A perspective on the fundamental quality of GPS radio occultation data

Abstract

Abstract. Radio occultation (RO) is a promising source of observation for weather and climate applications. However, the uncertainties arising from imperfect retrieval algorithms may weaken the overall confidence in the data and discourage their use. As an alternative approach of assessing the quality of RO data while avoiding the nuisance of retrieval errors, this study proposes to use minimally processed data (measurement) instead of derived RO data. This study compares measured phase paths with their model counterparts, simulated with an effective ray tracer for which the refractive indices along the complete ray path linking the transmitter and the receiver are realistically specified. The comparison of phase measurements with the European Centre for Medium-Range Weather Forecasts (ECMWF) data made in the observation space shows that the RO measurements are of sufficient accuracy to uncover regional-scale systematic errors in ECMWF's operational analysis and the 45-year reanalysis (ERA40), and to clearly depict the error growth of short-term ERA40 forecasts. In the southern hemispheric stratosphere, in particular, the RO measurements served as a robust reference against which both of the two analyses were significantly biased in opposite directions even though they were produced by the same center using virtually the same set of data. The measurement and ECMWF analyses showed a close agreement in the standard deviation except for the regions and heights that the quality of the ECMWF data is controversial. This confirms the high precision of RO measurements and also indicates that the main problem of the ECMWF analyses lies in their systematic error.

Assessment of NOAA NUCAPS upper air temperature profiles using COSMIC GPS radio occultation and ARM radiosondes

“…Typically, the vertical resolution of the RO observations ranges from 0.5 km in the lower troposphere to 1.4 km in the middle atmosphere [ Kursinski et al , ]. Wee and Kuo [] recently made use of the high‐quality RO‐measured phase paths and successfully used them to assess model forecasts and reanalyses.…”

Comparison of COSMIC and COSMIC-2 Radio Occultation Refractivity and Bending Angle Uncertainties in August 2006 and 2021

“…It is measured in v/v and is determined by antenna gain, the GNSS transmitter power, receiver noise, local multipath (signals scattered off the spacecraft near the antenna), and observational geometry [22,23]. SNR is a factor, but not the only one, in determining the total RO retrieval uncertainty (random error statistics) in bending angle (BA) and refractivity (N), which are calculated farther down in the processing chain [24,25].…”

Atmospheric GNSS RO 1D-Var in Use at UCAR: Description and Validation

“…1D-Var can also employ advanced observation operators, which might be too sophisticated for use in data assimilation. As an example, Wee et al [86] and Wee and Kuo [87] used a curved ray tracing to model phase measurements directly while extending the computational grid to the orbital height of GNSS satellites (~20,000 km). Our future work will focus on implementing these improvement and augmentation.…”