Abstract: RT86,a real time multi task operating system,is developed for the on board data handling system in accordance with ESA software engineering standards.Its hardware environment is 2 cool redundant Main Control Units(MCU)and several Remote Terminal Units(RTU)which are connected via a 1553B Serial Data Bus(SDB).The system performs telemetry,telecommand and various data management.This paper presents the main points in designing RT86 and the advantage of RT86 in the software organization,development,modification and maintenance.