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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,567 records · Page 87

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NASA NTRS1983

Siderophile, lithophile and mobile trace elements in the lunar meteorite Allan Hills 81005

The content of trace elements (siderophile Co, Au, As, Sb, Ga; chalcophile/mobile Se, Te, Bi, In, Ag, Zn, Tl, Cd; lithophile Rb, Cs, U) is investigated to ascertain whether the meteorite is of lunar origin. Five elements reflect lunar crustal processes, whereas the remaining 11 siderophile and mobile elements suggest 1.4 + or - 0.5 percent micrometeorite admixture or enrichment by thermal redistribution on the moon. It is found that the impact launching of ALH A81005 to the earth was not attended by substantial shock loading. A Martian origin for severely shocked SNC meteorites is therefore considered plausible.

Verkouteren, R. M.↗

NASA NTRS1983

Response of the mobile VLBI design to error sources

The response of Mobile VLBI design to error sources is addressed. The sensitivity of the hydrogen maser to variations in ambient temperature is discussed, with an example of drifts in the frequency system causing excursions in the time-delay observable exceeding + or - 200 cm. It is shown that baselines determined only from S-band data can contain errors in excess of 30 cm during periods of high ionospheric activity. The effect of the troposphere on baseline solutions is examined by comparing calibrations from the Water Vapor Radiometer (WVR) to those from a surface model. The apparent ability of the WVR to track relatively short-period fluctuations in water vapor is noted. Finally, consideration is given to the effects of source structure and the technique of monitoring closure of the time-delay observable around a closed figure of baselines.

Trask, D. W.↗

NASA NTRS1984

Highly mobile laser ranging facilities of the Crustal Dynamics Project

Technical specifications, performance, and applications of the NASA transportable laser ranging systems (TLRS-1 and -2) for use in the Crustal Dynamics Program are described. TLRS-1 is truck-mounted, with the laser deployed through the roof. Interacting with the LAGEOS satellite, TLRS has a photoelectric receiver for gathering data on the roundtrip time of the laser beam for calculations of the range gate. The laser has a 0.1 nsec pulse at 3.5 mJ/pulse. Range is measured to within an error of 9 cm. The TLRS-2 version is configured for ease of air transport and modular breakdown and assembly. It has been activated on Easter Island. TLRS-3 and -4 are in development to serve as mobile units in South America and the Mediterranean area.

Coates, R. J.↗

NASA NTRS1984

Mobile spectrometer measures radar backscatter

The present article is concerned with a helicopter-borne spectrometer (Heloscat), which has been developed to permit high-quality scattering measurements from a mobile platform at remote sites. The term 'spectrometer' referes to a class of scatterometers. The term 'scatterometer' is employed to denote a specialized radar for measuring scattering coefficients as a function of angle. A spectrometer, on the other hand, is a scatterometer which can measure backscatter at several frequencies. The Heloscat system is discussed, taking into account two antennas, RF hardware, and an externally mounted pendulum for angle encoding. A dual-antenna configuration is used for cross-polarized measurements, while a single-antenna system is used for like-polarized measurements. Attention is also given to oscillator characteristics, efficient data handling, and aspects of calibration.

Gogineni, S.↗

NASA NTRS1948

Concepts and cost trade-offs for land vehicle antennas in satellite mobile communications

Several antenna design concepts, operating at UHF (821 to 825 MHz transmit and 866 to 870 MHz receive bands), with gain ranging between 6 and 12 dBic, that are suitable for land mobile vehicles are presented. The antennas may be used within CONUS and ALASKA to communicate to and from a geosynchronous satellite. Depending on the type of steering mechanism, the antennas are broken down into three categories; (1) electronically scanned arrays with phase shifters, (2) electronically switched arrays with switchable power dividers/combiners, and (3) mechanically steered arrays. The operating characteristics of two of these design concepts, one a conformal antenna with electronic beam steering and the other a nonconformal design with mechanical steering, were evaluated with regard to two and three satellite system. Cost estimates of various antenna concepts were made and plotted against their overall gain performance.

Haddad, H. A.↗

NASA NTRS1984

Measurement results from a balloon experiment simulating land mobile satellite transmissions

A transmitter operating at 869.525 MHz was twice carried by a stratospheric balloon to an altitude of about 40 km. A motor vehicle was driven within the line-of-sight from the transmitter. Measurements of the received signal strength were made every 1/8 wavelength for an overall travelling distance of about seven hundred kilometers. This scenario was to simulate a satellite system providing mobile communications to rural areas. The statistics of the sampled field, consisting of a combination of direct wave, specular reflection and diffuse components, are presented as a function of elevation angle. Parameters such as type of road driven (mostly 2 lane) or type of landscape (rolling to flat) and vegetation (pine and mixed forest) encountered are described where possible. The power distribution function for all the data, at elevation angles from 10 to 35 degrees, is 1 dB below the free space mean at the 50% level, 7 dB below at the 90% level, and 18 dB below at the 99% level. In the elevation angle range of 30 to 35 degrees the corresponding values were found to be .5, 1.2, and 4.5 dB. The conditional fade duration and level crossing rate distribution functions are also presented. The former shows some dependence on the threshold level, the latter almost none.

Vogel, W. J.↗

NASA NTRS1985

Spacecraft configuration study for second generation mobile satellite system

A high power, high performance communicatons satellite bus being developed is designed to satisfy a broad range of multimission payload requirements in a cost effective manner and is compatible with both STS and expendable launchers. Results are presented of tradeoff studies conducted to optimize the second generation mobile satellite system for its mass, power, and physical size. Investigations of the 20-meter antenna configuration, transponder linearization techniques, needed spacecraft modifications, and spacecraft power, dissipation, mass, and physical size indicate that the advanced spacecraft bus is capable of supporting the required payload for the satellite.

Louie, M.↗

NASA NTRS1985

DMSK: A practical 2400-bps receiver for the mobile satellite service: An MSAT-X Report

The partical aspects of a 2400-bps differential detection minimum-shift-keying (DMSK) receiver are investigated. Fundamental issues relating to hardware precision, Doppler shift, fading, and frequency offset are examined, and it is concluded that the receiver's implementation at baseband is more advantageous both in cost and simplicity than its IF implementation. The DMSK receiver has been fabricated and tested under simulated mobile satellite environment conditions. The measured receiver performance in the presence of anomalies pertinent to the link is presented in this report. Furthermore, the receiver behavior in a band-limited channel (GMSK) is also investigated. The DMSK receiver performs substantially better than a coherent minimum-shift-keying (MSK) receiver in a heavily fading environment. The DMSK radio is simple and robust, and results in a lower error floor than its coherent counterpart. Moreover, this receiver is suitable for burst-type signals, and its recovery from deep fades is fast.

Davarian, F.↗

NASA NTRS1986

Trellis coded modulation for 4800-9600 bps transmission over a fading mobile satellite channel

The combination of trellis coding and multiple phase-shift-keyed (MPSK) signalling with the addition of asymmetry to the signal set is discussed with regard to its suitability as a modulation/coding scheme for the fading mobile satellite channel. For MPSK, introducing nonuniformity (asymmetry) into the spacing between signal points in the constellation buys a further improvement in performance over that achievable with trellis coded symmetric MPSK, all this without increasing average or peak power, or changing the bandwidth constraints imposed on the system. Whereas previous contributions have considered the performance of trellis coded modulation transmitted over an additive white Gaussian noise (AWGN) channel, the emphasis in the paper is on the performance of trellis coded MPSK in the fading environment. The results will be obtained by using a combination of analysis and simulation. It will be assumed that the effect of the fading on the phase of the received signal is fully compensated for either by tracking it with some form of phase-locked loop or with pilot tone calibration techniques. Thus, results will reflect only the degradation due to the effect of the fading on the amplitude of the received signal. Also, we shall consider only the case where interleaving/deinterleaving is employed to further combat the fading. This allows for considerable simplification of the analysis and is of great practical interest. Finally, the impact of the availability of channel state information on average bit error probability performance is assessed.

Divsalar, D.↗

NASA NTRS1985

Trap controlled minority-carrier mobility in heavily doped silicon

The activation behavior of the minority-carrier mobility and diffusivity in heavily doped (about 10 to the 20th per cu cm) Si(Si:As) was investigated in the temperature range, 20 - 350 K. Experimental results indicate that hole transitions between the valence band and localized shallow states give rise to the observed behavior. The activation energy is about 10 meV, which suggests that the localized states originate from band tails but does not rule out trapping at boron atoms in the compensated n(+) region.

Neugroschel, A.↗

NASA NTRS1986

A second-generation mobile satellite system

A design for a low-user-cost, 9000 channel capacity second generation mobile satellite system (Msat-2) for continental U.S., Alaska and Canada using two geostationary satellites at 90 and 130 deg west longitude, is presented. The increased capacity over the first generation system is obtained by use of a 20 m deployable antenna with an offset-fed antenna configuration, a high-power satellite bus, and by relaxing the north-south stationkeeping requirement to + or - 2 deg and the eclipse capability to 50 percent. Efficient frequency utilization is achieved for uplink and downlink spectra by a 7-frequency reuse scheme with 285 5-kHz channels per subband, and subband reuse of up to four times. Problems of interbeam interference and multipath fading contributed to the choice of a nonoverlapping feed for the Msat-2, and a proper modulation scheme using Gaussian baseband filtered minimum-shift-keying with differential detection.

Sue, M. K.↗

NASA NTRS1985

Land vehicle antennas for satellite mobile communications

Antenna designs applicable to future satellite mobile vehicle communications are examined. Microstrip disk, quadrifilar helix, cylindrical microstrip, and inverted V and U crossed-dipole low gain antennas (3-5 dBic) that provide omnidirectional coverage are described. Diagrams of medium gain antenna (9-12 dBic) concepts are presented; the antennas are classified into three types: (1) electronically steered with digital phase shifters; (2) electronically switched with switchable power divider/combiner; and (3) mechanically steered with motor. The operating characteristics of a conformal antenna with electronic beam steering and a nonconformal design with mechanical steering are evaluated with respect to isolation levels in a multiple satellite system. Vehicle antenna pointing systems and antenna system costs are investigated.

Haddad, H. A.↗

NASA NTRS1986

Minority-carrier mobility anomalies in low-resistivity silicon solar cells

Measurement of the minority-carrier mobility in the base region of a high-voltage metal-insulator-N-P solar cell (Green et al., 1984), as well as in other 0.1-ohm cm cells, provides direct proof that the high voltages measured for that cell are due not only to improved emitter characteristics but to an improved base region as well. The base characteristics are shown to be quite sensitive to the effects of diffusion-induced lattice stress originating in the emitter. The implications of these findings for the fabrication of high-efficiency cells are discussed.

Weizer, V. G.↗

NASA NTRS1987

Mobile Platform for Large Structures

Proposed platform moves forward and in reverse, turns left and right, and changes planes. Mobile-platform concept proposed to move remote manipulators, workers, or other loads over truss panels on large structures. Platform moves at constant speed so does not cause swinging motion in hanging loads and overstress remote-manipulator arms. Transferred around corners to adjacent panels. Platform rides on sprocketed guide pins extending from structure at truss joints. Set of orthogonal tracks under platform slides on pins, which have enlarged heads to interlock with tracks. At least three tracks engage at least three pins at any position on panel so platform adequately and stably supported.

Wesselski, C. J.↗

NASA NTRS1986

A Meteorological (humidity, temperature, aerosols)) mobile dial system: Concepts and design

Since 1982 a program was conducted to develop a mobile meteorological (humidity, temperature, aerosols) Differential Absorption Lidar (DIAL) devoted to the studies of the nuclear power plant atmospheric surroundings. The measurement objectives are defined according to the user needs and the lidar feasibility. The concepts and design adopted to meet both the requirement and the measurement objectives are described. Each sub-system is addressed sequentially: transmitting system, receiving system, detection system, and post detection.

Cahen, C.↗

NASA NTRS1987

The design and development of a mobile transporter system for the Space Station Remote Manipulator System

The analyses, selection process, and conceptual design of potential candidate Mobile Transporter (MT) systems to move the Space Station Remote Manipulator System (SSRMS) about the exposed faces of the Space Station truss structure are described. The actual requirements for a manipulator system on the space station are discussed, including potential tasks to be performed. The SSRMS operating environment and control methods are analyzed with potential design solutions highlighted. Three general categories of transporter systems are identified and analyzed. Several design solution have emerged that will satisfy these requirements. Their relative merits are discussed, and unique variations in each system are rated for functionality.

Carroll, Thomas W.↗

NASA NTRS1986

Design of a digital flexible mobile transceiver

A flexible, modular transceiver design for digital voice and data transmissions in the proposed Mobile Satellite Experimental (MSAT-X) System is presented. Major system design issues are addressed, including frequency stability control, modulation method, linear predictive coding vocoder algorithm, and error control technique. Detailed design aspects of the modular and flexible transceiver, including the core, RF/IF, modem, vocoder, forward error correction codec, and I/O modules, as well as the flexible interface, are also discussed.

Fang, R. J. F.↗

NASA NTRS1987

Trellis coded modulation for 4800-9600 bits/s transmission over a fading mobile satellite channel

The combination of trellis coding and multiple phase-shift-keyed (MPSK) signaling with the addition of asymmetry to the signal set is discussed with regard to its suitability as a modulation/coding scheme for the fading mobile satellite channel. For MPSK, introducing nonuniformity (asymmetry) into the spacing between signal points in the constellation buys a further improvement in performance over that achievable with trellis coded symmetric MPSK, all this without increasing average or peak power, or changing the bandwidth constraints imposed on the system. Whereas previous contributions have considered the performance of trellis coded modulation transmitted over an additive white Gaussian noise (AWGN) channel, the emphasis in the paper is on the performance of trellis coded MPSK in the fading environment. The results will be obtained by using a combination of analysis and simulation. It will be assumed that the effect of the fading on the phase of the received signal is fully compensated for either by tracking it with some form of phase-locked loop or with pilot tone calibration techniques. Thus, results will reflect only the degradation due to the effect of the fading on the amplitude of the received signal. Also, we shall consider only the case where interleaving/deinterleaving is employed to further combat the fading. This allows for considerable simplification of the analysis and is of great practical interest. Finally, the impact of the availability of channel state information on average bit error probability performance is assessed.

Divsalar, Dariush↗