| dc.description.abstract | Global Navigation Satellite System-Reflectometry, or GNSS-R, is a method that involves receiving reflected signals from the Earth′s surface, transmitted by the GNSS. The aim is to measure the Normalized Bistatic Radar Cross Section (NBRCS), which enables the acquisition of geophysical information about the scattering surface. Following the technical theory demonstration from 1998 to 2004 and the initial establishment of the radar scattering cross-sectional area correction algorithm, the GNSS-R technique has been widely applied to the retrieval of geophysical information such as sea surface wind speed, sea surface heat flux, inland water extend detection, and soil moisture. These are significant implications for disaster prevention and mitigation, such as typhoon forecasting, inland flood inundation or seawater inundation. GNSS-R became a new candidate for microwave sea surface wind remote sensing for its low risk of being influenced by precipitation, high revisit period, low mass and low-cost properties. United Kingdom (2003, 2014), America (2017) and Mainland China (2019) have already launched their GNSS-R satellite, and they have all been applied to retrieve the sea surface wind speed, u_10. Wind speed-wave height-wave frequency has been proven to be a coupled variable in high wind conditions and is called a wind-wave triplet. Wave age which represents the wave development stage have been proved to vary with wind speed and ocean surface roughness, mean square slope. The wind speed retrieving algorithm for GNSS-R has been explored for years. Most studies directly link Delay Doppler Map observable to wind speed to generate a one-step geophysical model function (GMF). In Cyclone Global Navigation Satellite System (CYGNSS) mission, they have built a wind speed retrieving algorithm for high wind speed conditions (15~70 m/s) by linking DDM observable to hurricane penetrate-measured collocated data and mentioned that wave age and fetch length in the state of development of the long wave portion near or far from a hurricane would account for the high uncertainty in high wind speed GMF establishment. Significant wave height has been used to retrieve wind speed in NOAA′s algorithm, however, no study has reported the usage of wave age or wave frequency to retrieve the wind speed. The GNSS-R satellite TRITON will be launched in September, 2023. This study aims to build a pre-launch DDM calibration and retrieving system using the published operational products from CYGNSS system for TRITON.
There are two aims of this study. The first aim is to establish the Level 1b calibration module to obtain the qualified normalized bistatic radar cross section (NBRCS) from GNSS-R measured Delay Dopper Map (DDM). The second aim is to develop a wind speed retrieving module applying the latest theories based on theoretical models of wind and wave interactions under high wind speeds to get wind products that better represent the natural condition. For the first time, this research will test the use of dimensionless wave frequency -- wave age as a second variable for wind speed retrieval and evaluate its difference in quality from DDMA-u_10 GMF method. Furthermore, the results will be compared with the quality of wind speed retrieval based on GNSS-R technology from different countries using the same reference wind speed. The established system can provide NBRCS, wind speed and ocean surface roughness products for the data assimilation system.
The contribution of this study is, firstly, we have established a Level 1b calibration system base on the published literature and have produced Delay Doppler Map Average (DDMA) product in which the residual to the reference product is within 5%. The wind speed retrieved from the latest-built DDMA-Mean Square Slope-Wave age GMF improved the root-mean-square-difference to the degree of 50% compared to the traditional method, and the bias is within 0.1 m/s for wind speed within 0 to 12 m/s. The quality is competitive with the products from other teams. | en_US |