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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 505 records · Page 28

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NASA NTRS1987

A near-earth optical communications terminal with a corevolving planetary sun shield

The umbra of a planet may serve as a sun shield for a space based optical communications terminal or for a space based astronomical observatory. An orbit that keeps the terminal or observatory within the umbra is desirable. There is a corevolution point behind every planet. A small body stabilized at the planet corevolution point will revolve about the sun at the same angular velocity as the planet, always keeping the planet between itself and the sun. This corevolution point is within the umbra of Mars but beyond the end of the umbra for Mercury, Venus, and earth. The Mars corevolution point is an ideal location for an astronomical observatory. There Mars obstruct less than 0.00024 percent of the sky at any time, and it shades the observatory completely from the sun. At the earth corevolution point, between 51 and 84 percent of the solar disk area is blocked, as is up to 92 percent of the sunlight. This provides a reduction from 3 dB to 11 dB in sunlight that could interfere with optical communications if scattered directly into the detectors. The variations is caused by revolution of the earth about the earth-moon barycenter.

Kerr, E. L.↗

NASA NTRS1988

Graphics software tool for VT terminals (VTGRAPH)

VTGRAPH is a graphics software tool for using DEC/VT or VT compatible terminals. It allows the user to deal with computer environments which use VT terminals for window management and graphics systems. VTGRAPH was developed using the Re'Gis Graphics set and it was written in FORTRAN language. It provides window management and a PLOT10-like package plus color or shade capability.

Wang, Caroline↗

NASA NTRS1987

Helicopter terminal approach using differential GPS with vertical-axis enhancement

The NAVSTAR Global Positioning System (GPS) in differential mode (DGPS) has been shown to be least accurate in the vertical axis. The vertical axis also has the most stringent accuracy requirements for aircraft precision approach and landing. A series of flight tests were conducted to evaluate a concept for improving the DGPS vertical axis navigation performance. These tests incorporated augmentation sensors to aid the DGPS navigation solution during terminal approach operations. A GPS receiver was installed on board a NASA helicopter and interfaced with a real-time digital computer system. A reconfigurable navigation filter programmed in the digital computer provided an augmented DGPS solution, with selectable inputs from a low-cost vertical accelerometer, a barometric altimeter, and the aircraft attitude gyros. The reference aircraft position was determined by a laser tracker. Extensive post-test analysis was done to optimize the filter performance during the terminal approach operation. Test results show that baro-altimeter aiding can significantly improve vertical axis performance. Follow-on tests are planned for the optimized configurations.

Edwards, F. G.↗

NASA NTRS1988

Scanning tunneling microscopy characterization of the geometric and electronic structure of hydrogen-terminated silicon surfaces

Scanning tunneling microscopy (STM) methods are used to characterize hydrogen-terminated Si surfaces prepared by a novel method. The surface preparation method is used to expose the Si-SiO2 interface. STM images directly reveal the topographic structure of the Si-SiO2 interface. The dependence of interface topography on oxide preparation conditions observed by STM is compared to the results of conventional surface characterization methods. Also, the electronic structure of the hydrogen-terminated surface is studied by STM spectroscopy. The near-ideal electronic structure of this surface enables direct tunnel spectroscopy measurements of Schottky barrier phenomena. In addition, this method enables probing of semiconductor subsurface properties by STM.

Kaiser, W. J.↗

NASA NTRS1987

Detection of radio-frequency modulated optical signals by two and three terminal microwave devices

An interdigitated photoconductor (two terminal device) on GaAlAs/GaAs heterostructure was fabricated and tested by an electro-optical sampling technique. Further, the photoresponse of GaAlAs/GaAs HEMT (three terminal device) was obtained by illuminating the device with an optical signal modulated up to 8 GHz. Gain-bandwidth product, response time, and noise properties of photoconductor and HEMT devices were obtained. Monolithic integration of these photodetectors with GaAs microwave devices for optically controlled phased array antenna applications is discussed.

Bhasin, K. B.↗

NASA NTRS1989

Simulation evaluation of TIMER, a time-based, terminal air traffic, flow-management concept

A description of a time-based, extended terminal area ATC concept called Traffic Intelligence for the Management of Efficient Runway scheduling (TIMER) and the results of a fast-time evaluation are presented. The TIMER concept is intended to bridge the gap between today's ATC system and a future automated time-based ATC system. The TIMER concept integrates en route metering, fuel-efficient cruise and profile descents, terminal time-based sequencing and spacing together with computer-generated controller aids, to improve delivery precision for fuller use of runway capacity. Simulation results identify and show the effects and interactions of such key variables as horizon of control location, delivery time error at both the metering fix and runway threshold, aircraft separation requirements, delay discounting, wind, aircraft heading and speed errors, and knowledge of final approach speed.

Credeur, Leonard↗

NASA NTRS1988

Plant microfossil record of the terminal Cretaceous event in the western United States and Canada

Plant microfossils, principally pollen grains and spores produced by land plants, provide an excellent record of the terminal Cretaceous event in nonmarine environments. The record indicates regional devastation of the latest Cretaceous vegetation with the extinction of many groups, followed by a recolonization of the earliest Tertiary land surface, and development of a permanently changed land flora. The regional variations in depositional environments, plant communities, and paleoclimates provide insight into the nature and effects of the event, which were short-lived but profound. The plant microfossil data support the hypothesis that an abruptly initiated, major ecological crisis occurred at the end of the Cretaceous. Disruption of the Late Cretaceous flora ultimately contributred to the rise of modern vegetation. The plant microfossils together with geochemical and mineralogical data are consistent with an extraterrestrial impact having been the cause of the terminal Cretaceous event.

Nichols, D. J.↗

NASA NTRS1989

Remote sensing of the termination of the solar wind via in situ plasma measurements

Since 1983 the Plasma Wave experiments on the two Voyager spacecraft have detected low frequency radio noise in the outer heliosphere which has been postulated to emanate from the terminal shock of the solar wind or, possibly, from the heliopause itself. The solar wind data from the Plasma Science experiment on the Voyager spacecraft are studied to search for correlations with these radio emissions. It is found that two anomalous high speed streams passed Voyager 2 in late 1982 and early 1983 and it is suggested that the interaction of the streams with the heliospheric terminal shock is responsible for the generation of the most intense radio noise observed later in the same year. If the stream speeds did not decrease in traveling to the interaction region, that region is about 135 AU from the sun. This is consistent with previous estimates of the distance to the inner heliospheric shock.

Mcnutt, R. L., Jr.↗

NASA NTRS1989

Design of a monitor and simulation terminal (master) for space station telerobotics and telescience

Based on Space Station and planetary spacecraft communication time delays and bandwidth limitations, it will be necessary to develop an intelligent, general purpose ground monitor terminal capable of sophisticated data display and control of on-orbit facilities and remote spacecraft. The basic elements that make up a Monitor and Simulation Terminal (MASTER) include computer overlay video, data compression, forward simulation, mission resource optimization and high level robotic control. Hardware and software elements of a MASTER are being assembled for testbed use. Applications of Neural Networks (NNs) to some key functions of a MASTER are also discussed. These functions are overlay graphics adjustment, object correlation and kinematic-dynamic characterization of the manipulator.

Lopez, L.↗

NASA NTRS1989

The winds of O-stars. II - The terminal velocities of stellar winds of O-type stars

The SEI method (Lamers et al., 1987) is used to obtain P Cygni profiles of the UV resonance lines of C IV, N V, and S IV and of the subordinate UV lines of N IV and C III observed in the spectra of 27 O-type stars. Theoretical profiles which include the turbulence effects agree well with the observations, and they can account for the deep absorption troughs, the shape of the violet absorption wings, and the wavelength of the emission peak. The resulting terminal velocities of the stellar winds are found to be systematically lower by about 400 km/s than previous estimates obtained using the Sobolev approximation (Castor and Lamers, 1979), suggesting that the narrow absorption components, observed in the UV resonance lines of O and B stars, reach the terminal velocity of the winds.

Groenewegen, M. A. T.↗

NASA NTRS1989

A near-earth optical communications terminal with a corevolving planetary sun shield

The umbra of a planet may serve as a sun shield for a space-based optical communications terminal or for a space-based astronomical observatory. An orbit that keeps the terminal or observatory within the umbra is desirable. There is a corevolution point behind every planet. A small body stabilized at the planet corevolution point will revolve about the sun at the same angular velocity as the planet, always keeping the planet between itself and the sun. This corevolution point is within the umbra of Mars but beyond the end of the umbra for Mercury, Venus, and earth. The Mars corevolution point is an ideal location for an astronomical observatory. There, Mars obstruct less than 0.00024 percent of the sky at any time, and it shades the observatory completely from the sun. At the earth corevolution point, between 51 and 84 percent of the solar disk area is blocked, as is up to 92 percent of the sunlight. This provides a reduction from 3 dB to 11 dB in sunlight that could interfere with optical communications if scattered directly into the detectors. The variations is caused by revolution of the earth about the earth-moon barycenter.

Kerr, E. L.↗

NASA NTRS1990

Experiments for Ka-band mobile applications: The ACTS mobile terminal

To explore the potential of Ka-band to support mobile satellite services, the Jet Propulsion Laboratory (JPL) has initiated the design and development of a Ka-band land-mobile terminal to be used with the Advanced Communications Technology Satellite (ACTS). The planned experimental setup with ACTS is described. Brief functional descriptions of the mobile and fixed terminals are provided. The inputs required from the propagation community to support the design activities and the planned experiments are also discussed.

Estabrook, Polly↗

NASA NTRS1990

ACTS propagation terminal prototype planning and design

The planning and design of a prototype propagation receiving terminal for beacon signals at 27 and 20 GHz bands are examined. The developmental plan is discussed, followed by technical design considerations including, the Advanced Communications Technology Satellite (ACTS) system salient features and frequency plan, beacon signal parameters and specifications, system calculations, and terminal hardware design issues.

Davarian, Faramaz↗

NASA NTRS1991

Progress toward a 30 percent-efficient, monolithic, three-junction, two-terminal concentrator solar cell for space applications

Component efficiencies of 0.2/sq cm cells at approximately 100x AMO light concentration and 80 C temperatures are not at 15.3 percent for a 1.9 eV AlGaAs top cell, 9.9 percent for a 1.4 eV GaAs middle cell under a 1.9 eV AlGaAs filter, and 2.4 percent for a bottom 1.0 eV InGaAs cell under a GaAs substrate. The goal is to continue improvement in these performance levels and to sequentially grow these devices on a single substrate to give 30 percent efficient, monolithic, two-terminal, three-junction space concentrator cells. The broad objective is a 30 percent efficient monolithic two-terminal cell that can operate under 25 to 100x AMO light concentrations and at 75 to 100 C cell temperatures. Detailed modeling predicts that this requires three junctions. Two options are being pursued, and both use a 1.9 eV AlGaAs top junction and a 1.4 eV GaAs middle junction grown by a 1 atm OMVPE on a lattice matched substrate. Option 1 uses a low-doped GaAs substrate with a lattice mismatched 1.0 eV InGaAs cell formed on the back of the substrate. Option 2 uses a Ge substrate to which the AlGaAs and GaAs top junctions are lattice matched, with a bottom 0.7 eV Ge junction formed near the substrate interface with the GaAs growth. The projected efficiency contributions are near 16, 11, and 3 percent, respectively, from the top, middle, and bottom junctions.

Partain, L. D.↗

NASA NTRS1991

InP/Ga0.47In0.53As monolithic, two-junction, three-terminal tandem solar cells

The work presented has focussed on increasing the efficiency of InP-based solar cells through the development of a high-performance InP/Ga(0.47)In(0.53)As two-junction, three-terminal monolithic tandem cell. Such a tandem is particularly suited to space applications where a radiation-hard top cell (i.e., InP) is required. Furthermore, the InP/Ga(0.47)In(0.53)As materials system is lattice matched and offers a top cell/bottom cell bandgap differential (0.60 eV at 300 K) suitable for high tandem cell efficiencies under AMO illumination. A three-terminal configuration was chosen since it allows for independent power collection from each subcell in the monolithic stack, thus minimizing the adverse impact of radiation damage on the overall tandem efficiency. Realistic computer modeling calculations predict an efficiency boost of 7 to 11 percent from the Ga(0.47)In(0.53)As bottom cell under AMO illumination (25 C) for concentration ratios in the 1 to 1000 range. Thus, practical AMO efficiencies of 25 to 32 percent appear possible with the InP/Ga(0.47)In(0.53)As tandem cell. Prototype n/p/n InP/Ga(0.47)In(0.53)As monolithic tandem cells were fabricated and tested successfully. Using an aperture to define the illuminated areas, efficiency measurements performed on a non-optimized device under standard global illumination conditions (25 C) with no antireflection coating (ARC) give 12.2 percent for the InP top cell and 3.2 percent for the Ga(0.47)In(0.53)As bottom cell, yielding an overall tandem efficiency of 15.4 percent. With an ARC, the tandem efficiency could reach approximately 22 percent global and approximately 20 percent AMO. Additional details regarding the performance of individual InP and Ga(0.47)In(0.53)As component cells, fabrication and operation of complete tandem cells and methods for improving the tandem cell performance, are also discussed.

Wanlaas, M. W.↗

NASA NTRS1989

Human factors in aviation: Terminal control area boundary conflicts

Air-to-air conflicts in the vicinity of Terminal Control Area (TCA) boundaries were studied to obtain a better understanding of the causal dynamics of these events with particular focus on human factor issues. The study dataset consisted of 381 Instrument Flight Rules/Visual Flight Rules (IFR/VFR) traffic conflicts in airspace layers above TCA ceiling and below TCA floors; 213 reports of incursions in TCA terminal airspace by VFR aircraft, of which 123 resulted in conflicts; and an additional set of reports describing problems with Air Traffic Control (ATC) services in and around TCAs. Results and conclusions are detailed.

Monan, William P.↗

NASA NTRS1991

Space Communications Artificial Intelligence for Link Evaluation Terminal (SCAILET)

A software application to assis end-users of the Link Evaluation Terminal (LET) for satellite communication is being developed. This software application incorporates artificial intelligence (AI) techniques and will be deployed as an interface to LET. The high burst rate (HBR) LET provides 30 GHz transmitting/20 GHz receiving, 220/110 Mbps capability for wideband communications technology experiments with the Advanced Communications Technology Satellite (ACTS). The HBR LET and ACTS are being developed at the NASA Lewis Research Center. The HBR LET can monitor and evaluate the integrity of the HBR communications uplink and downlink to the ACTS satellite. The uplink HBR transmission is performed by bursting the bit-pattern as a modulated signal to the satellite. By comparing the transmitted bit pattern with the received bit pattern, HBR LET can determine the bit error rate BER) under various atmospheric conditions. An algorithm for power augmentation is applied to enhance the system's BER performance at reduced signal strength caused by adverse conditions. Programming scripts, defined by the design engineer, set up the HBR LET terminal by programming subsystem devices through IEEE488 interfaces. However, the scripts are difficult to use, require a steep learning curve, are cryptic, and are hard to maintain. The combination of the learning curve and the complexities involved with editing the script files may discourage end-users from utilizing the full capabilities of the HBR LET system. An intelligent assistant component of SCAILET that addresses critical end-user needs in the programming of the HBR LET system as anticipated by its developers is described. A close look is taken at the various steps involved in writing ECM software for a C&P, computer and at how the intelligent assistant improves the HBR LET system and enhances the end-user's ability to perform the experiments.

Shahidi, Anoosh↗

NASA NTRS1991

Analysis of lossy composite terminating structures

A finite element solution and computer code for the electromagnetic scattering of inhomogeneous penetrable bodies is presented. The application for the code is for the analysis and design of leading and trailing edge terminations when conducting and nonconducting materials are used. Examples of simple triangular shaped terminations are also presented.

Andre, R.↗