Radar is an object-detecting device which uses a pulse of radio energy to identify the range, direction, altitude, direction, and speed of both moving and fixed objects such as aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish, or antenna, transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter.
Radar is based on the principle of sending very long wavelength radiation (called microwaves) from an antenna, and then detecting that energy after it bounces off a remote target. The wavelength of the microwave, its polarization (vertical or horizontal orientation) and strength can be controlled at the source and measured when it returns. Many common land-cover types and materials affect the polarity and strength of the radar return differently, which helps in their identification.
The term RADAR was coined in 1940 by the US Navy as an acronym for RAdio Detection And Ranging. The term radar has since entered the English and other languages as the common noun, radar, losing all of the capitalization. In the United Kingdom, this technology was initially called RDF (Range and Direction Finding), using the same acronym as the one for Radio Direction Finding to conceal its ranging capability.