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Communications via optical cross-link between two satellites require reciprocal spatial beam tracking. For intersatellite links (ISL) operating at narrow beamwidths, the line-of-sight pointing error at both satellite stations will have a significant impact on the system communication performance. Large pointing error on either end of the system can combine to impose higher power penalty at the transmitter, and can severely degrade the receiver performance. Tradeoffs in the required optical power and the size of the receiver aperture can be made while keeping the communication bit error rate constant. An approach to the selection of the optimal system parameters is presented. Given the tracking error statistics at each end of the link, and the desired probability of bit error rate (PBE), the combination of optical apertures are found that minimizes the required laser power. A tight error upper bound for direct-detection PPM system were developed for the bit error rate calculation. The spatial angle tracking error statistics modeled for each station are identical Rician distribution with static pointing errors. The product of optical apertures and the RMS tracking error, and the ratio of the required laser power and the RMS tracking error, are found to be constants, which are independent of the actual RMS tracking error for fixed PBE and static pointing error. Practical ISL parameters are used in the selection of the pair of optical apertures. It will be shown that in order to meet the size and weight constraint of the optical package and the requirement of the transmitting laser, suboptimal designs may be necessary for ISL applications. Sets of curves illustrating the impact of tracking error on the design parameters and the associated laser power penalty for different aperture gains and power requirement will be presented.
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