Protocols for Store-and-Forward Message Switching via Microsatellites

The authors have developed a suite of protocols specifically optimized for use on store-and-forward microsatellite communications missions. The protocols support networks in which user terminals directly access the store-and-forward satellite to transfer electronic mail. The authors' PACSAT Protocol Suite includes a message format standard, a virtual-circuit directory and file transfer service, and a datagram based point-to-multipoint "broadcast" protocol. An implementation of this suite has been operating on the UoSAT-3 PACSAT Communications Experiment (PCE) for twelve months and on the AMSAT Microsats for seven months. During this period, hundreds of small and medium-sized ground terminals in all parts of the world have accessed the satellites. On-board software monitors network activity closely, and usage statistics for UoSAT-3 are gathered regularly at the UoSAT Command Station. Based on this data and the reported experiences of regular system users, we compare the effectiveness of the various protocols. From these comparisons we make some recommendations which are generally applicable to store-and-forward microsatellite missions. This paper describes the design of the PACSAT Protocol Suite and the UoSAT-3 and Microsat implementations. It summarizes the in-orbit performance results, and concludes with recommendations for future store-and-forward microsatellite missions.