Free Space Optical (FSO) communication has been
recognized as a high-speed bridging technology to current
fiber optic networks [1], and a valuable technology in
commercial and military backbone networks [2]. However
its bright prospects depend on the performance of pointing,
acquisition, and tracking (PAT) technique, and autonomous
reconfiguration algorithms dealing with the effects of node
mobility and atmospheric obscuration.
Precise laser beam pointing requiring microradian to
milliradian accuracy is a challenging problem unless both
nodes are close to each other. In this case manual alignment
is straightforward, and can be guided by the use of optical
beacons, or image based pointing. However, if the link
distance is more than a few kilometers, then these
techniques become increasingly difficult to implement.
Instead, we need complete information as to where nodes
are (their position coordinates) as well as FSO transceiver
1-4244-1513-06/07/$25.00 C2007 IEEE
angular pointing coordinates (pointing vectors). The use of
various kinds of position and angular sensor devices is
therefore natural in pointing techniques, as described
previously [3]-[7].
References [3]-[4] have described a coarse pointing system
using a 180° field-of-view (FOV) fisheye camera and a 30°
FOV regular camera. The omnidirectional fisheye camera
first acquires the target (fixed or mobile node) of interest by
image extraction. Next, the 300 FOV camera is rotated
toward the target of interest based on an homographical
computation. The rotation angles are used to generate a
radial trifocal tensor, which is applied to estimate the
movement of the FSO transceiver on the target (tracking).
In this work a pointing error around 0.2° was reported.