David G. Tilley

Measuring ocean waves from Space : 1978 to 1988

Abstract Global estimates of directional ocean wave spectra from space can help to spur improvements in the accuracy of existing ocean wave forecasts. The results of three ocean wave experiments conducted since 1978 indicate that a low altitude (∼300km) free-flying SAR could accurately monitor the performance of global wave models, and thus could help to assess the impact of both scatterometer-aided wind fields and improved wave model physics. The first opportunity to demonstrate such a potential should occur around the mid-1990s, when the European ERS scatterometer and the US Shuttle Space Radar Laboratory should be operating simultaneously.

Amplitude Distributions for Seaset SAR and Skylab Data with Statistics for Classification and Correlation

ABSTRACT Seasat SAR and Skylab scenes of Nantucket Island, Mass., have been mapped to the same scale and orientation for a comparison of their reflectance distributions and for a classification of terrain. Rayleigh and Poisson statistics are employed in random Monte Carlo computer simulations to model the intensity distributions of the reflectance data. Two-dimensional autocorrelation functions for SAR and optical data and an amplitude normalized cross correlation of the multisensor scenes are computed with fast Fourier transform methods. Both positive and negative correlations are observed in the cross correlation, depending on the tilt modulation and texture of scene features. Six classes of terrain are identified using the Skylab green optical bandwidth, which is particularly sensitive to subsurface topography, and the Seasat L-band synthetic aperture radar, which is particularly Insensitive to smooth horizontal surfaces.

Ocean Data Reduction And Multi-sensor Fusion Studies Of The Chesapeake Region

The Chesapeake Bay estuary along the MidAtlantic Coast of The United States was imaged by synthetic aperture radar (SAR) systems aboard the Soviet Almaz satellite in 1991 and by the Seasat platform in 1978. The geologic and cultural features of this region were also imaged by the thematic mapper (TM) optical systems aboard the Landsat-4 platform in 1982. In this validation study, SAR and TM scenes of the Chesapeake region have been used to develop image processing methods for geometric registration, multisource fusion and data compression. Comparison of the two SAR scenes indicates superior spatial response characteristics for the single look Almaz data while radiometric response is much better for the Seasat four look data.

The Use of Speckle for Determining the Response Characteristics of Doppler Imaging Radars

The synthetic aperture radar (SAR) aboard Seasat in 1978 demonstrated a unique sensitivity to oceanic and geologic features imaged over a 100-kilometer swath with 25-meter resolution. The ability of the remote sensor to resolve the fine details of large environmental systems has resulted in the orbiting of a similar system, the shuttle imaging radar (SIR-B), aboard the space shuttle Challenger during October 1984. Coherent speckle noise observed for these Doppler imaging-radar systems is caused by random correlations of the illuminating radar chirp with the surface reflectance down range. Radar speckle is similar to optical speckle in that respect, but it is also influenced by along-track sampling statistics. The Ray-leigh statistics of coherent scattering and the Poisson statistics of radar pulse detection are employed to model the speckle observed in spatially random data samples. Speckle degradation of a radar scene may obstruct interpretations of scene detail, but it can also be useful in determining the spatial response of the remote sensor and scene correlator. Randomly speckled scenes that are otherwise featureless provide a white-noise input to the Doppler imaging process. Several such scenes have been processed with fast Fourier trans-form (FFT) methods to estimate the point-spread function and its Fourier-domain equivalent, the wavenumber response function. These measurements of spatial resolution are used to compare the Seasat SAR and the Challenger SIR-B remote sensors. In addition, two ground-scene correlators are compared in terms of the point-spread estimate of spatial resolution for common input data from the SIR-B sensor.

Ers-1 and Almaz Ocean Wave Monitoring Experiments

Preliminary results from two ocean wave monitoring experiments conducted in 1991 using the high-altitude ERS-1 synthetic aperture radar (SAR) and the low-altitude ex-USSR Almaz 1 SAR are presented. ERS-1 imagery of the Gulf Stream supports the idea that a future wide-swath scansar will be a valuable tool for monitoring large-scale ocean dynamics at high resolution. A direct comparison of ERS-1 and Almaz 1 ocean wave spectra shows major deficiencies in the ERS-1 high range-to-velocity ratio R/V sensor that are partially resolved with the lower-altitude Almaz platform. Optimum wave imaging from space will require both a low R/V and low off-nadir angle.

The Influence Of Platform Attitude Dynamics On The Doppler Frequency Synthesis Of Sir-b Ocean Data SIR-B Hurricane Josephine

The Space Shuttle Challenger was maneuvered into an SAR data-acquisition mode to capture a SIR-B scene of Hurricane Josephine, which presented itself as an unscheduled target of opportunity during NASA Mission 41-G during October 1984. Steeply sloped seas approaching 10 m in height were observed several hundred km in advance of the hurricane track. Mission attitude data indicate that the Shuttle pitch, roll, and yaw were unstable during the data take. The statistical clutterlock used by the ground SAR processor yields a Doppler frequency estimate that tracks the Shuttle yaw variation but is ambiguous with a single discontinuity equal to one multiple of the radar pulse repetition frequency. The SIR-B signal data have been compressed under several Doppler parameterizations to investigate the ambiguity of the clutterlock. Resultant ocean images are analyzed for their contrast and resolution properties.

Estimating Aircraft SAR Response Characteristics And Approximating Ocean Wave Spectra In The Labrador Sea

The data processing methods employed to compute estimates of two-dimensional wave height-variance spectra from the ocean imagery obtained in the Labrador Sea by a C-band airborne SAR system are described. The SAR spectra are compared for high and low altitude geometries with large and small elevation angles. A surface contour radar and a radar ocean wave spectrometer aboard an aircraft are used to verify the surface wave spectrum.