The first model of the Leopard to enter service with the Federal German Army was called the Leopard 1 of which 1,845 were built in the first, second, third and fourth batches. From 1971 they were fitted with a thermal sleeve forthe main armament, gun stabilisation system, new tracks and lateral skirts. The stabilisation system controls the gun in elevation and traverse so that the gunner is able to observe the terrain, acquire and then engage the target while moving across country, with a high degree of first round hit probability. With these modifications the Leopard 1 became the Leopard 1A1. These tanks have since been fitted with Blohm+Voss add-on armour on the turret and gun shield and are now known as the Leopard 1A1A1. The additional add-on armour consists of flexibly mounted, screwed-on armour steel plates with two-faced rubber lining. The armour also covers the turret bustle back and the gun shield is reinforced with armour steel plates. In addition, armour steel plates are welded on to the sloped front roof section. The extra weight caused by the fitting of this additional armour is about 760 kg, which makes the vehicle approximately the same weight as the Leopard 1A4. Another improvement on the Leopard 1A1A1 is the significantly enhanced combustion air intake filters. Leopard 1A1A1s with the PZB 200 LLLTV system are designated the Leopard 1A1A2.