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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Directional spectra of ocean waves from microwave backscatter

The paper presents an analysis of two proposed microwave radar techniques for measuring ocean wave directional spectra. Tomiyasu's (1971) short pulse idea and Barrick's (1972) two-frequency correlation idea are regarded - independent of transmitted waveform - as essentially two alternative detection systems for modulated noise. Together, the two systems constitute a general detection system for modulated noise described some years ago by Parzen and Shiren (1956). A frequency domain analysis for backscatter on arbitrary incident waveform is given, and an interesting physical optics solution for the generalized fourth-order moments of the scattering matrix is obtained. It is shown that the present narrowband version of Barrick's two-frequency idea is impractical, and that the proper application of Barrick's idea is to wide band signals.

Jackson, F. C.↗
NASA NTRS1976

Transformation of apparent ocean wave spectra observed from an aircraft sensor platform

The problem considered was transformation of a unidirectional apparent ocean wave spectrum observed from an aircraft sensor platform into the true spectrum that would be observed from a stationary platform. Spectral transformation equations were developed in terms of the linear wave dispersion relationship and the wave group speed. An iterative solution to the equations was outlined and used to transform reference theoretical apparent spectra for several assumed values of average water depth. Results show that changing the average water depth leads to a redistribution of energy density among the various frequency bands of the transformed spectrum. This redistribution is most severe when much of the energy density is expected, a priori, to reside at relatively low true frequencies.

Poole, L. R.↗
NASA NTRS1978

Ocean wave detection and direction measurements with microwave radars

The application of synthetic aperture microwave radar imagery to the measurement of ocean wave direction and wavelength is discussed and an airborne experiment conducted in preparation for the SEASAT-A satellite mission is presented. Models for radar backscattering are examined and it is concluded that the Bragg-Rice model, which represents the ocean surface as a combination of periodic surfaces so that the dominant backscatter matches some portion of the wave spectrum, appears most useful. The experiment was conducted with airborne X-band and L-band synthetic aperture radar (SAR) and data was compared with that obtained from a pitch-and-roll buoy and an instrument package mounted on a sea sled. SAR data was converted to wave spectra and it was found to match data from the buoy, while SAR data on wave direction agrees to within two degrees with the buoy and to within one degree with the sea sled sensors. Wave direction data taken by two real aperture radars is also presented and shown to agree with airborne photographs.

Teleki, P. G.↗
NASA NTRS1984

Measurements of ocean wave spectra and modulation transfer function with the airborne two frequency scatterometer

The directional spectrum and the microwave modulation transfer function of ocean waves can be measured with the airborne two frequency scatterometer technique. Similar to tower based observations, the aircraft measurements of the Modulation Transfer Function (MTF) show that it is strongly affected by both wind speed and sea state. Also detected are small differences in the magnitudes of the MTF between downwind and upwind radar look directions, and variations with ocean wavenumber. The MTF inferred from the two frequency radar is larger than that measured using single frequency, wave orbital velocity techniques such as tower based radars or ROWS measurements from low altitude aircraft. Possible reasons for this are discussed. The ability to measure the ocean directional spectrum with the two frequency scatterometer, with supporting MTF data, is demonstrated.

Weissman, D. E.↗
NASA NTRS1986

SIR-B observations of dominant ocean waves near hurricane Josephine

Radar images of ocean surface waves near hurricane Josephine were acquired with the Shuttle imaging radar B (SIR-B) system on October 12, 1984. Fast Fourier transform analyses of the images have been performed along most of the 600-km image track. These data reveal the presence of at least two dominant wave systems which undergo significant spatial variations in wavelength and direction.

Holt, B.↗
NASA NTRS1986

Measurements of ocean wave spectra and modulation transfer function with the airborne two-frequency scatterometer

The directional spectrum and the microwave modulation transfer function of ocean waves can be measured with the airborne two frequency scatterometer technique. Similar to tower based observations, the aircraft measurements of the Modulation Transfer Function (MTF) show that it is strongly affected by both wind speed and sea state. Also detected are small differences in the magnitudes of the MTF between downwind and upwind radar look directions, and variations with ocean wavenumber. The MTF inferred from the two frequency radar is larger than that measured using single frequency, wave orbital velocity techniques such as tower based radars or ROWS measurements from low altitude aircraft. Possible reasons for this are discussed. The ability to measure the ocean directional spectrum with the two frequency scatterometer, with supporting MTF data, is demonstrated.

Weissman, D. E.↗
NASA NTRS1979

Seasat synthetic aperture radar - Ocean wave detection capabilities

A preliminary assessment has been made of the capability of the Seasat synthetic aperture radar to detect ocean waves. Comparison with surface and aircraft measurements from five passes of the satellite over the Gulf of Alaska indicates agreement to within about 15 percent in wavelength and about 25 deg in wave direction. These results apply to waves 100 to 250 meters in length, propagating in a direction predominantly across the satellite track, in sea states with significant wave height in a range of 2 to 3.5 meters.

Gonzalez, F. I.↗
NASA NTRS1981

Optimal spatial filtering and transfer function for SAR ocean wave spectra

The Seasat Synthetic Aperture Radar (SAR) has proved to be an instrument of great utility in the sensing of ocean conditions on a global scale. An analysis of oceanographic and atmospheric aspects of Seasat data has shown that the features observed in the imagery are linked to ocean phenomena such as storm sources and their resulting swell systems. However, there remains one central problem which has not been satisfactorily solved to date. This problem is related to the accurate measurement of wind-generated ocean wave spectra. Investigations addressing this problem are currently being conducted. The problem has two parts, including the accurate measurement of the image spectra and the inference of actual surface wave spectra from these measurements. A description is presented of the progress made towards solving the first part of the problem, taking into account a digital rather than optical computation of the image transforms.

Goldfinger, A. D.↗
NASA NTRS1980

The Seasat SAR Wind and Ocean Wave Monitoring Capabilities: A case study for pass 1339m

A well organized low energy 11 sec. swell system off the East Coast of the U.S. was detected with the Seasat Synthetic Aperture Radar and successfully tracked from deep water, across the continental shelf, and into shallow water. In addition, a less organized 7 sec. system was tentatively identified in the imagery. Both systems were independently confirmed with simultaneous wave spectral measurements from a research pier, aircraft laser profilometer data, and Fleet Numerical Spectral Ocean Wave Models.

Beal, R. C.↗
NASA NTRS1976

The theory and applications of ocean wave measuring systems at and below the sea surface, on the land, from aircraft, and from spacecraft

Methods for measuring and analyzing ocean waves are described, including those presently in use on spacecraft and planned for SEASAT-A. Potential difficulties with synthetic aperture systems for a spacecraft are described and an alternate design is suggested. The different methods can yield different kinds of spectra and other kinds of imagery. Ways to compare different kinds of data are given. The scientific and practical applications of data from spacecraft are given.

Pierson, W. J.↗
NASA NTRSDate not supplied

NASA’s Orion Crew Module Seakeeping Simulation and Test Comparisons in Ocean Wave Environments

The NASA Orion Crew Module (CM) post-landing seakeeping and uprighting capabilities are being evaluated across a variety of ocean wave conditions. This paper provides an overview of quarter-scale and full-scale test data and how it compares with the CM seakeeping dynamic simulation results evaluated using Wave Energy Convertor Simulator (WEC-Sim).This includes a discussion of the status of the dynamic modeling effort, correlation of model inputs with the wave field from the test, and a comparison of the results against the physical test data with emphasis on the full-scale results. Analysis in the frequency domain showed similar response characteristics between the model and the test and shows energy peaks corresponding to both the natural response and the forced response due to the wave environment. Time domain comparisons showed that the linear WEC-Sim model under predicted the amplitude of the pitch and roll motion of the test article, with improved matching for smaller, longer crested waves. An attempt was made to correlate the pitch/roll response to incident wave slope to account for a possible delay in the BTA’s response. The results did not show a strong relationship in the response characteristics to incident wave slope, but better correlation was seen by comparing responses in the frequency domain. Evaluation of the simulation’s ability to match test results aided in understanding the current fidelity of the CM model in WEC-Sim and identifying areas for model improvement

Abigail Lockard↗
NASA NTRS1988

Ocean wave directional spectra and wave-current interaction in the Agulhas from the Shuttle Imaging Radar-B synthetic aperture radar

SIR-B synthetic aperture radar ocean-wave spectra for a 200-km pass crossing the Agulhas current off the coast of Africa are analyzed. A significant enhancement of one spectral peak along the northern edge of the current is attributed both to amplification of the waves by refraction-dominated wave-current interaction and to transient specular backscatter contributions spatially correlated to the waves.

Irvine, D. E.↗
NASA NTRS1979

Extraction of ocean wave height and dominant wavelength from Geos 3 altimeter data

Some data from the Geos 3 satellite altimeter are examined in detail to demonstrate the techniques used for extracting wave height and skewness of the sea surface. The approach used in determining the dominant wavelength of the ocean waves is discussed. Improvements incorporated into the Seasat 1 altimeter as the result of experience with Geos 3 are described.

Walsh, E. J.↗
NASA NTRS1985

A comparison of in situ and airborne radar observations of ocean wave directionality

The directional spectrum of a fully arisen, about 3 m sea as measured by an experimental airborne radar, the NASA K(u)-band radar ocean wave spectrometer (ROWS), is compared to reference pitch-roll buoy data and to the classical SWOP (stereo wave observations project) spectrum for fully developed conditions. The ROWS spectrum, inferred indirectly from backscattered power measurements at 5-km altitude, is shown to be in excellent agreement with the buoy spectrum. Specifically, excellent agreement is found between the two nondirectional height spectra, and mean wave directions and directional spreads as functions of frequency. A comparison of the ROWS and SWOP spectra shows the two spectra to be very similar, in detailed shape as well as in terms of the gross spreading characteristics. Both spectra are seen to exhibit bimodal structures which accord with the Phillips' (1958) resonance mechanism. This observation is thus seen to support Phillips' contention that the SWOP modes were indeed resonance modes, not statistical artifacts.

Jackson, F. C.↗
NASA NTRS1993

Measurement of ocean wave spectra using polarimetric AIRSAR data

A polarimetric technique for improving the visibility of waves, whose propagation direction has an azimuthal component, in RAR (real aperture radar) or SAR (synthetic aperture radar) images has been investigated. The technique shows promise as a means of producing more accurate 2-D polarimetric RAR ocean wave spectra. For SAR applications domination by velocity-bunching effects may limit its usefulness to long ocean swell. A modification of this technique involving measurement of polarization signature modulations in the image is useful for detecting waves in SAR images and, potentially, estimating RMS wave slopes.

Schuler, D. L.↗
NASA NTRS1974

The scientific justification for obtaining ocean wave data of various kinds with Seasat-A

Synthetic aperture radar images of sea surface in deep water as well as two candidate instruments that essentially scan along a line are considered for obtaining spectral wave information with SEASAT-A. An altimeter provides data on wave height for every 50 or 100 kilometers along subsatellite track routine. A combination of these instruments is proposed to obtain wind generated ocean waves and swells.

Pierson, W. J.↗
NASA NTRS1993

Radar Ocean Wave Spectrometer (ROWS) preprocessing program (PREROWS2.EXE). User's manual and program description

This Technical Memorandum is a user's manual with additional program documentation for the computer program PREROWS2.EXE. PREROWS2 works with data collected by an ocean wave spectrometer that uses radar (ROWS) as an active remote sensor. The original ROWS data acquisition subsystem was replaced with a PC in 1990. PREROWS2.EXE is a compiled QuickBasic 4.5 program that unpacks the recorded data, displays various variables, and provides for copying blocks of data from the original 8mm tape to a PC file.

Vaughn, Charles R.↗