## 1 Introduction  
Signals from the Global Navigation Satellite Systems (GNSS) are used in many modern devices and various applications, such as navigation, surveying and mapping, satellite communication, emergency response, location-based services, and precise time services, among many others. The robustness and resilience of the GNSS signals are very important for those applications, especially in safety-critical applications. There are particular concerns regarding the spoofing of signals, which means that a GNSS receiver can be deceived by fake GNSS signals in an attempt to modify the Position, Velocity, and Time (PVT) solution of the target receiver. One method for preventing spoofing is the authentication of signals, which enables ensuring that a message is identical to the one transmitted at its origin and that it was generated by a trusted source [1]. The Galileo Open Service Navigation Message Authentication (OSNMA) is the first service providing this authenticity check in the civilian sector. It has currently been in the Public Observation (PO) test phase for slightly longer than a year. It uses a navigation message authentication scheme based on the Timed Efficient Stream Loss-tolerant Authentication (TESLA) protocol [2] and enables authentication at the receiver end in a manner that virtually eliminates the need for a chain of trust with dependence on third-party services. This offers the opportunity for Galileo to become the reference for many diverse new applications requiring authenticated positions. Galileo OSNMA signals may also play a vital role in the field of road vehicle navigation. In this respect, the ESRIUM project, funded in the context of Horizon 2020, aims at creating road wear maps with accurate information about the position and shape of road damage, and to send prompt and real-time notifications to drivers and autonomous vehicles with instructions to avoid the damaged areas and route recommendations (e.g., change of lanes and driving offsets with respect to the lane center) to even out the road wear [3]. ESRIUM relies in this regard on Galileo services for increasing the positioning accuracy of vehicles receiving these notifications, and for authenticating the position estimates using Galileo OSNMA. The overall goal is to increase the safety and robustness of the localization solution and to reduce road maintenance costs as well as improve road safety and comfort at the same time [4]. Within the scope of ESRIUM, the Finnish Geospatial Research Institute (FGI) developed an open-source implementation of Galileo's OSNMA, called _FGI-OSNMA_[5], which distinguishes itself, e.g., by modularity and real-time usability. Other implementations also exist already, for example _OSNMA_[6] and _galileo-osnma_[7]. The Septentrio company included support for OSNMA in some of their products, such as the multi-constellation GNSS receiver module Mosaic-X5 [8]. Theoretical work regarding OSNMA is provided by several authors, e.g., [2; 9; 10; 11; 12; 13]. Practical performance assessments are reported by, e.g., [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. From an experimental performance assessment of Galileo OSNMA within the scope of ESRIUM, it was observed in a four day-long static observation that the receiver could produce authenticated signals 99.74% of the time under open sky conditions [26]. The authentication status could not be attained in cases with a low number of satellites transmitting OSNMA data to the receiver. Therefore, it was concluded that the authentication performance will significantly decrease in environments with degraded satellite visibility, such as urban environments. Hence, with regard to OSNMA, it is beneficial to also use data from satellites with low elevation, as this increases the chance to cross-authenticate visible satellites.