a radar transponder that identified the aircraft and helped improve radar performance. Since 1966, the responsible agency has been called the Federal Aviation Administration (FAA). A Terminal Radar Approach Control (TRACON) is an ATC facility usually located within the vicinity of a large airport. In the US Air Force it is known as RAPCON (Radar Approach Control), and in the US Navy as a RATCF (Radar Air Traffic Control Facility). Typically, the TRACON controls aircraft within a 30 to 50 nautical mile (56 to 93 km) radius of the airport at an altitude between 10,000 and 15,000 feet (3,000 to 4,600 m). This uses one or more Airport Surveillance Radars (ASR-8, 9 and 11, ASR-7 is obsolete), sweeping the sky once every few seconds. These Primary ASR radars are typically paired with secondary radars (Air Traffic Radar Beacon Interrogators, or ATCBI) of the ATCBI-5, Mode S or MSSR types. Unlike primary radar, secondary radar relies upon an aircraft based transponder, which receives an interrogation from the ground and replies with an appropriate digital code which includes the aircraft id and reports the aircraft's altitude. The principle is similar to the military IFF Identification friend or foe. The secondary radar antenna array rides atop the primary radar dish at the radar site, with both rotating at approximately 12 revolutions per minute. The Digital Airport Surveillance Radar (DASR) is a newer TRACON radar system, replacing the old analog systems with digital technology. The civilian nomenclature for these radars is the ASR-9 and the ASR-11, and AN/GPN-30 is used by the military. In the ASR-11, two radar systems are included. The primary is an S-Band (~2.8 GHz) system with 25 kW pulse power. It provides 3-D tracking of target aircraft and also measures rainfall intensity. The secondary is a P-Band (~1.05 GHz) system with a peak-power of about 25 kW. It uses a transponder set to interrogate aircraft and receive operational data. The antennas for both systems rotate atop a tall tower. Weather radar During World War II, military radar operators noticed noise in returned echoes due to weather elements like rain, snow, and sleet. Just after the war, military scientists returned to civilian life or continued in the Armed Forces and pursued their work in developing a use for those echoes. In the United States, David Atlas, for the Air Force group at first, and later for MIT, developed the first operational