Narrow-field-of-view bathymetrical lidar: theory and field test

Abstract

The purpose of this paper is to derive a reliable theory to predict the performance of a narrow -FOV bathymetric lidar. A fundamental discrepancy between the theoretical estimate and experimental results was the inspiration for the work presented here Meeting oceanographic mapping requirements is a critically important goal for littoral laser bathymetry. In contrast to traditional airborne lidar system which are optimized for recovering signals from the deepest possible waters , the above challenge may be met with a radical narrowing of the lidar transmit beam and receiver field of view (FOV) employed in EAARL (Experimental Advanced Airborne Research Lidar, NASA). In this paper we discuss theoretical analysis carried out on the basis of a sophisticated "multiple-forward scattering and single-backscattering model" for lidar return signals allows a quantitative estimation of the advantages of a narrow-FOV system over traditional bathymetric lidars (SHOALS-400, SHOALS-1000, LADS Mk II) when used in clear shallowwater cases. Some of those advantages are:• Increase in bottom definition (or reduced false-alarm probability) due to the enhanced contrast of the bottom return over the background backscatter from the water column, • Enhancement in depth measurement accuracy resulting from narrower bottom return pulse width, • Reduction of post-surface return effects in the lidar photo-multiplier detector due to a more rapid decay of water column backscatter, • Greatly improved rejection of ambient light permitting lidar operations in all zenith sun angles and flight directions. The model computations make it possible to estimate the maximal operational depth for the system under consideration by the implementation of statistical theory of detectability. These computations depend on the prevailing seawater optical properties and lidar parameters. The theoretical predictions are compared with results obtained in the field test of the EAARL system carried out in Florida Keys in 2001.

Semi-Analytic Monte Carlo Model for Oceanographic Lidar Systems: Lookup Table Method Used for Randomly Choosing Scattering Angles

“…However, when the field of view was large enough, K sys approaches α, since the only light lost at the receiver was what had been absorbed. These results agree with previous studies [23][24][25]. They indicate that the configuration of multiple FOV applied for lidar system is useful for optical properties' inversion in various types of water.…”

Shallow Bathymetric Mapping via Multistop Single Photoelectron Sensitivity Laser Ranging

“…This approach is unnecessary if a much lower power laser is used. Theoretical analysis that was carried out based on a multiple-forward scattering and single-backscattering model for narrow field-of-view LiDAR return signals indicated an increase in bottom definition, enhancement in depth measurement accuracy, reduction of post-surface return effects in the PMT, and greatly improved rejection of ambient light, permitting operations in all zenith sun angles and flight directions [25]. Given that producing low-power (< 5 μJ) pulses in short duration (< 1 ns) is easier, we can also expect the required components to be less expensive.…”

“…In the aforementioned static testing, employing maximum gain on the PMT produced very large signals over bare-earth surfaces, resulting in data flow difficulties when attempting to record 100 channels of multistop information. To reduce the data throughput, only channels on the second ranging board (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32) or the central 4 × 4 square) were enabled for a majority of the experiments.…”

Airborne LiDAR Methods Applied to Riverine Environments

“…The EAARL was initially used to survey coral reefs (Brock et al, 2004) and coastal areas (Nayegandhi et al, 2006;Sallenger et al, 2004;Nayegandhi et al, 2009), but it also found use along rivers and streams (Kinzel et al, 2007;McKean et al, 2008). The system offers a radically different design that sets it apart from other bathymetric LIDARs, as the green (532-nm) laser has a relatively low power (70 μJ/pulse compared with the 5 mJ of other bathymetric LiDARs), which enables eye-safe operation at a much narrower field of view than other systems (Wright and Brock, 2002;Feygels et al, 2003). The area illuminated by the EAARL at a typical surveying altitude (300 m) is approximately 0.20 m, in contrast to a spot size greater than 1 m of other bathymetric systems.…”