Abstract
The recent large real time systems have the needs of growing systems including heterogeneous functions and operations. High assurance system is very important for such systems. In order to realize the high assurance system, we research the step-by-step system construction technique with assurance technology. In this paper we propose the functional reliability and the online functional availability as the best index to indicate the assurance property for step-by-step construction. According to the new index, the best solution for the step-by-step construction is the construction pattern to put the large functional units into online at first, and to put the small functional units into online gradually.