One of the mysteries of the English language finally explained.
Designating a radar navigational system in which the position of aircraft is measured as a distance and bearing from a single ground station; relating to or involving such a system.
A position measured as a distance and bearing from a single ground station. Now rare.
1950s. From rho + theta, the names of the Greek letters ρ and θ, used symbolically in radar to denote distance and bearing respectively (probably in turn reflecting the use of the same letters in Mathematics to denote distance and bearing from the origin in polar coordinates).
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