An improved time division multiple access (TDMA) satellite transmission system uses N TDMA stations and employs a signal format in which successive time frames begin with a header containing synchronizing and control signals generated by a selected TDMA and followed by N time slots, and in which each TDMA station generates and transmits data packets having a high energy level during a unique time slot assigned to each TDMA and in which each TDMA transmits data packets only during its assigned time slot, and further in which at least some of the TDMA's do not transmit data packets in the available time during the assigned time slot, thereby leaving unpredictably occurring idle time. Apparatus is provided for using a substantial portion of the idle time. The apparatus includes a plurality of random access terminals (RATS) each constructed to generate and transmit a data packet, having a lower energy level then the TDMA energy level, to any given single time slot during each frame and logic apparatus is provided for selecting such given single time slot on a predetermined statistical basis. Each RAT is capable of receiving all successfully transmitted data packets and determines the successful or non-successful transmission of any data packet originating therefrom. Data is retransmitted by the RAT's during a subsequent frame when the originally scheduled RAT transmission is not successfully completed.