europe followed a similar path. Under Commission Implementing Regulation (EU) No 1207/2011, the ADS-B Out mandate took effect on December 7, 2020 — pushed back six months from an original June 7, 2020 date by Regulation (EU) 2020/587 as the industry absorbed the COVID-19 shock. The European rule applies to fixed-wing aircraft with a maximum takeoff mass above 5,700 kg or a maximum cruising true airspeed above 250 knots operating under IFR, and it standardizes on 1090ES. For any pilot working toward an airline career, understanding this regulatory landscape is part of the job; our own step-by-step pilot training book walks through how systems like this fit into modern airline operations.
The Open-Broadcast Trade-Off
There is one consequence of that fourth word — broadcast — worth being honest about. ADS-B transmissions are unencrypted and, by design, receivable by anyone with the right antenna. That openness is exactly what powers the public flight-tracking websites and apps that enthusiasts love. It also means aircraft movements are far more visible than they were in the radar era, which has prompted programs to limit the display of certain private aircraft data. For most of aviation, though, transparency is a feature: it is what lets a hobbyist on the ground and a controller in a center see the same sky. If you know an avgeek who would appreciate the technology, our gifts for pilots collection is full of things built around exactly this fascination.
Why ADS-B Matters
ADS-B is one of those rare upgrades that improves safety, efficiency, and access all at once. It gives controllers a position update every second instead of every several, extends surveillance into airspace radar never reached, and hands general aviation pilots free traffic and weather that used to cost a fortune. It is the connective tissue of NextGen and the foundation that emerging concepts — from more efficient routing to future automation — are being built on top of. The next time you watch a flight crawl across a tracking map in real time, you are watching ADS-B do its job: automatic, dependent, surveillance, broadcast — one transmission per second, all day, everywhere.
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