Performance of Photon counting Lidar in turbid water

Sammanfattning

Photon counting lidar has shown high resolution performance for target detection and imaging in air. However, in underwater applications, the scattering in turbid waters reduces the target signal and therefore the resolution and image quality. This thesis investigated the performance of a photon counting lidar system in turbid water environments and compared it to a simulated lidar system with longer instrument response function. This simulated system was used to generate results expected from a traditional analogue lidar system. This work included experimental measurements, carried out in a water tank. The measurements were performed using a lidar system that included a single-photon avalanche diode detector. The measurements were carried out at different turbidity levels and target ranges at approximately 0.2 m, 0.55 m and 0.9 m, corresponding to attenuation lengths between the lidar system and the targets. The sensor performance was evaluated by analysing the detection probability, the resolution analysis using the modulation transfer function, and the image reconstruction. The results show that the photon counting lidar system obtained high resolution even in highly scattering environments up to 5.14 AL. At higher attenuation length, the signal from the target was very weak and could no longer be detected. As turbidity increased, the contrast decreased for both the photon counting system and the simulated system. However, the simulated system with longer instrument response function showed a larger decrease in contrast in comparison to the photon counting system as the turbidity increased. The results of the image reconstruction showed that a cross-correlation method improved target detection in high attenuation environments where the signal-to-noise ratio was low. The simulated system was also applied in the image reconstruction, and the result showed that the simulated system performed worse, especially in low signal conditions. The study indicates that photon counting lidar can provide improved contrast and range estimation in turbid underwater environments compared to systems with longer temporal response. This shows that a short instrument response function is advantageous in turbid waters where the backscattering from the water volume is strong.