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In this paper, we investigate the uplink transmission in a satellite-aerial-terrestrial network (SATN), where an aerial platform acts as an amplify-and-forward relay assisting the communication between multiple users and satellite. The users send messages to the aerial relay via radio frequency (RF) links, which are then forwarded to the satellite through a free-space optical (FSO) link. By assuming that the angle-of-arrival based imperfect channel state information of each user is known at the aerial platform, we propose a beamforming scheme to maximize the minimum average signal-to-interference-plus-noise ratio of the users. Due to the mathematical intractability, we design an iterative algorithm to obtain the optimal beamforming vector for the RF link. Furthermore, by considering that the FSO link experiences the M&#x00E1;laga fading with non-zero boresight pointing error and the RF links follow Nakagami-<inline-formula><tex-math notation="LaTeX">$m$</tex-math></inline-formula> fading, we derive an analytical expression for the outage probability of the considered SATN. Finally, computer simulation is conducted to validate our theoretical analysis. It is shown that the proposed algorithm can improve the system performance and robustness compared to existing works.