The ASDE-3/AMASS radar system that spotted an errant Jeep on Philadelphia International's runways has one fatal flaw — it thinks raindrops are incoming jets and goes nuts during downpours. The ASDE Model X, however, monitors runways rain or shine.
Airport Surface Detection Equipment (ASDE) and its add-on, the Airport Movement Area Safety System (AMASS) are designed to provide automated audio and visual alerts if it detects a potential runway collision when a plane is landing or taking off. In service since 2001 — when it was installed at San Francisco International — it does so by predicting the movements of aircraft in relation to ground vehicles. However, the ASDE/AMASS system has to be shut off during storms because it mistakes raindrops for airplanes. This system was offline in 2005 when a passenger jet and cargo plane nearly collided at JFK, during a nighttime storm.
The ASDE-X system, however, is not affected by precipitation. The Model X employs a huge variety of data including surface movement radar, multilateration sensors, Automatic Dependent Surveillance-Broadcast sensors, terminal radar, the terminal's own automation system, and transponders affixed to aircraft and service vehicles. The system takes all this data, mashes it together and outputs it to a real-time, colour-coded map of the airports runways and taxiways. It keeps tabs on everything moving on the tarmac as well as all aircraft within 5 miles (and in some cases up to 60) of the airport.
The first Model X was installed in 2003 at General Mitchell International Airport in Milwaukee, Wisconsin, USA. Since then it has been installed in 35 of America's busiest transportation hubs including Chicago's O'Hare, Dallas-Ft Worth, JFK International, John Wayne Airport, Dulles International, Sky Harbor in Phoenix. And, since it is built from mostly off-the shelf components, the ASDE-X offers significant cost savings over the ASDE-3/AMASS system. [AMASS Wiki - ASDE-X Wiki - FAA 1 - FAA 2 - FAA 3 - NATCA]