Accurate spaceborne waveform simulation in heterogeneous forests using small-footprint airborne LiDAR point clouds

Spaceborne light detection and ranging (LiDAR) waveform sensors require accurate signal simulations to facilitate prelaunch calibration, postlaunch validation, and the development of land surface data products. However, accurately simulating spaceborne LiDAR waveforms over heterogeneous forests remains challenging because data-driven methods do not account for complicated pulse transport within heterogeneous canopies, whereas analytical radiative transfer models overly rely on assumptions about canopy structure and distribution. Thus, a comprehensive simulation method is needed to account for both the complexity of pulse transport within canopies and the structural heterogeneity of forests. In this study, we propose a framework for spaceborne LiDAR waveform simulation by integrating a new radiative transfer model – the canopy voxel radiative transfer (CVRT) model – with reconstructed three-dimensional (3D) voxel forest scenes from small-footprint airborne LiDAR (ALS) point clouds. The CVRT model describes the radiative transfer process within canopy voxels and uses fractional crown cover to account for within-voxel heterogeneity, minimizing the need for assumptions about canopy shape and distribution and significantly reducing the number of input parameters. All the parameters for scene construction and model inputs can be obtained from the ALS point clouds. The performance of the proposed framework was assessed by comparing the results to the simulated LiDAR waveforms from DART, Global Ecosystem Dynamics Investigation (GEDI) data over heterogeneous forest stands, and Land, Vegetation, and Ice Sensor (LVIS) data from the National Ecological Observatory Network (NEON) site. The results suggest that compared with existing models, the new framework with the CVRT model achieved improved agreement with both simulated and measured data, with an average R² improvement of approximately 2% to 5% and an average RMSE reduction of approximately 0.5% to 3%. The proposed framework was also highly adaptive and robust to variations in model configurations, input data quality, and environmental attributes. In summary, this work extends current research on accurate and robust large-footprint LiDAR waveform simulations over heterogeneous forest canopies and could help refine product development for emerging spaceborne LiDAR missions.