Short answers, pulled from the story.
Christian Hülsmeyer invented the first radar device in 1904. He called this device a telemobiloscope and demonstrated its ability to detect ships in dense fog. The German military rejected his invention as impractical despite his patent obtained in April 1904.
The Daventry Experiment proved radio waves could detect aircraft on the 26th of February 1935. This test was conducted by a team led by Robert Watson-Watt and Arnold Wilkins. The successful detection led to the immediate funding of the Chain Home system by Hugh Dowding.
The cavity magnetron allowed radar systems to become small enough for aircraft in 1940. This vacuum tube generated microwaves and enabled the creation of compact radar sets for night fighters and maritime patrol aircraft. Britain shared this technology with the United States during the Tizard Mission in 1940.
Multiple countries including the United States, France, Germany, Italy, Japan, the Soviet Union, and the Netherlands developed radar systems before World War II. France installed an obstacle-locating radio apparatus on the ocean liner Normandie in 1935. The Soviet Union produced its first mass production radars, the RUS-1 and RUS-2 Redut, in 1939.
Radar transitioned from military to civilian use in 1938 with the installation of a Bell Lab unit on United Air Lines aircraft. This was the first commercial device fitted to aircraft. Modern applications now include air traffic control, weather forecasting, and the guidance of self-driving cars.
Radar systems measure the speed of a target using the Doppler effect. This phenomenon changes the frequency of radio waves when the target moves toward or away from the transmitter. Modern systems use pulse-Doppler signal processing to separate clutter from desirable signals and detect fast-moving vehicles.