ADS-B replaced radar as the primary way ATC (and, increasingly, other pilots) knows exactly where an aircraft is, and it did it by flipping the whole model: instead of a ground station sweeping a beam and measuring an echo, every equipped aircraft simply announces its own precise GPS-derived position to everyone listening, many times a second.
Automatic Dependent Surveillance-Broadcast (ADS-B) is exactly what its name describes: automatic (no pilot action needed once installed and operating), dependent (it depends on the aircraft's own GPS-derived position, not radar), and surveillance-broadcast (it broadcasts that position openly, for anyone equipped to receive it, rather than only reporting to a specific ground radar). The result is a system that's more precise than radar, updates more frequently, and works in places radar coverage never reached.
| Component | What it does |
|---|---|
| ADS-B Out | Transmits the aircraft's own GPS position, altitude, velocity, and identity to ground stations and other aircraft |
| ADS-B In | Receives ADS-B broadcasts from other aircraft, plus ground-broadcast weather and traffic data, and displays them in the cockpit |
ADS-B Out is what's mandated in most controlled airspace; ADS-B In is optional but is what actually puts traffic and weather on a cockpit display, since Out-only equipment transmits but doesn't receive anything.
ADS-B uses two different radio frequencies in the U.S., which don't talk to each other directly. 1090ES (1090 MHz Extended Squitter) is required for aircraft operating above 18,000 ft and is common in higher-performance and turbine aircraft, sharing its frequency with Mode S transponders. UAT (Universal Access Transceiver, 978 MHz) is an option below 18,000 ft, popular in general aviation, and is the only link that carries the free FIS-B weather service directly.
Traffic Information Service-Broadcast (TIS-B) is how a ground station fills in the picture for an ADS-B In-equipped aircraft: it rebroadcasts nearby traffic detected by radar or the other data link, so a UAT-equipped aircraft can see a nearby 1090ES aircraft (and vice versa) even though their ADS-B Out broadcasts use different frequencies. TIS-B is only available within range of a ground station, and only for traffic near an ADS-B In aircraft, it's not a complete national traffic picture broadcast everywhere at once.
Flight Information Service-Broadcast (FIS-B) is a free weather and aeronautical information service broadcast over the UAT link: METARs, TAFs, NEXRAD radar mosaics, PIREPs, TFRs, and more, received automatically by any UAT ADS-B In receiver within range of a ground station, no subscription required. It's one of the more underrated practical benefits of choosing UAT-based equipment.
In the U.S., ADS-B Out is generally required in Class A, B, and C airspace, above 10,000 ft MSL (excluding airspace at and below 2,500 ft AGL), within 30 nm of a Class B primary airport (the "Mode C veil"), and in certain other designated airspace. The requirement is airspace-based, not aircraft-type based, which is why even simple, older general aviation aircraft flying into a busy Class B environment need it installed.
ADS-B ground stations are far more numerous and lower-cost to deploy than legacy radar, which is why ADS-B coverage, especially at lower altitudes, extends into many areas radar historically never covered well: mountain valleys, remote regions, and low-altitude airspace generally have noticeably better ADS-B-based traffic visibility than they ever had with radar alone.
An ADS-B In display typically shows nearby traffic as icons with relative altitude and trend (climbing, descending, level), plus, on the UAT/FIS-B side, weather overlays like NEXRAD radar and METAR flags. The traffic picture is genuinely useful situational awareness, but it's explicitly not a substitute for maintaining visual scanning for traffic, both because of the coverage and latency limitations below and because it's meant to supplement, not replace, see-and-avoid.
NEXRAD weather displayed via FIS-B is also inherently a mosaic composited from ground radar sites and can be several minutes old by the time it displays, the same staleness caveat that applies to any groundbased weather radar picture, ADS-B or otherwise.
An aircraft equipped with a UAT-based ADS-B In/Out system departs a non-towered airport below the Class B veil. In flight, it automatically transmits its GPS position (ADS-B Out, satisfying the airspace requirement nearby), receives free NEXRAD and METAR data over FIS-B without any subscription, and sees a 1090ES-equipped airliner on its traffic display only because a nearby ground station rebroadcasts that traffic via TIS-B onto the UAT link the aircraft is listening on.
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This guide is intended for educational and training purposes only. It is not a substitute for official FAA publications, an authorized ground school, or qualified flight instruction, and should not be used as a sole source for real-world flight planning, dispatch, or operational decisions.