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

Abstract

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

The FengYun-3C radio occultation sounder GNOS: a review of the mission and its early results and science applications

“…Currently, in the processing of GNOS data, the single-differencing method is applied to obtain the excess phase as a function of time in an Earthcentered inertial reference frame, for all the GPS RO events and the BDS RO events which have sufficient reference satellites. Under the condition of fewer reference satellites, a zero-difference technique is applied and more appropriate for BDS RO events, since it does not require a reference satellite for simultaneous observations but requires an ultra-stable oscillator on an LEO receiver (Beyerle et al, 2005), which is available for GNOS (cf. Table 1; Bai et al, 2018).…”

“…An ultra-stable oscillator (USO) is used as a reference oscillator with highly stable frequency (1 s Allan deviation of 10 −12 ), in order to retrieve atmospheric measurements with high accuracy (Du et al, 2016;Sun et al, 2017). It also allows data users to invert the excess phase by using the zero-difference method (Beyerle et al, 2005;Bai et al, 2018).…”

Response to Referee 1

“…ECMWF fields were chosen for their proven leading quality (Untch et al, 2006;Bauer et al, 2015) and thus high suitability for serving as zero-order state profiles; any other reasonable model profiles could be used as well since the retrieval results negligibly depend on the exactly chosen zero-order model profiles. For comparison we plotted L m (t) for the COSMIC example case into Fig.…”

Validation of Preliminary Results of Thermal Tropopause Derived from FY-3C GNOS Data

“…Until now, FY-3C has been in operation for more than 5 years, providing about ten thousand atmosphere profiles per month. As a sun-synchronize satellite, the local time distribution of FY-3C RO events are centered at 10 am and 10 pm, which provides time-centered observation [28]. After 5 years of data accumulation, FY-3C GNOS (FY-3C in later sections) data are ready to be used in the climate research field.…”