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Angle discrimination based pointing and tracking for free-space communication links

Laser based free-space-optical communication (FSOC) links are known to provide covert, secure, jam-proof and very high bandwidth performances. For mobile platforms, precision pointing and tracking schemes are critical for continuous guiding of a modulated laser beam to establish data link maintenance. In this paper, preliminary experiments of an angle-discrimination based smart pointing and tracking scheme suitable for high-speed, closed-loop, FSOC is discussed. A dual-axis, high-speed, galvo-mirror based scanner was utilized for conical scanning at 550 Hz. Greenwood frequency in the presence of moderate atmospheric turbulence over a range of 1 km at 1.5 μm was measured. It is shown that selection of a scan frequency much higher than the Greenwood frequency reduces scintillation effects on scan angle measurements for track loop maintenance. The measured scan angle value of the receiver with respect to transmit beam when fed back to the scanner through an optical transponder would allow pointing error estimation and correction. Based on our initial phenomenology study, it is shown that the scan-angle modulation based pointing and tracking scheme would provide data-link reliability for dynamic platforms traveling on rough terrains.