Reliable surface height observations over inland water bodies are useful for understanding the hydrological cycle.Satellite radar altimetry particularly contributed with its long-term archive and minimal cloud interfe...Reliable surface height observations over inland water bodies are useful for understanding the hydrological cycle.Satellite radar altimetry particularly contributed with its long-term archive and minimal cloud interference.Specialized inland water altimetry,developed from oceanography and geodesy,is still being extensively investigated.By synthesizing pioneering studies on“retracking algorithms”,this review demonstrates,from a user perspective,why optimizing conventional retracking is still important and how it can extend reliable historical water level retrieval over more ungauged sites.Numerous unrevealed inland water bodies have small sizes or complex surroundings,posing challenges to maintaining accuracy.Applications have shown that a critical key lies in the retracking correction during range retrieval(uncertainty likely on the order of meters),compared with other corrections(on the order of centimeters or decimeters).From multiple uncertainty factors in range retrieval,signal entanglements from land contamination and off-nadir effects are core issues.We evaluate and compared key strategies from prototype retrackers to improved retrackers,especially the empirical ones optimized for inland waters.Sub-waveform extraction and adjustment for Delay-Doppler modes has advanced range retrieval to a new stage.Four innovative inland-water-compatible retrackers are introduced in detail,with a highlight on their distinct approaches to robustly improve performance.Considering the selection of different data and retrackers in varying scenarios,a synthesis analysis is conducted based on results reported in previous literature.In conclusion,the empirical retracking has been enhanced to offer stable decimeter-level accuracy in intricate landscapes(e.g.,small lakes and rivers with varied surroundings).In comparison,the physical retracking has been upgraded to provide greater precision for homogeneous surface in large lakes.For future inland water altimetry,we articulate how additionally retracked results can benefit hydrological applications,and what difficulties would arise when extending study scales.展开更多
During the satellite pulse propagation and reception, the altimeter waveform is inevitably affected by noise. To reduce the noise level in Jason altimeter waveforms, we used singular spectrum analysis(SSA),empirical m...During the satellite pulse propagation and reception, the altimeter waveform is inevitably affected by noise. To reduce the noise level in Jason altimeter waveforms, we used singular spectrum analysis(SSA),empirical mode decomposition(EMD), and the combination of SSA and EMD to obtain the denoised waveforms. The advantages of the combined method were verified and the accuracy of the mean sea surface height(MSSH) model was improved. Comparing the denoising effect of the three methods, the results show that the signal-to-noise ratio(SNR), correlation coefficient and root-mean-square error are effectively improved by the combination of SSA and EMD. The sea surface heights(SSHs) were remeasured with a 50% threshold retracker of denoised waveforms, and the MSSH model of the Caspian Sea with a grid of 1’× 1’was established from the retracked SSHs of Jason-1/2/3. Taking the mean value of the four models as a control, it is found that the model calculated by the combined denoising method has the highest accuracy. This indicates that using the combined denoising method to reduce the noise level is beneficial to improve the accuracy of the MSSH model.展开更多
The quality of altimeter data and ocean tide model is critical to the recovery of coastal gravity anomalies. In this contribution, three retracking methods (threshold, improved threshold and Beta-5) are investigated w...The quality of altimeter data and ocean tide model is critical to the recovery of coastal gravity anomalies. In this contribution, three retracking methods (threshold, improved threshold and Beta-5) are investigated with the aim of improving the altimeter data over a shallow water area. Comparison indicates that the improved threshold is the best retracking method over China Sea. Two ocean tide models, NAO99b and CSR4.0, are analyzed. Results show that different tide models used in the processing of altimeter data may result in differences more than 10 mGal in recovered coastal gravity anomalies. Also, NAO99b is more suitable than CSR4.0 over the shallow water area of China Sea. Finally, gravity anomalies over China Sea are calculated from retracked Geosat/GM and ERS-1/GM data by least squares collocation. Comparison with shipborne gravimetry data demonstrates that gravity anomalies from retracked data are significantly superior to those from non-retracked data. Our results have the same order as the other two altimeter-derived gravity models: Sandwell&Smith(V16) and DNSC08.展开更多
The quality of satellite radar altimetric data is very important in studies of geodesy,geophysics,and oceanography.Over coastal oceans,altimeter waveforms are contaminated by the terrain and physical environments so t...The quality of satellite radar altimetric data is very important in studies of geodesy,geophysics,and oceanography.Over coastal oceans,altimeter waveforms are contaminated by the terrain and physical environments so that the accuracy of altimeter data is lower than that over open oceans.Here we develop a new multi-subwaveform parametric retracker(MSPR) to improve the quality of altimeter data for the recovery of gravity anomaly in coastal oceans.The least squares collocation method is used to recover the residual gravity anomaly over the coastal water from altimetric data.The waveform data records from Geosat/GM around Taiwan Island are practically retracked with MSPR.When compared with the Taiwan geoid height,the results retracked by MSPR are more accurate than those retracked by the well-known β-5-parmeter method and from the geophysical data records(GDRs).The gravity anomalies over Taiwan coastal waters are calculated from the retracked altimeter data with the least squares collocation.When we compared gravity anomalies computed using altimeter GDRs with the ship-borne gravity data over Taiwan coastal ocean,we found that the results from retracked data are more accurate than those from GDRs.展开更多
基金funded by the National Key Research and Development Program of China(2022YFF0711603)the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDA28020503)+2 种基金the National Natural Science Foundation of China(Grant No.42371399,42301431)Basic Research Program of Jiangsu(Grant No.BK20240112)the Science and Technology Planning Project of NIGLAS(Grant No.2022NIGLAS-CJH04).
文摘Reliable surface height observations over inland water bodies are useful for understanding the hydrological cycle.Satellite radar altimetry particularly contributed with its long-term archive and minimal cloud interference.Specialized inland water altimetry,developed from oceanography and geodesy,is still being extensively investigated.By synthesizing pioneering studies on“retracking algorithms”,this review demonstrates,from a user perspective,why optimizing conventional retracking is still important and how it can extend reliable historical water level retrieval over more ungauged sites.Numerous unrevealed inland water bodies have small sizes or complex surroundings,posing challenges to maintaining accuracy.Applications have shown that a critical key lies in the retracking correction during range retrieval(uncertainty likely on the order of meters),compared with other corrections(on the order of centimeters or decimeters).From multiple uncertainty factors in range retrieval,signal entanglements from land contamination and off-nadir effects are core issues.We evaluate and compared key strategies from prototype retrackers to improved retrackers,especially the empirical ones optimized for inland waters.Sub-waveform extraction and adjustment for Delay-Doppler modes has advanced range retrieval to a new stage.Four innovative inland-water-compatible retrackers are introduced in detail,with a highlight on their distinct approaches to robustly improve performance.Considering the selection of different data and retrackers in varying scenarios,a synthesis analysis is conducted based on results reported in previous literature.In conclusion,the empirical retracking has been enhanced to offer stable decimeter-level accuracy in intricate landscapes(e.g.,small lakes and rivers with varied surroundings).In comparison,the physical retracking has been upgraded to provide greater precision for homogeneous surface in large lakes.For future inland water altimetry,we articulate how additionally retracked results can benefit hydrological applications,and what difficulties would arise when extending study scales.
基金We acknowledge the National Natural Science Foundation of China(grant number 41974013)for financial support.
文摘During the satellite pulse propagation and reception, the altimeter waveform is inevitably affected by noise. To reduce the noise level in Jason altimeter waveforms, we used singular spectrum analysis(SSA),empirical mode decomposition(EMD), and the combination of SSA and EMD to obtain the denoised waveforms. The advantages of the combined method were verified and the accuracy of the mean sea surface height(MSSH) model was improved. Comparing the denoising effect of the three methods, the results show that the signal-to-noise ratio(SNR), correlation coefficient and root-mean-square error are effectively improved by the combination of SSA and EMD. The sea surface heights(SSHs) were remeasured with a 50% threshold retracker of denoised waveforms, and the MSSH model of the Caspian Sea with a grid of 1’× 1’was established from the retracked SSHs of Jason-1/2/3. Taking the mean value of the four models as a control, it is found that the model calculated by the combined denoising method has the highest accuracy. This indicates that using the combined denoising method to reduce the noise level is beneficial to improve the accuracy of the MSSH model.
基金Supported by the National Natural Science Foundation of China(No.40704003)the National 973 Program of China(No.2007CB714405)the Key Laboratory of Surveying and Mapping Technology on Island and Reef,State Bureau of Surveying and Mapping,China(No.2009B04)
文摘The quality of altimeter data and ocean tide model is critical to the recovery of coastal gravity anomalies. In this contribution, three retracking methods (threshold, improved threshold and Beta-5) are investigated with the aim of improving the altimeter data over a shallow water area. Comparison indicates that the improved threshold is the best retracking method over China Sea. Two ocean tide models, NAO99b and CSR4.0, are analyzed. Results show that different tide models used in the processing of altimeter data may result in differences more than 10 mGal in recovered coastal gravity anomalies. Also, NAO99b is more suitable than CSR4.0 over the shallow water area of China Sea. Finally, gravity anomalies over China Sea are calculated from retracked Geosat/GM and ERS-1/GM data by least squares collocation. Comparison with shipborne gravimetry data demonstrates that gravity anomalies from retracked data are significantly superior to those from non-retracked data. Our results have the same order as the other two altimeter-derived gravity models: Sandwell&Smith(V16) and DNSC08.
基金supported by National Natural Science Foundation of China (Grant No.40774009)National Hi-tech R & D Program of China (Grant No.2006AA12Z303)+2 种基金International S & T Cooperation Program of China (Grant No.2009DFB00130)Special Fund of Taishan Scholars Project of Shandong Province,China (Grant No.TSXZ0502)Key Laboratory of Mapping from Space of SBSM,China (Grant No.200801)
文摘The quality of satellite radar altimetric data is very important in studies of geodesy,geophysics,and oceanography.Over coastal oceans,altimeter waveforms are contaminated by the terrain and physical environments so that the accuracy of altimeter data is lower than that over open oceans.Here we develop a new multi-subwaveform parametric retracker(MSPR) to improve the quality of altimeter data for the recovery of gravity anomaly in coastal oceans.The least squares collocation method is used to recover the residual gravity anomaly over the coastal water from altimetric data.The waveform data records from Geosat/GM around Taiwan Island are practically retracked with MSPR.When compared with the Taiwan geoid height,the results retracked by MSPR are more accurate than those retracked by the well-known β-5-parmeter method and from the geophysical data records(GDRs).The gravity anomalies over Taiwan coastal waters are calculated from the retracked altimeter data with the least squares collocation.When we compared gravity anomalies computed using altimeter GDRs with the ship-borne gravity data over Taiwan coastal ocean,we found that the results from retracked data are more accurate than those from GDRs.
