RADAR short for Radio Detection and Ranging, Sir Robert Watson-Watt was the first person to develop a practical radar system and was granted a patent for Air Defence in April 1935.
In the 1880s, German physicist Heinrich Hertz first produced and transmitted radio waves, which were a form of electromagnetic radiation. He noticed that some materials reflected the waves and some absorbed it.
In 1901, Italian physicist G. Marconi sent first radio waves across the Atlantic Ocean.
In 1904, German engineer Christian Huelsmeyer invented a crude system that used radio waves to prevent boats and trains from colliding on foggy days.
In February 1935, Watson-Watt demonstrated to an Air ministry committee the first practical radio system for detecting aircraft.
In 1940, British engineers Harry Boot and John Randall invented the cavity magnetron; the magnetron was able to generate 400 watts of power at wavelength about 10cm. This device was able to be used in night and was compact enough to install in the airplanes. The magnetron could detect airplane echoes from many miles away.
The radar system has a transmitter that emits radio waves that are called radio signals. These signals when coming into contact with a surface which has considerable electrical conductivity are reflected back, these reflected signals are then detected by the transmitter.
Aircraft surveillance is done with the help of primary and secondary radars. Secondary radar cannot detect an aircraft if the transponder is in off mode. But a primary radar detects the aircraft because it is independent of the transponder. “The two waves simultaneously received from clouds and the aircraft can clutter the signal. But through technology, the signals from the clouds can be suppressed by the ATC and only the image of the aircraft can then be seen,” [Source: The Telegraph].