ADS-B is a surveillance system in which an aircraft works out its own position from satellite navigation and broadcasts it, unprompted, to anyone listening. The FAA has required ADS-B Out in most controlled U.S. airspace since January 1, 2020, and 14 CFR 91.227 obliges equipped aircraft to transmit at least once per second while airborne.
The acronym unpacks the design. Automatic: the FAA states the equipment "periodically transmits information with no pilot or operator involvement required." Dependent: the aircraft depends on its own onboard navigation source rather than on a ground sensor measuring it. Surveillance-Broadcast: the message goes out to every receiver in range at once, not to a single interrogator.
That last word is why the same technology underpins both an air traffic controller's display and the free flight-tracking map on a phone. One transmission, many listeners, no addressing.
What does an aircraft actually broadcast?
More than a dot on a map. Under 14 CFR 91.227(d), an ADS-B Out message set includes latitude and longitude, barometric pressure altitude, geometric altitude, a velocity indication, the aircraft's Mode A transponder code, its call sign, its ICAO 24-bit address, an emitter category, collision-avoidance system status, emergency or communication-failure status, and the aircraft's length and width.
The ICAO 24-bit address is the piece that makes public tracking work. It is a unique identifier tied to the airframe's registration, so a receiver that decodes it can match a moving target to a specific aircraft in a national registry without the flight crew transmitting anything extra.
The message also carries its own report card. Each transmission includes quality indicators - NACp for position accuracy, NACv for velocity accuracy, NIC for containment integrity, plus SDA and SIL values describing how much the data can be trusted. The rule sets floors for all of them: the regulation requires an NACp of less than 0.05 nautical miles, an NACv of less than 10 meters per second, and an NIC of less than 0.2 nautical miles, with SDA at or below 10-5 per flight hour and SIL at or below 10-7 per flight hour or per sample.
Timing is regulated just as tightly. The aircraft must transmit its geometric position no later than 2.0 seconds after the position is measured, with no more than 0.6 seconds of uncompensated latency. Broadcast cadence is at least once per second while airborne or moving on the airport surface, dropping to at least once every five seconds when the aircraft is stationary.
How is ADS-B different from radar?
Radar measures the aircraft from the outside; ADS-B lets the aircraft report itself. That single inversion changes update rate, infrastructure cost, and coverage. The FAA notes that ADS-B updates once per second against a radar that "sweeps for position information every 5 to 12 seconds."
The infrastructure difference is equally practical. According to the FAA, ADS-B ground stations "are smaller and more adaptable than radar towers and can be placed in locations not possible with radar," which extends surveillance into terrain where a rotating antenna was never viable.
The trade-off sits in the word dependent. A radar return exists whether or not the target cooperates. An ADS-B report exists because the aircraft chose to send it, sourced from its own navigation equipment - which is precisely why the integrity fields above are written into the rule rather than left to manufacturers.
Why does the United States use two ADS-B frequencies?
Because two different fleets needed two different answers. The FAA approves 1090 MHz Extended Squitter, known as 1090ES and built on the Mode S transponder, and a separate 978 MHz Universal Access Transceiver, or UAT. The choice is not free: per the FAA, 1090ES is required for operations in Class A airspace at and above FL180, while the 978 UAT link is for operations below FL180 in U.S. airspace.
| Link | Standard | Where it is used | Weather uplink |
|---|---|---|---|
| 1090ES (Mode S Extended Squitter) | TSO-C166b | Required in Class A airspace, FL180 and above | No |
| 978 MHz UAT | TSO-C154c | Below FL180 in U.S. airspace | Yes, via FIS-B |
The practical consequence for anyone comparing avionics quotes: the airline-standard link is the one that cannot receive the free weather service, and the general-aviation link is the one that can. Airlines already have datalink weather by other means; a light aircraft below FL180 usually does not.
Where is ADS-B Out required?
The mandate is drawn by airspace class rather than by aircraft type. The FAA requires ADS-B Out in Class A, B, and C airspace; in Class E airspace within the 48 contiguous states at and above 10,000 feet MSL; and within 30 nautical miles of the airports listed in 14 CFR part 91, Appendix D - the ring general aviation still calls the Mode C veil. The requirement took effect on January 1, 2020.
Read the inverse and the map makes sense: low-altitude flying away from busy terminal areas is largely outside the mandate. That is the airspace where an aircraft can still be legally invisible to ADS-B receivers, and it is why coverage on a public tracking map thins out over rural terrain even when the sky is not empty.
What does ADS-B In add in the cockpit?
ADS-B Out is a legal obligation; ADS-B In is an optional receiver, and the FAA supplies its services at no charge. The agency describes ADS-B In as giving operators of properly equipped aircraft "weather and traffic position information delivered directly to the cockpit."
Two services do the work. TIS-B, per the FAA, "increases pilots' situational awareness by providing traffic information on all transponder-based aircraft" - the ground network relays targets an aircraft cannot hear directly, including those seen only by radar. FIS-B carries weather and aeronautical data, and the FAA makes it available only on the 978 MHz link, which is the single most consequential line in the frequency comparison above.
The FAA lists graphical weather displays plus text-based advisories, including notices to airmen and significant weather activity, among the products delivered.
How does a position reach a public tracking map?
- The aircraft's navigation source fixes a geometric position, and the ADS-B Out unit assembles the message set required by 14 CFR 91.227(d).
- The transmitter sends it within 2.0 seconds of measurement, at least once per second in flight, per the same rule.
- The broadcast propagates line-of-sight on 1090 MHz or 978 MHz. It is not addressed to anyone.
- FAA ground stations receive it for air traffic control, and other suitably equipped aircraft receive it directly.
- Any other receiver in range decodes the same unencrypted message, matches the ICAO 24-bit address to a registry entry, and plots it.
Step five is not a loophole. It is what "broadcast" means, and it is the reason enthusiast receiver networks can build near-global coverage from hardware costing less than a headset.
Why can anyone legally track a flight?
Because the signal is public by construction and no law requires listeners to look away. NBC News reported that ADS-B "also allows airplanes to be easily tracked by anyone with a compatible receiver," and quoted ADS-B Exchange founder Dan Streufert saying of his site's data: "We publish everything, and it already is public data."
Mitigations exist, and they target identification rather than the broadcast. NBC News described an FAA program under which owners can request that registration numbers be withheld from public display - honored by major tracking sites - and a second program that lets aircraft encode their ADS-B signals so they cannot be matched against the publicly available civil aviation registry.
Neither switches the transmitter off. The aircraft still broadcasts, still shows up as a target, and still meets the mandate; what changes is how easily that target is tied to a named owner. Sites that build their coverage from their own volunteer receivers rather than FAA feeds are not bound by the blocking arrangements at all, NBC News reported.
What the technology does not do
ADS-B is a surveillance and situational-awareness system, not a collision-avoidance system. The rule treats them as separate: collision-avoidance status is one of the fields an ADS-B Out message must report, which only makes sense if the two are distinct pieces of equipment.
Nor does the mandate cover every aircraft everywhere. It is defined by the airspace listed in the rule, on the effective date the FAA set, using the two links the FAA approved - a specific regulatory shape, not a universal one.
For a related aviation perspective, read Why Long-Haul Flight Paths Curve on the Map Instead of Going Straight.
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For more context, read edison.