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

Abstract

Precise isolating atmospheric excess phase (AEP) is the fundamental work of Global Navigation Satellite System (GNSS) radio occultation technique towards reconstructing on the refractivity and other meteorological profiles. Currently, the COSMIC-2 satellites AEP products from University Corporation for Atmospheric Research (UCAR) were calculated by conventional single-differential approach at cost of complicating the processing and also amplified the excess phase noise. This study provided a strengthened un-differential method for COSMIC-2 AEP calculation such that the interpretation of raw occulting phase data could be enriched. The vital bias, receiver clock offset, was modeled and calibrated at 1s sampling via pre-processing of LEO precise orbit determination, which took both two antennas’ observations into estimator for intensifying orbit and clock precision. After combining the un-differential method with 1s LEO receiver clock corrections, COSMIC-2 AEP was calculated and collected to retrieve the refractivity profiles for statistical validation. Analyzation of 2579 RO profiles indicates that the mean bias of our results against COSMIC-2 official refractivity profiles were below 0.2% at height above 10km. Furthermore, external reference of European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data was adopted for revealing the characteristics of bias and investigating the advantage of un-differential approach. Validation indicates that the standard deviation (STD) of un-difference derived refractivity against co-located referred profiles were around 1.17% and 1.28% for GPS and GLONASS occultations at altitude below 35km, appearing bias less than 0.04% against single difference method derived. The un-difference processing also shows higher accuracy than single difference in the lower troposphere. Hence, the proposed UD processing for COSMIC-2 radio occultation retrieval is feasible when the 1s LEO clock offsets were estimated for correction.