Radar was a detection system based on radio waves.
On 26 March 1935, the first ever radar test took place at Daventry. According to the Seventh Doctor, the British did not yet have access to radar at that point and this day simply marked "the first time that a radar echo was ever detected." (PROSE: Just War)
A signal transmitted by the Game Station masked the Dalek Fleet from sonar, radar and scanners. After he was taken to the Game Station in 200100, the Ninth Doctor cancelled this signal, revealing who was manipulating mankind. (TV: Bad Wolf)
Torchwood Tower was built to reach the height of the radar black spot later determined by the Institute to constitute an anomaly in space and time, in reality a breach into the Void. (TV: Army of Ghosts)
On 7 June 1941, a group of Nazis planning to destroy Britain's radar defences landed on the beach near Little Malding and took over St Michael's Church. The Nazis' plans were ultimately foiled by a local boy named George Woods, with the help of a time-travelling Clyde Langer. (TV: Lost in Time)