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 single-difference 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.
Highlights
In this study we have introduced our single- and zerodifference excess phase processing of BeiDou System (BDS) radio occultation (RO) data of the FengYun 3 series C satellite (FY-3C) GNOS mission and evaluated the quality of atmospheric profiles derived from this single- and zerodifference processing
The advantage of the zero-difference algorithm is its independence from reference satellites, but it requires a receiver clock of very high quality to obtain a highly accurate receiver clock offset estimate, which can leave some residual errors after the clock offset correction
Because BDS is still a regional navigation system, we found that about 20 % of the GNOS BDS RO events do not have proper reference satellites for single differencing, providing another argument for a zero-difference alternative
Summary
The radio occultation (RO) technique (Melbourne et al, 1994; Ware et al, 1996) using signals from global navigation satellite systems (GNSSs), in particular from GPS, has been widely used to observe the Earth’s atmospheric parameters (e.g., bending angle, refractivity, temperature, pressure, and water vapor) for applications such as numerical weather prediction (NWP; e.g., Healy and Eyre, 2000; Kuo et al, 2000; Healy and Thepaut, 2006; Aparicio and Deblonde, 2008; Cucurull and Derber, 2008; Poli et al, 2008; Huang et al, 2010; Le Marshall et al, 2010; Harnisch et al, 2013) and global climate monitoring (GCM; e.g., Steiner et al, 2001, 2009, 2011, 2013; Schmidt et al, 2005, 2008, 2010; Loescher and Kirchengast, 2008; Ho et al, 2009, 2012; Foelsche et al, 2011a; Lackner et al, 2011). Zero-difference processing was started to be used (Beyerle et al, 2005; Wickert et al, 2005), which can compute excess phase data by applying prior estimated LEO and GNSS clock offsets without need of a reference satellite or ground station. It requires that the LEO receiver is equipped with an ultra-stable oscillator that, so far, was only available for the GRACE and MetOp missions (Beyerle et al, 2005; Luntama et al, 2008).
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