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At least 505 records · Page 28
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NASA NTRS1989

Structural optimization and recent large ground antenna installations

Within the past several years, the Jet Propulsion Laboratory has designed and built major ground antenna structures in Spain, Australia, and California. One of the antennas at each location is a 70 meter-diameter structure that is a retrofit of the existing 64 meter antenna. The 64 meter existing antennas were first stripped back to a 34 meter interior and then completely new construction with deeper trusses was added to extend the interior to 70 meters. The 70 meter project included the rare opportunity to collect field data to compare with predictions of the finite-element analytical models. The new quadripod design was tested for its lower mode natural frequencies and the main reflector was measured by theodolite to determine deflections of subsets of the backup-structure deformations under load. The emphasis here is to examine measurement results and possibly provide some appreciation of the relationship of predictions made from the design model to actual measurements.

Levy, Roy↗
NASA NTRS1989

The geological record of life 3500 Ma ago: Coping with the rigors of a young earth during late accretion

Thin cherty sedimentary layers within the volcanic portions of the 3,500 to 3,300 Ma-old Onverwacht and Fig Tree Groups, Barberton Greenstone belt, South Africa, and Warrawoona Group, eastern Pilbara Block, Western Australia, contain an abundant record of early Archean life. Five principal types of organic and probably biogenic remains and or structures can be identifed: stromatolites, stromatolite detritus, carbonaceous laminite or flat stromalite, carbonaceous detrital particles, and microfossils. Early Archean stromatolites were reported from both the Barberton and eastern Pilbara greenstone belts. Systematic studies are lacking, but two main morphological types of stromatolites appear to be represented by these occurrences. Morphology of the stromalites is described. Preserved early Archean stromatolites and carbonaceous matter appear to reflect communities of photosynthetic cyanobacteria inhabiting shallow, probably marine environments developed over the surfaces of low-relief, rapidly subsiding, simatic volcanic platforms. The overall environmental and tectonic conditions were those that probably prevailed at Earth's surface since the simatic crust and oceans formed sometime before 3,800 Ma. Recent studies also suggest that these early Archean sequences contain layers of debris formed by large-body impacts on early Earth. If so, then these early bacterial communities had developed strategies for coping with the disruptive effects of possibly globe-encircling high-temperature impact vapor clouds, dust blankets, and impact-generated tsunamis. It is probable that these early Archean biogenic materials represent organic communities that evolved long before the beginning of the preserved geological record and were well adapted to the rigors of life on a young, volcanically active Earth during late bombardment. These conditions may have had parallels on Mars during its early evolution.

Lowe, Donald R.↗
NASA NTRS1989

DSN radio science system design and testing for Voyager-Neptune encounter

The Deep Space Network (DSN) Radio Science System presently implemented within the Deep Space Network was designed to meet stringent requirements imposed by the demands of the Voyager-Neptune encounter and future missions. One of the initial parameters related to frequency stability is discussed. The requirement, specification, design, and methodology for measuring this parameter are described. A description of special instrumentation that was developed for the test measurements and initial test data resulting from the system tests performed at Canberra, Australia and Usuda, Japan are given.

Ham, N. C.↗
NASA NTRS1988

Radiosonde intercomparison

The largest amount of material ever collected from a radiosonde comparison was examined. Radiosondes from Australia, Finland, India, and the United States were involved. Data were received from 100 soundings, each of which was a simultaneous in situ test of four different instrument types. The simultaneous temperature comparison of participating operational radiosondes in daylight was about 1 C at the 100 hPa level and about 4 C at the 10 hPa level, while the corresponding comparison for geopotential was about 40 meters at 100 hPa and 100 meters at 10 hPa. Estimates of the reproducibility of standard level temperatures are given. The reproducibility obtained from the in situ comparisons is, in general, slightly better than corresponding results from monitoring measurements in a real-time mode at analysis centers. Conclusions from the intercomparison are many; the following call for particular attention: (1) fully automated radiosonde systems were able to reproduce geopotential measurements better than non-automated systems, mainly due to a decrease in observer mistakes; (2) observed temperature differences between radiosonde measurements were as large during the night as during the day; and (3) significant inconsistencies still exist between the night time and day time measurements, as well as significant bias errors in the pressure measurements of some radiosonde types.

Schmidlin, F. J.↗
NASA NTRS1989

Correlation of Puma airloads: Evaluation of CFD prediction methods

A cooperative program was undertaken by research organizations in England, France, Australia and the U.S. to study the capabilities of computational fluid dynamics codes (CFD) to predict the aerodynamic loading on helicopter rotor blades. The program goal is to compare predictions with experimental data for flight tests of a research Puma helicopter with rectangular and swept tip blades. Two topics are studied. First, computed results from three CFD codes are compared for flight test cases where all three codes use the same partial inflow-angle boundary conditions. Second, one of the CFD codes (FPR) is iteratively coupled with the CAMRAD/JA helicopter performance code. These results are compared with experimental data and with an uncoupled CAMRAD/JA solution. The influence of flow field unsteadiness is found to play an important role in the blade aerodynamics. Alternate boundary conditions are suggested in order to properly model this unsteadiness in the CFD codes.

Strawn, Roger C.↗
NASA NTRS1989

Seasonal variation of turbulence intensities in the upper mesosphere and lower thermosphere measured by radar techniques

Since February 1985, the 2 MHz narrow beam radar operated by the University of Adelaide in Australia has been used to measure the short term root-mean-square fluctuating velocities of radio wave scatterers in the upper middle atmosphere (80 to 100 km). These measured fluctuations are caused by a mixture of turbulence and gravity waves, and under certain reasonable assumptions the turbulent contribution can be extracted. The results of these measurements were discussed in detail by Hocking (1988). These results are summarized and the data set is extended to include 1987.

Hocking, W. K.↗
NASA NTRS1988

The long term stability of VLBI earth orientation measurements

Tectonic motions will, in general, change the orientation as well as the length of baselines used in Very Long Baseline Interferometry (VLBI), and will thus cause slow divergences between earth orientation results obtained with different VLBI networks, as well as between VLBI results and those obtained by satellite laser ranging and lunar laser ranging. Such drifts (on the order of a milliarcsecond/year) are inherently interesting as well as being significant in combinations of orientation results from different sources. The geodetic study of tectonic motions is also closely connected to research into the nature and causes of systematic errors in data from the modern techniques of space geodesy. Both a special coordinate system found to be of use in the analysis of VLBI data and tectonic motion estimates for a VLBI baseline stretching from California to Australia are described.

Eubanks, T. M.↗
NASA NTRS1988

Very long baseline interferometry observations using the tracking and data relay satellite as an orbiting radio telescope

An antenna in geostationary orbit was used for VLBI observations at 2.3 GHz, in combination with ground antennas in Australia and Japan. 23 of the 25 observed sources were detected on orbiter-ground baselines, with baseline lengths as large as 2.15 earth diameters. Brightness temperatures between 10 to the 12th K and 4 x 10 to the 12th K were measured for 10 sources.

Linfield, R. P.↗
NASA NTRS1988

Upper limits to the high-energy gamma-ray and hard X-ray flux from SN 1987A on day 55

A balloon-borne experiment was carried out on April 19, 1987 from Alice Springs, Australia to detect high-energy gamma-rays and hard X-rays from the supernova SN 1987A in the Large Magellanic Cloud. An optical spark chamber for measuring the high-energy gamma-rays and a multiwire proportional counter for measuring the hard X-rays shared a common balloon platform. No positive emission was observed by either detector. The gamma-ray upper limit obtained indicates that either the expanding supernova shell was opaque to gamma-rays at the time of the flight, or that the supernova cosmic ray luminosity was less than about 10 to the 40th erg/s. The X-ray upper limit obtained confirms the opacity of the shell to Comptonized hard X-rays.

Sood, R. K.↗
NASA NTRS1988

An imaging observation of SN 1987A at gamma-ray energies

The Caltech imaging gamma-ray telescope was launched by balloon from Alice Springs, Australia for observations of SN 1987A during November 1987. The preliminary results presented here are derived from 8200 s of instrument live time on the supernova and 2500 s on the Crab Nebula and pulsar at a float altitude of 36 km. Images of the SN 1987A region at gamma-ray energies confirming that the bulk of the gamma-ray emission comes from the supernova and not from LMC X-1. These flux measurements are compared to recent predictions concerning the distribution of Co-56 in the supernova ejecta. The data are consistent with models invoking moderate mixing of core material into the envelope.

Cook, W. R.↗
NASA NTRS1988

Spectral analysis of the Elatina series

The Elatina formation in South Australia, which provides a rich fossil record of presumptive solar activity in the late Precambrian, is of great potential significance for the physics of the sun because it contains luminae grouped in cycles of about 12, an appearance suggestive of the solar cycle. Here, the laminae are treated as varves laid down yearly and modulated in thickness in accordance with the late Precambrian sunspot activity for the year of deposition. The purpose is to present a simple structure, or intrinsic spectrum, that will be uncovered by appropriate data analysis.

Bracewell, R. N.↗
NASA NTRS1988

Natural radio lasing at Jupiter

Like the comparable AKR radio emissions from earth's magnetosphere, the well-known decametric radio S-bursts from Jupiter, observed in France and Australia at frequencies from 10 to 26 MHz, have been found to exhibit equally spaced discrete spectral components which can be attributed to the adjacent longitudinal oscillation modes of natural radio lasers. Implying sizes of only a few kilometers for the individual radio lasers producing the S-bursts, the frequency spacing of these modes was roughly constant with frequency and about 30 to 50 kHz. Their corresponding temporal spacings, however, varied inversely proportional to the observing frequency, suggesting that the radio lasers producing the S-bursts were expanding uniformly at a rate of about 4 km/s. Presumably caused by the projected motion of Io with respect to the planet, this expansion of the S-burst radio lasers would account for the downward frequency drifts of the S-bursts without the energetic electron bunches which have heretofore always been assumed necessary to account for such behavior.

Calvert, W.↗
NASA NTRS1988

Geometry and treatment of fiducial networks - Effects on GPS baseline precision in South America

A covariance analysis indicates that GPS baseline precision in northern South America is substantially improved when fiducial stations in North America are supplemented by stations in Hawaii, Australia, and New Zealand. The formal errors for a variety of fiducial networks are calculated. It is found that the systematic error of fiducial stations is dependent on the fiducial network geometry. Sensitivity analysis indicates that the baselines of northern South America are very sensitive to uncertainties in the locations of the closest fiducial stations.

Freymueller, Jeffrey T.↗
NASA NTRS1988

The gamma ray north-south effect

Theoretical calculations are presented that explain the balloon observations by O'Neill et al. (1987) of a strong north-south anisotropy of atmospheric gamma rays over the Southern Hemisphere, and to predict the north-south ratios. It is shown that the gamma rays that originate at the longest distances from the telescopes give the largest north-south ratios. Comparisons are made of the experimental north-south ratios measured on balloons launched from Alice Springs, Australia, and from Palestine, Texas, U.S., and predictions are made for ratios at other geomagnetic latitudes and longitudes. It is pointed out that observers who measure backgrounds for celestial sources may be misled unless they correct for the north-south effect.

White, R. S.↗
NASA NTRS1989

Recent results in the NASA research balloon program

The NASA Balloon Program has progressed from a total hiatus in the fall of 1985 to an unprecedented flight success rate in the fall of 1988. Using heavy-lift balloons being regularly supplied by two manufacturers, the program has provided a timely response for investigations of Supernova 1987A from Australia, low energy cosmic ray investigations from Canada during periods of near-solar-minimum, and routine domestic turnaround flights for a variety of investigations. Recent re-evaluation of balloon flight-safety have resulted in severe constraints on flights launched from the Palestine, Texas facility. The future program must rely heavily on the use of remote launch sites to meet the growing requirements for more frequent and longer duration flights being planned for the next 3 - 5 years.

Jones, W. Vernon↗
NASA NTRS1988

On air motion trajectories in cold fronts

This paper examines air parcel trajectories in the two-dimensional model for a cold front by Reeder and Smith (1987). These are found to be in close agreement with trajectories deduced from analyses of summertime 'cool changes' in southeastern Australia, adding further support to the applicability of the numerical model to this kind of cold front. The favorable comparison points also to the dynamical consistency of the conceptual model for the cool change, which has evolved from the analysis of data from observational experiments.

Reeder, Michael J.↗
NASA NTRS1989

Australian aerosol backscatter survey

This paper describes measurements of the atmospheric backscatter coefficient in and around Australia during May and June 1986. One set of backscatter measurements was made with a CO2 lidar operating at 10.6 microns; the other set was obtained from calculations using measured aerosol parameters. Despite the two quite different data collection techniques, there is quite good agreement between the two methods. Backscatter values range from near 1 x 10 to the -8th/m per sr near the surface to 4 - 5 x 10 to the -11th/m per sr in the free troposphere at 5-7-km altitude. The values in the free troposphere are somewhat lower than those typically measured at the same height in the Northern Hemisphere.

Gras, John L.↗
NASA NTRS1989

Calibration and performance of the energetic X-ray imaging telescope experiment

The final integration and testing of the Energetic X-ray Imaging Telescope Experiment (EXITE) are described. The detector and telescope as finally assembled are described, and problems as well as solutions are detailed. The preflight calibration of the position-sensitive NaI detector revealed unanticipated effects in the energy spectra. In particular, an effect due to surface roughness (lack of firm optical polishing) on the crystal was found and calibrated. The radial variation of gain and resolution in the detector was also measured, and so-called flat fielding methods have been derived to analyze the data. The integration of the EXITE detector and gondola systems is described. Several novel features were developed for the EXITE gondola, most notably a shock absorber system. The performance of the detector and gondola systems was evaluated on a first balloon flight from Australia (May 1988) and a second flight from Ft. Sumner, New Mexico (October 1988).

Braga, Joao↗