This paper addresses the concept of a 3D Geodata Web Service for the support of indoor navigation with satellite-based positioning systems like Galileo or GPS. The presented work is part of a project called INDOOR, which principally aims at the support of satellite positioning and navigation within buildings. The intended web service shall provide 3D building data to mobile clients, where it will be used to calculate the indoor signal dispersion and to provide location based information to the user. After an expose of the detailed requirements on the service CityGML has been selected as best fitting data format. It is shown how the inherent material properties of 3D building elements can be included in CityGML. Further 1 The work presented in this paper is part of the project INDOOR (http://www.indoor-navigation.de), funded by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR) with financial resources of the German Ministry of Education and Research (BMBF) u...