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At least 325 records · Page 18

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NASA NTRS1991

Statistical problems of wind-generated gravity waves arising in microwave remote sensing of surface winds

The basic geometric features of a developed sea surface affecting the accuracy of wind speed measurements by satellite instruments are reviewed. Error trends in the satellite winds are shown to be correlated with various measures of wave development. For a developed sea, the influence of the equilibrium range parameters characterizing the spectrum of wind-generated waves are derived and compared with field observations. Fluctuations in the actual number of steep wavelet events which occur within a finite surface area are shown to be an important factor of the radar backscatter if the surface area, i.e., the resolution cell, is much smaller than 1 sq km. The inner scale of the equilibrium range caused by energy dissipation by breaking waves is considered to be a major factor in determining the surface's statistical geometry and, hence, its microwave returns for all satellite sensors.

Glazman, Roman E.↗

NASA NTRS1980

Influence of soils on Landsat spectral signatures of corn

Landsat data have been investigated extensively to determine crop types and acreage. However, confounding site factors have been found to reduce accuracy. Soils data in a small, contiguous area in southeast South Dakota were used to stratify Landsat data. A June 5 and July 29 CCT were used in a statistical analysis of corn training data. Significant soil parameters causing differences in study area soils were slope and parent material. Implication of the results is that, in this region, stratification of CCT data along parent material boundaries would improve corn classification accuracy. Research expanding on the interaction of soils and crops is both in progress and scheduled for additional studies in east central South Dakota.

Dalsted, K. J.↗

NASA NTRS1977

Estimation of terrain iso-gradients from a stochastic range data measurement matrix

The problem of estimating terrain iso-gradients for an autonomous roving vehicle is complicated by the measurement error inherent in the range data matrix. In this paper, the in-path and cross-path slopes at each data point in the multivariable range matrix are expressed in terms of parameters at that data point and those at its surrounding points. The sensitivity in the change of these slopes - due to the errors in measurements in range, elevation angle and azimuth angle are formulated. These sensitivities are used to determine the appropriate statistics of the gradient, which are then used to describe the terrain with a known probability of accuracy.

Shen, C. N.↗

NASA NTRS1979

A parametric model for multispectral scanners

A set of test statistics are specified and the corresponding output quantities computed by the characteristic function. Two sets of classification accuracies, one at the input and one at the output are estimated. The scanner's instantaneous field of view is changed and the variation of the output classification performance is monitored.

Mobasseri, B. G.↗

NASA NTRS1981

Earth feature identification for onboard multispectral data editing: Computational experiments

A computational model of the processes involved in multispectral remote sensing and data classification is developed as a tool for designing smart sensors which can process, edit, and classify the data that they acquire. An evaluation of sensor system performance and design tradeoffs involves classification rates and errors as a function of number and location of spectral channels, radiometric sensitivity and calibration accuracy, target discrimination assignments, and accuracy and frequency of compensation for imaging conditions. This model provides a link between the radiometric and statistical properties of the signals to be classified and the performance characteristics of electro-optical sensors and data processing devices. Preliminary computational results are presented which illustrate the editing performance of several remote sensing approaches.

Aherron, R. M.↗

NASA NTRS1983

AN-A46. LANDSAT scene-to-scene registration assessment

Initial results obtained from the registration of LANDSAT-4 MSS data to LANDSAT-2 MSS data are documented and compared with results obtained from a LANDSAT-2 MSS-to-LANDSAT-2 MSS scene-to-scene registration (using the same LANDSAT-2 MSS data as the base data set in both procedures). RMS errors calculated on the control points used in the establishment of scene-to-scene mapping equations are compared to errors computed from independently chosen verification points. Models developed to estimate actual scene-to-scene registration accuracy based on the use of electrostatic plots are also presented. Analysis or results obtained indicates a statistically significant difference in the RMS errors for the element contribution. Scan line errors were not significantly different. It appears that a modification to the LANDSAT-4 MSS scan mirror coefficients is required to correct the situation.

Anderson, J. E.↗

NASA NTRS1990

Nonlinear damping model for flexible structures

The study of nonlinear damping problem of flexible structures is addressed. Both passive and active damping, both finite dimensional and infinite dimensional models are studied. In the first part, the spectral density and the correlation function of a single DOF nonlinear damping model is investigated. A formula for the spectral density is established with O(Gamma(sub 2)) accuracy based upon Fokker-Planck technique and perturbation. The spectral density depends upon certain first order statistics which could be obtained if the stationary density is known. A method is proposed to find the approximate stationary density explicitly. In the second part, the spectral density of a multi-DOF nonlinear damping model is investigated. In the third part, energy type nonlinear damping model in an infinite dimensional setting is studied.

Zang, Weijian↗

NASA NTRS2004

Characterizing the Preturbulence Environment for Sensor Development, New Hazard Algorithms and NASA Experimental Flight Planning

During the grant period, several tasks were performed in support of the NASA Turbulence Prediction and Warning Systems (TPAWS) program. The primary focus of the research was on characterizing the preturbulence environment by developing predictive tools and simulating atmospheric conditions that preceded severe turbulence. The goal of the research being to provide both dynamical understanding of conditions that preceded turbulence as well as providing predictive tools in support of operational NASA B-757 turbulence research flights. The advancements in characterizing the preturbulence environment will be applied by NASA to sensor development for predicting turbulence onboard commercial aircraft. Numerical simulations with atmospheric models as well as multi-scale observational analyses provided insights into the environment organizing turbulence in a total of forty-eight specific case studies of severe accident producing turbulence on commercial aircraft. These accidents exclusively affected commercial aircraft. A paradigm was developed which diagnosed specific atmospheric circulation systems from the synoptic scale down to the meso-y scale that preceded turbulence in both clear air and in proximity to convection. The emphasis was primarily on convective turbulence as that is what the TPAWS program is most focused on in terms of developing improved sensors for turbulence warning and avoidance. However, the dynamical paradigm also has applicability to clear air and mountain turbulence. This dynamical sequence of events was then employed to formulate and test new hazard prediction indices that were first tested in research simulation studies and then ultimately were further tested in support of the NASA B-757 turbulence research flights. The new hazard characterization algorithms were utilized in a Real Time Turbulence Model (RTTM) that was operationally employed to support the NASA B-757 turbulence research flights. Improvements in the RTTM were implemented in an effort to increase the accuracy of the operational characterization of the preturbulence environment. Additionally, the initial research necessary to create a statistical evaluation scheme for the characterization indices utilized in the RTTM was undertaken. Results of all components of this research were then published in NASA contractor reports and scientific journal papers.

Kaplan, Michael L.↗

NASA NTRS1996

Downlink Probability Density Functions for EOS-McMurdo Sound

The visibility times and communication link dynamics for the Earth Observations Satellite (EOS)-McMurdo Sound direct downlinks have been studied. The 16 day EOS periodicity may be shown with the Goddard Trajectory Determination System (GTDS) and the entire 16 day period should be simulated for representative link statistics. We desire many attributes of the downlink, however, and a faster orbital determination method is desirable. We use the method of osculating elements for speed and accuracy in simulating the EOS orbit. The accuracy of the method of osculating elements is demonstrated by closely reproducing the observed 16 day Landsat periodicity. An autocorrelation function method is used to show the correlation spike at 16 days. The entire 16 day record of passes over McMurdo Sound is then used to generate statistics for innage time, outage time, elevation angle, antenna angle rates, and propagation loss. The levation angle probability density function is compared with 1967 analytic approximation which has been used for medium to high altitude satellites. One practical result of this comparison is seen to be the rare occurrence of zenith passes. The new result is functionally different than the earlier result, with a heavy emphasis on low elevation angles. EOS is one of a large class of sun synchronous satellites which may be downlinked to McMurdo Sound. We examine delay statistics for an entire group of sun synchronous satellites ranging from 400 km to 1000 km altitude. Outage probability density function results are presented three dimensionally.

Christopher, P.↗

NASA NTRS1995

Vertical Scales of Turbulence at the Mount Wilson Observatory

The vertical scales of turbulence at the Mount Wilson Observatory are inferred from data from the University of California at Berkeley Infrared Spatial Interferometer (ISI), by modeling path length fluctuations observed in the interferometric paths to celestial objects and those in instrumental ground-based paths. The correlations between the stellar and ground-based path length fluctuations and the temporal statistics of those fluctuations are modeled on various timescales to constrain the vertical scales. A Kolmogorov-Taylor turbulence model with a finite outer scale was used to simulate ISI data. The simulation also included the white instrumental noise of the interferometer, aperture-filtering effects, and the data analysis algorithms. The simulations suggest that the path delay fluctuations observed in the 1992-1993 ISI data are largely consistent with being generated by refractivity fluctuations at two characteristic vertical scales: one extending to a height of 45 m above the ground, with a wind speed of about 1 m/ s, and another at a much higher altitude, with a wind speed of about 10 m/ s. The height of the lower layer is of the order of the dimensions of trees and other structures near the interferometer, which suggests that these objects, including elements of the interferometer, may play a role in generating the lower layer of turbulence. The modeling indicates that the high- attitude component contributes primarily to short-period (less than 10 s) fluctuations, while the lower component dominates the long-period (up to a few minutes) fluctuations. The lower component turbulent height, along with outer scales of the order of 10 m, suggest that the baseline dependence of long-term interferometric, atmospheric fluctuations should weaken for baselines greater than a few tens of meters. Simulations further show that there is the potential for improving the seeing or astrometric accuracy by about 30%-50% on average, if the path length fluctuations in the lower component are directly calibrated. Statistical and systematic effects induce an error of about 15 m in the estimate of the lower component turbulent altitude.

Treuhaft, Robert N.↗

NASA NTRS1973

Mariner Venus/Mercury 1973 navigation strategy

This paper presents the navigational aspects of the Mariner Venus/Mercury 1973 mission. Principal emphasis is on the development of the trajectory correction strategy, propellant costs, and delivery accuracies at Venus and Mercury. Key error sources and mission constraints are discussed. Of particular interest are the statistics of the first, post-Venus, maneuver which must correct for the magnification of errors in the Venus encounter. Finally, although not a primary objective of the mission, the analysis is extended to include a second Mercury encounter.

Mckinley, E. L.↗

NASA NTRS1982

Random element method for numerical modeling of diffusional processes

The random element method is a generalization of the random vortex method that was developed for the numerical modeling of momentum transport processes as expressed in terms of the Navier-Stokes equations. The method is based on the concept that random walk, as exemplified by Brownian motion, is the stochastic manifestation of diffusional processes. The algorithm based on this method is grid-free and does not require the diffusion equation to be discritized over a mesh, it is thus devoid of numerical diffusion associated with finite difference methods. Moreover, the algorithm is self-adaptive in space and explicit in time, resulting in an improved numerical resolution of gradients as well as a simple and efficient computational procedure. The method is applied here to an assortment of problems of diffusion of momentum and energy in one-dimension as well as heat conduction in two-dimensions in order to assess its validity and accuracy. The numerical solutions obtained are found to be in good agreement with exact solution except for a statistical error introduced by using a finite number of elements, the error can be reduced by increasing the number of elements or by using ensemble averaging over a number of solutions.

Ghoniem, A. F.↗

NASA NTRS1986

Single-dish high sensitivity determination of solar limb emission at 22 and 44 GHz

A large number of solar maps were obtained with the use of Itapetinga 45 ft antenna at 22 GHz and 44 GHz. A statistical study of these maps, reduced using original techniques, permitted the establishment of the solar radius with great accuracy at the two frequencies. It is found that 22 GHz and 44 BHz radiation originates at 16,00 km and 12,500 km above the photosphere, respectively. Excess emission due to active regions was clearly identified at lower solar latitudes above and below the equator, extending up to 26,000 km and 16,500 km above the photosphere, at 22 GHs and 44 GHz, respectively.

Costa, J. E. R.↗

NASA NTRS1984

A comparison of Landsat point and rectangular field training sets for land-use classification

Rectangular training fields of homogeneous spectroreflectance are commonly used in supervised pattern recognition efforts. Trial image classification with manually selected training sets gives irregular and misleading results due to statistical bias. A self-verifying, grid-sampled training point approach is proposed as a more statistically valid feature extraction technique. A systematic pixel sampling network of every ninth row and ninth column efficiently replaced the full image scene with smaller statistical vectors which preserved the necessary characteristics for classification. The composite second- and third-order average classification accuracy of 50.1 percent for 331,776 pixels in the full image substantially agreed with the 51 percent value predicted by the grid-sampled, 4,100-point training set.

Tom, C. H.↗

NASA NTRS1991

Oculomotor function during space flight and susceptibility to space motion sickness

Horizontal vestibulo-ocular reflex (VOR) and saccadic eye movements (SEM) were studied in 18 subjects before and during five Space Shuttle missions to evaluate the effects of weightlessness and correlations between results and susceptibility to and actual presence of space motion sickness (SMS). Active sinusoidal head oscillation was the stimulus for VOR tests with vision (VVOR), with eyes shaded (VOR-ES), and VOR suppression (VOR-S). Eye movements were recorded by electrooculography and head position by a potentiometer. No pathological nystagmus or other abnormal eye movements were seen. No significant in-flight changes were seen in the gain, phase shift or waveform of VVOR, VOR-ES or VOR-S. Statistically significant increases in saccadic latency and decreases in saccadic velocity were seen, with no change in saccadic accuracy. Preflight differences between SMS susceptible and nonsusceptible subjects were noted only in VOR-S, with less complete suppression in susceptible subjects, a finding also seen in flight. During flight, VVOR gain was significantly increased in three nonaffected subjects. Saccades of SMS-affected subjects showed increased latency and velocity and decreased accuracy compared to saccades of unaffected subjects.

Thornton, William E.↗

NASA NTRS1996

Signal Processing and Calibration of Continuous-Wave Focused CO2 Doppler Lidars for Atmospheric Backscatter Measurement

Two continuous-wave(CW)focused C02 Doppler lidars (9.1 and 10.6 micrometers) were developed for airborne in situ aerosol backscatter measurements. The complex path of reliably calibrating these systems, with different signal processors, for accurate derivation of atmospheric backscatter coefficients is documented. Lidar calibration for absolute backscatter measurement for both lidars is based on range response over the lidar sample volume, not solely at focus. Both lidars were calibrated with a new technique using well-characterized aerosols as radiometric standard targets and related to conventional hard-target calibration. A digital signal processor (DSP), a surface acoustic and spectrum analyzer and manually tuned spectrum analyzer signal analyzers were used. The DSP signals were analyzed with an innovative method of correcting for systematic noise fluctuation; the noise statistics exhibit the chi-square distribution predicted by theory. System parametric studies and detailed calibration improved the accuracy of conversion from the measured signal-to-noise ratio to absolute backscatter. The minimum backscatter sensitivity is approximately 3 x 10(exp -12)/m/sr at 9.1 micrometers and approximately 9 x 10(exp -12)/m/sr at 10.6 micrometers. Sample measurements are shown for a flight over the remote Pacific Ocean in 1990 as part of the NASA Global Backscatter Experiment (GLOBE) survey missions, the first time to our knowledge that 9.1-10.6 micrometer lidar intercomparisons were made. Measurements at 9.1 micrometers, a potential wavelength for space-based lidar remote-sensing applications, are to our knowledge the first based on the rare isotope C-12 O(2)-18 gas.

Rothermel, Jeffry↗

NASA NTRS2010

VLBI2010 Imaging and Structure Corrections

Simulations show that the next generation VLBI system is generally well suited for imaging extragalactic radio sources. In addition to revealing the morphology of the sources, simulated VLBI2010 images may also be used to generate structure correction maps, which characterize the impact of source structure on the VLBI measurements. By comparing structure corrections for a set of simulated images based on Monte-Carlo generated visibilities with theoretical structure corrections derived from the model, we assess the accuracy of VLBI2010 structure corrections. In the most favorable case (32-station network, source at declination +40 deg.), statistics demonstrate that the structure corrections obtained from simulated images are statistically close to the theoretical model ones, which seems very promising for the VLBI2010 system.

Collioud, Arnaud↗

NASA NTRS2018

Constraining the Thickness of Polar Ice Deposits on Mercury Using the Mercury Laser Altimeter and Small Craters in Permanently Shadowed Regions

Radar-bright deposits at the poles of Mercury are located in permanently shadowed regions, which pro- vide thermally stable environments for hosting and retaining water ice on the surface or in the near sub- surface for geologic timescales. While the areal distribution of these radar-bright deposits is well characterized, their thickness, and thus their total mass and volume, remain poorly constrained. Here we derive thickness estimates for selected water-ice deposits using small, simple craters visible within the permanently shadowed, radar-bright deposits. We examine two endmember scenarios: in Case I, these craters predate the emplacement of the ice, and in Case II, these craters postdate the emplacement of the ice. In Case I, we find the difference between estimated depths of the original unfilled craters and the measured depths of the craters to find the estimated infill of material. The average estimated infilled material for 9 craters assumed to be overlain with water ice is ∼41 +30 −14 m, where 1 −σ standard error of the mean is reported as uncertainty. Reported uncertainties are for statistical errors only. Additional systematic uncertainty may stem from georeferencing the images and topographic datasets, from the radial accuracy of the altimeter measurements, or from assumptions in our models including (1) ice is flat in the bowl- shaped crater and (2) there is negligible ice at the crater rims. In Case II, we derive crater excavation depths to investigate the thickness of the ice layer that may have been penetrated by the impact. While the absence of excavated regolith associated with the small craters observed suggests that impacts generally do not penetrate through the ice deposit, the spatial resolution and complex illumination geometry of images may limit the observations. Therefore, it is not possible to conclude whether the small craters in this study penetrate through the ice deposit, and thus Case II does not provide a constraint on the ice thickness. For Mercury’s polar deposits, we argue that Case I of the small craters predating the emplacement of the ice deposits is more likely, given other geologic evidence that suggests that these ice deposits are relatively young. Using the ice thickness estimates from Case I to calculate the total amount of water ice currently contained in Mercury’s polar deposits results in a value of ∼10 14 –10 15 kg. This is equivalent to ∼100–1000 km 3 ice in volume. This volume of water ice is consistent with delivery via micrometeorite bombardment, Jupiter-family comets, or potentially a single impactor.

Deutsch, Ariel N.↗