Challenges for GNSS-based high precision positioning - some geodetic aspects

In geodesy, GNSS-based methods are widely used for positioning and navigation. Since the precision (< some mm) and accuracy (< 1 cm) requirements are very high, sophisticated correction models and analysis strategies were developed, e.g. like the network RTK concept realised in different active permanent reference station networks. However, multipath and signal diffraction as well as the incomplete stochastic model for GNSS observations limits still the attainable accuracy and precision, especially for real time applications. In this paper, we focus on multipath and diffraction mitigation. The state of the art procedures for mitigation are revisited as well as the challenges for new strategies are addressed and explained using examples from the Satellite Positioning Service of the German National Survey (SAPOS) and from typical geodetic monitoring networks. Finally the importance of a complete stochastic model for GPS observations for statistically tests and meaningful interpretations of the results is underlined.