Discrepancies between time-based and real depth profiles in ocean lidar due to multiple scattering

Due to its ability to provide day-and-night profiling and high depth resolution, ocean lidar has become an important tool for marine remote sensing. However, a lidar system provides time-based measurements of backscattered photons, where the distance (or depth for vertical profiling) is a product of light speed in water and the time photons pass. When there are significant contributions of multiple scattering in the backscattered signals of ocean lidar, the perceived depth of these measured photons will be deeper than the real depth. Therefore, if the objective of a lidar system is to sense the vertical profile of particles, the present time-based depth profile will not match the real depth profile of particles in the water column. To address this discrepancy, we carried out semi-analytical Monte Carlo simulations for a wide range of water properties (represented by scattering coefficient, b), focusing on Case-1 water, with platforms including spaceborne, airborne, shipborne, and underwater. In the simulation process, it is assumed that the water column is vertically homogeneous, and the influence of sea surface fluctuations is ignored. Based on the simulated data, relationships between the discrepancy and b, as well as the radius of the received footprint on the water surface (r_s), are established. Sensitivity analysis indicates that the discrepancy is more sensitive to b than to r_s. Further, the impact of the absorption coefficient, scattering phase function, rough sea surface, and vertically non-uniform inherent optical properties on this discrepancy is discussed. Our results not only highlight the significance of considering multiple scattering, particularly for airborne and spaceborne platforms, in sensing the vertical profiles of particles but also provide guidance for interpreting backscattered signals in ocean lidar applications.