Satellite systems will be employed to extend the terrestrial cellular networks and to provide a global coverage, for both mobile and fixed users. In these networks, the resource capacity is shared among handoffs and new calls. Forced termination has a significant impact on the user's quality of service. Several mechanisms such us guard channels or handoff queuing have been proposed in order to reduce handoff failure probability. The guard channel scheme consists in giving a prioritized access to the handoff calls. The penalty is the reduction of the total carried traffic. This paper proposes a dynamic reservation scheme for LEO satellite system, which adapts the number of reserved channel according to the current number of ongoing calls and on the localisation of users in the considered area.