Undifferenced processing of COSMIC-2 radio occultation measurements with 1-Hz LEO clock corrections

Calculation of the atmospheric excess phase (AEP) is the fundamental work of Global Navigation Satellite System (GNSS) radio occultation toward refractivity and other meteorological profile retrievals. At present, the COSMIC-2 satellite AEP products from the University Corporation for Atmospheric Research (UCAR) are calculated by a conventional single-differenced approach, which increases the computational complexity and introduces random errors from the reference linkage. This study proposed a strengthened indifference method with high-rate LEO clock corrections for COSMIC-2 AEP calculation. Combining observations from both POD antennas on COSMIC-2 satellites, the LEO receiver clock offsets were estimated at 1-s intervals during the process of LEO precise orbit determination. Allan deviation of the estimated clock offsets was verified at 2 × 10 –12 s to 3 × 10 –12 s at 1-s and 10-s intervals, respectively. After applying 1-s LEO receiver clock corrections, the COSMIC-2 undifferenced AEP was calculated, and the corresponding refractivity profiles were retrieved for validation. Analysis of 2579 COSMIC-2 RO profiles shows that the mean bias between the undifferenced refractivity profiles and UCAR refractivity profiles was below 0.2% at heights above 10 km. The reference profiles from the European Centre for Medium-Range Weather Forecasts (ECMWF) were also used for external validation. The results indicated that the standard deviation (STD) of refractivity derived from undifferenced processing was approximately 1.2% for GPS and 1.4% for GLONASS occultations at altitudes below 35 km. There is almost no difference between the results derived from the undifferenced method and the single-differenced method at altitudes of 10–35 km. The refractivity from the undifferenced processing also showed higher accuracy than the results of the single-differenced strategy in the low troposphere. Hence, the proposed UD processing for COSMIC-2 radio occultation retrieval was feasible when LEO clock corrections could be provided at a 1 Hz sampling rate.