Store and Forward Communication Payload Design for LEO Satellite Systems

This paper presents the design and implementation of store and forward (S&F) communication payload for Sina- 1satellite. The S&F subsystem uses VHF and UHF radio links for receiving and transmitting messages. The main purpose of the system is to provide the handling of transmitting/receiving of packet type messages via satellite. A message from one ground terminal is stored in satellite on-board memory and delivered as the satellite passes over the destination ground terminal. The two ground terminals do not have to see the satellite at the same time, and a single satellite can provide global communications coverage. The main specifications of store and forward subsystem are calculated and presented. Data transmission on downlink is performed in time division multiplex (TDM) mode. Multiple access to receiving channel of on-board transceiver (uplink) is performed according to split channel reservation multiple access (SRMA) protocol. The effect of different parameters such as the number of users, packet length and visibility time is analyzed. Phase shift keying (PSK) modulation is used for downlink and uplink transmissions. In order to enhance the performance of satellite communications link, error correction technique is used in this payload. Link calculation reveals that a packet service of 4.8 kbps can be provided without satellite tracking mechanisms. Finally the structure of the on-board S&F transceiver is presented.