In this paper, we investigate the physical layer security of a multiuser satellite communication system in the presence of multiple eavesdroppers. Particularly, we propose a threshold-based scheduling scheme among the multiple legitimate users, where the geographically clustered eavesdroppers with both the colluded and collaborated eavesdropping scenarios are assumed. Specifically, the closed-form expression for the secrecy outage probability (SOP) is derived for the passive eavesdropping scenario when the channel state information (CSI) of the eavesdroppers is unavailable. In order to get insights of the proposed scheduling scheme at high signal-to-noise ratios (SNRs), the asymptotic analysis for the SOP is also obtained. Moreover, the reduced percentage with respect to number of user examination is also given, which validates the simplicity and efficiency of our proposed scheme compared to the traditional approaches. Numerical results suggest that with the proposed scheme, a comparable system performance with regard to the maximal selection (MS) scheme can be achieved.

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