Development of RTCM SC-134 Messages for High-Integrity Precise Positioning

Special Committee 134 (SC-134) “Integrity for GNSS-based High Accuracy Applications” was established by the Radio Technical Commission for Maritime Services (RTCM) in 2018 to define message messages and data formats to include information for the implementation of integrity monitoring services for high accuracy applications. Such messages, to be exchanged between Augmentation Service Providers and users or among them through collaborative approaches, allow users to calculate their integrity (safety) performance in real-time and implement their own Fault Detection and exclusion algorithms, e.g. through the calculation of the real-time position Protection Levels (PLs) that bound errors under faulty conditions. This approach to integrity starts from the established background for Satellite-based and Ground-based Augmentation Systems (SBAS and GBAS) in the ICAO SARPs and RTCA standards for aviation. However, unlike SBAS and GBAS, SC-134 standard needs to support a wide variety of user applications, augmentation system technologies, and existing and future GNSS applications. This requires a “multimodal”, “multiservice,” and “technology agnostic” approach with message formats that support all envisioned means of providing high-accuracy and high-integrity services. More than 250 members of SC-134 have been working to achieve these goals, organized into five working groups divided by application and ad-hoc subgroups focusing on specific technical issues. At his aim, the maximum reuse of RTCM SC 104 heritage, as well as liaison activities with single domain organization is carried out. This paper provides an overview of the SC-134 message formats included in the draft standard, provides several examples of key messages, and describes the considerations and trade-offs that went into their selections. The impact of quantization loss on localarea DGNSS for candidate fault probabilities and mean fault durations (MFDs) are shown based on simulations of worldwide user performance using the Stanford Matlab Algorithm Assessment Simulation Tool (MAAST). These messages and accompanying technical guidance are now being finalized and tested in accordance with RTCM policies in preparation for a first release of the SC134 standard in early 2025.