Research On Retrieval Method Of Sea Surface Significant Wave Height And Wind Field For Shipborne GNSS-R

In recent years,since the rise of the concept of smart ships and the increasing development of smart ship technology,the intelligent navigation has become the development trend of global shipping.The information perception technique of navigation environment is the primary issue to be solved to realize the intelligentization of ships,and it is also one of the key technologies to ensure the safety of maritime traffic.At present,traditional marine information detection methods have various limitations,including small data volume and high cost,limited spatial and temporal resolution,and lack of systematicness and integrity.However,with the gradual enrichment and diversified development of the Global Navigation Satellite System(GNSS),a new remote sensing technology based on GNSS-R(GNSS-Reflectometry)has gradually developed.With the advantages of low cost,all-weather,high temporal and spatial resolution,and wide coverage,GNSS-R technology has been applied in sea surface wind field,significant wave height,sea surface oil spill,sea ice,and other researches.Since the relevant researches and experiments of this technology are predominantly concentrated on spaceborne and airborne platforms,whereas research and experiments related to shipborne platforms are still in the blank stage.Besides,the ocean state information is a key parameter to ensure the safety of ship navigation,this thesis aims to study the scattering characteristics of GNSS-R through actual ship experiments,and establish the retrieval method of sea surface wind field and significant wave height for shipborne GNSS-R.The relevant studies are as follows:(1)Aiming at the issue that traditional GNSS-R wind speed retrieval methods can not effectively retrieve the wind speed for shipborne GNSS-R,this study proposes a sea surface wind speed retrieval method using curve slopes from zero frequency shift of Delay Doppler Map(DDM)for shipborne GNSS-R.For the low altitude of GNSS-R equipment under the ship platform,the Glistening Zone(GZ)of reflected signals on the sea surface is concentrated in a small chip range.This study uses the scattered signals with integrated coherently and accumulated incoherently by multi-channel correlator to construct shipborne DDM.Therefore,based on the characteristics of Doppler frequency in GZ and the different slopes change in the front and rear stages of delay waveforms,the sensitivity of the slope of each stage to different wind speeds was analyzed,and a GNSS-R wind speed retrieval model using mixed slopes was established.Experimental results using an actual ship show that the proposed algorithm produces smaller root mean square error,and the accuracy is improved by about 28%compared with the traditional methods applied to the ship platform.Therefore,this study can fill the gap of wind speed retrieval method for shipborne GNSS-R platform.(2)In order to improve the sensitivity of shipborne GNSS-R to wind direction,the delay window area of delay Doppler maps in scattering signal is defined,and a wind direction retrieval method for shipborne GNSS-R is also proposed to constract the relationship betweent delay window area and wind direction.However,in the process of wind direction inversion using GNSS-R technology,a same observation value of delay window area will present multiple wind direction results.To eliminate this ambiguity,this thesis uses satellite observations at different azimuths to determine the final wind direction information.The shipborne experiments show that the multi-azimuth information can be used to solve the multi-solution problem in wind direction retrieval process,and the estimated results based on the proposed algorithm are closer to the distribution of true value data,with a root mean square error of ± 21°,which can effectively realize the wind direction retrieval for shipborne GNSS-R.(3)Aiming at the low estimated accuracy of significant wave height(SWH)for shipborne GNSS-R,this study proposes an SWH retrieval method using accumulated delay Doppler correlation power,based on energy distribution characteristics of scattered signals for shipborne GNSS-R.Besides,this study introduces an offset correction that combines the differences between the elevation angles of multi-satellites.The correction reduces the impact of satellite altitude angle information on the estimation of SWH and improves the correlation between accumulated delay Doppler correlation power and SWH.And the proposed algorithm is verified by shipborne experiments.The results show that the proposed algorithm,with a mean absolute percentage error of 5.88%,a root mean square error of 0.12 m,and a maximum error of 0.28 m,are highly consistent with the true values of buoy.The method is intended to improve the retrieval accuracy of SWH based on the GNSS-R technique,and to provide a new method for sea-state information monitoring during ship navigation.This thesis has carried out relevant research and experiments on the sea surface wind field and significant wave height around shipborne GNSS-R technology.It can provide technical support for the information perception of ship navigation environment,which is of great significance for ensuring maritime traffic safety,and can provide a new information platform for the marine integrated environmental monitoring.