In a radundant switchover system for a TDMA satellite communication system for relaying messages from earth station to earth station via a communication satellite, each earth station comprises an initially active TDMA terminal unit (1) and an initially stand-by TDMA terminal unit (2). Determination of loss of transmit burst synchronization in the initially stand-by terminal unit is carried out more than one round trip time period later than the determination of loss of transmit burst synchronization in the initially active terminal unit. Switching of the initially active and stand-by terminal units to the stand-by mode and the active mode, respectively, is effected when loss of transmit burst synchronization is determined in the initially active terminal unit.