Surface Movement Radar (SMR)
A rotating X-band radar mounted on the airport tower or a nearby mast. It scans the entire movement area — runways, taxiways, aprons — with updates every second. The returns are small because aircraft and vehicles are close to the ground, but the range is short so the signal is strong. SMR shows controllers a map of everything moving.
ASDE-X: sensor fusion
The Airport Surface Detection Equipment, Model X, doesn't rely on radar alone. It fuses SMR with multilateration (MLAT) using aircraft transponder signals, ADS-B broadcasts, and terminal area radar. The result is a single fused picture with identity: not just 'something at intersection A', but 'Delta 1847 at intersection A'.
Runway incursion alerts
When the system predicts that two targets will occupy the same runway within 30 seconds, it flashes a visual and audible alert. Controllers can issue hold-short instructions. Pilots with cockpit moving-map displays see the conflict too. At Los Angeles International, ASDE-X has prevented dozens of potential collisions since 2008.
Low-visibility operations
In Category III weather — fog so thick you can't see the wingtip — surface radar is the only way to manage traffic. Combined with GPS-based ground guidance and stop-bar lights, airports can maintain operations down to 75 m runway visual range. Without surface radar, they would close.