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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

The potential of SAR directional spectra in operational wave forecasting

Digitally processed Seasat SAR ocean wave imagery, when digitally transformed, smoothed, and corrected for known instrument biases, can accurately track multiple (at least three) wave systems in the open ocean, across major currents, and into shallow water. Furthermore, the relative backscatterer of the digitally processed SAR image, when sampled within a single pass and along a constant range, responds to the local wind through a simple power law relationship. The results indicate that SAR spectra, if obtained from a low altitude satellite, could be an important supplement to global winds and non-directional wave heights obtained through other methods.

Beal, R. C.↗
NASA NTRS1985

Aircraft and satellite measurement of ocean wave directional spectra using scanning-beam microwave radars

A microwave radar technique for remotely measuring the vector wave number spectrum of the ocean surface is described. The technique which employs short-pulse, noncoherent radars in a conical scan mode near vertical incidence, is shown to be suitable for both aircraft and satellite application, the technique was validated at 10 km aircraft altitude, where we have found excellent agreement between buoy and radar-inferred absolute wave height spectra.

Jackson, F. C.↗
NASA NTRS1985

Observations of the polar regions from satellites using active and passive microwave techniques

Observations of the ocean ice, ice sheets, ice edges, and ocean waves in the Antarctic, performed by the Seasat and Nimbus-7 satellites are described. Specific features of the Seasat altimeter, the SAR, the Seasat-A Scatterometer System, and the Scanning Multichannel Microwave Radiometer (SMMR) are discussed, together with the advantages and limitations of each instrument as applied to the study of ice dynamics. The Nimbus-7 SMMR algorithm for calculating sea-ice concentrations is presented. In addition, the Seasat altimeter-derived data on wind speed and wave heights in the Southern Ocean, and on the ice sheets of the Antarctica and Greenland were evaluated.

Swift, C. T.↗
NASA NTRS1986

An overview of the NSCAT/N-ROSS program

The NASA Scatterometer (NSCAT) to fly on the U.S. Navy Remote Ocean Sensing System (N-ROSS) mission is presented. The overall N-ROSS mission, the NSCAT flight instrument and groundbased data processing/distribution system, and NASA-supported science and verification activities are described. The N-ROSS system is designed to provide measurements of near-surface wind, ocean topography, wave height, sea-surface temperature, and atmospheric water content over the global oceans. The NSCAT is an improved version of the Seasat scatterometer. It will measure near surface vector winds.

Martin, B. D.↗
NASA NTRS1987

The physical basis for estimating wave energy spectra from SAR imagery

Ocean surface waves are imaged by synthetic aperture radar (SAR) through a combination of the effects of changes in the surface slope, surface roughness, and surface motion. Over a limited range of conditions, each of these effects can be described in terms of a linear modulation-transfer function. In such cases, the wave-height spectrum can be estimated in a straightforward manner from the SAR image-intensity spectrum. The range of conditions over which this assumption of linearity is valid is investigated using a numerical simulation model, and the implications of various departures from linearity are discussed.

Lyzenga, David R.↗
NASA NTRS1988

Radar returns from ground clutter in vicinity of airports

The objective of this project is to develop a dynamic simulation of the received signals from natural and man-made ground features in the vicinity of airports. The simulation is run during landing and takeoff stages of a flight. Vugraphs of noteworthy features of the simulation, ground clutter data bases, the development of algorithms for terrain features, typical wave theory results, and a gravity wave height profile are given.

Raemer, H. R.↗
NASA NTRS1987

A review of sea-state bias in altimetry

The accuracy of satellite altimetric measurements of sea level is limited in part by the influence by ocean waves on the altimeter signal reflected from the sea surface. This error, termed the sea-state bias, is poorly known; yet, for such altimetric satellite missions as Topex/Poseidon, it is the largest source of error exclusive of those resulting from calculation of the satellite's ephemeris. Previous observations of sea-state bias have had a large, apparently random scatter, in the range of 1-5 percent of significant wave height; and the observations are not consistent with theoretical calculations of the bias. Further theoretical and experimental studies will be required for understanding the error with an accuracy required by future altimetric satellite missions.

Stewart, Robert↗
NASA NTRS1989

Seasonal variability in global sea level observed with Geosat altimetry

Time changes in global mesoscale sea level variances were observed with satellite altimetry between November 1986 and March 1988, showing significant, geographically coherent seasonal patterns. The NE Pacific and NE Atlantic variances show the most reliable patterns, higher than their yearly averages in both the fall and winter. The response to wind forcing appears as the major contributor to the NE Pacific and Atlantic signals; errors in the estimated inverse barometer response due to errors in atmospheric pressure, residual orbit errors, and errors in sea state bias are evaluated and found to be negligible contributors to this particular signal. The equatorial regions also show significant seasonal patterns, but the uncertainties in the wet tropospheric correction prevent definitive conclusions. The western boundary current changes are very large but not statistically significant. Estimates of the regression coefficient between sea level and significant wave height, an estimate of the sea state bias correction, range between 2.3 and 2.9 percent and vary with the type of orbit correction applied.

Zlotnicki, V.↗
NASA NTRS1990

Determination of the ocean circulation using Geosat altimetry

A spherical harmonic model of the sea surface topography complete to degree and order 10 and a model of the earth's geopotential field complete to degree and order 50 have been obtained in a simultaneous solution using Geosat altimeter data and tracking data from 14 different satellites. The sea surface topography model compares well with oceanographic models computed using hydrographic data and ship drift data. Currently, errors in the estimated gravity field model limit the determination of the spherical harmonic coefficients of the general ocean circulation to degrees 10 and lower, corresponding to a minimum wavelength of 4000 km. Error analysis indicates that the correlation between the geoid and the sea surface topography model is less than 0.2, indicating good separation of the geoid and the sea surface topography at wavelengths of 4000 km or longer. Estimates of the scale factor for the significant wave height (H1/3), which is used to compute the electromagnetic bias correction and the bias for the Geosat altimeter, are obtained. The estimate of the H1/3 correction is 3.6 + or - 1.5 percent, and the height bias estimate is zero.

Nerem, R. S.↗
NASA NTRS1990

Project summaries

Lunar base projects, including a reconfigurable lunar cargo launcher, a thermal and micrometeorite protection system, a versatile lifting machine with robotic capabilities, a cargo transport system, the design of a road construction system for a lunar base, and the design of a device for removing lunar dust from material surfaces, are discussed. The emphasis on the Gulf of Mexico project was on the development of a computer simulation model for predicting vessel station keeping requirements. An existing code, used in predicting station keeping requirements for oil drilling platforms operating in North Shore (Alaska) waters was used as a basis for the computer simulation. Modifications were made to the existing code. The input into the model consists of satellite altimeter readings and water velocity readings from buoys stationed in the Gulf of Mexico. The satellite data consists of altimeter readings (wave height) taken during the spring of 1989. The simulation model predicts water velocity and direction, and wind velocity.

Source record↗
NASA NTRS1989

An airborne C-band scatterometer for remote sensing the air-sea interface

An airborne C-band scatterometer system (C-Scat) has been developed for remote sensing of the air-sea interface. The sensor has been designed to fly on a number of research aircraft, beginning with the NASA Ames Research Center's C-130B, on which test flights were conducted in August of 1988. The scatterometer utilizes a 10-W solid-state power amplifier and a frequency-steered microstrip array antenna which is installed beneath the fuselage of the airplane. The antenna is electrically scanned in elevation from 20 to 50 deg off nadir, and it is mechanically rotated 360 deg in azimuth. The system is fully computer controlled and is capable of accurately measuring ocean-surface normalized radar cross section (NRCS) from altitudes as high as 25,000 feet. It has been developed to study the relationship between NRCS and ocean-surface roughness influences such as wind speed and direction, wave height and slope, and air-sea temperature difference.

Mclaughlin, D. J.↗
NASA NTRS1991

A C-band scatterometer for remote sensing the air-sea interface

An airborne C-band scatterometer system (C-Scat) has been developed to remotely sense ocean surface winds and improve upon the present understanding of the relationship between normalized radar cross section (NRCS) and ocean surface roughness influences such as wind speed and direction, wave height and slope, and the air-sea temperature difference. The scatterometer utilizes a unique frequency-steered microstrip array antenna that is installed beneath the fuselage of an airplane. The antenna is electronically scanned in elevation, from 20 deg to 50 deg off-nadir, and mechanically spins in azimuth. The system is capable of measuring ocean surface NRCS from altitudes as high as 25,000 ft. The transmitter and receiver operate from 4.98 to 5.7 GHz. System parameters such as transmitter pulse width, pulse repetition frequency, output power level, and receiver bandwidth are programmable. Received signals can be averaged and displayed in real time and are stored on a Winchester disk drive for post-flight analysis. Preliminary flight data that demonstrates the instrument's performance is presented.

Mclaughlin, David J.↗
NASA NTRS1991

Satellite observations of polar lows by SSM/I, Geosat and TOVS

Satellite observations in high-latitude regions were used to detect polar lows and track their propagation and evolution. The Special Sensor Microwave/Imager (SSM/I) provided estimates of surface wind speed, integrated cloud liquid water, and water vapor contents over ocean; the Geosat radar altimeter measured surface wind speed and significant wave height; the TIROS-N Operational Vertical Sounder (TOVS) allowed the determination of temperature and humidity profiles in the atmosphere, as well as cloud information (cloud top pressure and temperature). The detection of polar lows at a very early stage, and their development, propagation, and decay is presented for a specific case occurring in the Norwegian Sea on 23-24 January 1988, using a combination of these three instruments, namely a passive and an active microwave sensor, and a vertical sounder (19 infrared, 1 visible, and 4 microwave channels). Advantage is taken of the time and coverage differences between these three satellite sensors for the observation of polar lows. A discussion of the results is given.

Claud, C.↗
NASA NTRS1992

Diffraction from sharply peaked waves as an ocean surface scattering mechanism

The role of sharply peaked waves as a major ocean scattering mechanism for radar is investigated. A prototype three-dimensional wedgelike wave shape was constructed, and its scattering properties were analyzed. Using results from the theory of the statistical geometry of the ocean surface, it is estimated how many such wedges there are per unit area, as a function of sea conditions. Taking into account a directional distribution of the wedges, the total radar cross section due to wedge diffraction effects is estimated. At large incidence angles, wedge diffraction appears to account for a significant amount of the radar cross section on the ocean surface. The wedgelike wave shape used is a more realistic representation of sharplypeaked waves. The scale-size and spatial density of the wedgelike waves are computed directly from the wave-height spectrum.

Jensen, Glenn A.↗
NASA NTRS1993

Satellite observations of a polar low over the Norwegian Sea by Special Sensor Microwave Imager, Geosat, and TIROS-N Operational Vertical Sounder

Many polar lows are generated at the boundary between sea ice and the ocean, in regions of large temperature gradients, where in situ observations are rare or nonexistent. Since satellite observations are frequent in high-latitude regions, they can be used to detect polar lows and track their propagation and evolution. The Special Sensor Microwave/Imager (SSM/I) providing estimates of surface wind speed, integrated cloud liquid water content, water vapor content, and precipitation size ice-scattering signal over the ocean; the Geosat radar altimeter measuring surface wind speed and significant wave height; and the TIROS-N Operational Vertical Sounder (TOVS) allowing the determination of temperature and humidity profiles in the atmosphere have been used in synergy for a specific case which occurred in the Norwegian Sea on January, 23-24 1988. All three instruments show sharp atmospheric gradients associated with the propagation of this low across the ocean, which permit the detection of the polar low at a very early stage and tracking it during its development, propagation, and decay. The wind speed gradients are measured with good qualitative agreement between the altimeter and SSM/I. TOVS retrieved fields prior to the formation of the low confirm the presence of an upper level trough, while during the mature phase baroclinicity can be observed in the 1000-500 hPa geopotential thicknesses.

Claud, Chantal↗
NASA NTRS1991

Fundamental studies of radar scattering from water surfaces: The Lake Washington experiment

The University of Kansas and the University of Washington conducted a series of experiments during July and August of 1989, and July and August of 1990, to study the effects of various geophysical parameters on radar backscatter. The experiments were conducted from a platform in Lake Washington. Measurements of backscattered power and radar range were made by the University of Kansas, and environmental data such as wind speed, wind direction, and air and water temperature were measured by the University of Washington. Results of preliminary data processing are described. Radar data were acquired using two radars, one that operated at C and X bands and another at Ka band. Measurements were made at W and HH antenna polarizations, at different angles of incidence and under various wind conditions. Plots of backscattered power, normalized radar cross section, and wave height, and the Modulation Transfer Functions of selected data are presented.

Salam, A.↗
NASA NTRS1993

A low noise 665 GHz SIS quasi-particle waveguide receiver

Recent results on a 565-690 GHz SIS heterodyne receiver employing a 0.36 micron(sup 2) Nb/AlOx/Nb SIS tunnel junction with high quality circular non-contacting back short and E-plane tuners in a full height wave guide mount are reported. No resonant tuning structures were incorporated in the junction design at this time, even though such structures are expected to help the performance of the receiver. The receiver operates to at least the gap frequency of Niobium, approximately 680 GHz. Typical receiver noise temperatures from 565-690 GHz range from 160K to 230K with a best value of 185K DSB at 648 GHz. With the mixer cooled from 4.3K to 2K the measured receiver noise temperatures decreased by approximately 15 percent, giving roughly 180K DSB from 660 to 680 GHz. The receiver has a full 1 GHz IF pass band and was successfully installed at the Caltech Submillimeter Observatory in Hawaii.

Kooi, J. W.↗
NASA NTRS1994

Assessment of the TOPEX altimeter performance using waveform retracking

To assess the accuracy of the TOPEX altimeter data, we have reprocessed the raw altimeter waveform data using more sophisticated algorithms than those implemented in the altimeter hardware. We discuss systematic contamination of the waveform which we have observed and its effect on very long wavelength errors. We conclude that these systematic errors are responsible for a very long wavelength error whose peak-to-peak magnitude for the Ku band altimeter is of the order of 1 cm. We also examine the ability of retracked data to reduce the repeat pass variance and correct for significant wave height (SWH) and acceleration dependent errors. We find that the ground postprocessing contains SWH dependent biases which depend on the altimeter fine height correction.

Rodriguez, Ernesto↗