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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Application of Statistical Methods of Rain Rate Estimation to Data From The TRMM Precipitation Radar

The TRMM Precipitation Radar is well suited to statistical methods in that the measurements over any given region are sparsely sampled in time. Moreover, the instantaneous rain rate estimates are often of limited accuracy at high rain rates because of attenuation effects and at light rain rates because of receiver sensitivity. For the estimation of the time-averaged rain characteristics over an area both errors are relevant. By enlarging the space-time region over which the data are collected, the sampling error can be reduced. However. the bias and distortion of the estimated rain distribution generally will remain if estimates at the high and low rain rates are not corrected. In this paper we use the TRMM PR data to investigate the behavior of 2 statistical methods the purpose of which is to estimate the rain rate over large space-time domains. Examination of large-scale rain characteristics provides a useful starting point. The high correlation between the mean and standard deviation of rain rate implies that the conditional distribution of this quantity can be approximated by a one-parameter distribution. This property is used to explore the behavior of the area-time-integral (ATI) methods where fractional area above a threshold is related to the mean rain rate. In the usual application of the ATI method a correlation is established between these quantities. However, if a particular form of the rain rate distribution is assumed and if the ratio of the mean to standard deviation is known, then not only the mean but the full distribution can be extracted from a measurement of fractional area above a threshold. The second method is an extension of this idea where the distribution is estimated from data over a range of rain rates chosen in an intermediate range where the effects of attenuation and poor sensitivity can be neglected. The advantage of estimating the distribution itself rather than the mean value is that it yields the fraction of rain contributed by the light and heavy rain rates. This is useful in estimating the fraction of rainfall contributed by the rain rates that go undetected by the radar. The results at high rain rates provide a cross-check on the usual attenuation correction methods that are applied at the highest resolution of the instrument.

Meneghini, R.↗
NASA NTRS2003

Spurious States-Free Solution of the k dot p Hamiltonian for Heterostructures

A method for eliminating spurious solution in the k dot p Hamiltonian has been proposed. Introduction of additional off-diagonal alpha k(exp 2) term converts spurious solution with large real wave vector to evanescent solution with large imaginary wave vector. This modification keeps the same effective masses at Gamma point and introduces negligible deviation from original nonparabolicity. A set of unphysical fast oscillation eigenfunctions in confined states of heterostructures are removed.

Kolokolov, Konstantin I.↗
NASA NTRS2011

Tissue Doppler Imaging Combined with Advanced 12-Lead ECG Analysis Might Improve Early Diagnosis of Hypertrophic Cardiomyopathy in Childhood

Optimization of early diagnosis of childhood hypertrophic cardiomyopathy (HCM) is essential in lowering the risk of HCM complications. Standard echocardiography (ECHO) has shown to be less sensitive in this regard. In this study, we sought to assess whether spatial QRS-T angle deviation, which has shown to predict HCM in adults with high sensitivity, and myocardial Tissue Doppler Imaging (TDI) could be additional tools in early diagnosis of HCM in childhood. Methods: Children and adolescents with familial HCM (n=10, median age 16, range 5-27 years), and without obvious hypertrophy but with heredity for HCM (n=12, median age 16, range 4-25 years, HCM or sudden death with autopsy-verified HCM in greater than or equal to 1 first-degree relative, HCM-risk) were additionally investigated with TDI and advanced 12-lead ECG analysis using Cardiax(Registered trademark) (IMED Co Ltd, Budapest, Hungary and Houston). Spatial QRS-T angle (SA) was derived from Kors regression-related transformation. Healthy age-matched controls (n=21) were also studied. All participants underwent thorough clinical examination. Results: Spatial QRS-T angle (Figure/ Panel A) and septal E/Ea ratio (Figure/Panel B) were most increased in HCM group as compared to the HCM-risk and control groups (p less than 0.05). Of note, these 2 variables showed a trend toward higher levels in HCM-risk group than in control group (p=0.05 for E/Ea and 0.06 for QRS/T by ANOVA). In a logistic regression model, increased SA and septal E/Ea ratio appeared to significantly predict both the disease (Chi-square in HCM group: 9 and 5, respectively, p less than 0.05 for both) and the risk for HCM (Chi-square in HCM-risk group: 5 and 4 respectively, p less than 0.05 for both), with further increased predictability level when these 2 variables were combined (Chi-square 10 in HCM group, and 7 in HCM-risk group, p less than 0.01 for both). Conclusions: In this small material, Tissue Doppler Imaging and spatial mean QRS-T angle deviation, particularly when combined, appear to be sensitive in predicting the risk for developing childhood HCM. Large-scale, prospective studies are needed to confirm this hypothesis.

Femlund, E.↗
NASA NTRS2012

Interval Management with Spacing to Parallel Dependent Runways (IMSPIDR) Experiment and Results

An area in aviation operations that may offer an increase in efficiency is the use of continuous descent arrivals (CDA), especially during dependent parallel runway operations. However, variations in aircraft descent angle and speed can cause inaccuracies in estimated time of arrival calculations, requiring an increase in the size of the buffer between aircraft. This in turn reduces airport throughput and limits the use of CDAs during high-density operations, particularly to dependent parallel runways. The Interval Management with Spacing to Parallel Dependent Runways (IMSPiDR) concept uses a trajectory-based spacing tool onboard the aircraft to achieve by the runway an air traffic control assigned spacing interval behind the previous aircraft. This paper describes the first ever experiment and results of this concept at NASA Langley. Pilots flew CDAs to the Dallas Fort-Worth airport using airspeed calculations from the spacing tool to achieve either a Required Time of Arrival (RTA) or Interval Management (IM) spacing interval at the runway threshold. Results indicate flight crews were able to land aircraft on the runway with a mean of 2 seconds and less than 4 seconds standard deviation of the air traffic control assigned time, even in the presence of forecast wind error and large time delay. Statistically significant differences in delivery precision and number of speed changes as a function of stream position were observed, however, there was no trend to the difference and the error did not increase during the operation. Two areas the flight crew indicated as not acceptable included the additional number of speed changes required during the wind shear event, and issuing an IM clearance via data link while at low altitude. A number of refinements and future spacing algorithm capabilities were also identified.

Baxley, Brian T.↗
NASA NTRS1985

Ariel 6 measurements of ultra-heavy cosmic ray fluxes in the region Z or = 48

For this re-analysis of the Ariel VI data, the contribution of non Z square effects to the restricted energy loss and to Cerenkov radiation in the Bristol sphere has been evaluated using the Mott cross section ratios and the non-relativistic Bloch correction. Results obtained were similar in form to those derived for HEAO3 but with maximum deviations approximately 10% rather than 15% for the Mott term, corresponding to a thinner detector. Because of the large uncertainties in the parameters involved, no relativistic Bloch term was included. In addition the experiments on the HEAO detector make the application of a correction to the Cerenkov response of doubtful justification and none was applied in this analysis. An energy dependent correction was made using an effective energy calculated from the vertical cut-off for a given event. The maximum value of this correction was about 0.6% in Z for low cut-offs, declining to approximately zero by 10 GV.

Fowler, P. H.↗
NASA NTRS1986

An analysis on the shapes of ultraviolet extinction curves. I - The 2175 A bump

IUE data are used to investigate the properties of the 2175 A interstellar extinction bump toward 45 reddened Milky Way stars. Using analytic fitting functions, parameters are derived which describe the central position, FWHM, and strength of the bump. The bump position is very stable, with an extreme deviation which is still significantly larger than the measurement uncertainty, indicating true variability. The bump FWHM has a large range of intrinsic values, 0.77/micron to 1.25/micron. The normalized bump strength varies by more than a factor of three. There are no convincing correlations among the three bump parameters, but the width of the bump is strongly correlated with the dust grain environments. Dense quiescent regions yield broad bumps, while diffuse interstellar medium and regions of recent star formation yield narrower bumps. The absence of any correlation between bump width and central position argues strongly against graphite grains in the size range prescribed by the Mathis, Rumpl, and Nordsieck (1977) model as carriers of the bump.

Fitzpatrick, E. L.↗
NASA NTRS1987

Cryogenic wind tunnels. I

The basic principles of cryogenic wind-tunnel operation are presented, and a typical cryogenic wind tunnel is described. Low-speed cryogenic wind tunnels are considered, along with some experiments conducted in these tunnels. Focus is placed on real-gas effects including condensation and thermal and caloric imperfections. The operating limits set by the saturation boundaries of nitrogen and air are analyzed, and the consequences of thermal and caloric imperfections on both isentropic and normal-shock flow in nitrogen are considered. The extension of analytical real-gas studies, including both laminar and turbulent boundary layers is covered. It is concluded that the deviation of the nitrogen boundary-layer parameters from their ideal-gas values are small, and that large real-gas effects will not be a factor in cryogenic tunnel simulation of the complex flows met in flight.

Kilgore, Robert A.↗
NASA NTRS2022

Winds at the Mars 2020 Landing Site. 2. Wind Variability and Turbulence

Wind speeds measured by the Mars 2020 Perseverance rover in Jezero crater were fitted as a Weibull distribution. InSight wind data acquired in Elysium Planitia were also used to contextualize observations. Jezero winds were found to be much calmer on average than in previous landing sites, despite the intense aeolian activity observed. However, a great influence of turbulence and wave activity was observed in the wind speed variations, thus driving the probability of reaching the highest wind speeds at Jezero, instead of sustained winds driven by local, regional, or large-scale circulation. The power spectral density of wind speed fluctuations follows a power-law, whose slope deviates depending on the time of day from that predicted considering homogeneous and isotropic turbulence. Daytime wave activity is related to convection cells and smaller eddies in the boundary layer, advected over the crater. The signature of convection cells was also found during dust storm conditions, when prevailing winds were consistent with a tidal drive. Nighttime fluctuations were also intense, suggesting strong mechanical turbulence. Convective vortices were usually involved in rapid wind fluctuations and extreme winds, with variations peaking at 9.2 times the background winds. Transient high wind events by vortex-passages, turbulence, and wave activity could be driving aeolian activity at Jezero. We report the detection of a strong dust cloud of 0.75–1.5 km in length passing over the rover. The observed aeolian activity had major implications for instrumentation, with the wind sensor suffering damage throughout the mission, probably due to flying debris advected by winds.

D. Viúdez-moreiras↗
NASA NTRS1978

Effect of wind turbulence and shear on landing performance of jet transports

Computer simulations of a Boeing 727 class aircraft landing in turbulence were developed by programming the longitudinal aircraft equations of motion into a digital computer with various input values of vertical and horizontal wind speeds. Turbulent wind data was fed to the computer in one-second intervals. The computer computed in one-second intervals the aircraft speed, altitude, horizontal distance traveled, rate-of-descent, pitch attitude, glide path angle (from edge of runway) and elevator angle. All computer runs were made in the 'stick-fixed' mode. The RMS values of altitude and velocity perturbations (from equilibrium) were found to be large when horizontal wind gusts had sinusoidal components at or near the phugoid (long period) frequency. Maximum RMS altitude deviations occurred when the vertical wind had sinusoidal components which were 1/10 to 1/5 of the phugoid frequency. When real wind data (obtained from NCAR Queen Air) were used as input winds good correlations were found to exist between RMS velocity perturbations and both horizontal and vertical wind shears.

Blick, E. F.↗
NASA NTRS1991

Jitter model and signal processing techniques for pulse width modulation optical recording

A jitter model and signal processing techniques are discussed for data recovery in Pulse Width Modulation (PWM) optical recording. In PWM, information is stored through modulating sizes of sequential marks alternating in magnetic polarization or in material structure. Jitter, defined as the deviation from the original mark size in the time domain, will result in error detection if it is excessively large. A new approach is taken in data recovery by first using a high speed counter clock to convert time marks to amplitude marks, and signal processing techniques are used to minimize jitter according to the jitter model. The signal processing techniques include motor speed and intersymbol interference equalization, differential and additive detection, and differential and additive modulation.

Liu, Max M.-K.↗
NASA NTRS1993

A zonal CFD method for three-dimensional wing simulations

The primary objective of this work is to demonstrate the feasibility of a 3D potential/viscous flow coupling procedure for reducing computational effort while maintaining solution accuracy. The closed-loop, overlapped, velocity-coupling concept has been developed in a new code, ZAP3D, that couples a potential flow panel code with a Navier-Stokes method. The current ZAP3D calculation for an aspect ratio 5 wing with an outer domain radius of about 1.2 chords represents a speed-up in CPU time over the ARC3D large domain calculation by about a factor of 2.5. This improvement is achieved for less than a 0.5 percent deviation in C(L), 10 counts change in C(D), and 0.0015 variation in C(My). Additional reductions in the required computational domain for ZAP3D are expected as the method is further developed and refined.

Summa, J. M.↗
NASA NTRS2022

KMT-2021-BLG-0912Lb: A Microlensing Super Earth Around A K-Type Star

Aims . The light curve of the microlensing event KMT-2021-BLG-0912 exhibits a very short anomaly relative to a single-lens single-source form. We investigate the light curve for the purpose of identifying the origin of the anomaly. Methods . We model the light curve under various interpretations. From this, we find four solutions, in which three solutions are found under the assumption that the lens is composed of two masses (2L1S models), and the other solution is found under the assumption that the source is comprised of binary stars (1L2S model). The 1L2S model is ruled out based on the contradiction that the faint source companion is bigger than its primary, and one of the 2L1S solutions is excluded from the combination of the poorer fit, blending constraint, and lower overall probability, leaving two surviving solutions with the planet/host mass ratios of q ∼ 2.8 × 10 −5 and ∼1.1 × 10 −5 . A subtle central deviation supports the possibility of a tertiary lens component, either a binary companion to the host with a very large or small separation, or a second planet lying near the Einstein ring, but it is difficult to claim a secure detection due to the marginal improvement of the fit, lack of consistency among different data sets, and difficulty in uniquely specifying the nature of the tertiary component. Results . With the observables of the event, it is estimated that the masses of the planet and host are ∼(6.9 M ⊕ , 0.75 M ⊙ ) according to one solution and ∼(2.8 M ⊕ , 0.80 M ⊙ ) according to the other, indicating that the planet is a super Earth around a K-type star, regardless of the solution. The fact that 16 (including the one reported in this work) out of 19 microlensing planets with M ≲ 10 M ⊕ were detected during the last 6 yr nicely demonstrates the importance of high-cadence global surveys in detecting very low-mass planets.

gravitational lensing↗
NASA NTRS1984

A statistical evaluation and comparison of VISSR Atmospheric Sounder (VAS) data and corresponding rawinsonde measurements

Three mesoscale sounding data sets from the VISSR Atmospheric Sounder (VAS) produced using different retrieval techniques were evaluated of corresponding ground truth rawinsonde data for 6-7 March 1982. Mean, standard deviations, and RMS differences between the satellite and rawinsonde parameters were calculated over gridded fields in central Texas and Oklahoma. Large differences exist between each satellite data set and the ground truth data. Biases in the satellite temperature and moisture profiles seem extremely dependent upon the three dimensional structure of the atmosphere and range from 1 deg to 3 deg C for temperature and 3 deg to 6 deg C for dewpoint temperature. Atmospheric gradients of basic and derived parameters determined from the VAS data sets produced an adequate representation of the mesoscale environment but their magnitudes were often reduced by 30 to 50%.

Source record↗
NASA NTRS1986

Analysis of Jimsphere pairs for use in assessing Space Vehicle ascent capability

A data base of paired detailed wind profiles was developed for use in evaluating Shuttle Transportation System (STS) ascent capability. Since launch decision is based on a wind measured about 3.5 hr before launch, a data base of paired detailed profiles is needed. Method and technique on the reduction process and analysis is also presented. Guidelines used in selecting the pairs of profiles were established to insure a valid and representative data base. Sigma-U values for 3.5 hr at 12 km altitude show 8 percent increase from the transition case to the winter case and 18 percent decrease from the transition case to the summer case. Sigma-Values for 3.5 hr at 12 km altitude shows 12 percent increase from the transition case to the winter case and 17 percent decrease from the transition case to the summer case. A special feature of the 7- and 10.5-hr cases is that sigma-V increases by as much as 30 percent from the transition to the winter profiles. This large increase does not appear in the sigma-U data. Comparisons of the calculated values of 3.5 hr standard deviations of U and V with actual component deviations measured during space shuttle launch conditions confirm that the statistical values are representative.

Hill, C. K.↗
NASA NTRS1990

Proposal for a conceptually new compact collecting optics for powerful light sources

Testing of a spacecraft under simulated outer space conditions is the best guarantee for a successful mission, and for almost all of the parameters a good match (to within a few percent) may be reached in relevant test facilities (e.g., IR background, earthshine and albedo radiation, residual atmosphere, intensity stability, distribution and spectrum of the solar radiation). The collimation angle of the sun (32 arc min) is, however, a drastic exception, here only + or - 1.5 to 2 deg are achieved which means a relative deviation by nearly one order of magnitude. The main reason for this is the presently used collecting optics having unnecessary large diameters. Here a lens-mirror combination is proposed which allows the reduction of the diameter by nearly a factor of two and to achieve with the present Xenon arc lamps a collimation angle of + or - 0.8 deg.

Saenger, G.↗
NASA NTRS2006

Using SCIAMACHY and Ground-based FTS Measurements to Test the OCO X(sub CO2) Retrieval and Validation Approach

Global, space-based observations of atmospheric CO2 with precision, resolution, and coverage needed to monitor sources and sinks: a) Spectra of reflected/scattered sunlight in NIR CO2 and O2 bands used to estimate X(sub CO2) with large sensitivity to surface; b) A-train orbit (1:15 PM polar sun sync); c) 16 day repeat cycle samples seasonal cycle on semi-monthly intervals; and d) NASA ESSP (Earth Space System Pathfinder) scheduled for Sept 2008 launch; 2 yrs lifetime. Initial comparison of SCIAMACHY and FTS retrievals for Park Falls: a) Positive bias in X(sub CO2) of approx. 10 ppm; and b) Negative bias in surface pressure After correction of spectral artifacts in O2A band: a) Largely improved agreement between SCIAMACHY and FTS X(sub CO2) (without clear bias) and in surface pressure; and b) Standard deviation of SCIAMACHY X(sub CO2 approx. 6 ppm. Good qualitative agreement with GEOS-CHEM, with GEOS-CHEM underestimating seasonal cycle. OCO is a dedicated CO2 instrument and will achieve much higher accuracy and precision: a) much higher spectral resolution (by factor of 20); and b) smaller ground pixels (by factor of 600).

CO2 measurements↗
NASA NTRS2023

Direct Measurement of the Cosmic-Ray Helium Spectrum from 40 GeV to 250 TeV with the Calorimetric Electron Telescope on the International Space Station

We present the results of a direct measurement of the cosmic-ray helium spectrum with the CALET instrument in operation on the International Space Station since 2015. The observation period covered by this analysis spans from October 13, 2015, to April 30, 2022 (2392 days). The very wide dynamic range of CALET allowed for the collection of helium data over a large energy interval, from ∼40 GeV to ∼250 TeV, for the first time with a single instrument in low Earth orbit. The measured spectrum shows evidence of a deviation of the flux from a single power law by more than 8σ with a progressive spectral hardening from a few hundred GeV to a few tens of TeV. This result is consistent with the data reported by space instruments including PAMELA, AMS-02, and DAMPE and balloon instruments including CREAM. At higher energy we report the onset of a softening of the helium spectrum around 30 TeV (total kinetic energy). Though affected by large uncertainties in the highest energy bins, the observation of a flux reduction turns out to be consistent with the most recent results of DAMPE. A double broken power law is found to fit simultaneously both spectral features: the hardening (at lower energy) and the softening (at higher energy). A measurement of the proton to helium flux ratio in the energy range from 60 GeV=n to about 60 TeV=n is also presented, using the CALET proton flux recently updated with higher statistics.

O Adriani↗
NASA NTRS1972

Large spacecraft antennas: Conical ring-membrane reflectors

A 1.83-m diameter furlable conical Gregorian antenna based on a novel spoke-supported ring-membrane concept has been successfully demonstrated. Mechanical measurements of the conical reflecting surface, as well as BE gain measurements at Ku-band, show an rms surface deviation from the proper conical surface of 0.3 mm and a repeatability after multiple furling-unfurling cycles of + or - 0.05 mm. Design features and performance characteristics of this antenna indicate that the spoke-supported ring-membrane concept is a promising approach for producing large, furlable, lightweight, conical reflectors for spacecraft high-gain antennas.

Oliver, R. E.↗