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At least 649 records · Page 36

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NASA NTRS1972

An error criterion for determining sampling rates in closed-loop control systems

The determination of an error criterion which will give a sampling rate for adequate performance of linear, time-invariant closed-loop, discrete-data control systems was studied. The proper modelling of the closed-loop control system for characterization of the error behavior, and the determination of an absolute error definition for performance of the two commonly used holding devices are discussed. The definition of an adequate relative error criterion as a function of the sampling rate and the parameters characterizing the system is established along with the determination of sampling rates. The validity of the expressions for the sampling interval was confirmed by computer simulations. Their application solves the problem of making a first choice in the selection of sampling rates.

Brecher, S. M.↗

NASA NTRS1973

Effects of cesium ions and cesium vapor on selected ATS-F samples

Thermal control coating samples were subjected to cesium ion beam and vapor exposures. Degradation of solar absorptance and infrared emittance were measured. Solar cells and samples selected from surfaces on the ATS-F spacecraft likely to experience ion or vapor impingement were bombarded by 10-volt cesium ions. Other samples were subjected to high levels of cesium vapor. Aluminum and white paint were backsputtered by 550-volt cesium ions onto selected samples. For direct bombardment, the threshold for ion-induced property changes was above five-thousand trillion ions/sq cm. With material sputtered from a 450-sq cm target onto samples 36 cm distant, the threshold for noticeable effects was above 5 times 10 to the 17-th power ions/sq cm.

Kemp, R. F.↗

NASA NTRS1974

Quantizing and sampling considerations in digital phased-locked loops

The quantizer problem is first considered. The conditions under which the uniform white sequence model for the quantizer error is valid are established independent of the sampling rate. An equivalent spectral density is defined for the quantizer error resulting in an effective SNR value. This effective SNR may be used to determine quantized performance from infinitely fine quantized results. Attention is given to sampling rate considerations. Sampling rate characteristics of the digital phase-locked loop (DPLL) structure are investigated for the infinitely fine quantized system. The predicted phase error variance equation is examined as a function of the sampling rate. Simulation results are presented and a method is described which enables the minimum required sampling rate to be determined from the predicted phase error variance equations.

Hurst, G. T.↗

NASA NTRS1973

Volatile element chemistry of selected lunar, meteoritic, and terrestrial samples

Using vacuum pyrolysis and high resolution mass spectrometry, a study is made of the gas release patterns of representative lunar samples, meteorites, terrestrial samples, and synthetic samples doped with various sources of carbon and nitrogen. The pyrolytic gas evolution patterns were intercorrelated, allowing an assessment of the possible sources of the volatilizable material in the lunar samples to be made. Lightly surface adsorbed species and more strongly chemisorbed species are released from ambient to 300 C and from 300 to 500 C, respectively. The low-temperature volatiles (less than 500 C) derived from various chondrites correlate well with the gas evolution patterns of volatile-rich samples, as for example 74220 and 61221. Solar wind entrapped species and molecules derived from reactions probably in the grain surfaces are evolved from about 500 to 700 C, respectively. Solar wind implanted C, N, and S species are generated from 750 to 1150 C, probably by reaction with the mineral matrix during the annealing process. Possible indigenous and/or refractory carbide, nitride, and sulfide C, N, and S are released in the region from 1200 C to fusion.

Simoneit, B. R.↗

NASA NTRS1973

Evaluations of lunar samples for the presence of viable organisms

Samples from the six successful Apollo lunar exploration missions were examined for the presence of biological formed elements and were used to inoculate a variety of culture media designed to promote growth of a broad spectrum of microorganisms. No evidence of viable organisms was obtained from any of these analyses. Following incubation of the lunar material-culture medium complexes, microbial growth dynamics studies were conducted with known test species to evaluate the possible presence of toxic factors. Only extracts of culture media which had been in contact with a mixture of lunar material from both Apollo 11 core tubes proved to be toxic to all species tested. Attempts to reproduce this toxic effect with individual Apollo 11 core samples obtained at other parts of the core and analyzed under somewhat different conditions were unsuccessful. In all, 48 different lunar samples were examined. These samples were collected at the lunar surface, in trenches, and in core samples to a depth of 297 cm.

Taylor, G. R.↗

NASA NTRS1974

Preparation, testing and analysis of zinc diffusion samples, NASA Skylab experiment M-558

Transport mechanisms of zinc atoms in molten zinc were investigated by radiotracer techniques in unit and in near-zero gravity environments. Each melt in the Skylab flight experiments was maintained in a thermal gradient of 420 C to 790 C. Similar tests were performed in a unit gravity environment for comparison. After melting in the gradient furnace followed by a thermal soak period (the latter was used for flight samples only), the samples were cooled and analyzed for Zn-65 distribution. All samples melted in a unit gravity environment were found to have uniform Zn-65 distribution - no concentration gradient was observed even when the sample was brought rapidly to melting and then quenched. Space-melted samples, however, showed textbook distributions, obviously the result of diffusion. It was evident that convection phenomena were the dominant factors influencing zinc transport in unit gravity experiments, while diffusion was the dominant factor in near-zero gravity experiments.

Braski, D. N.↗

NASA NTRS1975

Investigation to develop a multistage forest sampling inventory system using ERTS-1 imagery

The author has identified the following significant results. The annotation system produced a RMSE of about 200 m ground distance in the MSS data system with the control data used. All the analytical MSS interpretation models tried were highly significant. However, the gains in forest sampling efficiency that can be achieved by using the models vary from zero to over 50 percent depending on the area to which they are applied and the sampling method used. Among the sampling methods tried, regression sampling yielded substantial and the most consistent gains. The single most significant variable in the interpretation model was the difference between bands 5 and 7. The contrast variable, computed by the Hadamard transform was significant but did not contribute much to the interpretation model. Forest areas containing very large timber volumes because of large tree sizes were not separable from areas of similar crown cover but containing smaller trees using ERTS image interpretation only. All correlations between space derived timber volume predictions and estimates obtained from aerial and ground sampling were relatively low but significant and stable. There was a much stronger relationship between variables derived from MSS and U2 data than between U2 and ground data.

Langley, P. G.↗

NASA NTRS1976

The Surface Chemical Composition of Lunar Samples and Its Significance for Optical Properties

The surface iron, titanium, calcium, and silicon concentration in numerous lunar soil and rock samples was determined by Auger electron spectroscopy. All soil samples show a large increase in the iron to oxygen ratio compared with samples of pulverized rock or with results of the bulk chemical analysis. A solar wind simulation experiment using 2 keV energy alpha -particles showed that an ion dose corresponding to approximately 30,000 years of solar wind increased the iron concentration on the surface of the pulverized Apollo 14 rock sample 14310 to the concentration measured in the Apollo 14 soil sample 14163, and the albedo of the pulverized rock decreased from 0.36 to 0.07. The low albedo of the lunar soil is related to the iron + titanium concentration on its surface. A solar wind sputter reduction mechanism is discussed as a possible cause for both the surface chemical and optical properties of the soil.

Gold, T.↗

NASA NTRS1978

Technology for return of planetary samples, 1977

Recent progress on the development of a basic warning system (BWS) proposed to assess the biohazard of a Mars sample returned to earth, an earth orbiting spacecraft, or to a moon base was presented. The BWS package consists of terrestrial microorganisms representing major metabolic pathways. A vital processes component of the BWS will examine the effects of a Mars sample at terrestrial atmospheric conditions while a hardy organism component will examine the effects of a Mars sample under conditions approaching those of the Martian environment. Any deleterious insult on terrestrial metabolism effected by the Mars sample could be indicated long before the sample reached earth proximity.

Source record↗

NASA NTRS1978

Optimal sampling and quantization of synthetic aperture radar signals

Some theoretical and experimental results on optimal sampling and quantization of synthetic aperture radar (SAR) signals are presented. It includes a description of a derived theoretical relationship between the pixel signal to noise ratio of processed SAR images and the number of quantization bits per sampled signal, assuming homogeneous extended targets. With this relationship known, a solution may be realized for the problem of optimal allocation of a fixed data bit-volume (for specified surface area and resolution criterion) between the number of samples and the number of bits per sample. The results indicate that to achieve the best possible image quality for a fixed bit rate and a given resolution criterion, one should quantize individual samples coarsely and thereby maximize the number of multiple looks. The theoretical results are then compared with simulation results obtained by processing aircraft SAR data.

Wu, C.↗

NASA NTRS1977

The Viking X ray fluorescence experiment - Sampling strategies and laboratory simulations

Ten samples of Mars regolith material (six on Viking Lander 1 and four on Viking Lander 2) have been delivered to the X ray fluorescence spectrometers as of March 31, 1977. An additional six samples at least are planned for acquisition in the remaining Extended Mission (to January 1979) for each lander. All samples acquired are Martian fines from the near surface (less than 6-cm depth) of the landing sites except the latest on Viking Lander 1, which is fine material from the bottom of a trench dug to a depth of 25 cm. Several attempts on each lander to acquire fresh rock material (in pebble sizes) for analysis have yielded only cemented surface crustal material (duricrust). Laboratory simulation and experimentation are required both for mission planning of sampling and for interpretation of data returned from Mars. This paper is concerned with the rationale for sample site selections, surface sampler operations, and the supportive laboratory studies needed to interpret X ray results from Mars.

Baird, A. K.↗

NASA NTRS1975

Pb loss from Apollo 17 glassy samples and Apollo 16 revisited

U-Th-Pb analyses of bulk samples of soil breccia 70019, when compared to KREEP data, indicate that different lunar samples are sometimes characterized by distinctly different initial Pb isotopic compositions. U-Th-Pb analyses on nonglassy samples and a glassy separate of soil breccia 70019, and bulk and agglutinate samples of soil 72701 showed that the glassy samples did lose considerable Pb relative to U less than 200 million years ago. These data support the contention of other investigators that vaporization of volatile elements owing to impact events significantly alters the chemistry of the lunar regolith. Metaclastic rock 60315 was reanalyzed and was found to be concordant at 3.93 billion years rather than slightly discordant, as was previously reported.

Nunes, P. D.↗

NASA NTRS1978

Meteoritic material in lunar highland samples from the Apollo 11 and 12 sites

Radiochemical neutron activation analyses for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Te, Tl, U and Zn were carried out on 11 samples from the Apollo 11 and 12 sites (two samples of rock 12013, one granitic and one KREEPy; 4 KREEP fragments from 2-4 mm soil 12033,2; 4 anorthositic fragments from 1-3 mm soil 10085,104; 1 sample of 'Luny' rock-felsite-KREEP breccia 12013 10085,31 LR-1). Most Apollo 12 samples were found to have an Imbrium meteoritic component; others, typified by the Luny breccia, apparently contain a new meteoritic component of low Re content. Components of higher Ir/Au ratio (3L, 5L and 7) were found in Apollo 11 anorthositic samples; the fact that their enrichment in meteoritic siderophiles parallels that of Fe, Mg, REE and other KREEP elements is consistent with progressive contamination of an anorthositic crust. The Luny rock was tentatively classified Group 2, supporting the assignment of this group to Serenitatis.

Janssens, M.-J.↗

NASA NTRS1980

Ferrographic and spectrographic analysis of oil sampled before and after failure of a jet engine

An experimental gas turbine engine was destroyed as a result of the combustion of its titanium components. Several engine oil samples (before and after the failure) were analyzed with a Ferrograph as well as plasma, atomic absorption, and emission spectrometers. The analyses indicated that a lubrication system failure was not a causative factor in the engine failure. Neither an abnormal wear mechanism, nor a high level of wear debris was detected in the oil sample from the engine just prior to the test in which the failure occurred. However, low concentrations of titanium were evident in this sample and samples taken earlier. After the failure, higher titanium concentrations were detected in oil samples taken from different engine locations. Ferrographic analysis indicated that most of the titanium was contained in spherical metallic debris after the failure.

Jones, W. R., Jr.↗

NASA NTRS1979

Paleointensity determinations at elevated temperatures - Sample preparation technique

A technique has been developed for the encapsulation of rock samples in order to prevent the chemical alterations which commonly accompany paleointensity measurements at elevated temperatures. The technique involves vacuum pumping at about 100 C of the sample as placed in a silica tube. The tube containing the sample and a Ti 'getter' are sealed under vacuum. Measurements can be made at 200 and 300 C. Immediately after this, the sample is sealed-off from the getter. The sample is now ready for measurements at higher temperatures.

Taylor, L. A.↗

NASA NTRS1981

The effect of interstitial gaseous pressure on the thermal conductivity of a simulated Apollo 12 lunar soil sample

The thermal conductivity of a simulated Apollo 12 soil sample is measured as a function of interstitial gas density, and implications for the thermal properties of lunar and Martian regolith are discussed. Measurements were performed for samples consisting of a mixture of Knippa and Berkely basalt powders with a grain size distribution identical to that of Apollo 12 lunar soil samples by the needle probe technique at interstitial pressures of He, N2, Ar and CO2 from 133,000 to 0.0133 Pa. It is shown that sample thermal conductivity decreases with decreasing interstitial gas pressure down to 1.0 Pa, due to the decreasing effective thermal conductivity of interstitial gas with decreasing gas pressure. Constant thermal conductivity values of 8.8 mW/m per K and 10.9 mW/m per K are obtained for sample densities of 1.70 and 1.85 g/cu cm, respectively, in agreement with in situ lunar regolith measurements. The results, which are greater than those obtained in previous soil studies, are explained by the dense packing of soil particles and enhanced intergranular thermal contact in the present experimental configuration, rather than the influence of interstitial gas pressure. The differences in conductivity between loose soils and packed regolith may also be used to account for the two peaks observed in Martian surface thermal inertia data.

Horai, K.-I.↗

NASA NTRS1982

Construction of a remotely sensed area sampling frame for Southern Brazil

A remotely sensed area sampling frame was constructed for selected areas in Southern Brazil. The sampling unit information was stored in digital form in a latitudinal/longitudinal characterized population. Computerized sampling procedures were developed which allow for flexibility in sample unit specifications and sampling designs.

Fecso, R.↗

NASA NTRS1982

Sampling theory for asynoptic satellite observations. I Space-time spectra, resolution, and aliasing. II - Fast Fourier synoptic mapping

An evaluation of the information content of asynoptic data taken in the form of nadir sonde and limb scan observations is presented, and a one-to-one correspondence is established between the alias-free data and twice-daily synoptic maps. Attention is given to space and time limitations of sampling and the orbital geometry is discussed. The sampling pattern is demonstrated to determine unique space-time spectra at all wavenumbers and frequencies. Spectral resolution and aliasing are explored, while restrictions on sampling and information content are defined. It is noted that irregular sampling at high latitudes produces spurious contamination effects. An Asynoptic Sampling Theorem is thereby formulated, as is a Synoptic Retrieval Theorem, in the second part of the article. In the latter, a procedure is developed for retrieving the unique correspondence between the asymptotic data and the synoptic maps. Applications examples are provided using data from the Nimbus-6 satellite.

Salby, M. L.↗