Store-and-Forward Message Relay using Microsatellites: The UoSAT-3 PACSAT Communications Payload

One of the most promising applications for small satellites in the 10-50 kg class is store-and-forward message relay. A single store-and-forward message relay satellite in a polar orbit can provide a global communications network carrying electronic mail, digitized voice, images or computer data. With appropriate choice of link characteristics, small, low cost ground terminals can be used. When designing an inexpensive microsatellite system to provide store-and-forward communications to small ground terminals, the engineer must challenge the standard assumptions made concerning such things as link frequency, modulation techniques, error-control, and multiple-access arbitration. Beginning with experiments on its UoSAT-2 satellite, the Spacecraft Engineering Research Unit at the University of Surrey (UK) - in collaboration with AMSAT and VITA - has been investigating store-and-forward communications using microsatellites. The UoSAT-2 store-and-forward transponder used a relatively slow 8-bit CPU with only 96 kbytes of message store, but it has been used by stations world-wide, demonstrating system feasibility. The latest experiments undertaken by Surrey will qualify a commercial-capacity microsatellite store-and-forward system. The onboard transponder is based on a 8-MHz, 16-bit, 80C186 CPU, multitasking operating software and 4 Mbytes of RAM message store. The UoSAT/SST modular microsatellite bus provides 9600 baud FSK communications links and other support facilities for the store and forward mission. This payload was launched on the UoSAT-3 satellite in January 1990, and is now successfully operating in orbit.