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A method is developed to determine station-keeping maneuvers for a fleet of satellites collocated in a geostationary slot. The method is enabled by a linear time-varying formulation of the satellite orbit dynamics in terms of non-singular orbital elements. A leader-follower control hierarchy is used, such that the motion of the follower satellites is controlled relative to the leader. Key objectives of the station-keeping method are to minimize propellant consumption and to limit the number of maneuvers, while guaranteeing safe separation between the satellites. The method is applied in a realistic simulation scenario, including orbit determination, actuation and modeling errors. The method is demonstrated to work for fleet of four satellites with differences in mass, surface area and propulsion system for a maneuver cycle of one week. It is then demonstrated that by reducing the maneuver cycle duration to one day, the method allows to collocate 16 satellites in a single slot, without penalties on propellant consumption.

We have reported on a single-exposure dual-energy system based on computed radiography (CR) technology. In a clinical study conducted over a two year period, the dual-energy system proved to be highly successful in improving the detection (p=0.0005) and characterization (p=0.005) of pulmonary nodules when compared to conventional screen-film radiography. The basic components of our dual-energy detector system include source filtration with gadolinium to produce a bi-modal x-ray spectrum and a cassette containing four CR imaging plates. The front and back plates record the low-energy and high-energy images, respectively, and the middle two plates serve as an intermediate filter. Since our initial report, a number of improvements have been made to make the system more practical. An automatic registration algorithm based on image features has been developed to align the front and back image plates. There have been two improvements in scatter correction: a simple correction is now made to account for scatter within the multiplate detector; and a correction algorithm is applied to account for scatter variations between patients. An improved basis material decomposition (BMD) algorithm has been developed to facilitate automatic operation of the algorithm. Finally, two new noise suppression techniques are under investigation: one adjusts the noise filtering parameters depending on the strength of edge signals in the detected image in order to greatly reduce quantum mottle while minimizing the introduction of artifacts; a second routine uses knowledge of the region of valid low-energy and highenergy image data to suppress noise with minimal introduction of artifacts. This paper is a synthesis of recent work aimed at improving the performance of dual-energy CR conducted at three institutions: