A Monte Carlo radiative transfer model of satellite waveform LiDAR

We present a method and results for a model of the interaction of waveform Light Detection And Ranging (LiDAR) with a three-dimensional vegetation canopy. The model is developed from the FLIGHT radiative transfer model based on Monte Carlo simulations of photon transport. Foliage is represented by structural properties of leaf area, leaf-angle distribution, crown dimensions and fractional cover, and the optical properties of leaves, branch, shoot and ground components. The model represents multiple scattering of light within the canopy and with the ground surface, simulates the return signal efficiently at multiple wavebands and includes the effects of topography. LiDAR-emitted pulse and spatial and temporal sampling characteristics of the instrument are explicitly modelled. Agreement is found between the integrated waveform energy and directly derived bidirectional reflectance factors from FLIGHT (root mean square error < 0.01), and between simulated and observed Ice, Cloud and land Elevation Satellite (ICESat) Geoscience Laser Altimeter System (GLAS) waveforms for a complex forest site. A sensitivity analysis gives expected effects of canopy parameters on the waveform, and indicates potential for retrieval of the canopy properties of fractional cover and leaf area, in addition to height. Where canopy and ground pulses can be separated, an index derived from the waveform shows theoretical retrieval of vertically projected plant area index with correlation coefficient R-2 = 0.87.