High accuracy GNSS-based positioning and navigation is required by an increasing number of applications in urban environments. The development is to a large degree driven by the advent of self-driving and autonomous vehicles, increasing use of unmanned aerial vehicles (UAV), and increasing use of outdoor robots. Further, the network real-time kinematic (RTK) positioning methodology is well known as a mean to obtain high accuracy GNSS-based positioning in real-time using GNSS carrier-phase data. The Testbed in Aarhus for Precision Positioning and Autonomous Systems (TAPAS) is built around an RTK system based on a high-density network of 11 reference stations in the city of Aarhus in Denmark. Thus, TAPAS is an augmentation system providing cm-level position accuracy for GNSS-based applications. Using the TAPAS testbed, this paper presents analyses of jamming and multipath in order to characterize the robustness of high accuracy GNSS based positioning in urban environments, when a high-density network RTK is used for augmentation. Analyses of jamming incidents registered at the 11 reference station of TAPAS throughout the year of 2019 show that a total of 1934 jamming incidents were detected. The jamming incidents were further analyzed and it is revealed that most incidents occur during business hours on weekdays, more incidents are found for reference stations located close to larger roads, and when analyzing the impact on positioning performance for high accuracy multi-frequency carrier phase-based positioning it is shown that the effect on end-user position accuracy is very small. Further, multiple dynamic vehicle-based tests using the TAPAS platform were carried out with the purpose of analyzing positioning performance and the effects of multipath. Data was collected using three different GNSS antennas, three different GNSS receivers and two different GNSS/INS systems. Results showed that high-end geodetic equipment outperformed new evolving multi-band multi-constellation mass-market receivers. Despite substantial differences in performance between the antennas and receivers, multipath still severely degraded the positioning results for all equipment grades.