Carrier‐phase differential GPS is an ideal sensor for formation flying missions in low earth orbit since it provides a direct measure of the relative positions and velocities of the vehicles in the fleet. This paper presents results for a relative navigation filter that achieves centimeter‐level precision using a customized GPS receiver. A precise, robust extended Kalman filter estimates the relative states based on simple measurement models and a linear Keplerian propagation model. To increase the filter robustness in the face of environment uncertainty, adaptive method of maximum likelihood estimation (MMLE) algorithms are implemented. The adaptive filter accurately identifies the process and sensor noise covariances for the system. Despite the simplicity of the filter, hardware‐in‐the‐loop simulations performed on the Formation Flying Testbed at Goddard Space Flight Center demonstrated that this filter can achieve ≅2 cm relative position accuracy and <0.5 mm/s relative velocity accuracy for a range of low‐earth‐orbit formation.

This paper presents the¯rst set of hardware-in-theloop results that experimentally demonstrate precise relative navigation for true formation-°ying spacecraft applications. Carrier Di®erential-phase GPS provides an ideal navigation sensor for formation°ying missions because it can be used to directly measure the relative positions and velocities of the vehicles in the°eet. A decentralized algorithm for formation estimation is presented, using an Adaptive Extended Kalman Filter to achieve unprecedented levels of accuracy. Four modi¯ed GPS receivers were used in the NASA Goddard Formation Flying Testbed facility to demonstrate formation°ying in low Earth orbit. Results from these hardware-in-theloop tests show accuracies on the order of » 1cm position and » 0.5mm/s for a 1 km elliptical formation. These results validate both the use of a decentralized architecture and the adaptive EKF.

Project Orion-Emerald has been designed to demonstrate the use of a lowast, low-power, single frequency GPS receiver (with previous space heritage) for performing abolute and (precise) relative navigation m 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 m the Orion-Emerald formation. The paper b r i e descrbes the GPS receiver design and presents the bias estimation and relative navigation algorithms that have been developed. The receiver has been used m 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 m low Earth orbit. Estimation performance results from these "hardwart+in-tht+loop" tests are presented and compared to the ground-based tests. The results of these preliminary tests are promising, but they also iudicate that further iuning of the various filters will be required.

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