Multiple access to the radio channel is a very important aspect that determines the efficiency of wireless networks. In this paper, we propose a packet multiple access protocol for uplink satellite channels characterized by their long propagation delay. The Generalized Retransmission Announcement Protocol, GRAP, relies on a framed channel structure, allowing users' access by either direct contention or reservation. This hybrid access scheme is proven to be more efficient in following the dynamic load variations of the satellite system. The different features and operation of GRAP are explained in this paper. An analytical model is developed to calculate the numerical channel throughput achieved by GRAP for different traffic loads.