This paper deals with the problem of real-time onboard relative positioning of low-Earth-orbit spacecraft over longudbaselines using the Global Positioning System. Large intersatellite separations, up to hundreds of kilometers, are ofudinterest to multistatic and bistatic synthetic-aperture radar applications, in which highly accurate relative positioningudmay be required in spite of the long baseline. To compute the baseline with high accuracy, the integer nature of dualfrequency,uddouble-difference carrier-phase ambiguities can be exploited. However, the large intersatellite separationudcomplicates the integer-ambiguities determination task due to the presence of significant differential ionosphericuddelays and broadcast ephemeris errors. To overcome this problem, an original approach is proposed, combining anudextended Kalman filter with an integer least-square estimator in a closed-loop scheme, capable of fast on-the-flyudinteger-ambiguities resolution. These integer solutions are then used to compute the relative positions with a singleepochudkinematic least-square algorithm that processes ionospheric-free combinations of debiased carrier-phaseudmeasurements. Approach performance and robustness are assessed by using the flight data of the Gravity Recoveryudand Climate Experiment mission. Results show that the baseline can be computed in real time with decimeter-leveludaccuracy in different operating conditions.
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