attack jet aircraft at low altitudes. The vehicle cannot perform aimed fire on the move. Although the ZSU-57-2 had the highest firepower among production SPAAGs of its time, the anti-aircraft fire efficiency of a battery of four vehicles was even lower than that of a battery of six towed 57 mm S-60 anti-aircraft guns controlled by the PUAZO-6 anti-aircraft artillery director with the SON-9 fire control radar or later by the RPK-1 Vaza radar. It became obvious that the hit probability on a jet aircraft of the era was very low using only determination of target speed by aircraft type and determination of distance to the target by eye or by rangefinder; the ZSU-57-2 was designed to defend tank units against NATO attack aircraft flying at subsonic speeds, but it entered service ten years too late. Aircraft technology had improved to the point that a SPAAG required a much higher rate of fire, turret traverse speeds of 50–100° per second and a fully automatic radar-controlled fire control system. Works No. 174 started a modernization programme for the ZSU-57-2 in parallel with the beginning of its serial production in 1957, but this programme was rejected due to the development of new radar-guided SPAAGs armed with small-bore autocannons and another tracked chassis. The ZSU-57-2 still retained some of the features of its predecessor, the ZSU-37. One of them was the lack of an armoured roof on the turret. The advantages of an open turret for SPAAGs, such as very high elevation angle for AA autocannons, excellent visibility of the combat situation by the gunners and no need for induced ventilation of the fighting compartment during intense fire were significantly over-shadowed by the disadvantages. The open turret of the ZSU-57-2 made it vulnerable from above, and prevented operations under NBC conditions. This flaw was partially nullified in the modified Bosnian Serb ZSU-57-2s, which had improvised overhead armour protection. Nevertheless, its Western counterparts that were operationally available in the 1950s, such as the US M19 Multiple Gun Motor Carriage (based on the M24 Chaffee light tank) and the M42 Duster SPAAGs (both armed with the famous 40 mm Bofors M2A1 twin AA gun), had similar problems and were armed with less powerful weapons. The M42 Duster was tested with a T50 radar system in 1956 though it was a failure (production of the ZSU-57-2 had not started at this point). The Soviet Union would