激光测高卫星波形饱和识别与测高误差改正初步研究

Preliminary study on echo saturation identification and altimetry error correction of laser altimetry satellite waveform

激光测高仪回波波形饱和现象客观存在,为增加可用激光点数目、提高饱和波形测高精度,本文提出了一种波形饱和识别与测高误差改正方法,首先,利用回波波形峰度系数对饱和波形进行识别,然后,针对饱和现象对波形高斯拟合的影响,计算高斯拟合波形与原始波形相交区域的形心位置,以形心位置差异确定因波形饱和导致的测高误差并改正。最后,采用ICESat/GLAS(Ice,Cloud and land Elevation Satellite/Geo-scienceLaser Altimeter System)在青海湖、纳木错、色林错采集的波形数据进行实验。结果表明,经本文算法改正后数据误差均值为0.03 m,大型湖泊区域可实现约0.05 m的测高精度,结合峰度的饱和识别方法可以对波形进行有效筛选,可发现GLAS遗漏的饱和波形,饱和改正算法可以有效改正波形饱和引起的测高误差,改正后精度明显优于GLAS提供的饱和改正结果,相关结论对高分七号卫星激光波形处理有一定参考价值。

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.