Study On Satellite Radar Altimeter Retrieval Algorithms Over Coastal Seas And Under High Sea State Events

Radar altimeter is an active microwave sensor, which measured the instantaneous range between the satellite and sea surface, SWH (Significant Wave Height) and radar cross section of the ocean surface at the nadir point of satellite. The instantaneous SSH (Sea Surface Height), ocean surface wind and wave field, mean sea level, ocean gravitational field, tidal system, etc. can be derived, which are widely used by oceanography community. There are two limiting factors of altimetry, one is that most coastal altimetry data are flagged as useless; the another is that altimetry data under extreme weather events such as tropical cyclones, typhoons and hurricanes are affected by various factors associated with high sea states, such as atmospheric rains and ocean surface foams. The object of this thesis is to solve those two problems. Main achievements are as following:(1) Based on evaluation of large amount of coastal return waveform of JASON1 and ENVISAT altimeter, the problems and affected distances of altimetry waveform at China and adjacent coastal areas are presented.(2) The raw SSH and SWH are derived from JASON1 waveform measurements by using the standard Ocean retracking algorithm at open ocean, which are consist with JASON1 GDR products. Four specialized retrackers: Beta5/9, OCOG (Offset Centre of Gravity), Ice-2, and Threshold retracking algorithms are employed at coastal sea. In order to quantitative compare retracker, the bias, root mean square and standard deviation of the SLA(Sea Level Anomaly) difference between the ascending and descending passes at the crossovers are calculated; the SSH derived from five retracking algorithms are compared with in-situ gauge station measurements; the SWH derived from five retracking algorithms are compared with in-situ gauge station and buoy measurements. Based those studies, we proposed the retracking strategy based on waveform classification and sub-waveform extraction for coastal altimetry. The corresponding validations are given.(3) Using a simplified stratified foam model and scattering theory of electromagnetic wave, the attenuation effect of foam is investigated, which should not be neglected under high sea state. We proposed an iterative method to correct both rain and sea foam effects aided by the liquid water content measurements from Jason Microwave Radiometer. Therefore, the accurate wave height, wind speed, rain rate, and the additional foam coverage from dual frequency altimeter measurement in typhoon can be obtained considering both effects of rain and sea foam. In summary, the waveform retracking algorithm and correction method proposed in this these are helpful to solve the problems of coastal altimeter waveform processing and altimeter data under high sea state, respectively. The quantity and quality of satellite altimetry data are improved.The Wave Spectrometer is new sensor, which employs five non-nadir beams with beam scanning in azimuth (0-360°) in addition to nadir beam of altimeter. The principle of Wave Spectrometer to measure wave spectrum at small incidence is analysis and compared to the real aperture radar and Synthetic Aperture Radar. The simulation and retrieval of Wave Spectrometer are performed based on the non-linearity of the relation between image spectrum and wave spectrum.