How ADS-B works
Each aircraft determines its own position via GPS and broadcasts it twice per second on 1090 MHz (or 978 MHz UAT for general aviation). Ground stations, satellites and other aircraft pick it up. Accuracy: a few metres. Cost per ground station: about $100k vs. tens of millions for radar.
Why radar still matters
ADS-B requires the aircraft to be cooperative, powered, GPS-locked and not spoofing. A primary surveillance radar sees anything that reflects — including a hijacked transponder-off airliner, a swarm of birds, a stray drone, or a balloon. After 9/11 the FAA quietly raised investment in primary radar.
Secondary surveillance radar (Mode S, Mode C)
The middle ground: ground radar interrogates the aircraft's transponder, which replies with altitude and ID. Faster than primary, more trustworthy than ADS-B, used everywhere as backup.
The future
Space-based ADS-B (Aireon's 66-satellite constellation) now covers oceans where ground radar can't reach. Most controllers run a fused display — ADS-B as the primary picture, radar as the truth check, space-based ADS-B for the bits in between.