From the early colidar systems all laser rangefinders, laser altimeters and lidar units are derived. Stadiametric range-finders were successively replaced by coincidence and laser rangefinders. Shakey combined multiple sensor inputs, including TV cameras, laser rangefinders, and "bump sensors" to navigate. Special laser rangefinders are used in forestry. Four new rangefinders and eight searchlights were installed. The order also included rangefinders and searchlights. Other improvements included a new engine, ballistic computers, and laser rangefinders. Turrets 2 and 3 had optical rangefinders and ballistics computers. APS-C size sensors are also used in a few digital rangefinders. There are two basic types of optical rangefinders, stereoscopic and coincidence. They are used in various measuring instruments, such as rangefinders. They were replaced by either the British rangefinders or domestically built instruments of length. The secondary armament was controlled by four directors equipped with rangefinders. Four KDP-4t-II directors, with two rangefinders each, controlled the secondary armament. They used range data received from three coincidence rangefinders.