The research field “Theory” involves the investigation and testing of mathematical models in order to improve existing positioning methods. The adaptation of classical parameter estimation methods and the development of new mathematical algorithms are key elements for innovative navigation applications. In this context the main focus lies on the realization of real-time solutions in order to facilitate mobile navigation applications (e.g. smartphones) or commercial high-precision applications. Investigations concerning the optimal weighting of sensor data are also a field of research. In addition, non-linear systems and their peculiarities are studied as well. The research output from this topical focus is a precursor to other research topics but also benefits from feedback from application-oriented fields of research.