A receiver for the Decca navigation system. The set has three dials on front, each marked from 0-1 with divisions of 0.1, subdivided to 0.01, with an additional digital readout in the centre of each. A panel beneath the dials swings down to reveal further controls, including chain selection. The Mark 30 was a semi-microprocessor controlled receiver, which was introduced in 1980 in response to recent developments in electronics.