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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

An initial assessment of the performance achieved by the Seasat-1 radar altimeter

This paper describes the results of an initial on-orbit engineering assessment of the performance achieved by the radar altimeter system flown on Seasat-1. Additionally, the general design characteristics of this system are discussed and illustrations of altimeter data products are provided. The instrument consists of a 13.5-GHz monostatic radar system that tracks in range only using a 1-m parabolic antenna pointed at the satellite nadir. Two of its unique features are a linear FM transmitter with 320-MHz bandwidth, which yields a 3.125-ns time-delay resolution, and microprocessor-implemented closed-loop range tracking, automatic gain control (AGC), and real-time estimation of significant wave height (SWH). Results presented herein show that the altimeter generally performed in accordance with its original performance requirements of measuring altitude to a precision of less than 10-cm rms, SWH to an accuracy of plus or minus 0.5 m or 10 percent whichever is greater, and ocean backscatter coefficient to an accuracy of plus or minus 1 dB, all over an SWH range of 1 to 20 m.

Townsend, W. F.↗
NASA NTRS1980

Comparisons between wave directional spectra from SAR and pressure sensor arrays

Simultaneous directional wave measurements were made at Torrey Pines Beach, California, by a synthetic aperture radar (SAR) and a linear array of pressure sensors. The measurements were conducted during the West Coast Experiment in March 1977. Quantitative comparisons of the normalized directional spectra from the two systems were made for wave periods of 6.9-17.0 s. The comparison results were variable but generally showed good agreement of the primary mode of the normalized directional energy. An attempt was made to quantify the physical criteria for good wave imaging in the SAR. A frequency band analysis of wave parameters such as band energy, slope, and orbital velocity did not show good correlation with the directional comparisons. It is noted that absolute values of the wave height spectrum cannot be derived from the SAR images yet and, consequently, no comparisons of absolute energy levels with corresponding array measurements were intended.

Pawka, S. S.↗
NASA NTRS1979

Simulation studies of the application of SEASAT data in weather and state of sea forecasting models

The design and analysis of SEASAT simulation studies in which the error structure of conventional analyses and forecasts is modeled realistically are presented. The development and computer implementation of a global spectral ocean wave model is described. The design of algorithms for the assimilation of theoretical wind data into computers and for the utilization of real wind data and wave height data in a coupled computer system are presented.

Cardone, V. J.↗
NASA NTRS1980

On the pressure field of nonlinear standing water waves

The pressure field produced by two dimensional nonlinear time and space periodic standing waves was calculated as a series expansion in the wave height. The high order series was summed by the use of Pade approximants. Calculations included the pressure variation at great depth, which was considered to be a likely cause of microseismic activity, and the pressure distribution on a vertical barrier or breakwater.

Schwartz, L. W.↗
NASA NTRS1981

Flight performance of SEASAT-1 attitude control system

The paper describes the attitude control flight performance of SEASAT-A, (launched in June 1978), until failure of the power subsystem terminated the mission in Oct. 1978. Information provided by the satellite pertained to surface winds and coastal storm activity, currents and wave heights, ocean topography and temperature. The ACS used three-axis momentum exchange and magnetic momentum containment and was set in a nose-down configuration. Attitude control sensing was via horizon sensors and attitude anomalies, resulting from solar interference with the sensors, is covered in detail. Remedial action was taken by reconfiguring the IR scanners, but failure resulted nevertheless. Recommendations for similar future missions include consideration of the effect of light sources and circuit time constants and scan rates. A block diagram of the ACS is included.

Beach, S. W.↗
NASA NTRS1981

Commercial applications of satellite oceanography

It is shown that in the next decade the oceans' commercial users will require an operational oceanographic satellite system or systems capable of maximizing real-time coverage over all ocean areas. Seasat studies suggest that three spacecraft are required to achieve this. Here, the sensor suite would measure surface winds, wave heights (and spectral energy distribution), ice characteristics, sea-surface temperature, ocean colorimetry, height of the geoid, salinity, and subsurface thermal structure. The importance of oceanographic data being distributed to commercial users within two hours of observation time is stressed. Also emphasized is the importance of creating a responsive oceanographic satellite data archive. An estimate of the potential dollar benefits of such an operational oceanographic satellite system is given.

Montgomery, D. R.↗
NASA NTRS1981

Remote sensing of sea state by laser altimeters

The reflection of short laser pulses from the ocean surface was analyzed based on the specular point theory of scattering. The expressions for the averaged received signal, shot noise and speckle induced noise were derived for a direct detection system. It is found that the reflected laser pulses have an average shape closely related to the probability density function associated with the surface profile. This result is applied to estimate the mean sea level and significant wave height from the receiver output of the laser altimeter.

Tsai, B.↗
NASA NTRS1982

The Seasat altimeter data and its accuracy assessment

A description is given of the Seasat satellite radar altimeter, which was designed to measure (1) the altitude of the satellite above the ocean surface, (2) surface wave height, and (3) the ocean-surface backscatter coefficient from which wind speed may be inferred. The atmospheric and geophysical effects influencing radar altimeter measurement accuracies and the attendant correction models adopted for the altimeter geophysical data record are summarized, along with Seasat Altimeter/Precision Orbit Determination Experiment Team activities directed towards the validation and improvement of these models and investigations assessing the accuracy of both the altimeter measurements and the computed satellite altitude ephemeris.

Tapley, B. D.↗
NASA NTRS1982

The observation of ocean surface phenomena using imagery from the Seasat synthetic aperture radar - An assessment

The principles governing synthetic aperture radar (SAR) and its use on the Seasat spacecraft are reviewed. The way in which wind stress, surface currents, long gravity waves, and surface films modulate the scattering properties of resonant (approximately 30-cm-wavelength) waves is discussed, with particular emphasis placed on the mechanisms that could produce images of long gravity waves. Doppler effects by ocean motion are also described. Measurements of long (wavelength more than about 100 m) gravity waves made using Seasat SAR imagery are compared with surface measurements during several experiments. Combining these results, it is found that dominant wavelength and direction are measured by Seasat SAR within + or - 12% and + or - 15 deg, respectively. It is noted, however, that ocean waves are not always visible in SAR images, and detection criteria are discussed in terms of wave height, length, and direction.

Vesecky, J. F.↗
NASA NTRS1982

Oceanography from space

Active and passive spaceborne instruments that can observe the sea are discussed. Attention is given to satellite observations of ocean surface temperature and heating, wind speed and direction, ocean currents, wave height, ocean color, and sea ice. Specific measurements now being made from space are described, the accuracy of various instruments is considered, and problems associated with the analysis of satellite data are examined. It is concluded that the satellites and techniques used by different nations should be sufficiently standard that data from one satellite can be directly compared with data from another and that accurate calibration and overlap of satellite data are necessary to confirm the continuity and homogeneity of the data.

Stewart, R. H.↗
NASA NTRS1981

Satellite radar altimeters - Present and future oceanographic capabilities

Satellite radar altimeters have the ability to provide information related to ocean wave heights, wind speed, and currents. The present investigation has the objective to demonstrate the current capabilities and to indicate ways to increase the information content of the altimeter return through the use of wider bandwidth, higher pulse repetition frequencies (PRF), and multibeam. Altimeters aboard Skylab, Geos-3, and Seasat-1 have provided investigators with valuable experience in translating the radar observables into oceanic parameters. Basically, an altimeter transmits a narrow pulse and measures the time interval until the return energy from the ocean surface is received. That direct measure of the satellite altitude can be interpreted in terms of surface topography. Attention is given to altimeter oceanographic measurements, altimeter return pulse characteristics, satellite pointing and surface slopes, the adaptive tracker for terrain mapping, and a multibeam altimeter.

Townsend, W. F.↗
NASA NTRS1982

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 NTRS1983

Satellite altimetry

Progress in satellite altimetry accuracy, together with geoid surface measurements, is reviewed, and future programs are outlined. The Skylab, Geos-3, and Seasat spacecraft all carried altimeters for observing ocean topography, wind speed, and wave height from space. Data were gathered on the geoid and gravity anomalies over ocean areas, leading to determinations of the mesoscale variability of the oceans and a preliminary data base on ocean circulation. Satellite altimeters are currently accurate to 2 cm over a period of several years, and radial errors have been reduced to 50 cm with Seasat. Mean sea surfaces are measured with decimeter accuracy, a level of precision expected for altimetry when improved Doppler tracking, laser tracking, and the introduction of a GPS-based tracking system are implemented. The Shuttle is regarded as a prime vehicle for testing new altimetry techniques.

Marsh, J. G.↗
NASA NTRS1984

Response of small amplitude slosh to time-dependent effective gravity

Small amplitude, linearized wave theory is used to derive a differential equation for the sloshing wave height under conditions in which a sloshing liquid drains out of a tank of circular but variable cross sectional area, allowing for changes of effective gravity with time. A few illustrative examples are given in which analytical results can be displayed and interpreted. Numerical integrations are presented for various examples of interest with respect to slosh behavior in the bottom dome of the Space Shuttle External Tank during thrust termination, particularly as this bears on scavenging of cryogens.

Traugott, S. C.↗
NASA NTRS1985

Advanced 2-frequency ocean sensing radar using high resolution antenna beams

The opportunity to use a large space antenna system for remote sensing applications permits the creation of an advanced ocean sensing radar that combines the abilities of previously developed techniques. The 15 meter antenna will permit much higher angular and spatial resolution at the surface that will lead to techniques of observing ocean wave heights and the directional spectrum that had not previously been feasible from space. At the same time, sensors to measure ocean surface winds can be in operation and the data from both can be combined to increase the accuracy of each individual sensor. The existing capabilities and sensor techniques with typical data characteristics for the individual measurement of sea surface quantities are outlined.

Weissman, D. E.↗
NASA NTRS1984

The COSPAS/SARSAT demonstration and evaluation. Results of the first year in the United States

The COSPAS/SARSAT location accuracy, coverage, location probability, 406 MHz message transmission, and image rejection were tested using a commercially available beacon (EPIRB) with modulated, coherent, 121.5 and 243.0 MHz distress signals emitted at a minimum of 75 mW, a 406 MHz EPIRB with electronics designed to meet the international specification for experimental beacons, and a beacon with the same electronics as the other 406 MHz EPIRB, incorporating a floating antenna design. The COSPAS/SARSAT system contributed to 15 out of 33 rescue missions, in which 35 of the 85 lives were rescued. In 10% of the cases the COSPAS/SARSAT system provided the only means of notification, and in 18% it was the first means. Tests show that 121.5 MHz accuracy is adequate, but 406 MHz must improve. The 121.5 MHz location probability is 82%, but ambiguity resolution is inferior to 406 MHz. Performance of both 406 MHz beacons at low elevation angles and in high wave heights is good.

Gilbert, M. E.↗
NASA NTRS1985

Air-sea interaction with SSM/I and altimeter

A number of important developments in satellite remote sensing techniques have occurred recently which offer the possibility of studying over vast areas of the ocean the temporally evolving energy exchange between the ocean and the atmosphere. Commencing in spring of 1985, passive and active microwave sensors that can provide valuable data for scientific utilization will start to become operational on Department of Defense (DOD) missions. The passive microwave radiometer can be used to estimate surface wind speed, total air column humidity, and rain rate. The active radar, or altimeter, senses surface gravity wave height and surface wind speed.

Source record↗
NASA NTRS1984

Turbulent roughness drag due to surface waviness at low roughness Reynolds numbers

The current investigation has the objective to provide data which will make it possible to obtain a better estimate regarding the roughness drag for surface waviness. The data employed for this investigation were acquired in connection with a wavy wall study which was conducted as part of an overall program to reduce the skin friction of turbulent boundary layers in external flows. The results of the present investigation show that the low-speed roughness drag of small-amplitude sinusoidal wave trains having wavelengths of the order of the boundary-layer thickness is not only a function of h/lambda (h = total wave height, lambda = wavelength), but, in addition, is also a function of the roughness Reynolds number.

Lin, J. C.↗