A close-form expression for the bit error rate of inter-HAPs optical communication system under Γ turbulence channel was deduced and the effect of platforms instability on system performance was analyzed.In order to reduce the BER,the acquisition,pointing and tracking(APT) system was adopted in the system.The design requirement of APT system under the condition of platforms instability was also analyzed.The results indicate that the roll of the transmitter and the vertical movement have signific...