Future Spaceborne Oceanographic Lidar: Exploring the Effects of Large Off-Nadir Angles on Signal Dynamic Range and Depth Aliasing

Abstract

The large signal dynamic range affecting the profile recognition of refined structures is one of the major challenges for future spaceborne oceanographic light detection and ranging (lidar) systems. Reduce the intensity of the sea surface signal and ensure that the detector operates in a linear response region, which helps reduce the subsurface signal error and improves the capability to detect weak signals in deep water. As a solution, the off-nadir pointing could reduce the photon counts from the sea surface but leads to depth aliasing. This reduces the vertical resolution and makes it difficult to determine the sea surface’s position and retrieve the thin chlorophyll layer. The lidar signal’s dynamic range is simulated to improve the detection accuracy. Based on the oceanographic lidar simulator, the laser transmission characteristics are analyzed, taking into account various different environmental parameters (including wind speed, sea surface roughness, concentration of whitecaps and bubbles) and lidar specifications (including laser off-nadir angle, divergence angle, and pulsewidth). The results show that increasing the off-nadir angle to 7°–15° can effectively reduce the dynamic range of the sea surface signal by about one order of magnitude, while increasing the aliasing depth by about 4–8 m. Reducing the beam divergence angle is beneficial for accurate inversion of profiles within the limits of engineering realization. Other parameters, such as pulsewidth, wind speed, and sea surface roughness, have little influence on depth aliasing and depth estimation errors.