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High precision acquisition, tracking, pointing (ATP) system is one of the key techniques of laser
communication. The spot-detecting camera is used to detect the direction of beacon in laser communication link, so that
it can get the position information of communication terminal for ATP system. The positioning accuracy of camera
decides the capability of laser communication system directly. So the spot-detecting camera in satellite-to-earth laser
communication ATP systems needs high precision on target detection. The positioning accuracy of cameras should be
better than ±1μ rad . The spot-detecting cameras usually adopt centroid algorithm to get the position information of
light spot on detectors. When the intensity of beacon is moderate, calculation results of centroid algorithm will be precise.
But the intensity of beacon changes greatly during communication for distance, atmospheric scintillation, weather etc.
The output signal of detector will be insufficient when the camera underexposes to beacon because of low light intensity.
On the other hand, the output signal of detector will be saturated when the camera overexposes to beacon because of
high light intensity. The calculation accuracy of centroid algorithm becomes worse if the spot-detecting camera
underexposes or overexposes, and then the positioning accuracy of camera will be reduced obviously. In order to
improve the accuracy, space-based cameras should regulate exposure time in real time according to light intensity. The
algorithm of adaptive-exposure technique for spot-detecting camera based on metal-oxide-semiconductor (CMOS)
detector is analyzed. According to analytic results, a CMOS camera in space-based laser communication system is
described, which utilizes the algorithm of adaptive-exposure to adapting exposure time. Test results from imaging
experiment system formed verify the design. Experimental results prove that this design can restrain the reduction of
positioning accuracy for the change of light intensity. So the camera can keep stable and high positioning accuracy
during communication.
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