Carrier-phase differential GPS (CDGPS) is a promising technology for accurate relative navigation in LEO formations of cooperating satellites, but navigation filter robustness against poor GPS geometry and noisy measurements has to be improved. This can be performed by augmenting the navigation filter with intersatellite local ranging measurements, as the ones provided by ranging transponders or GNSS-like systems. In this paper, an augmented CDGPS navigation filter is proposed for the formation of two satellites characterized by a short, varying baseline, relevant to next generation Synthetic Aperture Radar missions. Specifically, a cascade-combination of dynamic and kinematic filters which processes double-differenced code and carrier measurements on two frequencies, as well as local inter-satellite ranging measurements, is used to get centimeter-level baseline estimates. The augmented filter is validated by numerical simulations of the formation orbital path. Results demonstrate that the proposed approach is effective in preserving the centimeter-level accuracy achievable by a CDGPS-only filter also in the presence of a poor GDOP or a limited number of GPS satellites in view. 1. Introduction In the recent years, the interest towards the use of satellite formations to perform many space missions, such as Earth remote sensing, observation of the universe, and Earth gravity field mapping, is increased. In these applications, the platforms can either act coordinately to synthesize a unique payload or a complement to each other so as to magnify mission scientific goals. Formation flying offers many performance and operational advantages with respect to traditional space systems based on a single platform, but at the same time, the maintenance of a prefixed formation geometry is crucial to achieve specific mission goals. This is a challenging task, requiring the implementation of a robust communication interlink between the various platforms, as well as autonomous relative navigation and control. Many applications require the determination of the satellite separations with centimeter-level accuracy to control the satellite relative positions, especially when they come closer as a result of the relative orbital path or at the subcentimeter level to achieve scientific goals [1]. For determining the relative position, advanced technologies based on GPS, vision, and laser systems have been proposed, and a number of missions are being performed for inflight demonstration of these technologies [2, 3]. With reference to LEO formations, carrier-phase differential
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