ADS-B stands for Automatic Dependent Surveillance–Broadcast, and the name describes it precisely. It is automatic — no crew action required. It is dependent — it relies on the aircraft's own navigation systems rather than an independent measurement. It is surveillance. And it is broadcast: transmitted openly to anyone in range.
What it sends
The aircraft determines its position from GNSS — GPS and equivalents — and broadcasts a message roughly twice a second containing its identity, position, altitude, ground speed, vertical rate and heading.
The transmission is on 1090 MHz, and it is not encrypted. That is a deliberate design choice: the purpose is for everyone nearby to know where the aircraft is, including other aircraft.
Why it replaced radar
Radar is expensive to install and maintain, has limited range, updates only as fast as the antenna rotates — typically every four to twelve seconds — and cannot be placed over oceans or mountains.
ADS-B updates twice a second, is far more accurate because it comes from GNSS rather than from a bearing and range measurement, and the ground infrastructure is a receiver that costs a tiny fraction of a radar head. Most of the world has mandated it for commercial aircraft.
ADS-B In, and traffic awareness
Aircraft can also receive other aircraft's broadcasts. This gives a flight deck a picture of surrounding traffic without any ground involvement, which supports more efficient oceanic spacing and in-trail procedures.
The satellite change
The original weakness was the same as everything else in aviation surveillance: it needed a receiver within a few hundred kilometres, which meant no coverage over oceans.
That changed with satellite-based ADS-B reception, deployed on a low-earth-orbit constellation from around 2018. Receivers in orbit pick up the same 1090 MHz broadcasts, giving genuinely global coverage including oceans and polar regions.
The operational effect has been significant. Oceanic separation standards, which were built around aircraft reporting position by voice every half hour, have been reduced where satellite ADS-B is available — which means more aircraft on optimal tracks and altitudes, and measurably less fuel burned.
The limitation worth knowing
ADS-B is dependent, and it is broadcast. The aircraft is telling you where it is. If the transponder is switched off, ADS-B stops with it — which is exactly what happened to MH370, and no amount of satellite reception helps with an aircraft that is not transmitting.
This is why primary radar has not been abandoned, and why the autonomous distress tracking requirement introduced after MH370 is designed to operate independently of the transponder.
The open nature of ADS-B is also why flight tracking websites exist. They aggregate feeds from thousands of volunteer receivers plus satellite data. Some military and state aircraft filter their broadcasts, but civil traffic is generally visible to anyone with an antenna.