Gary C. Guenther

Design considerations for achieving high accuracy with the SHOALS bathymetric lidar system

The ultimate accuracy of depths from an airborne laser hydrography system depends both on careful hardware design aimed at producing the best possible accuracy and precision of recorded data, along with insensitivity to environmental effects, and on post-flight data processing software which corrects for a number of unavoidable biases and provides for flexible operator interaction to handle special cases. The generic procedure for obtaining a depth from an airborne lidar pulse involves measurement of the time between the surface return and the bottom return. In practice, because both of these return times are biased due to a number of environmental and hardware effects, it is necessary to apply various correctors in order to obtain depth estimates which are sufficiently accurate to meet International Hydrographic Office standards. Potential false targets, also of both environmental and hardware origin, must be discriminated, and wave heights must be removed. It is important to have a depth confidence value matched to accuracy and to have warnings about or automatic deletion of pulses with questionable characteristics. Techniques, procedures, and algorithms developed for the SHOALS systems are detailed here.

New Capabilities of the “SHOALS” Airborne Lidar Bathymeter

Abstract The technology and methodology of airborne laser bathymetry are far from mature, and new capabilities continue to be attained. The SHOALS lidar system was recently augmented with the ability to utilize kinematic GPS (KGPS) with on-the-fly (OTF) phase ambiguity resolution to permit the system to be used more extensively over land and for shoreline and other topographic mapping, in addition to underwater bathymetry. The seamless integration of these capabilities permits continuous surveying across the land/water boundary. Importantly, this also permits the production of sea bottom and topographic elevations without the need for concurrently measured water-level data. Algorithms and procedures associated with the use of OTF KGPS are presented along with examples of performance.

Effects Of Propagation-Induced Pulse Stretching In Airborne Laser Hydrography

Monte Carlo simulation techniques have been applied to underwater light propagation to calculate the magnitudes of propagation-induced depth measurement bias errors as well as spatial beam spreading and signal attenuation for airborne laser hydrography. The bias errors are caused by the spatial and subsequent temporal dispersion of the laser beam by particulate scattering as it twice traverses the water column. Beam spreading results dictate spatial resolution at the bottom and the receiver field-of-view requirement. Sample temporal response functions are presented. The peak power attenuation relationships developed can be used to predict maxim um penetration depths, Predicted depth measurem ent biases are reported as functions of scanner nadir angle, physical and optical depths, scattering phase function, single-scattering albedo, and receiver field of view for several diverse signal processing and pulse location algorithms. Bias variations as a function of unknown in the field) water optical parameters are seen to be minimized for limited ranges of nadir angles whose values depend on the processing protocol. Bias correctors for use on field data are reported as functions of nadir angle and depth.

System Design And Performance Factors For Airborne Laser Hydrography

Methods are being sought to improve the efficiency of shallow-water hydrography. Reductions in cost, manpower, and data collection time are desired. Studies have indicated that airborne laser hydrography has the potential to provide five-fold reductions in cost and manpower requirements over conventional launch-based sonar systems, while at the same time increasing productivity and adding rapid response reconaissance capability. Analyses have shown that typical penetration depths are adequate in many coastal waters where extensive survey requirements now exist. The critical performance factor is depth measurement accuracy. Airborne laser hydrography is prone to a number of depth measurement error sources whose net magnitude could exceed standards if not carefully constrained by restricting system design and operational parameters. The interrelationships and compromises among these parameters necessary to meet operational requirements and goals are discussed in detail.

Analysis Of Airborne Laser Hydrography Waveforms

The U.S. Navy is presently constructing and flight testing the Airborne Bathymetric Survey System which contains as one of its sensors a lidar system. Automated post-flight waveform processing software has been developed around several sets of heuristic rules to reliably extract accurate depths while exhibiting a low false alarm rate. In 1987, flight tests were conducted in the Key West, Florida area to develop hardware and to provide data for exercising the processing procedures. The performance of the waveform processor, including noise immunity, target discrimination, and limiting cases, is presented. Interesting environmental effects on the surface returns, volume backscatter, and bottom returns are detailed. Work in progress, including comparisons of different pulse location algorithms, is noted.

Laser Applications For Near-Shore Nautical Charting

Flight testing of an airborne, scanning lidar bathymetric system has been conducted to determine vertical accuracy, operational constraints, and the effects of system variables. Test results are described, and an analytic performance model based on optical interactions is presented.

Water surface detection strategy for an airborne laser bathymeter

An algorithm is described for estimating wave heights and correcting measured local depths to the mean water level for an airborne laser bathymetry system utilizing collinear infrared and green beams. The wave heights are referenced to a mean surface derived from laser slant ranges plus vertical accelerometer data to isolate long period swell from aircraft motion. Performance of the technique under nominal operating conditions is characterized through detailed error analyses. It is seen that filters as long as several hundred seconds can be used to determine mean water level - as might be needed, for example, in the presence of swell crests parallel to the flight line. Use of the accelerometer also permits a sufficiently accurate mean water surface to be maintained for about 6 seconds without update by local surface slant range detections. This allows depths to be calculated for pulses whose surface returns are not detected. The use of slant ranges to calibrate system alignment parameters is demonstrated. Comparisons are made with an alternative approach used in the Australian LADS system.

Laser Bathymetry for Near Shore Charting Application (Preliminary Field Text Results)

An airborne lidar(light detection and ranging at optical frequencies) system has been extensively flight tested to study the operational feasibility of using a scanning, rapidly pulsed laser beam, projected into water from a fixed wing aircraft, for near-shore hydrographic applications. Field test results for vertical accuracy, environmental constraints, and effects of system parameters are discussed. Detailed utilization studies indicate that such a system should yield significantly reduced cost as well as increased volume of near-shore bathymetric data for charting purposes.

Optimum Utilization of Positioning Data in SDS III

A new, computerized hydrographic data acquisition and processing system, Shipboard Data System III (SDS III), is being designed and built for use by the National Ocean Service. An integrated positioning and navigation system is a critical element of this development. Design features include the ability to benefit from time-deskewed multiple lines of position from mixed sensor types (both electronic and manual), raw data quality evaluation including blunder removal and the use of signal strength data, high precision geodetic calculations, corrections for control and sensor offsets as well as for rare but difficult geometries, and the use of auxiliary speed and heading data in the application of advanced filtering and smoothing techniques for reduction of random noise and recognition of bias errors. Performance has been assessed for a variety of maneuvers via a track simulator which adds both vessel motions and sensor measurement noise. Results are extremely stable and robust. Measurement noise can be reduced by as much as a factor of three without adding significant biases, even on turns, while retaining actual random vessel motions. Operations can continue during complete losses of positioning data for limited but significant periods of time, including during maneuvers.