基于足印探测的激光测高仪在轨标定

On-orbit calibration of satellite laser altimeters based on footprint detection

高精度的地表目标三维观测结果需要卫星激光测高仪对其系统误差进行定期的在轨标定工作,这包含系统误差的估计和校正以及标定结果的精度检验,现有方式分别通过姿态机动法和足印探测法予以实现.然而,姿态机动方式不适用于我国的卫星平台,传统足印探测方式没有针对系统误差的估计模型,仅能用于标定结果的精度检验.本文推导了基于足印探测方式的激光指向角系统误差估计模型,使得足印探测法能完成包含在轨误差校正以及精度检验的工作闭环,同时对用于激光足印获取的地面能量探测器进行了改进设计.通过设计仿真实验对所推导的误差估计模型进行验证,并量化分析了探测器阵列激光入射角度、标定场地表粗糙度及探测器布设间距等因素对系统误差校正精度的影响.结果表明,若要实现1.8 m的水平定位精度(对应0.6 arcsec激光指向精度),探测器阵列间距达到20 m即可,探测器阵列面的入射角需高于3°,标定场地表粗糙度需小于10 cm.以上结论对我国未来发射GF-7号光学/激光立体测绘卫星具有重要参考价值.

The positioning accuracy of the footprint of a satellite laser altimeter is primarily dependent on the accuracy of its laser pointing, e.g., a 30 arcsec pointing bias will induce 87 m horizontal error and 1.5 m vertical error when the altitude is 600 km and the laser incident angle is 1°. In order to achieve the three-dimensional high-precision observation on the Earth surface, on-orbit calibration is needed to remove the systematic pointing bias mainly arising from the thermal effect. The current methods of on-orbit calibration and verification for laser altimeters are the attitude maneuvering and the footprint detection, respectively. However, the attitude maneuvering is not applicable to the existing satellite platform of China, which uses the large platform with a three-axis attitude stabilization system. The current footprint detection method can only achieve on-orbit verification task, i.e., the horizontal and vertical errors can be evaluated by analyzing the captured laser footprints but the systematic pointing bias cannot be estimated and removed. An improved design scenario of energy detector that is used for capturing laser footprint is given in this paper. The quantification level of the captured laser energy is equal to 8, which is bigger than that of the energy detector designed for geoscience laser altimeter systems corresponding to level 2. Benefiting from the new design scenario, fewer detectors are needed to achieve the same precision when calculating the centroid geolocations of captured footprints. A new systematic misalignment estimation model in the laser direction cosines is deduced, and it is used to estimate the systematic bias by using the detected footprints based on the Gauss-Markoff criterion. With the new detectors and bias estimation model, the footprint detection method now can achieve on-orbit calibration, as well as on-orbit verification. According to the presented calculation model, simulation experiments are operated to analyse three effects that may influence the performance of the footprint detection on-orbit calibration, i.e., the laser incident angle on the detector array, the surface roughness of the site where detectors lay out, and the grid density of the detector array. The simulation results indicate that, when the horizontal positioning accuracy of the captured footprint centroid demands better than 1.8 m which corresponds to 0.6 arcsec laser pointing accuracy when the altitude of the satellite is 600 km, the grid distance of the detector array can be 20 m, the laser incident angle on the detector array should be larger than 3°, and the surface roughness of the calibration site should be less than 0.1 m. The designed detectors and calibration method will be used to capture laser footprints and remove the systematic bias for the laser altimeter on China GF-7 satellite, which is one of the upcoming high-resolution satellites for Earth observation.