## 1 Introduction  
Precise orbit determination (POD) has become indispensable for many space-borne applications that monitor the Earth's climate system, one of which is Global Navigation Satellite System (GNSS) [1; 2] radio occultation (RO) [3; 4; 5]. The RO remote sensing technique is considered highly valuable for atmosphere and climate sciences since it provides high-vertical-resolution measurements over the troposphere and stratosphere with global coverage, high accuracy, long-term stability, and virtually all-weather capability [6; 7]. During an occultation measurement GNSS signals scan the atmosphere in limb sounding geometry and arrive with a time delay at the receiving RO satellite in low Earth orbit (LEO), which is due to the signal's refraction in the Earth's atmosphere. A vertically rising or setting occultation event is observed depending on whether the GNSS transmitter satellite rises or sets behind the Earth's horizon from the viewpoint of the rapidly moving LEO receiver satellite. The atmospheric excess phase path, which denotes the difference between the geometric straight-line distance between transmitter and receiver satellite and the signal's bended propagation path, can be derived from the measured time delays and reliable POD results. This traceability to fundamental time standards ensures long-term stability and no need for calibration [8; 9; 10]. The short-term stability of an individual RO event of about 1 to 2 min duration is satisfied by highly stable clock oscillators. Finally, essential climate variables (ECVs) [11], such as temperature, pressure, and tropospheric water vapor, can be derived from the raw measurements utilizing a dedicated RO _retrieval_. Resulting, the GNSS RO space-geodetic observing system delivers an abundance of geographically distributed vertical profiles of ECVs all over the globe, which can be averaged and used for climate studies. Due to the unique properties of occultation soundings introduced above and global availability since 2001, records of basic RO measurements (i.e., atmospheric excess phase or derived Doppler shift) have the potential to serve as _Fundamental Climate Data Record_ (FCDR): A well-characterized, global, and long-term stable data record for the derivation of accurate and stable ECVs, globally and covering timespans from days to decades [12; 13]. However, in order to fully exploit this potential and provide climate benchmark data for meteorology, climate research, climate monitoring as well as for calibration and validation, the accuracy of RO data needs to be assessed. That includes quantification of remaining uncertainties throughout the entire retrieval, starting from raw measurement data with the modeling of the observation geometry, to derivation of atmospheric excess phase data, to the final thermodynamic ECVs (Figure 1). While the quality and high accuracy of RO in the upper-troposphere and lower-stratosphere regions is well-acknowledged (e.g., [14; 15; 16]) and high consistency has been assessed between different RO retrievals of leading international processing centers [17; 10; 18], a rigorous uncertainty estimation and propagation throughout the entire RO retrieval remains an important but incomplete task. In addition, the former occultation processing system at WEGC [16] lacks this utility since the retrieval starts from external excess phase data and thus does not tie to the raw measurements and the physical unit of time. The new Reference Occultation Processing System (rOPS) [19; 20] developed at WEGC, on the contrary, aims to establish such a fully traceable processing comprising all retrieval steps [19; 21; 22]. For this reason, the rOPS features the integration of RO low-level data processing starting from the raw satellite observation data. This low-level processing comprises the RO observation geometry modeling within the daily system modeling (DSM) and the level 1a (L1a) excess phase processing [23] as part of the occultation data processing (ODP) chain (cf. Figure 1).