Abstract
The radio occultation technique provides stable atmospheric measurements that can work as a benchmark for calibrating and validating satellite-sounding data. Launched on 25 June 2019, the Constellation Observing System for Meteorology, Ionosphere, and Climate 2 and Formosa Satellite Mission 7 (COSMIC-2/FORMOSAT-7) are expected to produce about 5000 high-quality RO observations daily over the tropics and subtropics. COSMIC-2 constellation consists of 6 Low Earth Orbit (LEO) satellites in 24° inclination orbits at 720 km altitude and distributed mainly between 45°N to 45°S. The COSMIC-2 observations have uniform temporal coverage between 30°N to 30°S. This paper presents an independent inversion algorithm to invert COSMIC-2 geometry and phase data to bending angle and refractivity. We also investigate the quality of Global Navigation Satellite System (GNSS) and LEO position vectors derived from the UCAR COSMIC Data Analysis and Archive Center (CDAAC). The GNSS and LEO position vectors are stable with LEO position variations < 1.4 mm/s. The signal-to-noise ratio (SNR) on the L1 band ranges from 300–2600 v/v with a mean of 1600 v/v. The inversion algorithm developed at NOAA Center for Satellite Applications and Research (STAR) uses the Full Spectrum Inversion (FSI) method to invert COSMIC-2 geometry and phase data to bending angle and refractivity profiles. The STAR COSMIC-2 bending angle and refractivity profiles are compared with in situ radiosonde, the current COSMIC-2 products derived from CDAAC, and the collocated European Center for Medium-Range Weather Forecasts (ECMWF) climate reanalysis data ERA5. The mean bias at 8–40 km altitude among the UCAR, ERA5, and STAR is <0.1% for both bending and refractivity, with a standard deviation in the range of 1.4–2.3 and 0.9–1.1% for bending angles refractivity, respectively. In the lowest 2 km, the RO bias relative to ERA-5 shows a strong latitudinal and SNR dependence.
Highlights
Radio occultation (RO) techniques measure the time delay of carrier radio signals passing through the atmosphere between the Global Navigation Satellite System (GNSS)satellite and the receiver onboard low earth orbit (LEO) satellite [1]
The inversion algorithm developed at NOAA Center for Satellite Applications and Research (STAR) uses the Full Spectrum Inversion (FSI) method to invert COSMIC-2 geometry and phase data to bending angle and refractivity profiles
The study finds that the COSMIC-2 position vectors have high stability with the incremental rate of change of the position vectors < 0.1 mm/s for the GNSS positions and
Summary
Radio occultation (RO) techniques measure the time delay of carrier radio signals passing through the atmosphere between the Global Navigation Satellite System (GNSS). The approach to inverting geometry and phase data to bending angle profile is called the geometric optics (GO) inversion method [24,25,26]. [31] demonstrated that for a uniformly circular orbital geometry, the Fourier transform phase derivative to frequency gives the central angle formed by the radius vectors to the LEO and the GNSS satellites, and the total frequency of each ray is proportional to the impact parameter. We apply the FSI method to develop an independent RO processing package to invert COSMIC-2 excess phase to bending angle and refractivity profiles. Based on the FSI method, we convert COSMIC-2 occultation geometry and phase data into bending angle and refractivity profiles (Figure 1).
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