A new terrain matching method for estimating laser pointing and ranging systematic biases for spaceborne photon-counting laser altimeters

Abstract

Precisely estimating on-orbit laser ranging and pointing systematic bias plays a significant role in obtaining high accuracy data product for spaceborne laser altimeters. Current terrain matching methods were mainly used to estimate the systematic bias for analog (or full waveform) laser altimeters or to evaluate the geolocation accuracy for both analog and photon-counting laser altimeters. The new generation photon-counting laser altimeter provides unprecedented high-density laser points on the ground and can obtain the along-track surface profiles even when observing terrains with complex topography. In this study, we propose a terrain matching algorithm to iteratively estimate the laser pointing and laser ranging systematic biases for spaceborne photon-counting laser altimeters. With the help of the ground truth topography, the systematic biases are obtained by matching the observed ranging profiles from a spaceborne photon-counting laser altimeter with the expected ranging profiles calculated from a reasonable model. Meanwhile, the theoretical model of the estimation precision is derived and denoted as confidence intervals. The data products and system parameters of the only current spaceborne photon-counting laser altimeter, i.e. Ice, Cloud, and Land Elevation Satellite-2 Advanced Topographic Laser Altimeter System (ICESat-2 ATLAS), are used to test the performance of the proposed method in Lianyungang City, China with local ground truth Digital Elevation Model (DEM). This method is proved that it can be further used to validate the geolocation accuracy, and the results show that the mean horizontal accuracy of ICESat-2 is approximately 1.7 m in the study site. It indicates that ICESat-2 has very good geolocation accuracy, even though the study site is a mountainous region with complex topography. In addition, the ranging model difference between analog and photon-counting laser altimeters is derived and quantitatively analyzed in this study. The proposed method can be not only used in the validation of geolocation accuracy but also as a potential calibration method for spaceborne photon-counting laser altimeters.