## 1 Introduction  
Accurate temperature and water vapor measurements are crucial for understanding the atmospheric energy balance, hydrological cycle, and climate change. Using active global navigation satellite system (GNSS) radio occultation (RO) technology, the bending of rays passing through the atmosphere provides information on the vertical distribution of density variation, which can be converted into atmospheric bending angle and refractivity profiles [1; 2; 3; 4]. In the neutral atmosphere, the refractivity is a function of atmospheric pressure, temperature, and moisture. Using an optimal inversion algorithm and a priori atmospheric thermal information, the refractivity profiles can be inverted into accurate temperature and water vapor profiles [5; 6]. Numerous studies have demonstrated that because RO measurements are not affected by clouds and precipitation, the RO-derived water vapor profiles in the neutral atmosphere are complemented by those from satellite infrared (IR) and microwave (MW) sounders and provide water vapor information within and below clouds [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. Launched in June 2006, the constellation observing system for meteorology, ionosphere, and climate 1 mission and Formosa satellite mission 3 (COSMIC/FORMOSAT-3, COSMIC-1 hereafter) was the first RO mission that provided nearly uniform spatial and temporal coverage daily [14]. Although designed with a lifetime of two years, COSMIC-1 provided atmospheric occultations until the end of 2019. The COSMIC-2 is a COSMIC-1 follow-on mission, which was launched on 25 June 2019. COSMIC-2 measurements are of increased signal-noise ratio (SNR) compared to other RO missions [15]. Several studies discussed how high SNR COSMIC-2 data may affect the retrieval results in the lower troposphere [15; 28]. To optimize the usage of RO data in the numerical weather prediction (NWP) system through data assimilation (DA), it is necessary to quantify the accuracy and the uncertainty of the RO-derived atmospheric profiles, especially in the lower troposphere. Studies using COSMIC-1 data for NWP, climate applications, atmospheric applications, and space weather are summarized in [14]. Recognizing the importance of GNSS RO data on atmospheric applications and NWPs, the NOAA decided to include RO as a long-term core observable as those from satellite IR and MW sounder measurements. COSMIC-2 is now the NOAA's backbone RO mission. Currently, the COSMIC-2 near-real-time data are processed by the University Corporation for Atmospheric Research (UCAR) COSMIC Data Archive Center (CDAAC) and distributed to global NWP operational centers through the global telecommunication system (GTS). The challenges in using RO data, including COSMIC-2, for NWP and atmospheric applications, especially in the lower tropospheric topic region, were summarized in [14]. [PERSON] et al. [29; 30; 31] stated that independent processing is the best way to quantify RO processing errors and structural uncertainty. The Center for Satellite Applications and Research (STAR) is NOAA's dedicated GNSS RO Satellite processing and science Application Center (RO-SAAC). The general goal for the STAR RO-SAAC is to build a long-term RO data processing and science center to support all RO-related operation, validation, processing, re-processing, and science applications for NOAA users and international science communities, as we do for MW and IR sounders. To better quantify the observation uncertainty and understand how the observation uncertainty from clock errors and geometry determination may propagate to excess phase, bending angle, and refractivity profiles, the STAR RO-SAAC has developed the GNSS RO data processing and validation system (ROPVS). This system includes the following functions (Figure 1):  
1. RO data processing and re-processing (for Level 1a (L1a) to Level 1b (L1b)and L1b to Level 2 (L2) processing and re-processing). We generated independently developed RO products for multiple RO missions.