Imaging performance with turret aero-optical wavefront disturbances

Abstract

We modeled the imaging performance of an acquisition, tracking, and pointing sensor when operating on a high-speed aircraft platform through a turreted laser beam director/telescope. We applied standard scaling relations to wavefront sensor (WFS) data collected from the Airborne Aero-Optics Laboratory test platform operating at Mach 0.5 to model aero-optical aberrations for a λ=1 μm wavelength laser system with a D ap =30 cm aperture diameter and a 90-cm turret diameter on a platform operating at 30 kft and for speeds of Mach 0.4 to 0.8. Using these data, we quantified the imaging point spread function for each aircraft speed. Our simulation results show Strehl ratios from 0.1 to 0.8 with substantial scattering of energy out to 7.5× the diffraction-limited core. Analysis of the imaging MTF shows a rapid reduction of contrast for low-to-mid range spatial frequencies with an increasing Mach number. Low modulation contrast at higher spatial frequencies limits imaging resolution to >2× diffraction-limit at Mach 0.5 and 5× diffraction-limit at Mach 0.8. Practical limits to usable spatial frequencies require higher image signal to noise ratio in the presence of aero-optical disturbances at a high Mach number. Illuminator laser propagation through aero-optical aberrations produces target illumination modulation at scale sizes near the diffraction-limit of the transmitting laser aperture, thereby producing illumination artifacts that can degrade image-contrast-based tracking algorithms.