The Galileo In-Orbit Validation Element (GIOVE) is an experiment led by the European Space Agency (ESA) aimed at supporting the on-going implementation of Galileo, the European global navigation satellite system (GNSS). Among the objectives of the GIOVE Mission are the validation and characterization of the on-board clock technologies. The current baseline technologies for on-board clocks are the rubidium atomic frequency standard (RAFS) and the passive hydrogen maser (PHM). Both technologies have been validated and qualified on ground and are now being further validated in a representative in-orbit environment aboard 2 spacecrafts, GIOVE-A and GIOVE-B. This paper presents the results obtained in the frame of the GIOVE experimentation. The behavior and performances of the clock technologies on board both spacecrafts has been investigated and analyzed in terms of operation, frequency stability, and clock prediction error after more than 3 years of operation for GIOVE-A and almost one year for GIOVE-B. In addition, relativistic frequency shifts of GIOVE spacecrafts have been investigated.
Within the GIOVE Mission (GIOVE-M), two experimental satellites called GIOVE-A and GIOVE-B have been launched by the European Space Agency. This paper analyses the different issues involved in GPS/GIOVE interoperability for positioning and timing, including GGTO (the GPS to Galileo time offset) and timing biases, and presents practical experience and results related to EGGTO, the GIOVE-M experimental version of GGTO broadcast within the GIOVE navigation messages.
In intensity modulated radiation therapy (IMRT), intensity maps are computed from prescribed doses to target volumes, adding dose restrictions to the surrounding tissues. Those intensity (fluence) maps are discretized into matrices of natural numbers and translated to sequences of multileaf collimator (MLC) leaf movements, which will finally deliver the computed x-ray intensities. A unidirectional leaf sequencing algorithm that controls the shape of the segments and reduces leaf motion time for step-and-shoot dose delivery is presented. The problem of constructing segments in controlling its shape was solved by synchronizing right leaves motion. This is done without increasing the number of segments, or the total number of monitor units, and taking into account the unidirectional leaf motion and the interleaf collision constraints. The method was tested using random matrices and a clinical case planned with the PCRT 3D(R) planning system. Compared to other unidirectional leaf sequencing methods, the proposed algorithm performs very similarly. But, in addition, the segment shape control produces segments with smoother outlines and more compact shapes, which may help to reduce MLC-specific effects when delivering the planned fluence map. Finally, as a result of imposing unidirectionality, this algorithm can be extended for sliding window segment generation.
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