A powerful onboard navigation processor dedicated to the GPS- based guidance, navigation, and control of satellite formations in real-time has been built up. The system features an industrial Power PC 823e processor supporting multiple Global Positioning System (GPS) receiver interfaces and thus allows flexible hardware-in-the-loop simulations of complex satellites formations comprising more than two spacecraft. The real-time operating system BOSS separates the kernel runtime system and a hardware dependant layer of the navigation processor, which significantly speeds up software development and implementation on the hardware platform. The same processor type and the operating system have successfully been flown in space, onboard the BIRD small satellite. A Real-time Formation Flying software has been developed and implemented on the processor. It applies a complex dynamic model of the spacecraft motion to estimate the absolute as well as the relative spacecraft states, based on GPS pseudoranges and carrier phase measurements. Making use of a float integer ambiguity resolution of the GPS carrier phases, a hardware-in-the- loop relative position reconstruction at the range of some centimeters has been demonstrated. In addition, the navigation processor has been utilized to demonstrate a closed-loop hardware-in-the-loop simulation of autonomous formation control. To this end, two GPS receivers, the navigation processor and a GPS signal simulator were coupled to demonstrate a real-time control accuracy of the satellite formation in the meter range.
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