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. The Global Navigation Satellite System (GNSS) Occultation Sounder (GNOS) is one of the new-generation payloads onboard theChinese FengYun 3 (FY-3) series of operational meteorological satellites for sounding the Earth's neutral atmosphere andionosphere. The GNOS was designed for acquiring setting and rising radio occultation (RO) data by using GNSS signals fromboth the Chinese BeiDou System (BDS) and the US Global Positioning System (GPS). An ultra-stable oscillator with 1 sstability (Allan deviation) at the level of 10−12 was installed on the FY-3C GNOS, and thus both zero-difference andsingle-difference excess phase processing methods should be feasible for FY-3C GNOS observations. In this study we focuson evaluating zero-difference processing of BDS RO data vs. single-difference processing, in order to investigate thezero-difference feasibility for this new instrument, which after its launch in September 2013 started to use BDS signalsfrom five geostationary orbit (GEO) satellites, five inclined geosynchronous orbit (IGSO) satellites and four medium Earth orbit(MEO) satellites. We used a 3-month set of GNOS BDS RO data (October to December 2013) for the evaluation and comparedatmospheric bending angle and refractivity profiles, derived from single- and zero-difference excess phase data, againstco-located profiles from European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. We also compared againstco-located refractivity profiles from radiosondes. The statistical evaluation against these reference data shows that theresults from single- and zero-difference processing are reasonably consistent in both bias and standard deviation, clearly demonstratingthe feasibility of zero differencing for GNOS BDS RO observations. The average bias (and standard deviation) of the bending angle andrefractivity profiles were found to be about 0.05 to 0.2 % (and 0.7 to 1.6 %) over the upper troposphere and lowerstratosphere. Zero differencing was found to perform slightly better, as may be expected from its lower vulnerability tonoise. The validation results indicate that GNOS can provide, on top of GPS RO profiles, accurate and precise BDS ROprofiles both from single- and zero-difference processing. The GNOS observations by the series of FY-3 satellites are thusexpected to provide important contributions to numerical weather prediction and global climate change analysis.

atmospheric measurement techniques

Evaluation of atmospheric profiles derived from single- and zero-difference excess phase processing of BeiDou radio occultation data from the FY-3C GNOS mission