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, color-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. Since then it has been installed in 35 of the nation'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]