There is a growing interest on small satellites for Earth observation and communication services. Nevertheless, these low cost systems have shown a certain number of limitations, they may be overcome through the use of available technology and on-board data handling capabilities. One of the most important limitations is the attitude and orbit determination and control system (AODCS). PoSAT-1, launched in September 1993, has a passive ADCS system, with magnetometers for attitude determination (sun sensors and earth horizon sensors are implemented only for redundancy), a 4.5 m gravity gradient boom and magnetorquers for attitude control. The satellite also carries a star imaging system (SIS) and a GPS receiver for more accurate and standalone attitude and position determination. Both experimental subsystems are working according to specifications. The authors present their current work on an integrated model for AODCS, to be implemented on PoSAT-1, which is based on the SIS the GPS and the parallel nature of on-board transputer processing, and discuss it as a powerful and efficient navigation unit for small and low earth orbiting satellites.