Research progress of water level extraction methods based on satellite altimeter

Due to climate and topography and there are few water level surface monitoring sites of rivers,lakes and reservoirs,poor continuity of monitoring data and high cost of monitoring,which brings great inconvenience to the scientific assessment of water reserves and the study of the impact of climate change.The development of multitemporal resolution remote sensing technology brings a new opportunity for longterm and rapid monitoring of water level in a large range.The satellite altimeters with low cost,high efficiency,and wide monitoring range allows for extracting the water level which is particularly important for areas where there are lack of in-situ observations. Comparing the methods of traditional radar,synthetic aperture radar(SAR),and other altimeters,this paper reviews advances in applying fullwaveform and wavelet retracking algorithm for various inland water extracting in order to study the merits and demerits of the existing water level extraction method and guide its future application in different scenes,as well puts forward the proposal of improved method for future of satellite altimeters. The results revealed that the subsequent launches of conventional radar altimeter has been gradually improved in spatial and temporal resolution,the measurement precision has been gradually improved,and the time series is longer than before,but it is difficult for the relatively large footprint to monitor small and mediumsized water bodies,and the operation task has basically ended.SAR altimeter is the main satellite altimeter currently in orbit and will be launched，which can enhance the accuracy，reduce the footprint,and make the echo characteristics of water body more obvious.But it has the short operation time and the low data temporal and spatial coverage.It can be combined with the traditional radar altimeter through precision correction to extend the water level time series.Laser altimeter has the absolute advantage of small footprint,high measuring accuracy,more water bodies that can be monitored than most satellite altimeter,and low time resolution.The design of the wide swath altimeter can improve spatial and temporal coverage of data significantly.It can make up for the deficiency of the existing satellite altimeter trajectory only linear monitoring after launch,and promote the application of satellite altimeter water level extraction data in production practice.And it is expected to become the mainstream satellite altimeter in the future.The narrow primary peak retracker and multiple waveform persistent peak retracker have become available for large and mediumsized rivers and lakes with flat terrain.The target waveform retracker based on waveform classification is suitable for large lakes.The bimodal correction algorithm is suitable for lakes with frozen period in cold regions of high latitude.The threshold retracker with multisubwave and multiweight is applicable for large and mediumsized rivers with complex terrain.The TIC algorithm is applicable to detect the level changes in narrow mountain rivers. At present,the water level extraction based on satellite altimeters have limitations in monitoring mountain rivers and,small and mediumsized water bodies due to the complex topographic conditions around the water body and the limitation of the width of the altimeter transit track in the water body.It will be the key development directions in the future to launch new satellite altimeters such as widesweep altimeter,build satellite constellations,improve the continuity and coverage of monitoring data,and develop the high-precision intelligent algorithms.

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