Cluster is a four-spacecraft mission launched in 2000 to study the Earth's magnetic field and its interaction with the solar wind. Originally supposed to last two and a half years, the mission is now extended until the end of 2014 provided a successful midterm review in June 2012. In time, the Cluster ground segment has undergone a thorough modernization process. Lately, the mission planning system has been the object of the biggest changes. The initial strategy was to optimize the utilization of a single ground station by all four of the spacecraft. This was soon modified to account for a second ground station, in order to increase the science data return. Each station had to be allocated to a specific pair of spacecraft. Such a rigid planning approach became, year after year, more inadequate as the evolving orbit changed the visibility, and the spacecraft power subsystem degraded. Many more ground stations are needed today, and the increased complexity of the spacecraft operations requires the capability for short term re-scheduling. Currently, an interactive and fast adapting mission planning system is used. A key-role is played by Cluster Web, a software package entirely developed by the Cluster flight control team to offer a visual overview of the mission plan and an elegant interface to implement conflict-free changes. Its description is the objective of this paper, together with the discussion of the transition from an 18-years old VMS architecture to a Linux system that took advantage from the on-going development of several mission planning facilities within the European Space Operations Centre. The goal is to underline how a flexible approach and the ability to design and develop ad-hoc tools could cope with continuously evolving operational demands and budget constraints.
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