Radar was a detection system based on radio waves.
The Guerner species never developed visual transmission technology, but had advanced radar systems to compensate. They were, according to the Fourth Doctor, capable of "finding a beetle in a haystack". (PROSE: The Sleeping Beast [+]Doctor Who Annual 1978 (Doctor Who annual, 1977).)
By 1903, Nikola Tesla had invented a prototype of radar. He called it an exploring ray. (TV: Nikola Tesla's Night of Terror [+]Nina Métivier, Doctor Who series 12 (BBC One, 2020).)
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 [+]Lance Parkin, Virgin New Adventures (Virgin Books, 1996).)
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 [+]Russell T Davies, Doctor Who series 1 (BBC One, 2005).)
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 [+]Russell T Davies, Doctor Who series 2 (BBC One, 2006).)
UNIT operated a radar station from Perth that was capable of detecting spacecraft. (TV: The End of Time [+]Russell T Davies, Doctor Who Christmas Special 2009 and New Year Special 2010 (BBC One, 2009-2010).)
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 [+]Rupert Laight, The Sarah Jane Adventures series 4 (CBBC, 2010).)
When Alex Bannen caused waves of reality-altering energy to emanate throughout Project Eden's base, Satvindha Mudan's face changed into a bony dome that picked up radar signals. (PROSE: Lucifer Rising [+]Jim Mortimore and Andy Lane, Virgin New Adventures (Virgin Books, 1993).)