The spread presence of GNSS Permanent Station Networks makes satellite surveys simpler, quicker and cheaper, and the achievement of high precisions easier. However, the result of a GNSS campaign benefits from an accurate planning, especially in presence of natural or artificial obstructions that make satellite observations difficult. Most of the GNSS office suites provide planning tools to analyze the satellite's visibility; visibility plots, sky plots and DOP charts are commonly built from the GNSS almanac. They usually allow planning on single points, taking into account the obstructions due to terrain morphology or buildings by hand-drawing polar diagrams after a visit of each location to be surveyed. This task can be made fully automatic by using a digital surface model to build the obstruction plot and the almanac to evaluate satellites' positions, hence their visibility and not only availability, in a GIS. The procedure may be applied to whole areas to find the best time for the survey campaign. Taking a step further, this approach has been made available as a service by creating a Web Processing Service, which allows also non GIS specialists to access this tool through a simple WebGIS interface. The user can select the satellites constellation, GPS or GPS and GLONASS, the cut-off elevation angle, the day, hour and temporal window of the survey. The service returns raster maps describing the minimum number of visible satellites and the maximum PDOP index in every pixel for the temporal window. Moreover, the service implements some features of the UNAVCO TEQC software, which can help to choose the location of new permanent stations. The implementation follows the OGC specifications and relies fully on FOSS software, combining different systems to provide the user web interface, the data storage and the processing engine. The service features, the underlying technology and some examples will be presented in this paper.
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