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Preliminary study on echo saturation identification and altimetry error correction of laser altimetry satellite waveform
resumen
Laser altimeter rec waveform saturation commonly exists in Polar Regions, lakes and other areas, but studies about it are few. This study summarizes the previous research results and analyzes the causes of waveform saturation and the influence of waveform saturation on measurement accuracy. A method for waveform saturation recognition and height measurement error correction is proposed to increase the number of available laser points and improve the accuracy of saturation waveform measurement.The waveform saturation recognition algorithm uses a combination of saturation threshold and waveform kurtosis. The minimum saturation threshold is used to remove waveform data whose amplitude is low and unsaturated. Then, the kurtosis of the original waveform data is compared with that of the average distribution data. If the waveform kurtosis is less than the average distribution kurtosis, then it is considered t saturated; otherwise, it is a normal waveform. The saturation correction algorithm uses the centroid position difference to calculate the height measurement error. Gaussian fitting is performed on the original waveform data, the intersection point of the Gaussian fitting curve and the original data is calculated, the intersection point is selected according to the principle of selecting points, the two selected intersection points are connected to form a line segment, and the closed area surrounded by the line segment and the Gaussian fitted waveform is computed. The difference between the centroid position of the Gaussian fitting and that of the closed area is the time deviation of the height measurement caused by the waveform saturation. The time deviation is multiplied by one-half the speed of light to obtain the saturation correction value.Data collected by Ice, Cloud, and Land Elevation Satellite/Geo-science Laser Altimeter System (ICESat/GLAS) in Qinghai Lake, Nam Co, and Selin Co are used for experiments. Experimental results show that the average error of the data after algorithm correction is 0.03 m, and the large lake area can achieve an accuracy of about 0.05 m. The proposed saturation recognition algorithm is more accurate than that in the previous studies. GLAS uses a fixed threshold to identify missing saturated waveforms. The proposed saturation correction algorithm is easier to implement and has higher accuracy than the algorithm provided by GLAS. This proposed algorithm only relies on the shape characteristics of waveform data for calculation; thus, it has more universal applicability. Different from previous studies, the experiments also found that the waveform saturation does not only cause low conditions on elevation measurement, but also high conditions. The comparison between the corrected elevation and the actual elevation proves that this situation does exist.The number and accuracy of available laser points can be greatly increased by the effective correction of saturation waveform data. The proposed method can provide some references for saturation processing of echo data of domestic laser altimeter, but it is mainly suitable for effective identification and correction of single waveform saturation data, such as lake, flat land, and polar ice sheet. How to identify and correct multiwaveform overlapping saturation data in complex areas such as forest should be studied.
palabra clave
satellite laser altimeter;Saturation waveform;error correction;ICESat/GLAS;GF-7 satellite
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