The SS Explorer’s type 21 Decca Navigator. In the days before satellite navigation, Decca ruled the roost for short range navigation. Land based transmitters, a master and three slaves would form a “chain” located around the area. These would transmit on discrete frequencies from 70 to 130 kHz. Special charts with a Decca lattice overlay would then be used to transfer readings from the Decca navigator to obtain the vessel or aircraft’s position. Also a plotter was fitted but it is currently unclear how this was used.