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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

The singly averaged differential equations of satellite motion for e greater than or equal to 0 and less than 1

The singly-averaged differential equations of motion of a satellite are developed in terms of parameters valid for all eccentricities less than one. The perturbations included in the acceleration model are due to an aspherical central planet (zonal harmonics up to degree 20 and resonant harmonics up to degree and order 20), atmospheric drag for a time-varying atmosphere, third-body gravity (the sun and moon for an earth satellite), solar radiation pressure with shadowing, and impulsive maneuvers. Analytic averaging is used to remove short-period terms due to the aspherical central planet and third-body gravity. Numerical averaging is used to remove short-period terms due to atmospheric drag and solar radiation pressure.

Dallas, S. S.↗
NASA NTRS1976

Terrestrial kilometric radiation: 3-average spectral properties

A study is presented of the average spectral properties of terrestrial kilometric radiation (TKR) derived from observations made by radio astronomy experiments onboard the IMP-6 and RAE-2 spacecraft. As viewed from near the equatorial plane, TKR is most intense and most often observed in the 21-24 hr local time zone and is rarely seen in the 09-12 hr zone. The peak flux density usually occurs near 240 kHz, but there is evidence that the peak occurs at a somewhat lower frequency on the dayside. The frequency of the peak in the average flux spectrum varies inversely with increasing substorm activity as inferred from the auroral electrojet index (AE) from a maximum near 300 kHz during very quiet times to a minimum below 200 kHz during very disturbed times. The absolute flux levels in the 100-600 kHz TKR band increase significantly with increasing AE. The average power associated with a particular source region seems to decrease rapidly with increasing source altitude.

Kaiser, M. L.↗
NASA NTRS1977

On the high correlation between long-term averages of solar wind speed and geomagnetic activity

Six-month and yearly averages of solar-wind speed from 1962 to 1975 are shown to be highly correlated with geomagnetic activity as measured by averages of the Ap index. On the same time scale the correlation between the southward component of the interplanetary magnetic field and geomagnetic activity is poor. Previous studies with hourly averages gave opposite results. The better correlation with the southward component on an hourly time scale is explained by its large variation compared with the relatively constant solar-wind speed. However, on a yearly time scale the magnitude of the variations in both parameters are about the same. This problem can be solved by invoking an energy transfer mechanism which is proportional to the first power of the southward component and a higher power of the solar-wind speed.

Crooker, N. U.↗
NASA NTRS1978

A note on the diurnal averaging of aeronomical models

An approximate technique for diurnally time-averaging atmospheric photochemical-dynamical models which eliminates the need for a detailed numerical resolution of sunrise and sunset transitions is developed. The scheme is equivalent to scaling certain chemical rate constants and photodissociation coefficients by appropriate aeronomical factors. To calculate the scaling factors, diurnal variations are parameterized with a step-function behavior, assuming that each species has a constant day-time and night-time concentration whose ratio can be determined by analyzing the chemical interactions occurring after sunset. The solution accounts for the effects of night-time reactions on the 24 h average values of species abundances, and on the average daily rates of the catalytic processes consuming ozone in the stratosphere. The accuracy of the technique is demonstrated by comparing its predictions to those of a full diurnal simulation; typically, the precision is better than 10%. By contrast, it is shown that the use of some other well-known computational schemes can result in significantly larger predictive errors.

Turco, R. P.↗
NASA NTRS1978

A note on the averaging method

A simple problem of a two-body system perturbed by the disturbing function epsilon/r-squared is considered to show that the time-averaged equations of the true and mean anomaly are not necessarily equal. Secular effects due to perturbations in the mean and true anomalies are different not only in value but also in sign. A more general case where the disturbing function is periodic with respect to the mean anomaly is also considered. Here a discrepancy in the time-averaged solutions is due to the fact that the mean anomaly is an action-angle variable conjugate to the action variable L, while the true anomaly is not. When the action-angle variables are chosen as dependent variables, the Hamiltonian is a function only of action variables. One must first derive first-order solutions, substitute them into the right-hand side of the equations of motion, and then take time averages.

Kinoshita, H.↗
NASA NTRS1979

Improvement of scanning radiometer performance by digital reference averaging

Most radiometers utilize a calibration technique in which measurements of a known reference are subtracted from measurements of an unknown source so that common-mode bias errors are cancelled. When a radiometer is scanned over a varying scene, it produces a sequence of outputs, each being proportional to the difference between the reference and the corresponding input. A reference averaging technique is presented that employs a simple digital algorithm which exploits the asymmetry between the time-variable scene inputs and the nominally constant reference input by averaging many reference measurements to decrease the statistical uncertainty in the reference value. This algorithm is, therefore, optimized by an asymmetric chopping sequence in which the scene is viewed for more than one-half of the duty cycle (unlike the analog Dicke technique). Reference averaging algorithms are well within the capabilities of small microprocessors. Although this paper develops the technique for microwave radiometry, it may be beneficial for any system which measures a large number of unknowns relative to a known reference in the presence of slowly varying common-mode errors.

Bremer, J. C.↗
NASA NTRS1979

On the angle between the average interplanetary magnetic field and the propagation direction of plane large amplitude Alfven waves

The paper shows that the experimentally observed close alignment of magnetic field minimum variance direction with the average magnetic field for Alfven waves in the solar wind is consistent with theoretically predicted properties of plane large amplitude Alfven waves in the MHD approximation. The theoretical properties of these Alfven waves constrain the time averaged magnetic field to cluster around the direction of minimum variance, which is aligned with the wave normal. Thus, spacecraft magnetometer observations in the solar wind of minimum variance directions strongly peaked about the average magnetic field direction are consistent with plane large amplitude Alfven waves which have wave normals aligned with the directions of minimum variance. This does not imply that geometrical hydromagnetic calculations for Alfven wave propagation direction in the solar wind are incorrect, but there is a discrepancy between geometrical hydromagnetics theory and observations that IMF minimum variance directions tend to be aligned with the ideal Parker spiral instead of the radial direction.

Lichtenstein, B. R.↗
NASA NTRS1981

A comparative statistical study of long-term agroclimatic conditions affecting the growth of US winter wheat: Distributions of regional monthly average precipitation on the Great Plains and the state of Maryland and the effect of agroclimatic conditions on yield in the state of Kansas

A histogram analysis of average monthly precipitation over 30 and 84 year periods for both Maryland and Kansas was made and the results compared. A second analysis, a statistical assessment of the effect of average monthly precipitation on Kansas winter wheat yield was made. The data sets covered the three periods of 1941-1970, 1887-1970, and 1887-1921. Analyses of the limited data sets used (only the average monthly precipitation and temperature were correlated against yield) indicated that fall precipitation values, especially those of September and October, were more important to winter wheat yield than were spring values, particularly for the period 1941-1970.

Welker, J.↗
NASA NTRS1981

Synoptic observations of Jupiter's radio emissions - Average statistical properties observed by Voyager

Observations of Jupiter's low-frequency radio emissions collected over one-month intervals before and after each Voyager encounter are analyzed to provide a synoptic view of the average statistical properties of the emissions. Compilations of occurrence probability, average power flux density, and average sense of circular polarization are given as a function of central meridian longitude, phase of Io, and frequency. The results are then compared with ground-based observations. The necessary geometric conditions and preferred polarization sense for Io-related decametric emission observed by Voyager from above both the dayside and nightside hemispheres are found to be basically the same as those observed in earth-based studies.

Alexander, J. K.↗
NASA NTRS1983

Average sedimentary rock rare Earth element patterns and crustal evolution: Some observations and implications from the 3800 Ma ISUA supracrustal belt, West Greenland

Rare earth element (REE) data is given on a set of clastic metasediments from the 3800 Ma Isua Supracrustal belt, West Greenland. Each of two units from the same sedimentary sequence has a distinctive REE pattern, but the average of these rocks bears a very strong resemblance to the REE pattern for the North American Shale Composite (NASC), and departs considerably from previous estimates of REE patterns in Archaean sediments. The possibility that the source area for the Isua sediments resembled that of the NASC is regarded as highly unlikely. However, REE patterns like that in the NASC may be produced by sedimentary recycling of material yielding patterns such as are found at Isua. The results lead to the following tentative conclusions: (1) The REE patterns for Isua Seq. B MBG indicate the existence of crustal materials with fractionated REE and negative Eu anomalies at 3800 Ma, (2) The average Seq. B REE pattern resembles that of the North American Shale Composite (NASC), (3) If the Seq. B average is truly representative of its crustal sources, then this early crust could have been extensively differentiated. In this regard, a proper understanding of the NASC pattern, and its relationship to post-Archaean crustal REE reservoirs, is essential, (4) The Isua results may represent a local effect.

Dymek, R. F.↗
NASA NTRS1984

Average flow between about 70 and 220 earth radii in the geomagnetic tail

A detailed statistical analysis is made of the flow magnitude and direction in the geomagnetic tail between about 70 and 220 earth radii derived from suprathermal protons measured with the ultra-low energy charge analyzer on the ISEE-3 spacecraft. It is found that the average tailward flow velocity between 203 and 220 earth radii is 567 km/s, and earthward flow occurs in less than 1 percent of all the cases. At a radial distance of 117 and 153 earth radii, the result is similar: the average tailward flow velocity is 665 km/s, and the earthward flow occurs in less than 6 percent of all measurements. In contrast, at radial distances of 61 to 76 earth radii, equal probability for earthbound and tailward flow has been found, and average flow velocities are 336 km/s in both directions. These results indicate that magnetic neutral lines rarely move or form beyond about 100 earth radii.

Scholer, M.↗
NASA NTRS1985

Average and worst-case specifications of precipitating auroral electron environment

The precipitation electrons in the auroral environment are highly variable in their energy and intensity in both space and time. As such they are a source of potential hazard to the operation of the Space Shuttle and other large spacecraft operating in polar orbit. In order to assess these hazards both the average and extreme states of the precipitating electrons must be determined. Work aimed at such a specification is presented. First results of a global study of the average characteristics are presented. In this study the high latitude region was divided into spatial elements in magnetic local time and corrected geomagnetic latitude. The average electron spectrum was then determined in each spatial element for seven different levels of activity as measured by K sub p using an extremely large data set of auroral observations. Second a case study of an extreme auroral electron environment is presented, in which the electrons are accelerated through field aligned potential as high as 30,000 volts and in which the spacecraft is seen to charge negatively to a potential approaching .5 kilovolts.

Hardy, D. A.↗
NASA NTRS1984

Phase averaged acoustic measurements for a Mach number 0.6 jet

Acoustic phase (ensemble) averaged measurements were performed in a cold, axisymmetric, Mach number 0.6 jet of air. These measurements show that the noise directly radiated by the coherent structure in the jet flow field was responsible for the directivity of the acoustic field. Comparison of phase averaged acoustic spectra with phase averaged radial velocity fluctuations within the jet illustrate the direct link between the two and show that different regions of the jet are responsible for producing the noise measured at various locations in the acoustic field.

Morrison, G. L.↗
NASA NTRS1987

Atmospheric trends in methylchloroform and the global average for the hydroxyl radical

ALE-GAGE (Atmospheric Lifetime Experiment-Global Atmospheric Gases Experiment) data obtained over the seven-year period from July 1978 to June 1985 are presented and interpreted. The data, combined with knowledge of industrial emissions, are used in an optimal estimation inversion scheme to deduce a globally average methylchloroform atmospheric lifetime of 6.3(+ 1.2, - 0.9) years (1 sigma uncertainty) and a globally averaged tropospheric hydroxyl radical concentration of (7.7 + or - 1.4) x 10 to the 5th radicals/cu cm (1 sigma uncertainty). These results provide the most accurate estimates yet of the trends and lifetime of methylchloroform and of the global average for tropospheric hydroxyl radical levels.

Prinn, R.↗
NASA NTRS1988

Lagged average predictions in a predictability experiment

Lagged average predictions are examined here within the context of an idealized predictability experiment. Lagged predictions contribute to making better forecasts than the forecasts obtained from using only the latest initial state. Analytic models suggest that lagged predictions contribute the greatest amount when the error growth rates are small. Little dependence upon the magnitude of the intial error is found if the growth rates remain constant. It is also shown how lagged average forecasts can be used to predict the error. Discriminating forecasts made only when the error is predicted to be small are shown to have much better than average skill.

Roads, John O.↗
NASA NTRS1988

Average areal water equivalent of snow in a mountain basin using microwave and visible satellite data

Satellite microwave data were used to evaluate the average areal water equivalent of snow cover in the mountainous Rio Grande basin of Colorado. Areal water equivalent data for the basin were obtained from contoured values of point measurements and from zonal water volume values generated by a snowmelt runoff model. Comparison of these snow water equivalent values shows the model values to consistently exceed the contoured values, probably because of the narrow elevation range in the lower part of the basin where the point measurements are concentrated. A significant relationship between the difference in microwave brightness temperatures at two different wavelengths and a basin-wide average snow water equivalent value is obtained. The average water equivalent of the snow cover in the basin was derived from differences of the microwave brightness temperatures.

Rango, A.↗
NASA NTRS1990

Designing Digital Control Systems With Averaged Measurements

Rational criteria represent improvement over "cut-and-try" approach. Recent development in theory of control systems yields improvements in mathematical modeling and design of digital feedback controllers using time-averaged measurements. By using one of new formulations for systems with time-averaged measurements, designer takes averaging effect into account when modeling plant, eliminating need to iterate design and simulation phases.

Polites, Michael E.↗
NASA NTRS1989

Evolution of the Orszag-Tang vortex system in a compressible medium. I - Initial average subsonic flow

The results of fully compressible, Fourier collocation, numerical simulations of the Orszag-Tang vortex system are presented. The initial conditions for this system consist of a nonrandom, periodic field in which the magnetic and velocity field contain X points but differ in modal structure along one spatial direction. The velocity field is initially solenoidal, with the total initial pressure field consisting of the superposition of the appropriate incompressible pressure distribution upon a flat pressure field corresponding to the initial, average Mach number of the flow. In these numerical simulations, this initial Mach number is varied from 0.2-0.6. These values correspond to average plasma beta values ranging from 30.0 to 3.3, respectively. It is found that compressible effects develop within one or two Alfven transit times, as manifested in the spectra of compressible quantities such as the mass density and the nonsolenoidal flow field. These effects include (1) a retardation of growth of correlation between the magnetic field and the velocity field, (2) the emergence of compressible small-scale structure such as massive jets, and (3) bifurcation of eddies in the compressible flow field. Differences between the incompressible and compressible results tend to increase with increasing initial average Mach number.

Dahlburg, R. B.↗