sentences:  However, as a result of an order from the Ministry of Defense to add an armoured roof (the initial vehicles were still open-topped), series production could only begin in 1961.By then, the configuration was already out of date and in the second half of the 1960s, the VDV became the main operator of the SU-85 and renamed it the ASU-85.The ASU-85 is based on the PT-76 tank chassis, but without the amphibious capabilities and fitted with a new engine.