Recent AAOL In-Flight Wavefront Measurements of Aero-Optics and Implications for Aero-Optics Beam Control in Tactical Laser Weapon Systems

Aero-optics disturbances were measured inight with the Airborne Aero-Optics Laboratory (AAOL) 1 ft (4 in. clear aperture) at-windowed turret at an altitude of 15 kft altitude and at a Mach number of 0.5 for 47 az., 42 el. (forward-looking) and 139 az., 70 el. (rearlooking) turret angles. Wavefronts were collected using a FasCam CCD array camera with an AMO lenslet array in a Shack-Hartman con guration at a 20 kHz frame-rate. 2,000 consecutive wavefronts of each of the two turret pointing angles measured were scaled to a tactical high energy laser (HEL) laser weapon scenario and their wavefront error statistics analyzed. The two wavefront sequences were decomposed using proper orthogonal decomposition (POD) modes and their spatial frequency content evaluated. From this, the minimum number of deformable mirror (DM) actuators needed to compensate these disturbances was estimated to be approximately 18 actuators across the aperture diameter. Finally, a the performance of a simple integrator-type adaptive optics (AO) control system against the two scaled wavefront sequences was investigated. It was found that the forward-looking aero-optics aberrations were about three-times less in magnitude than the rear-looking aero-optics aberrations. Consequently, the open-loop Strehl ratio of the forward-looking case was considerably greater than the open-loop Strehl ratio of the rear-looking case. Nevertheless, in for both the forwardlooking and rear-looking cases, in order to achieve compensation better than open-loop, it was found necessary to have AO sample rates greater than 10 kHz with one frame of latency. These results suggest that signi cant AO compensation improvements will be achieved mainly by reducing the latency as much as possible, perhaps with adaptive/predictive AO controllers.