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  • 1
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    Synthetic aperture radar imaging of ocean waves - Comparison with wave measurements (1980)
    In:  Other Sources
    Details
    Publication Date: 2011-08-17
    Description: Synthetic aperture radar images of ocean waves were obtained in conjunction with reference wave data near Marineland, Florida, December 14, 1975. Each of the various types of measurements were processed into a form that allowed direct comparisons with the others. Maxima of radar spectra occurred at the same frequencies as the maxima of reference wave height spectra. In a comparison of a radar spectrum with observed spectra of wave height, wave orbital velocity, and surface slope the high-frequency portion of the radar spectrum lay near and between the wave height and the orbital velocity spectra but differed significantly from the surface slope spectrum. The radar-derived mean directions and model-fitted directional spreads of wave energy were close to the values from a directional wave buoy and indicated the accuracy of radar measurements of wave direction. However, a directional plot of a radar spectrum near shore at the frequency of the maximum showed a sharper peak than such a plot of a fitted spectrum derived from reference data.
    Keywords: OCEANOGRAPHY
    Type: Journal of Geophysical Research; 85; Sept. 20
    Format: text
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    NASA TECHNICAL REPORTS
  • 2
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    Comparison of in situ and remotely sensed ocean waves off Marineland, Florida (1978)
    In:  Other Sources
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    Publication Date: 2011-08-17
    Description: Comparison between X-band and L-band radar imagery of sea waves is made. Sea truth was obtained by in situ pitch-and-roll buoy measurements. Under the low wind and wave conditions of the tests, superior wave imagery and more useful Fourier transforms were obtained with X-band SAR as compared to L-band with equivalent signal-to-noise ratio and synthetic aperture resolution. A comparison between X-band wave images and the in situ measurements shows agreement in the dominant wave direction to within a few degrees. The normalized spectra show a striking resemblance in spectral shape.
    Keywords: OCEANOGRAPHY
    Format: text
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    NASA TECHNICAL REPORTS
  • 3
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    On the relationship of radar backscatter to wind speed and fetch (1978)
    In:  Other Sources
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    Publication Date: 2011-08-17
    Description: The physics of the interaction of electromagnetic waves with the ocean surface has been an active area of research for a number of years. This paper contains the results of satellite and aircraft experiments to investigate the ability of active microwave radars to infer surface wind speeds remotely. Data obtained from the recent National Aeronautics and Space Administration (NASA) Skylab experiment are compared with surface wind speeds measured by low-flying aircraft and ships-of-opportunity and found to give useful estimates of the ocean wind field. Also investigated was the influence of varying wave height on radar measurements of wind speed by measuring the backscattering cross-section for constant wind speed but variable wave conditions. It is found that this effect is of little importance.
    Keywords: OCEANOGRAPHY
    Format: text
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    NASA TECHNICAL REPORTS
  • 4
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    Spectral characteristics of the microwave emission from a wind-driven foam-covered sea (1976)
    In:  Other Sources
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    Publication Date: 2011-08-16
    Description: Aircraft observations of the microwave emission from the wind-driven foam-covered Bering Sea substantiate earlier results and show that the combination of surface roughness and white water yields a significant microwave brightness temperature dependence on wind speed over a wide range of microwave wavelengths, with a decreasing dependence for wavelengths above 6 cm. The spectral characteristic of brightness temperature as a function of wind speed is consistent with a foam model in which the bubbles give rise to a cusped surface between the foam and the sea. In the fetch-limited situation the contribution of the wave structure at the surface appears to increase as the foam coverage decreases. Although the data show that the thin streaks are the most important part of the white water signature, there is some evidence for the contribution of whitecaps.
    Keywords: OCEANOGRAPHY
    Type: Journal of Geophysical Research; 81; June 20
    Format: text
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    NASA TECHNICAL REPORTS
  • 5
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    Spectral variation in the microwave emissivity of the roughened seas (1974)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Recently acquired microwave data obtained from the NASA CV 990 research aircraft have yielded variation of sea surface emissivity as a function of various parameters. Data acquired at a wavelength of 1.5 cm, horizontal polarization, agree with data obtained earlier by Nordberg et al. and Hollinger at nadir and 50 deg viewing angles respectively; the ratio of brightness temperature change to wind speed change was found to be approximately 1 K per meter per second over a wind speed range of 5 to 26 meters per second. Combining these recent measurements with the earlier measurements, it is evident that microwave radiometry can be used as a remote-sensing anemometer over all wind speed ranges of interest. Data analysis revealed that for nadir-viewing instruments, the ratio of brightness temperature change to wind speed change was approximately constant for the 0.8-2.8 cm wavelength range, about three-quarters of that value at 6 cm, and nearly zero at 21 cm. A model is proposed that is consistent with observations.
    Keywords: OCEANOGRAPHY
    Type: Specialist Meeting on Microwave Scattering and Emission from the Earth; Sep 23, 1974 - Sep 26, 1974; Berne; Switzerland
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    NASA TECHNICAL REPORTS
  • 6
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    Ocean wave detection and direction measurements with microwave radars (1978)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: 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.
    Keywords: OCEANOGRAPHY
    Type: Oceans ''78: The ocean challenge; Fourth Annual Combined Conference; Sep 06, 1978 - Sep 08, 1978; Washington, DC
    Format: text
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    NASA TECHNICAL REPORTS
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