Influence of aero-optical disturbances on acquisition, tracking, and pointing performance characteristics in laser systems

Abstract

We have modeled the imaging performance of an acquisition, tracking, and pointing (ATP) 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 (AAOL) 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 0.4-0.8. Using these data, we quantified the imaging point spread function (PSF) for each aircraft speed. Our simulation results show Strehl ratios between 0.1-0.8 with substantial scattering of energy out to 7.5× the diffraction-limited core. Analysis of the imaging modulation transfer function (MTF) shows a rapid reduction of contrast for low-to-mid range spatial frequencies with increasing Mach number. Low modulation contrast at higher spatial frequencies limits imaging resolution to > 2× diffraction-limit at Mach 0.5 and approximately 5× diffraction-limit at Mach 0.8. Practical limits to usable spatial frequencies require higher image signal-to-noise ratio (SNR) in the presence of aero-optical disturbances at high Mach number. Propagation of an illuminator laser through these aero-optical aberrations produces intensity modulation in the incident target illumination on scale sizes near the diffraction-limit of the transmitting laser aperture, thereby producing illumination artifacts which can degrade image-contrast-based tracking algorithms.