Abstract
The elevation angle influence on coastal GNSS-R ocean code-based altimetry for GPS signals (L1 C/A and L5) and BDS B1 signals is investigated, and the corresponding correction method is presented. The study first focuses on the coastal ocean altimetry method, including the general experiment geometry and the code delay estimation using the single-point tracking algorithm. The peak power and the maximum first derivative are used as the location of the specular point. Then, the sensitivity of the height retrieved using the above coastal ocean altimetry method to elevation angle is analyzed based on the Z-V model. It can be seen that the elevation angle has a significant influence on the height retrieval, which will affect the precision of the coastal GNSS-R ocean altimetry. Finally, two correction methods, the model-driven method and the data-driven method, are proposed. The coastal altimetry experiments demonstrate that the correction methods can correct the elevation angle influence, and the data-driven method is more effective. The experimental results show that, after correcting the elevation angle influence, the code-based altimetry precision of the GPS L1 C/A signal, L5 signal, and BDS B1 signal can be up to the meter level, decimeter level (less than 4 decimeters), and meter level with respect to a reference tide gauge (TG) data set, respectively, without smoothing over time. These results provide information to guide the sea surface height retrieval using coastal GNSS-R, especially multi-satellite observation and GNSS signal with a higher chipping rate.
Highlights
Global navigation satellite system reflectometry (GNSS-R) that was primarily proposed for ocean mesoscale altimetry by Martin-Neira in 1993 [1] has been significantly extended to a wide range of applications as an emerging remote sensing technology
Ocean altimetry; ρ p is the propagation distance deviation caused by non-specular scattering and electromagnetic bias, which is sensitive to sea surface roughness and satellite elevation angle [44]; ρe is the residual measurement error induced by the instrumental delays; ∆ρ is the geometric path delay between the reflected signal and the direct signal
In order to evaluate the performance of the data-driven method, the training and testing datasets are randomly split as 50% and 50% of the whole dataset, respectively, which is similar to the deep learning or artificial neural networks
Summary
Global navigation satellite system reflectometry (GNSS-R) that was primarily proposed for ocean mesoscale altimetry by Martin-Neira in 1993 [1] has been significantly extended to a wide range of applications as an emerging remote sensing technology. The cGNSS-R retrieves the sea surface height through the time delay between reflected and direct GNSS signals. That the coherent components of the reflected signal should be sufficient enough; that is, the reflective surface should be smooth (such as sea ice or inland water), which limits the use of ocean altimetry based on the GNSS-R carrier-based method [35]. The precision of sea surface height based on the code-based method is limited by the code chip length of the GNSS signals [36]. The elevation angle influence on coastal GNSS-R ocean altimetry using the GPS signals (L1 C/A and L5) and BDS B1 signals based on code phase is theoretically and experimentally evaluated. The influence of elevation angles on coastal sea surface height retrieval is corrected by the theoretical model simulation and the data-driven model function, respectively.
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