The POPI (Post Office Position Indicator) of 1946 was another phase-comparison low-frequency (40 kHz) system that had some features of Consol and of Decca. It was to use aerials situated on a short baseline radiating the same frequency but not simultaneously, alternating between the two aerials at five times a second. Because of the short baseline, the resultant hyperbolae degenerated into radials very rapidly. A third aerial equispaced from the other two served to resolve ambiguities somewhat, and there was a novel method of actually measuring the phase differences, using a fourth transmitter in the middle of the other three. The system was never actually put into use, but the work done on it was later acquired by Decca, after an American system, Navarho, used some of its ideas.
The Journal Of Navigation - Chapter 4 ; W.F. Blanchard, Royal Institute of Navigation; Vol. 44, No. 3; Sept. 1991. Used with permission.