Abstract
The accurate modeled GNSS-R reflection delay, which is indispensable for the quantification, modeling, and correction of the GNSS-R altimetry sea-state bias, can be obtained based on the accurate modeled position of the specular point. At present, the reflection surface model of the specular point positioning still has the mean dynamic topography (MDT) error and the deviation of the vertical (DOV) error relative to the instantaneous sea surface. In this study, the following studies have been carried out. Based on the ship-borne GNSS-R observations in China seas, we introduced various elevation parameters including the MDT to correct the elevation error of the reflection surface. We introduced the DOV based on the elevation correction, and the DOV correction positioning method was proposed to correct the slope error of the reflection surface. The specular point was positioned on the instantaneous sea reflection surface. We verified the instantaneous sea reflection surface model and the specular point positioning results, analyzed the relationship between the position correction distance and the reflection incident angle, and discussed the spatial distribution characteristics of the MDT correction distance. The results showed that the reflection surface modeling and the specular point positioning were accurate. The positioning error increased to varying degrees with the increase of the reflection incident angle. The MDT correction improved the positioning by 0.91 m, and the DOV correction further improved the positioning by 0.12 m. Based on the combined application of the two kinds of correction positioning, the positioning was comprehensively improved by 0.99 m. The MDT correction of China seas gradually increased from the north to south. While in the regional sea areas, it gradually decreased from the north to south and showed randomness. The relative position between the antennas and their random changes introduced uncertainty, which can be reduced by integration. The new instantaneous sea reflection surface model and the corresponding specular point positioning method can provide accurate modeled reflection delay for the sea-state bias correction of ship-borne GNSS-R observations, and they can be extended to satellite-borne global observations.
Highlights
Based on the geoid and the ocean tidal reflection surface model constructed in our previous research, we sequentially introduced the mean dynamic topography (MDT) and the deviation of the vertical (DOV) to correct the elevation error and the slope error with the corresponding specular point positioning method
We calculated the difference between the positions of the specular points before and after adding the MDT error and the DOV error, and the mean value is 5.23 cm
In the DTU15 MDT error map, the error near Xiamen does not increase significantly compared with other parts of the study area, and the number of samples in Xiamen is small, so this will not be the main source of error
Summary
GNSS-R altimetry can provide global high-coverage sea surface height (SSH) observations for research on global ocean mesoscale and sub-mesoscale processes and global climate change as an auxiliary means to traditional altimeters (Martín-Neira, 1993; Wu et al, 1998; Stammer et al, 2000; Hajj and Zuffada, 2003; Saynisch et al, 2015; Zuffada et al, 2015; Li et al, 2016; Xie et al, 2018). On the other hand, obtaining accurate reflection delay based on model calculation is indispensable in the quantification of the SSB This requires correcting the specular point geometric positioning error introduced by the difference between the modeled sea surface and the instantaneous sea surface. Based on the geoid and the ocean tidal reflection surface model constructed in our previous research, we sequentially introduced the MDT and the DOV to correct the elevation error and the slope error with the corresponding specular point positioning method. The used geophysical models included the geoid undulation of the EGM2008 model, the ocean tidal heights of the TPXO model, and the DTU15 MDT elevations On this basis, we introduced the DOV from the GGMPlus gravitational field to correct the sea surface slope errors.
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