Radar

Radar is a system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for radio detection and ranging. The term has since entered the English language as a standard word, radar, losing the capitalization. Radar was originally called RDF (Radio Direction Finder) in the United Kingdom.

A radar system has a transmitter that emits either microwaves or radio waves that are reflected by the target and detected by a receiver, typically in the same location as the transmitter. Although the signal returned is usually very weak, the signal can be amplified. This enables radar to detect objects at ranges where other emissions, such as sound or visible light, would be too weak to detect. Radar is used in many contexts, including meteorological detection of precipitation, measuring ocean surface waves, air traffic control, police detection of speeding traffic, and by the military.

Source: Wikipedia

E-3 Sentry

The Boeing E-3 Sentry is an American military airborne warning and control system (AWACS) aircraft that provides all-weather surveillance, command, control and communications, to the United States, United Kingdom, France, Saudi Arabia, and NATO air defense forces. Production ended in 1992 after 68 had been built.

The E-3 Sentry is a modified Boeing 707-320B Advanced commercial airframe. Modifications included a rotating radar dome, single-point ground, and air refueling points. The dome is 30 feet (9.1 m) in diameter, six feet (1.8 m) thick at the center, and is held 14 feet (4.2 m) above the fuselage by two struts.

The dome weighs approximately 1.5 tons and provides 1.5 tons of lift. It contains a hydraulically rotated antenna system that permits the AN/APY-1/2 passive electronically scanned array radar system to provide surveillance from the Earth's surface up into the stratosphere, over land or water. Generators on each of the four engines provide the one megawatt of power required by the radar. The Pulse Doppler radar has a range of more than 250 miles (400 km) for low-flying targets at its operating altitude (essentially to the radar horizon), and the Pulse(BTH) beyond the horizon radar has a range of approximately 400 miles (650 km) for aerospace vehicles flying at medium to high altitudes (essentially above the radar horizon). The radar combined with an SSR subsystem can look down to detect, identify and track enemy and friendly low-flying aircraft by eliminating ground clutter returns.

Source: Wikipedia