# Evaluation of Forward Models for GNSS Radio Occultation Data Processing and Assimilation
Qifei Du  
1 National Space Science Center, Chinese Academy of Sciences (NSSC/CAS), Beijing 100190, China; [EMAIL_ADDRESS] (N.D.); [EMAIL_ADDRESS] (Y.S. ); [EMAIL_ADDRESS] (Q.D. ); [EMAIL_ADDRESS] (J.X. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (C.L. ); [EMAIL_ADDRESS] (Y.C. ); [EMAIL_ADDRESS] (C.M. ); [EMAIL_ADDRESS] (C.Y. ); [EMAIL_ADDRESS] (C.H. ); [EMAIL_ADDRESS] (F.H. ); [EMAIL_ADDRESS] (G.T. ); [EMAIL_ADDRESS] (G.T. ); [EMAIL_ADDRESS] (C.L.) Qifei Du  
1 National Space Science Center, Chinese Academy of Sciences (NSSC/CAS), Beijing 100190, China; [EMAIL_ADDRESS] (N.D.); [EMAIL_ADDRESS] (Y.S. ); [EMAIL_ADDRESS] (Q.D. ); [EMAIL_ADDRESS] (J.X. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (C.L. ); [EMAIL_ADDRESS] (Y.C. ); [EMAIL_ADDRESS] (C.M. ); [EMAIL_ADDRESS] (C.Y. ); [EMAIL_ADDRESS] (C.H. ); [EMAIL_ADDRESS] (P.H. ); [EMAIL_ADDRESS] (G.T. ); [EMAIL_ADDRESS] (C.L.) Qifei Du
1 National Space Science Center, Chinese Academy of Sciences (NSSC/CAS), Beijing 100190, China; [EMAIL_ADDRESS] (N.D.); [EMAIL_ADDRESS] (Y.S. ); [EMAIL_ADDRESS] (Q.D. ); [EMAIL_ADDRESS] (J.X. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (C.L. ); [EMAIL_ADDRESS] (Y.C. ); [EMAIL_ADDRESS] (C.M. ); [EMAIL_ADDRESS] (C.L. ); [EMAIL_ADDRESS] (C.L. ); [EMAIL_ADDRESS] (G.T. ); [EMAIL_ADDRESS] (C.L.) QifeiIonosphere, and Climate (COSMIC/COSMIC-2), the Meteorological Operational satellite Programme-A/B (MetOpA/B), FengYun-3C/D (FY-3C/D) [2], GRACE/GRACE-FO, Scientific Application Satellite-C/D (SAC-C/D), Korea Multi-Purpose Satellite-5 (KOMPSAT-5), and Spanish PAZ ('peace' in Spanish) [3; 4; 5]. By 2027, these missions will provide approximately 18,400 observations per day which theoretically is the number threshold of the positive effect that RO could bring to NWP [6; 7]. It will reduce error in prediction by 25% if all these observations are assimilated into the NWP assimilation system [8]. However, the actual situation does not coincide with theoretical expectations because almost half of the RO data either cannot be received or cannot pass quality control before assimilation [9]. There are two strategies to increase the RO observation available. The first one focus on increasing the soundings received per day via more missions, more receivers on the cube satellite, e.g., the cube satellite constellation of Spire company [10], and more multi-system-compatible RO receiver such as GNSS Radio Occultation Sonder (GNOS) onboard FY3 [2; 11; 12; 13]. The second strategy is to improve the RO data processing technique. The current RO data processing center includes the Danish Meteorological Institute (DMI), German Research Centre for Geosciences (GFZ), Jet Propulsion Laboratory (JPL), University Corporation for Atmospheric Research (UCAR), the National Oceanic and Atmospheric Administration (NOAA)'s National Environmental Satellite, Data, and Information Service (NESDIS), and Wegener Center/the University of Graz (WEGC). Each center developed different schemes to process RO data. The quality of their RO data is accessed by the international RO working group (IROWG) every other year [14]. The consistency and structural uncertainty of their RO data records are promising to step up to another level. Thus, improvements are expected in the RO data processing and RO data assimilation techniques. The forward model (FM) plays a critical role during these two processes. Firstly, the more accurate the FM, the more accurate these two processes. Secondly, the error of FM should be quantified, so that the other errors during these two processings can be accurately evaluated. The FM mainly consists of Abel integral algorithm and interpolation methods before Abel integral.