The constant irruption of new sensors is a challenge for software systems that do not rely on generic data models able to manage change or innovation. Several data modeling standards exist. Some of these address the problem from a generic perspective but are far too complex for the kind of applications targeted by this work, while others focus strictly on specific kinds of sensors. These approaches pose a problem for the maintainability of software systems dealing with sensor data. This work presents ASTROLABE, a generic and extensible data model specifically devised for trajectory determination systems working with sensors whose error distributions may be fully modeled using means and covariance matrices. A data model relying on four fundamental entities (observation, state, instrument, mathematical model) and related metadata is described; two compliant specifications (for file storage and network communications) are presented; a portable C++ library implementing these specifications is also briefly introduced. ASTROLABE, integrated in CTTC's trajectory determination system NAVEGA, has been extensively used since 2009 in research and production (real-life) projects, coping successfully with a significant variety of sensors. Such experience helped to improve the data model and validate its suitability for the target problem. The authors are considering putting ASTROLABE in the public domain