Real time hardware in the loop simulation testbed of spacecraft formation flying

Abstract

The great potential benefits associated with SFF (satellite formation flying) have led to considerable research in this concept around the world. As it is a new field of research and the implementation of formation flying in practice brings some inherent challenges and risks It is therefore particularly useful to develop a real time platform, where formation flying theories, technologies, algorithms and their coordination in open and close loop can be tested in scenarios close to those actually expected. In this research a low cost, real time hardware in the loop (RTHIL) test bed for low earth orbit autonomous SFF consisting of nano satellites has been developed which can integrate original hardware in the close or open loop. The proposed real time close loop test bed consists of a real time SFF model, a real time relative navigation system, guidance and control algorithm, and GPS simulator. Different SFF models already developed by researchers have been studied for their potential use in a real time test bed. Due to the limitations associated with them a novel real time SFF model and algorithm has been developed for real time hardware in the loop test bed. This real time SFF model was downloaded to a setup of two single board real time computers connected to each other through a serial port. The hardware of the relative navigation system has been designed and developed in this research. Single frequency GPS has been used as the relative navigation sensor. A model has been developed to estimate the dynamic characteristics of the GPS receiver for LEO orbits. This model simulates the relative dynamics of the GPS constellation and the LEO satellite. S3C2410 has been selected as the navigation processor. Radio transceivers are used to exchange data between the satellites. A uC/OS-II real time operating system is used in the navigation software code. The Hardware and software structure has been developed to simulate both centralized and decentralized approaches. A novel relative navigation algorithm is discussed. A Graphical Interface Software (GIS) has also been developed to initialize hardware and software prior any simulation and test run. It synchronizes the start of simulation on all real time computers, relative navigation systems and other components/simulator. It captures real time data from the deputy satellite. It also provides different debug and test options. Offline simulations were run for three models Hill’s, COEPOKE and RTSFF models by considering gravitational and atmospheric perturbation. The comparison of the results showed that the RTSFF model is able to simulate formations both in circular and elliptical orbits satisfactorily. The analysis of these models on the basis of their mathematical derivations showed that the RTSFF model gives better results than other models. The worth of this model for the real time test bed has been shown by running real time simulations onto a network of two synchronized single board real time computers and when the results were compared with offline simulation results they were found to be quite satisfactory. The hardware of the relative navigation system has been checked and tested. A basic infrastructure for navigation software has been developed. Different open loop tests have been run to verify the working of the hardware and software and these have verified the correct functioning of the system. The navigation algorithm could not be implemented in the embedded environment, Due to the unavailability of GPS simulator, no close loop simulation or test has been performed. The functionality of all the software, models, hardware and their operational control under GIS software has been checked in real time environment and found to be working correctly and are ready to be run in any open loop or closed loop simulation test.

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