ICESat-2 ATL03数据预处理及校正方法

ICESat-2 ATL03 data preprocessing and correction method

2018年9月15日,美国航空航天局成功发射了ICESat-2卫星,ICESat-2搭载了地形激光测高仪系统(ATLAS),已经广泛应用于极地冰盖高程测量、海冰厚度估算、陆地高程测量、地表植被测量等多个研究领域。不同于GLAS/ICESat,ATLAS在信号探测系统中采用更为灵敏的单光子探测器,每秒发射10 000个脉冲(GLAS/ICESat系统每秒发射40个脉冲),可以获取到重复频率更高、光斑更小的高密度光子计数回波数据,进而实现高精度全球观测。由于ATLAS采用的探测器光电倍增管(PMT)after-pulse的影响,在回波信号脉冲输出后常会出现小幅度脉冲干扰,不能准确反映原始信号波形,所以在实际应用过程中,必须对测量信号进行校正。首先基于ATL03产品,对光子计数回波数据进行预处理,获得了海冰、海洋、陆地和沙漠的廓线信息,结果显示在主信号回波之后均存在两个小的脉冲回波(约2.3 m和4.2 m处),此外陆地和沙漠在10~45 m间存在一个更长时间延迟的脉冲回波,证实了不同反射表面均会受到PMT after-pulse影响。然后利用ICESat-2卫星过境撒哈拉沙漠地区的夜间观测数据,计算了不同波束下的脉冲响应函数,6个波束得到的脉冲响应函数整体形态相似,在主信号回波后出现三个不同幅度的脉冲回波(约2.3 m、4.2 m和6.5 m处)。最后利用6个波束的脉冲响应函数对同一海洋剖面进行校正,结果显示除ATLAS Beam2外,其余波束均可有效滤除主脉冲之后的小幅度脉冲,校正得到真实的水下回波信号。

On September 15,2018,NASA successfully launched the ICESat-2 satellite,which carries the Advanced Topographic Laser Altimeter System(ATLAS).ATLAS/ICESat-2 has been widely used in many research fields,such as polar ice-sheet elevation measurement,sea-ice thickness estimation,land elevation measurement,and vegetation measurement.Unlike GLAS/ICESat,ATLAS uses a more sensitive single-photon detector in the signal detection system.The system emits 10000 pulses per second(GLAS/ICESat system emits 40 pulses per second),which enables high-density photon data with higher repetition frequency and smaller light spots,thus enabling high-precision global observation.Due to the influence of the detector photomultiplier tube(PMT)after-pulse used by ATLAS,small amplitude pulse interference often occurs after the output of echo signal pulse,which cannot accurately reflect the original signal waveform.Therefore,the measured signal must be corrected in the practical application process.Firstly,based on ATL03 product,the photon counting echo data were preprocessed to obtain the profiles of sea ice,ocean,land and desert.The results show that there are two small pulse echoes(about 2.3 m and 4.2 m)after the main signal echo,and there is a longer delay pulse echo between land and desert(about 10-45 m).It is confirmed that different reflective surfaces are affected by after-pulse of PMT.Secondly,the impulse response functions under different beams were calculated by using the night observation data of ICESat-2 over the Sahara Desert.The overall shape of the impulse response functions obtained from the six beams was similar.After the main signal echo,there were three pulse echoes of different amplitude(about 2.3 m,4.2 m and 6.5 m).Finally,the impulse response functions under six beams were used to correct the same ocean profile.The results show that except ATLAS Beam2,all the other beams can effectively filter out the small amplitude pulse after the main pulse,and the real underwater echo signal can be picked up.