The original ROTOR was replaced by Linesman in stages, starting in 1967. A similar expedient system in the United States was the Lashup Radar Network. Post-war situation UK radar operations were wound down late in the war, and by the time the war ended were already largely unused. It was assumed that another war was at least ten years away, and the need for any improvements in the cobbled-together system seemed remote. Thinking changed dramatically in 1949 with the Soviet test of their first atom bomb. It was known that the Soviets had made exact copies of the B-29 Superfortress as the Tupolev Tu-4, and these aircraft had the performance needed to reach the UK with a nuclear payload. Studying the problem, the 1949 Cherry Report suggested that the 170 existing Royal Air Force radar stations be reduced to 66 sites and the electronics extensively upgraded. Most of the new network would be made up of 28 rebuilt Chain Home systems, while the rest were taken from the existing selection of Chain Home Low, Chain Home Extra Low and the various Ground-controlled interception (GCI) radars that had formerly served special purposes. This was, in part, a stop-gap measure anticipating the availability of the dramatically improved radar known as the Microwave Early Warning, which was expected in the 1957 time-frame. Interception guidance would still be handled by existing systems in either case. All of the radars were to be improved in terms of siting, with the addition of hardened control bunkers to protect the operators from a conventional attack. On the east coast, where a Soviet attack would be most likely, the bunkers were underground in the 'R' series (R1, R2, R3 and R4 etc.), while those on the western side of the UK were generally semi-sunken hardened structures ('R6') or above ground 'Secco' type huts (Hartland Point etc.). The R-series bunkers themselves were otherwise similar, featuring concrete walls with all equipment, operations generators and air conditioning located inside. Additionally, ROTOR re-arranged the existing RAF Fighter Command structure into six "Sector Operational Commands" (SOC) with their own command bunkers (three level 'R4' protected accommodation). Only four of these were built. Additional "Anti-Aircraft Operations Rooms" were built to coordinate the British Army's AA defences in the same overall system. The entire network of bunkers, radars, fighter control and command centres used up 350,000 tons of concrete, 20,000 tons of steel and thousands