Robert W. L. Thomas

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.

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.

Ozone estimates derived from Dobson direct sun measurements: effect of atmospheric temperature variations and scattering

We have performed an analysis of the impact of the temperature sensitivity of the ozone absorption coefficients on estimates of total atmospheric ozone obtained by the Dobson spectrophotometer operating in direct sun mode. In general, the higher the mean ozone temperature the greater will be the tendency to overestimate the ozone amount. The spreads in ozone residuals over the temperature models we investigated were 3%, 4%, and 6% for the A, C, and D line pairs, respectively, whereas for coupled line pairs the spread was only about 2%. The A-C-D triplet showed a very small temperature effect, the spread being probably less than 2%. For the A-D system, currently recommended by the WMO, the computed spread was 2.4 +/- 0.5%. A Monte Carlo model was applied to investigate the potential impact of scattered radiation entering the system. The effect has been computed for various conical fields of view. For clear sky conditions with no aerosols present, the error introduced appears to be less than 1%. When a tropospheric aerosol model was inserted, however, significant errors were observed. For the models we studied aerosol attenuation resulted in overestimates of total ozone up to 8%, but the impact of scattered radiation was to reduce the overestimate, and, in some cases, the scattering and attenuation effects may balance for a realistic Dobson system. Both effects increased from the A to the C to the D line pairs. The results indicate that line pair coupling reduces the combined error due to both sources to less than 1%.

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.

Information content of sky intensity and polarization measurements at right angles to the solar direction

This paper presents the results of a Monte Carlo simulation study of the brightness and polarization at right angles to the solar direction both for ground-based observations (looking up) and for satellite-based systems (looking down). Calculations have been made for a solar zenith angle whose cosine was 0.6 and wavelengths ranging from 3500 A to 9500 A. We have succeeded in demonstrating a sensitivity of signatures to total aerosol loading, aerosol particle size distribution and refractive index, and the surface reflectance albedo. For Lambertian type surface reflection the albedo effects enters solely through the intensity sensitivity, and we have found very high correlations between the polarization term signatures for the ground-based and satellite-based systems. Potential applications of these results for local albedo predictions and satellite imaging systems recalibrations are discussed.

Validation of the BUV satellite ozone sensor using the rocket ozonesonde.

This paper reviews aspects of the rocket ozonesonde (ROCOZ) performance pertaining to the validation of satellite ozone sensing systems, particularly the backscattered ultraviolet (BUV) sensor. It is shown that while the BUV system relates cumulative vertical ozone to pressure, the ROCOZ can measure cumulative ozone as a function of radar altitude. As a consequence some method of relating atmospheric pressure and altitude is required, with a concomitant error introduced. A Monte Carlo simulation was performed to evaluate the contribution of scattered flux on the ROCOZ result. The scattered component always causes an underestimate in cumulative ozone that increases almost linearly with pressure and is only weakly dependent on the solar zenith angle. For most practical operating conditions, however, the error in cumulative ozone arising from the scattered flux is of the order of 1% or less.

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.