Lockheed Martin has begun flight testing a new sensor for the F-35's targeting system, developed by the company's missiles and fire-control division. The test aircraft is a highly modified Boeing 737 operated by BAE Systems and is known as the CATBird (cooperative avionics testbed). The F-35's electro-optical targeting system (EOTS) is being flight tested as the final step before integration with the passive targeting system on test aircraft BF-4, the STOVL variant of the F-35 dedicated to mission systems testing. The EOTS is mounted under the F-35's cockpit inside a low-observable turret, and its purpose is to lock onto targets visually when the fighter's radar is unavailable. The flight testing is being conducted using Block 1.0 software, allowing the targeting system to align its sensors using aircraft navigation data. Previous flight testing on a Sabreliner test aircraft was accomplished without the Block 1.0 software, which meant navigation data was not available to the targeting system. Rich Hinkle, EOTS program director for the F-35, said, "The CATBird's dynamic flight environment provides the first opportunity to test and evaluate how EOTS integrates into the F-35's fused sensor architecture."