Comparative Analysis of Clock Synchronization Schemes for Onboard Computers in Launch Vehicles

The avionics system in aerospace vehicles consists of different processor based subsystems and the associated embedded software. To enhance the reliability multiple independent computing chains are also used. The clocks in these real-time systems should meet high accuracy specifications and also be closely time-synchronized. In the Indian context, there are two and four independent computing chains for unmanned and man-rated launch vehicles respectively. Normally the outputs from the primary computing chain will be selected. If any failure is detected, the system will be reconfigured to operate with the outputs from a redundant computing chain. In order to make a smooth system reconfiguration, the master processors in all the chains should be tightly time synchronized. This work describes the study, evaluation, comparison and testing of different clock synchronization algorithms for aerospace systems. Our aim is to design a suitable scheme for meeting the tight synchronization requirements in onboard computers of aerospace systems with triple or quadruple redundancy. Based on the literature survey carried out on the various clock synchronization schemes used in different applications, four schemes were chosen for evaluation. The performance of the various synchronization algorithms was compared by implementing in software and testing in a simulation environment as well as hardware. The results show that the averaging scheme is best suited for our application. A novel, modified version of this scheme is proposed to ensure tight synchronization of the clocks in all possible failure scenarios.