(covers information from several alternate timelines)
RADAR was an acronym for 'RAdio Detection And Ranging' or 'Remote Aerial Detection And Ranging'. It was a sensing technique used on Earth in the 20th and 21st centuries. By projecting radio waves and analyzing any reflected waves, RADAR devices could attempt to identify the heading, speed, size, altitude, distance, and even shape of the object which reflected them.
In 2267, the USS Enterprise became ensnared in the gravitational field of a black star while en route to Starbase 9 for resupply. By using the warp engines in full reverse, the ship managed to escape but was subsequently flung back in time to 1969 Earth, where it was detected by Technician Webb who worked at a United States Air Force RADAR installation. (TOS: "Tomorrow is Yesterday")
Following a brief battle with the Federation timeship Aeon in 2373, the USS Voyager was pulled into a temporal rift whereby it was transported to Earth in the year 1996. Upon realizing this, Captain Kathryn Janeway instructed Lieutenant Tom Paris to maintain a high orbit and modulate the shields to scatter any RADAR beams emanating from the surface or orbital satellites.
Lieutenant B'Elanna Torres later configured the shields of a Class 2 shuttle to disguise its visual profile from RADAR using an interferometric dispersion when she and Commander Chakotay conducted a low altitude flight in an attempt to beam up Henry Starling. To any observer, the shuttle appeared to be a small 20th century aircraft. (VOY: "Future's End", "Future's End, Part II")
In 2374, in an early version of the alternate timeline known as the Year of Hell, Commander Chakotay noted at the dedication of Voyager's new astrometrics lab "before there were maps and globes, let alone RADAR and subspace sensors, mariners navigated by the stars." (VOY: "Year of Hell")
- SONAR, a similar technique that utilized sound waves projected underwater
- LIDAR, a similar technique that utilized light waves projected through space