An analysis of optical scintillation and fade on long slant-path atmospheric channels is presented via a direct comparison between wave-optics-based numerical simulations and experimental flight data from a ground-to-aircraft optical communication link. In addition to physically modeling the propagation through slant-path atmospheric turbulence, the numerical simulations include simultaneously the effects of mechanical pointing jitter, aperture-averaging, and first-order scattering/absorption models. The power spectral density, fade probability, and mean fade time of the simulated power fluctuations are studied and validated against measurements taken at slant-path distances ranging from 60 to 113 km and aircraft speeds up to 70 m / s.
As pointing system performances continue to increase while gimbal and vehicle weights decrease, structural
considerations of the gimbal-base interface become more problematic and important. We begin by presenting
the generalized structural transfer function equation in modal superposition form. We emphasize that the
reaction torque of an actuator must be included in practice. From this theoretical basis, we assume stiff gimbal
structures and evaluate the structural coupling due only to mount flexure. We show that in some inertially
stabilized precision pointing systems the effect of the actuator reaction torques cannot be ignored. We show
that structural stiffness is less important than properties of the mount symmetry. In particular, asymmetries in
the system mount (the structure between the reaction torque and the base) allow dangerous coupling between
gimbal axes. The observed coupling may depend on the angles of the gimbals, making experimental testing and
validation more challenging.