Flight Measurements of Aero-Optical Distortions from a Flat-Windowed Turret on the Airborne Aero-Optics Laboratory (AAOL)

Abstract

The optical environment of a flat-window hemisphere-on-cylinder turret during flight tests in Notre Dame's Airborne Aero-Optics Laboratory was evaluated. Aero-optical aberrations around the turret were measured using a high-speed (up to 20 kHz) Shack-Hartmann wavefront sensor providing the most complete aero-optical mapping to date. The primary data was acquired at Mach 0.5 at an altitude of 15,000 ft, with a subset of the data collected at Mach 0.4 for verification of scaling relationships. During each flight, data were acquired holding the elevation/azimuthal angle of the aperture constant. Additional data sets were acquired allowing the two planes to slew by, providing statistical data over a large range of aperture angles. The flight test data were also compared to wind tunnel measurements using the identical turret. Finally, using the spatial statistics of the flight data, the use of the Large Aperture Approximation (LAA) to estimate the resulting timeaveraged far-field Strehl ratio is revisited.

Statistical analysis of Airborne Aero-Optical Laboratory optical wavefront measurements

“…Porter et al 8 showed that, for a three dimensional (3-D) turret, the same phenomenon depended on both elevation and azimuth, even for the same LOS. However, Porter's analysis also suggested that the fluid dynamics should not be terribly sensitive at these particular combinations of azimuth and elevation, which are viewing fully separated flow, but in the region where higher-order OPD is declining due to aperture effects.…”

Receding-horizon adaptive control of aero-optical wavefronts

“…18 The original aero-optical wavefronts were produced by turbulence over a flat-windowed turret 18,[27][28][29] during a flight test in which a continuous-wave laser was transmitted between two planes flying in a constant formation at an altitude of 4570 m. The planes were separated by approximately 50 m to ensure aero-optical turbulence was the primary source of wavefront aberrations. Other significant parameters for the flight test are give in Table 1.…”

“…A detailed analysis of the statistics of the AAOL data has been published. 18 Since the control loops in this paper attempt to minimize the total phase error that would be encountered in similar airborne environments, no attempts were made to distinguish between the sources of the measured turbulence.…”

“…Results from an AO experiment are used to compare the performance of the new adaptive controller to the performance of the minimum-variance adaptive controller previously used for AO. 4,5,17 In the experiment, aero-optical wavefronts measured in the University of Notre Dame's Airborne Aero-Optics Laboratory (AAOL) 18 were mapped to the geometry of the University of California Los Angeles (UCLA) AO experiment and added to the laser beam to provide wavefront disturbance.…”

Predictive dynamic digital holography

“…In Chapter 2, the simulation model and theory for digital holography recording and reconstruction is presented. Chapter 3 discusses the aero-optical wavefront data [2,3] used in this dissertation, while Chapter 4 explains the image sharpness algorithm and the model representation of the atmospheric wavefronts.…”