Simultaneous Radio Occultation for Inter-satellite Comparison of Bending Angles towards More Accurate Atmospheric Sounding

Abstract

Global Navigation Satellite System (GNSS) radio occultation (RO) is a remote sensing technique that uses International System of Units (SI) traceable GNSS signals for atmospheric limb soundings. The retrieved atmospheric temperature profile is believed to be more accurate and stable than those from other remote sensing techniques, although rigorous comparison between independent measurements is difficult due to time and space differences between individual RO events. Typical RO comparisons are based on global statistics with relaxed matchup criteria (within 3 hours and 250 km) which are less than optimal given the dynamic nature and spatial non-uniformity of the atmosphere. This study presents a novel method which allows for direct comparison of bending angles when simultaneous RO measurements occur near the Simultaneous Nadir Overpasses (SNO) of two LEO satellites receiving the same GNSS signal passing through approximately the same atmosphere, within minutes in time and less than 125 km in distance. Using this method, we found very good agreement between FORMOSAT-7/COSMIC-2 satellite measurements and those from Metop-A/B/C, COSMIC-1, Kompsat-5, and PAZ, although systematic biases are also found in some of the inter-comparisons. Instrument and processing algorithm performances at different altitudes are also characterized. It is expected that this method can be used for the validation of GNSS RO measurements for most missions and would be a new addition to the tools for inter-satellite calibration. This is especially important given the large number of RO measurements made available both publicly and commercially, and the expansion of receiver capabilities to all GNSS systems.