that far exceeded the traditional, 'range and direction". The secondary radar was a dual military and civil system that worked by interrogating a transponder on board the aircraft, receiving and translating a coded reply. Decoding SSR signals was carried out at L1 with the decoders supplied by Elliott Automation. Height finding There were three height finding facilities within the Linesman system. One was built into the Type 85 radar, the second was a stand-alone system using the HF200 radar and the third was based on the SSR interrogating the aircraft's systems. The Type 85 working with the Auto Height Computer Type 12493 enabled an operator to select a target and, by triangulation in the radar's 12 beams, obtain a height. The HF200 provided another means of finding the height of a target. This radar, rather than continuously rotating, nodded on a bearing selected by the operator. The bearing derived from the main primary radar would enable a reading of the height to be sent back to the operator. The secondary radar system SSR750 as part of the joint military civil functionality could interrogate a transponder on board as aircraft for the height. This height, automatically derived from the onboard altimeter, was in the late 1960s and early 1970s shown to the operator on a box mounted above his screen. Later, with the advent of plot extraction it could be displayed alongside the aircraft on the traditional Plan position indicator (PPI) display. Passive detection A third system was later added to the network, the RX12874 "Winkle" passive jamming-detection system or PD system. PD consisted of a series of high-speed rotating antennas mounted on R15 buildings separated by many miles, combined with similar signals captured from a Type 85 radar. The three Type 85 radars located at Neatishead, Staxton Wold, and Boulmer used the associated high speed PD aerials to make a PD baseline. For this the two aerials (T85 and PD) 100 miles apart had to be in sync to ensure they swept the same area of sky at the same time, sync and turning information was sent over microwave link. For example, the Type 85 at Neatishead used the signals captured from the high speed aerial at Staxton Wold to detect jamming aircraft. Each Type 85 with its associated high speed aerial was able to provide a PD baseline of several hundred miles to the north and south. There was