Abstract
Abstract. In the GPS radio occultation technique, the atmospheric excess phase (AEP) can be used to derive the refractivity, which is an important quantity in numerical weather prediction. The AEP is conventionally estimated based on GPS double-difference or single-difference techniques. These two techniques, however, rely on the reference data in the data processing, increasing the complexity of computation. In this study, an undifferenced (ND) processing strategy is proposed to estimate the AEP. To begin with, we use PANDA (Positioning and Navigation Data Analyst) software to perform the precise orbit determination (POD) for the purpose of acquiring the position and velocity of the mass centre of the COSMIC (The Constellation Observing System for Meteorology, Ionosphere and Climate) satellites and the corresponding receiver clock offset. The bending angles, refractivity and dry temperature profiles are derived from the estimated AEP using Radio Occultation Processing Package (ROPP) software. The ND method is validated by the COSMIC products in typical rising and setting occultation events. Results indicate that rms (root mean square) errors of relative refractivity differences between undifferenced and atmospheric profiles (atmPrf) provided by UCAR/CDAAC (University Corporation for Atmospheric Research/COSMIC Data Analysis and Archive Centre) are better than 4 and 3 % in rising and setting occultation events respectively. In addition, we also compare the relative refractivity bias between ND-derived methods and atmPrf profiles of globally distributed 200 COSMIC occultation events on 12 December 2013. The statistical results indicate that the average rms relative refractivity deviation between ND-derived and COSMIC profiles is better than 2 % in the rising occultation event and better than 1.7 % in the setting occultation event. Moreover, the observed COSMIC refractivity profiles from ND processing strategy are further validated using European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data, and the results indicate that the undifferenced method reduces the noise level on the excess phase paths in the lower troposphere compared to the single-difference processing strategy.
Highlights
The radio occultation (RO) technique was first applied in the field of astronomy to detect the state of the planet’s atmosphere (Kursinski et al, 1997)
This study focuses on the extraction of the atmospheric excess phase (AEP) using the undifferenced processing strategy
The COSMIC precise orbit determination (POD) processing is used to accurately determine the position and velocity of the satellite mass centre and the receiver offset based on the PANDA software
Summary
The radio occultation (RO) technique was first applied in the field of astronomy to detect the state of the planet’s atmosphere (Kursinski et al, 1997). The ND processing can potentially obtain AEPs with lower noise by utilizing previously estimated LEO and GPS clocks (Beyerle et al, 2005; Schreiner et al, 2011). Beyerle et al (2005) firstly proposed the idea of using an ND technique to estimate the AEP and successfully analysed the GRACE-B satellite, correcting for the effect of receiver clock by interpolating the temporal resolution of 30 s GRACE-B’s receiver clock solutions into 20 ms Their results show that there is a good agreement of the refractivity between ND and SD techniques in the upper troposphere and lower stratosphere.
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