Study Of The GNSS-R Ocean Remote Sensing Technique

In this thesis,the technique of using GNSS-R signals to extract ocean surface geophysical information has been discussed in detail,It is an alternative purpose of the GNSS other than navigation or positioning that brings about an unprecedented source of data in the field of ocean remote sensing.The history of GNSS-R research is introduced in advance. On the assumption which the distribution of the sea surface height is Gaussian and the surface reflection is satisfied with the limit of the Kirchhoff Geometric Optics approximation, scattering cross-section coefficient is derived and a theoretical GNSS-R signal power waveform model has been developed which is depended on GNSS-R scattering geometry, sea surface roughness (characterized by mean-square slope) and so on. In order to calculate the mean-square slope, the Elfouhaily Model have been introduced, therefore, a theoretical model has been derived to retrieve wind field ,which can also be used for retracking in the airborne and space-borne altimetry applications by using GNSS-R signals. Furthermore, the shape and size of the iso-range annuli, iso-Doppler stripes and glistening zone have been discussed in details.The theoretical model and the method to inverse sea surface wind have been expatiated systemically by using microwave scattering theory and sea spectra, and the retrieve flow-chart , block diagram and operating principle has been discussed. Several retrieved examples have been introduced, which shows that the retrieval accuracys of wind speed and direction are about 2m/s and 20o respectively.Altimetry in GNSS-R can be carried out in two general ways, depending on the ranging principle used. In code altimetry, the code is used for ranging with the direct and reflected signals. In phase altimetry, the phase is used. Because Code altimetry is robust and applicable for ground-borne, air-borne and space-borne applications, the inverse methods are addressed in different receiver heights. Accuracy in code altimetry was demonstrated during the coastal Xiao Mai island Experiment (the results RMS error of the 5 minutes averaged GNSS-R absolute altimetric solution was of 39 cm) and the airborne Eddy Experiment (the results RMS error of the 20 km averaged GNSS-R absolute altimetric solution with respect to Jason-1 and GPS buoy measurements was of 10 cm, with a 2 cm mean difference). Base on the coastal experiment, the GNSS-R software receiver pre-processing algorithm is developed, which lays solid foundation for the futher research of oceanographic remote sensing. Furthermore, the potential capability of coverage and time-space resolution by GNSS-R from Low Earth Orbiters (LEOs) is studied, and the applicability of the space-borne GNSS-R to detect tsunami is discussed.As a new remote sensing tool, the GNSS-R technique is imperfect and developing, so the future work has been outlined in the end of the text.