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Proceedings Paper

Fully optical real-time pointing, acquisition, and tracking system for free space optical link
Author(s): Tzung-Hsien Ho; Stuart D. Milner; Christopher C. Davis
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Paper Abstract

A free space optical (FSO) link requires precise pointing, acquisition and tracking (PAT) for reliable communication. Among the various tracking schemes, optical tracking with an imaging system offers great precision. For a point-to-point FSO link, the tracking problem has six degree of freedom (DOF), including position coordinates and orientation angles. By using imaging systems at both end of the link this can be reduced to a problem with three DOF. By converting from Cartesian to spherical coordinates, we can further reduce the problem to two DOFs. Thus, a single camera at each end of the link is sufficient. In this paper, we propose an outside-in, real-time tracking system, which is based on blob extraction and two-dimensional prediction. The system is inspired by the stereo vision algorithm from the computer vision community. It can be divided into two parts: (1) the tracker, and (2) the pointer. They are operated in a closed-loop, which stabilizes performance and accuracy. The tracker is used for extracting target information. Its accuracy has to be in the milliradian range, which provides a first constraint. To operate in real-time, an optical beacon is placed at each end of the link, which is imaged as a "blob". The size of this "blob" imposes a second constraint on the tracker. To work within these constraints, a 1.4Mpixel digital camera is used. The pointing system consists of two stepping motors with resolution of 0.125 mrads and slew speed up to 800,000 steps per second. The overall system covers a whole sphere in less then 1 second.

Paper Details

Date Published: 18 April 2005
PDF: 12 pages
Proc. SPIE 5712, Free-Space Laser Communication Technologies XVII, (18 April 2005); doi: 10.1117/12.590982
Show Author Affiliations
Tzung-Hsien Ho, Univ. of Maryland/College Park (United States)
Stuart D. Milner, Univ. of Maryland/College Park (United States)
Christopher C. Davis, Univ. of Maryland/College Park (United States)

Published in SPIE Proceedings Vol. 5712:
Free-Space Laser Communication Technologies XVII
G. Stephen Mecherle, Editor(s)

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