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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Limitations imposed by ionospheric turbulence on satellite-to-satellite Doppler measurement accuracy

For some time the possibility has been considered to perform an accurate survey from orbit of the earth gravity field by making use of low-low, satellite-to-satellite Doppler tracking with a radio link which operates in the frequency band in the range from 50 to 100 GHz. It is, therefore, of interest to discuss the upper bound in Doppler measurement accuracy imposed by the effects of ionospheric turbulence. The present investigation is concerned with the measurement error induced by ionospheric turbulence. The assumptin is made that the so-called ionospheric refractive 'bias' can be removed with one of the multifrequency methods of the current practice.

Grossi, M. D.↗
NASA NTRS1983

Classification of journal surfaces using surface topography parameters and software methods to compensate for stylus geometry

Measurements made with a stylus surface tracer which provides a digitized representation of a surface profile are discussed. Parameters are defined to characterize the height (e.g., RMS roughness, skewness, and kurtosis) and length (e.g., autocorrelation) of the surface topography. These are applied to the characterization of crank shaft journals which were manufactured by different grinding and lopping procedures known to give significant differences in crank shaft bearing life. It was found that three parameters (RMS roughness, skewness, and kurtosis) are necessary to adequately distinguish the character of these surfaces. Every surface specimen has a set of values for these three parameters. They can be regarded as a set coordinate in a space constituted by three characteristics axes. The various journal surfaces can be classified along with the determination of a proper wavelength cutoff (0.25 mm) by using a method of separated subspace. The finite radius of the stylus used for profile tracing gives an inherent measurement error as it passes over the fine structure of the surface. A mathematical model is derived to compensate for this error.

Li, C. J.↗
NASA NTRS1983

Time maintenance of user clocks via the tracking and data relay satellite system

A system is described which uses the Tracking and Data Relay Satellite System (TDRSS) itself to compare the user satellite clock with a clock at the White Sands station that is referenced to Universal Time Coordinated (UTC). No command of the spacecraft by the system is required, and actual on-board clock corrections are made by the spacecraft control center at its discretion. Computer models were constructed using basic orbital parameters for user and TDRS satellites. With only first-order corrections and simple averaging techniques for constant clock rates, error measurement precision of better than one microsecond was obtained. More sophisticated computations should allow considerable improvement over this.

Whitworth, G.↗
NASA NTRS1981

Ice-sheet dynamics by satellite laser altimetry

Glaciological applications of a satellite laser altimeter system with a range precision of approximately 10 cm and a footprint diameter of 70 m are discussed. Available data on ice-sheet dynamics are reviewed, and the measurement errors associated with the laser altimeter are examined. It is shown that information collected with the system could make it possible to detect changes in average elevation (i.e., in average ice thickness) of less than 15 cm for areas of hundreds of sq km to a few thousand sq km. Other possible applications of the system are briefly considered, including the mapping of the positions of ice-sheet margins, the mapping of the junction between a grounded ice sheet and a floating ice shelf, and the estimation of iceberg calving rates.

Zwally, H. J.↗
NASA NTRS1983

Some constraints on the color-magnitude diagram of giants in the galactic spheroid

The color-magnitude diagram of giants in the Galactic spheroid is shown to be important in determining the number-color histogram of stars brighter than B = 19 mag. This result is demonstrated by comparing a standard Galaxy model with observations of 391 stars in a field in the direction of Aquarius (l = 36.5 deg, b = -51.1 deg). More than 80 percent of the spheroid stars and 40 percent of all stars in this magnitude range and direction are predicted to be giants. At most, a few percent of the spheroid stars in the current sample can lie on the main sequence bluer than the turn-off onto the giant branch near B - V approximately 0.4. The available observations suggest that the blue tip of the horizontal branch of the spheroid must be sparsely populated about a factor of 10 less than would be expected if the color-magnitude diagram of the spheroid were the same as diagrams for any of the globular clusters M3, M13, or M92. The total dispersion in colors (measurement errors and intrinsic dispersion) has a standard deviation in B - V color that is less than 0.2 mag.

Bahcall, J. N.↗
NASA NTRS1983

Ozone and water vapor monitoring using a ground-based lidar system

Measurements of atmospheric ozone and water vapor profiles obtained using a differential absorption lidar (DIAL) technique at Haute Provence Observatory are reported. The DIAL technique is based on the comparison of differences in attenuation in backscattered signals at wavelengths centered on strong absorptions or absorption lines of the species in question, and wavelengths corresponding to weaker or no absorption. The system in use is based on three lasers - a Nd:YAG Q-switched laser frequency doubled to emit at 532 nm, and two tunable narrowline dye lasers used to convert this emission into the IR or UV spectral regions. Ozone concentration profiles have been recorded up to a height of 30 km, and reveal large day-to-day variations in number density which were correlated with a decrease in tropopause height. Measurements of water vapor concentrations show a variability of the mixing ratio at heights up to 2.5 km representative of dynamic processes, and a variability above 2.5 km that is within the measurement error. Day-to-day variations in tropospheric mixing ratio were also observed.

Megie, G.↗
NASA NTRS1984

Scanning wind-vector scatterometers with two pencil beams

A scanning pencil-beam scatterometer for ocean windvector determination has potential advantages over the fan-beam systems used and proposed heretofore. The pencil beam permits use of lower transmitter power, and at the same time allows concurrent use of the reflector by a radiometer to correct for atmospheric attenuation and other radiometers for other purposes. The use of dual beams based on the same scanning reflector permits four looks at each cell on the surface, thereby improving accuracy and allowing alias removal. Simulation results for a spaceborne dual-beam scanning scatterometer with a 1-watt radiated power at an orbital altitude of 900 km is described. Two novel algorithms for removing the aliases in the windvector are described, in addition to an adaptation of the conventional maximum likelihood algorithm. The new algorithms are more effective at alias removal than the conventional one. Measurement errors for the wind speed, assuming perfect alias removal, were found to be less than 10%.

Kirimoto, T.↗
NASA NTRS1984

The PDS for space telescope guide star selection system: Status and improvements

Pointing the ST (Space Telescope) requires that the onboard guidance system be provided with the coordinates and magnitudes of at least two guide stars for each observation region. The accuracy and limitations of the model 2020G PDS microdensitometer are assessed in view of the guide star requirements of the space telescope. Modifications of the PDS to correct for measuring errors are reviewed.

Kinsey, J. H.↗
NASA NTRS1983

Inversion of vegetation canopy reflectance models for estimating agronomic variables. I - Problem definition and initial results using the Suits model

An important but relatively uninvestigated problem in remote sensing is the inversion of vegetative canopy reflectance models to obtain agrophysical parameters, given measured reflectances. The problem is here formally defined and its solution outlined. Numerical nonlinear optimization techniques are used to implement this inversion to obtain the leaf area index using Suits' canopy reflectance model. The results for a variety of cases indicate that this can be done successfully using infrared reflectances at different views or azimuth angles or a combination thereof. The other parameters of the model must be known, although reasonable measurement errors can be tolerated without seriously degrading the accuracy of the inversion. The application of the technique to ground based remote-sensing experiments is potentially useful, but is limited to the degree to which the canopy reflectance model can accurately predict observed reflectances.

Goel, N.↗
NASA NTRS1983

A comparison of geodetic Doppler satellite receivers

A great number of geodetic Doppler satellite receivers of various types have been employed since early 1970s. In October and November 1979, a comparison was conducted of three more recent types of receivers, including the CMA-751, the JMR-1A, and the MX-1502. A description of the studies involved in conducting the comparison is presented, taking into account the preliminary results obtained on the basis of the processing of the collected data. Attention is given to tracking statistics, chord difference results, range rate measurement errors, residual tests, station position repeatability, and absolute differences to ground control. Recommendations for future comparisons are also provided.

Archinal, B.↗
NASA NTRS1984

The S201 far-ultraviolet imaging survey. II - A field in Cygnus

Far-ultraviolet imagery of a region in Cygnus, a 20 deg diameter field centered near (1950) R. A. 21 h 31.2 m decl. +37 deg 25 arcmin, was obtained by the S201 far-ultraviolet camera during the Apollo 16 mission. In a 10 minute exposure covering the 1250-1600 A wavelength range (effective wavelength 1400 A), 730 star images can be detected, corresponding to a limiting ultraviolet magnitude of about m (1400) = 10. Assuming nominal interstellar extinction values in this region near the galactic plane, this result corresponds to the detection of A0 V stars to a distance of 300 pc and of B0 V stars to 1500 pc. Uncertainties in spectral classification and interstellar extinction for individual objects are probably more significant than calibration or measurement errors. Most of the objects detected are identified with stars in the Smithsonian Astrophysical Observatory Star Catalog (1966), or the Catalog of Stellar Identifications (1979) or both, but 87 objects remain unidentified (or are identified with late-type stars).

Carruthers, G. R.↗
NASA NTRS1984

Performance of a dedicated VLBI system for TDRSS navigation

Results of a covariance study to evaluate the performance of a proposed Very Long Baseline Interferometric (VLBI) system for TDRSS orbit determination (OD) are presented. This dedicated VLBI system consists of 4 one meter antennas for each TDRS positioned on orthogonal 6000 km baselines and uses a GPS signal for clock synchronization and calibration of measurement error sources. The performance of the VLBI system is compared with the current BRT system for support of TDRSS navigation. The criteria for evaluating the system performance is the TDRSS OD accuracy and the resulting navigation accuracy for two typical TDRSS users represented by a TOPEX type orbit and a Space Shuttle orbit. Based on the anticipated characteristics of a 1990 era VLBI system a factor of 20 improvement can be expected in the TDRSS OD accuracy. This translates into accuracy improvements ranging from factors of 3 to 10 for typical TDRSS users.

Ellis, J.↗
NASA NTRS1984

An analysis of Landsat-4 Thematic Mapper geometric properties

Landsat-4 Thematic Mapper data of Washington, DC, Harrisburg, PA, and Salton Sea, CA were analyzed to determine geometric integrity and conformity of the data to known earth surface geometry. Several tests were performed. Intraband correlation and interband registration were investigated. No problems were observed in the intraband analysis, and aside from indications of slight misregistration between bands of the primary versus bands of the secondary focal planes, interband registration was well within the specified tolerances. A substantial number of ground control points were found and used to check the images' conformity to the Space Oblique Mercator (SOM) projection of their respective areas. The means of the residual offsets, which included nonprocessing related measurement errors, were close to the one pixel level in the two scenes examined. The Harrisburg scene residual mean was 28.38 m (0.95 pixels) with a standard deviation of 19.82 m (0.66 pixels), while the mean and standard deviation for the Salton Sea scene were 40.46 (1.35 pixels) and 30.57 m (1.02 pixels), respectively. Overall, the data were judged to be a high geometric quality with errors close to those targeted by the TM sensor design specifications.

Walker, R. E.↗
NASA NTRS1984

On the implementation of modal filters for control of structures

The most common technique for the control of structures is modal control. In modal control, the differential equations in terms of actual coordinates are replaced by a set of ordinary differential equations in terms of the modal coordinates known as modal equations. In designing feedback controls in conjunction with the modal equations, one must know the modal states for the modes targeted for control. The sensors measure actual states, however. The modal states can be estimated by means of a Luenberger observer or modal filters. The modal filters produce estimates of the modal states from distributed measurements of the states. If distributed measurements are not available, then they can be reconstructed from measurements at discrete points via interpolation. This paper examines various questions associated with the implementation of modal filters, such as the effect of choice of interpolation functions and sensors locations, as well as of measurement errors, on the state estimation process. The method is demonstrated by means of two numerical examples.

Meirovitch, L.↗
NASA NTRS1984

Geosynchronous orbiter tracking by VLBI - Demonstration design

A demonstration has been designed to determine the three dimensional position of a satellite in geosynchronous earth orbit, with 5 meter accuracy, using tracking techniques based on very-long-base-line interferometry (VLBI). Two experiments are analyzed: the first uses tracking stations in California, Australia, and Guam, and the second uses stations in California, Australia, and Japan. Satellite VLBI observables are defined and measurement errors are predicted. Both experiments employ alternate observations between the satellite and extra-galactic radio sources to determine the satellite plane-of-sky position. Positional accuracy resulting from various combinations of data acquired over the satellite's orbital period is discussed. A strategy is devised for resolving integer cycle phase ambiguities, inherent in VLBI, without relying on an externally provided reference trajectory.

Border, J. S.↗
NASA NTRS1984

The v = 0 - 0 spectrum of H2

Ground-state rotational parameters for molecular hydrogen are evaluated using a combination of laboratory and observational data. High-accuracy v = 0 - 0 S(0)-S(5) frequencies from infrared laboratory measurements are analyzed simultaneously with available observed frequencies for the S(8), S(9), S(12), S(13), S(14), and S(15) Orion emission lines. The laboratory and Orion measurements are consistent with each other within their measurement errors. The combined data set yields a determination of five v = 0 rotational parameters and an improvement in the high-J calculated line positions. The analysis produces the parameters (in per cm): B(0) = 59.334627(30), D(0) = 0.0456912(30), H(0) = 4.8974(90) x 10 to the -5th, L(0) = -6.320(110) x 10 to the -8th, M(0) = 7.19(30) x 10 to the -11th, where a correction is made for the estimated contribution of sixth and higher orders.

Jennings, D. E.↗
NASA NTRS1983

Weighted least squares formulas for multiple Doppler radar analysis

An algorithm is developed for the analysis of multiple Doppler radar data in meteorological applications. The algorithm is based on the least squares linear solutions for the variable parameters of a local particle velocity field. The computational sequence for the algorithm is described, and some factors influencing the covariance of Doppler measurement errors are discussed.

Wilson, F. W., Jr.↗
NASA NTRS1985

A two-layer diagnostic model of the long-term physical evolution of warm-core ring 82B

The present shipboard data in the 1982 time series on a single Gulf Stream warm core ring are composited on a cylindrical coordinate system following ring motion; 10 C-isotherm depth measurements are used in a two-layer model of the ring's structure to compute gradient current, kinetic energy, available potential energy (APE), and potential vorticity. The ring's evolution can be divided into two periods: April-late June, while the ring is isolated from strong Gulf Stream interaction, during which the ring loses APE at a low rate, and from July onward, following ring interactions with the Gulf Stream and topography with much higher energy loss. Within measurement errors, the potential vorticity at the center of the ring is conserved from April through August, implying nearly inviscid dynamics.

Olson, D. B.↗