WEATHER RADARS * Radars are commonly used to track weather, with ground-based weather radars used by weathermen, and aircraft weather radars used by airliners and the like to avoid storm fronts. One of the better-known of the ground-based systems is "Nexrad", a network of 158 radar stations that was set up in the 1990s by the US National Oceanographic & Atmospheric Administration (NOAA). The Nexrad radars are mounted on towers ranging from 4.6 to 30.5 meters (15 to 100 feet) tall, topped by a rotating radar dish 8.5 meters (28 feet) in diameter inside a protective spherical fiberglass radome. The panels that make up the radome are irregularly shaped, giving the radome something of the look of a cracked-up eggshell, since a more regular pattern would interfere more with radio waves. In clear weather, the dish performs five rotations in ten minutes, with the angle of the sweeps ranging from 0.5 to 4.5 degrees. In nasty weather, the dish picks up the pace, performing 14 sweeps in five minutes, with the angle of the sweeps ranging from 0.5 to 19.5 degrees. The radar doesn't work well at angles lower than 0.5 degrees, since ground clutter ruins the measurements, and there is a "cone of silence" above the radar sweeps that isn't observed. Sweeps alternate between ranging or "reflectivity" sweeps that spot precipitation, and Doppler or "velocity" sweeps that determine wind speed and direction. A Nexrad radar operates in the S band, at a frequency of 3 GHz / 10 centimeters, sending out horizontally-polarized radio pulses with a PRF of 860 to 1,300 hertz. It is sensitive enough to pick up a single bee from a range of 29 kilometers (18 miles), and measurements can be thrown off by swarms of insects or flocks of birds. Nexrad developers are working on an improved weather radar that uses both horizontally and vertically polarized pulses to obtain more weather data, and are considering phased array radars for the next generation that do away with the rotating dish. * Airliner weather radars are a well-established item of technology and continue to be refined. The new Airbus A380 super-jumbo airliner uses the Honeywell RDR-4000 weather radar. The RDR-4000 can map a three-dimensional "volumetric" block of sky to obtain rain and turbulence data up to 600 kilometers (320 NMI) ahead of the aircraft, from the ground to 18,300 meters (60,000 feet). This data is stored in a database, to be accessed and processed as needed. The RDR-4000 also provides terrain-mapping capabilities and stores a digital terrain database. 3D weather profiles can be displayed relative to the ground track, the flight plan route, or the current position and bearing. Both a 2D top and side view can be displayed simultaneously if that is preferred. In addition, the highly automated radar provides automatic warning of "wind shear" conditions ahead. The RDR-4000 can be fitted with an antenna dish 30 or 45 centimeters (12 and 18 inches) in diameter, smaller than the antennas of previous airliner weather radars, and is also much more compact and lightweight than its predecessors, allowing it to be used in a wide range of aircraft. The US Air Force has obtained the RDR-4000 for use in the Boeing C-17 cargolifter, with this variant of the radar also providing rendezvous tracking for mid-air refueling.