Definition
AIRPORT SURFACE DETECTION EQUIPMENT (ASDE)
Surveillance equipment specifically designed to detect aircraft, vehicular traffic, and other objects, on the surface of an airport, and to present the image on a tower display. Used to augment visual observation by tower personnel of aircraft and/or vehicular movements on runways and taxiways. There are three ASDE systems deployed in the NAS:
Source: FAA Pilot/Controller Glossary (verbatim from the Federal Aviation Regulations).