The Determination Of Marine Gravity Anomalies Over Antarctic Oceans From Satellite Altimetry

Due to the limitations of poor geographical conditions and a lack of in-situ observations, knowledge about Antarctic Oceans (AOs) has not been explored well for a long time. However, these ocean areas are closely connected to global climate and ecological environment. Thus, it is important to improve our knowledge over these areas.Gravity anomaly (GA) is one of the basic physical quantities over the oceans. Highly accurate GA can help to improve the knowledge of internal structure and marine resources exploration, and satellite altimeter is the currently best approach to obtain the data over the oceans.Sea surface height (SSH) is the basic parameter derived from satellite altimeter. Normally, in the open ocean, it is relatively precise as the altimeter radar return follow ocean model of Brown, making it possible to derive precise gravity anomalies. However, the accuracy of SSH is often degraded due to a variety of factors, and may lead to false results in the resulting GAs. In order to improve the determination of GAs, SSH needs to be ameliorated by waveform retracking, updated altimeter corrections and data processing.This thesis introduces the ocean waveform model of Brown with a detailed waveform classification. It develops a new algorithm, called sub-waveform threshold, and presents an optimal waveform retracker. Next this thesis discusses data processing of SSH at different stages and deflection of the vertical (DOV), and derives 2'×2' gridded GAs over AOs. The quality of the derived gravity anomalies is evaluated by comparison with ship gravity data, resulting in a RMS agreement of about 7 mGal over AOs. This result is close to or even better than those of the DNSC08 and Sandwell and Smith gravity models. Finally this thesis discusses data fusing using altimeter-only and ship gravity value, and derives a new GA model around AOs.The major findings of thesis are summarized below:1. Sub-waveform threshold and the optimal waveform retracker selection This thesis presents a sub-waveform threshold retracker to determine sub-waveform with good sample waveforms, and improve the accuracy of gravity anomalies. A criteria parameter, which is correlated with quality of DOV, is present to determine the optimal retracker for the gravity anomaly derivation. The performances of P-5, threshold and sub-waveform threshold are assessed over waters around AOs. The sub-waveform threshold outperforms the other two. Use of the sub-waveform threshold leads to the improvement percentage of about 38%,78.8% and 90%, for Geosat/GM, diffuse and specular waveforms of ERS-1/GM over AOs, respectively.2. Data processing of altimeter SSHIn order to improve the accuracy of high-frequency SSH, the standard remove-restore procedure is firstly used at the stage of SSH processing. Different data processing methods, such as Guassian filter, average-moving of SSH and a modified spline polynomial function of Maus' (Maus et al,1998), are used at the corresponding stages, and a re-sampling 2-Hz SSHs are derived. Along some tracks, there exists large SSH jumps caused by the errors in altimeter data, incorrect corrections or bad waveforms.3. The optimal strategy determination for gravitySeveral factors, such as reference gravity field and the methods of gravity determination and deflection of the vertical, may affect the quality of satellite-only gravity anomalies. Altimeter-only gravity anomalies on a 2'×2'grid are derived based on several strategies. The altimeter-derived gravity anomalies are compared with ship gravity to determine the optimal strategy for gravity. The standard deviation of the differences between altimeter-derived gravity and ship gravity improves 30% as the 2160 degree of EGM08 the reference gravity field, compared to that of 360 degree. The accuracy of inverse Verning-Meinsz is close to that of least-square collocation, while the computation time of the former is just 40% of that of the latter.4. Marine gravity anomaly derivation over AOFollowing optimal strategy for gravity, gravity anomalies over sub-regions of Antarctic Oceans are derived from altimeter data on a 2'×2' grid. The gravity anomaly grids from this thesis (current model), DNSC08 model and Sandwell and Smith, are compared with ship gravity. The standard deviation of the differences between altimeter-derived gravity and ship gravity are about 3.2～11.2 mGal over AOs. The accuracy of current model is close to those of the other two, and better over several oceans. Over AOs, the current model improves by 2.2 mGal and 1.1 mGal compared to Sandwell and Smith model and DNSC08 model, respectively.5. Data fusing of ship and altimeter-only gravityTaken high-accuracy ship gravity as the control points, two methods of combining ship and altimeter-only gravity have been experimented over coastal waters around Taiwan. One is least square collocation and the other is Draping. Draping outperforms the LSC in the accuracy and efficiency and is used to get the marine gravity anomalies over AOs on a 2'×2' grid, which improves over the altimeter-only gravity and ship-only gravity anomaly grids.6. Waveform classification and altimeter application in sea iceReturned waveforms of altimeter depend largely on the property of the reflecting surface. This thesis presents methods of waveform identification and classification. An altimeter application on sea ice concentration (SIC) and sea ice distribution (SIS) is investigated, with an introduction to the principle of sea ice depth determination. The altimeter-derived SIC is compared with that of remote sensing, and the former shows a higher concentration of sea ice in the studied area. Altimeter data can not only be used to derive SIC and SIS, but also identify floating sea ice.