David R. Lyzenga

Passive remote sensing techniques for mapping water depth and bottom features

Ratio processing methods are reviewed, and a new method is proposed for extracting water depth and bottom type information from passive multispectral scanner data. Limitations of each technique are discussed, and an error analysis is performed using an analytical model for the radiance over shallow water.

Remote sensing of bottom reflectance and water attenuation parameters in shallow water using aircraft and landsat data

Abstract The reflectance of shallow water areas to solar illumination is a function of the water depth, the water optical properties and the bottom reflectance. Assuming the water optical properties to be uniform over a given scene area, the signals recorded by a multispectral scanner system may be combined to obtain information on the water attenuation and bottom reflectance parameters without knowledge of the water depth. These techniques are described and evaluated for a test site near North Cat Cay in the Bahamas.

Shallow-water bathymetry using combined lidar and passive multispectral scanner data

Abstract This paper describes a method of measuring water depths using a hybrid airborne sensor which incorporates both a lidar system and a passive multispectral scanner system. The methods of processing the data collected by this sensor are described and results are presented for data from two test sites in the Bahama Islands. Errors in the depth measurement are quantified, and an analysis of the probable source of these depth errors is presented

Multispectral bathymetry using a simple physically based algorithm

A simple method for estimating water depths from multispectral imagery is described and is applied to several IKONOS data sets for which independent measurements of the water depth are available. The methodology is based on a physical model for the shallow-water reflectance, and is capable of correcting for at least some range of water-quality and bottom-reflectance variations. Corrections for sun-glint effects are applied prior to the application of the bathymetry algorithm. The accuracy of the depth algorithm is determined by comparison with ground-truth measurements, and comparisons between the observed and calculated radiances are presented for one case to illustrate how the algorithm corrects for water-attenuation and/or bottom-reflectance variations

On the Estimation of Wave Slope-and Height-Varnance Spectra from SAR Imagery

A procedure is described for using synthetic aperture radar (SAR) imagery to estimate two-dimensional ocean wave slope-and height-variance spectra. The logic underpinning the procedure is based both on the results of the numerical simulation of SAR wave imagery and analytic descriptions of the SAR imaging process. The procedure, when applied to SAR imagery of waves acquired during the recent Shuttle Imaging Radar Mission (SIR-B), is shown to produce spectra that agree with independent measures of both the two-dimensional slope-and height-variance spectra. The implications of these results for future SAR missions aimed at measuring ocean waves are considered.

The Contribution of Wedge Scattering to the Radar Cross Section of the Ocean Surface

This letter considers the contribution to the radar cross section of the ocean surface due to scattering from edges for which the local radius of curvature is small compared with the radar wavelength. An analytic expression based on the method of equivalent currents is given for such scattering and is evaluated for several assumed sets of parameters. This contribution is shown to augment the Bragg scattering cross section in regions where the latter underestimates the measured radar cross section, while remaining smaller than the Bragg component elsewhere.

Synthetic aperture radar imaging of ocean-bottom topography via tidal-current interactions: theory and observations

Abstract A numerical model has been developed for the purpose of explaining and quantifying the relationship between the SEASAT synthetic aperture radar (SAR)signatures and the bottom topography of the ocean in the Southern Bight of the North Sea and Nantucket Shoals. The model uses environmental data (wind, current and depth changes)and radar system parameters (frequency, polarization, incidence angle and resolution cell size)as inputs and predicts SAR-observed backscatter changes over topographic changes in the ocean floor. The model results compare favourably with the actual SEASAT SAR-observed backscatter values. The comparisons between the model and the actual data are all within 1·5 dB except for one limiting geometry. The model suggests that for bottom features to be visible on SAR imagery, a moderate to high current velocity (0·4 m/s or greater) and a low to moderate wind (between 1·5 and 7·5 m/s) must be present.

Numerical Simulation of Synthetic Aperture Radar Image Spectra for Ocean Waves

A numerical model for predicting the synthetic aperture radar (SAR) image of a moving ocean surface is described, and results are presented for two SIR-B data sets collected off the coast of Chile. Wave height spectra measured by the NASA radar ocean wave spectrometer (ROWS) were used as inputs to this model, and results are compared with actual SIR-B image spectra from orbits 91 and 106. Additional parametric variations are presented to illustrate the effects of nonlinearities in the imaging process.

Synthetic aperture radar imaging of upper ocean circulation features and wind fronts

a sharp transition from dark to brighter backscatter regions. The corresponding profiles of backscatter modulation across these features are expressed by a peak of about 2 dB in contrast to a steplike increase of 5-8 dB. This suggests that SAR image expressions of upper ocean circulation features and wind fronts can be distinguished and classified. On the basis of this classification, we attempt to quantify the dominating marine geophysical variables. This method for systematic interpretation of SAR images should be further validated with the use of airborne or satellite data such as from the first

In situ measurements of capillary‐gravity wave spectra using a scanning laser slope gauge and microwave radars

Capillary-gravity wave spectra are measured using a scanning laser slope gauge (SLSG), and simultaneously by X and K band Doppler radars off the Chemotaxis Dock at the Quissett campus of the Woods Hole Oceanographic Institution at Woods Hole, Massachusetts. Wave spectral densities estimated from the radar measurements using the Bragg theory agree with those measured using the SLSG at the Bragg wavenumber to within a few decibels, suggesting that Bragg scattering theory is valid for the conditions of this experiment. The observed degree of saturation of capillary-gravity waves is in reasonable agreement with measurements by Jahne and Riemer (1990) obtained from measurements in a large wind-wave flume at intermediate wind speeds, but our data indicate a higher degree of saturation at very low wind speeds. The rate at which the slope-frequency spectrum falls off, however, is much lower in the field than in laboratories, even at moderate winds, suggesting long waves are responsible for a large Doppler shift of capillary-gravity waves. Close examination of combined wavenumber-frequency slope spectra also reveals significant smearing of the spectra in the frequency domain due to long waves. These observations confirm that spatial measurements (wavenumber spectra measurements) are essential for characterizing short capillary-gravity waves, since this strong Doppler shift will dramatically change apparent frequency spectra.