## 1 Introduction  
The Constellation Observing System for Meteorology, Ionosphere, Climate (COSMIC) 2 mission and Formosa Satellite Mission 7 (COSMIC-2/FORMOSAT\(-\)7, hereafter COSMIC-2) is the backbone Global Navigation Satellite System (GNSS) Radio Occultation (RO) mission in National Oceanic and Atmospheric Administration (NOAA). Launched on June 25, 2019, the COSMIC-2 is a six-satellite constellation at 720 km altitude. Each COSMIC-2 satellite is equipped with a Tri-band GNSS (TriG) Receiver System (TGRS), which can receive RO signals emitted from GPS (Global Positioning System), GLONASS (GLObal NAVigation Satellite System), and GALILEO systems. The COSMIC-2 TGRS's high gain antennas have a superior signal-to-noise ratio (SNR) to previous RO missions [1]. The early analysis demonstrated that COSMIC-2 data have a deeper penetration depth in the lower troposphere than other RO missions [1,2]. Numerical studies have demonstrated that the neutral atmospheric parameters derived from active GNSS RO technology are crucial for atmospheric applications [1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. However, the RO observation errors in the moisture lower troposphere are significant [1], and residual ionospheric errors are usually not completely removed with the first-order approximation [20]. A previous study indicated that the impacts of RO observations on numerical weather prediction (NWP) are mainly from 10 to 25 km altitudes and decrease sharply to atmospheres above and below, respectively [21]. COSMIC-2 bending angle assimilation into operational NWPs also showed positive impacts on the similar height as other RO missions [22]. With the precise positions and velocity of the Low Earth Orbit (LEO) and GNSS satellites, the time and path delay signal caused by the atmospheric refraction can be converted to atmospheric parameters, including bending angle, refractivity, temperature, moisture, and pressure in the neutral atmosphere [23]. To derive bending angle and refractivity profile from raw occultation measurements, we need to perform complicated L0-L1 (from raw data to excess phase) and L1-L2 (converting excess phase to bending angle) processing. The inversion procedures (see Figure 1) include: (i) precise orbit determination (POD) for the LEO satellite, (ii) occultation excess phase calculation with the clock bias removal and geometric range removal, and the accurate estimate of antenna phase center position in Earth-Centered Inertial (ECI) Coordinate System, (iii) inversion of bending angle from excess phase, (iv) conversion of bending angle to refractivity and other high-level products, and (v) quality control of the data products. A previous study showed that the uncertainty from excess phase conversion might propagate to bending angle retrievals [24]. The uncertainty in each step requires a detailed analysis of the error propagation and impacts on the next step. Figure 1: The steps to convert RO from L0 data to L1 excess phase and converting excess phase to bending angle (L2) processing. The COSMIC-2 implementation-related uncertainties may primarily arise from the following three factors:  
1. POD calculation for the LEO satellite The POD process determines the LEO satellites' mass center and the estimation of the receiver clock bias and dynamically interpolates the GNSS satellite position. The mass center position determines the antenna position and the geometric distance between receiver and transmitter to derive the geometric free excess phase later. The receiver clock bias from the POD process is used to correct the time tag in the excess phase calculation [25]. The POD (orbital) errors may introduce velocity errors, affecting the accuracy of excess phase calculation (see Section 3). Note that the orbital estimates can differ from processing software and configurations. Previous studies showed that the COSMIC-1 orbital difference could have a 10 cm mean root mean square (RMS) difference among different processing centers [26].