Orion-Emerald: carrier differential GPS for LEO formation flying

Project Orion-Emerald has been designed to demonstrate the use of a low-cost, low-power, single-frequency GPS receiver (with previous space heritage) for performing absolute and (precise) relative navigation in LEO. The carrier-phase differential GPS measurements from these receivers will also be used to actively control the relative position of the three micro-satellites in the Orion-Emerald formation. The paper briefly describes the GPS receiver design and presents the bias estimation and relative navigation algorithms that have been developed. The receiver has been used in numerous ground tests to determine the baseline performance and to analyze the expected noise levels. Several simulations of the LEO performance are presented using these predicted noise values. The receiver has also been tested with a GSS GPS Signal Generator to provide a better simulation of the performance in low Earth orbit. Estimation performance results from these "hardware-in-the-loop" tests are presented and compared to the ground-based tests. The results of these preliminary tests are promising, but they also indicate that further tuning of the various filters is required.