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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Mobile very long baseline interferometry and Global Positioning System measurement of vertical crustal motion

Mobile Very Long Base Interferometry (VLBI) and Global Positioning System (GPS) geodetic measurements have many error sources in common. Calibration of the effects of water vapor on signal transmission through the atmosphere, however, remains the primary limitation to the accuracy of vertical crustal motion measurements made by either technique. The two primary methods of water vapor calibration currently in use for mobile VLBI baseline measurements were evaluated: radiometric measurements of the sky brightness near the 22 GHz emission line of free water molecules and surface meteorological measurements used as input to an atmospheric model. Based upon a limited set of 9 baselines, it is shown that calibrating VLBI data with water vapor radiometer measurements provides a significantly better fit to the theoretical decay model than calibrating the same data with surface meteorological measurements. The effect of estimating a systematic error in the surface meteorological calibration is shown to improve the consistency of the vertical baseline components obtained by the two calibration methods. A detailed error model for the vertical baseline components obtained indicates current mobile VLBI technology should allow accuracies of order 3 cm with WVR calibration and 10 cm when surface meteorological calibration is used.

Kroger, Peter M.↗
NASA NTRS1987

Crustal dynamics project data analysis, 1987. Volume 2: Mobile VLBI geodetic results, 1982-1986

The Goddard VLBI group reports the results of analyzing 101 Mark III data sets acquired from mobile observing sites through the end of 1986 and available to the Crustal Dynamics Project. The fixed VLBI observations at Hat Creek, Ft. Davis, Mojave, and OVRO are included as they participate heavily in the mobile schedules. One large solution GLB171 was used to obtain baseline length and transverse evolutions. Radio source positions were estimated globally, while nutation offsets were estimated from each data set. The results include 28 mobile sites.

Ma, C.↗
NASA NTRS1988

NASA's mobile satellite development program

A Mobile Satellite System (MSS) will provide data and voice communications over a vast geographical area to a large population of mobile users. A technical overview is given of the extensive research and development studies and development performed under NASA's mobile satellite program (MSAT-X) in support of the introduction of a U.S. MSS. The critical technologies necessary to enable such a system are emphasized: vehicle antennas, modulation and coding, speech coders, networking and propagation characterization. Also proposed is a first, and future generation MSS architecture based upon realized ground segment equipment and advanced space segment studies.

Rafferty, William↗
NASA NTRS1988

Mobile satellite service for Canada

The Mobile Satellite (MSAT) system and a special program designed to provide interim mobile satellite services (IMSS) during the construction phase of MSAT are described. A mobile satellite system is a key element in extending voice and and data telecommunications to all Canadians.

Sward, David↗
NASA NTRS1988

An overview of the OmniTRACS: The first operational mobile Ku-band satellite communications

The service features of the OmniTRACS system developed by Omninet Communications Services of Los Angeles, California are described. This system is the first operational mobile Ku-band satellite communications system that provides two-way messaging and position determination and reporting services to mobile users on a nationwide basis. The system uses existing Ku-band satellites under a secondary international allocation for mobile satellite services.

Salmasi, Allen↗
NASA NTRS1988

Mobile satellite services: International co-ordination, co-operation and competition

In the context of a discussion of international cooperation, coordination and competition regarding mobile satellite services, it is asserted that: there will be more than one civil mobile satellite service in the 1990's; competition between these separate mobile satellite systems is inevitable; no system should enjoy monopoly protection or subsidies; and coordination and cooperation are desirable and necessary, since the available L-band spectrum is in short supply.

Lundberg, Olof↗
NASA NTRS1988

Development of a mobile satellite communication unit

A compact 210(W) x 280(H) x 330(D) mm mobile terminal capable of transmitting voice and data through L-band mobile satellites is described. The Voice Codec can convert an analog voice to or from digital codes at rates of 9.6, 8 and 4.8 kb/s by an MPC algorithm. The terminal functions with a single 12 V power supplied vehicle battery. The equipment can operate at any L-band frequency allocated for mobile uses in a full duplex mode and will soon be put into a field test via Japans's ETS-V satellite.

Suzuki, Ryutaro↗
NASA NTRS1988

The JPL MSAT mobile laboratory and the pilot field experiments

A Mobile Laboratory/Propagation Measurement Van (PMV) was developed to support the field experiments of the Mobile Satellite Experiment (MSAT-X) Project. This van was designed to provide flexibility, self-sufficiency and data acquisition to allow for both measurement of equipment performance and the mobile environment. The design philosophy and implementation of the PMV are described. The Pilot Field Experiments and an overall description of the three experiments in which the PMV was used are described.

Berner, Jeff B.↗
NASA NTRS1988

Mobile satellite service in the United States

Mobile satellite service (MSS) has been under development in the United States for more than two decades. The service will soon be provided on a commercial basis by a consortium of eight U.S. companies called the American Mobile Satellite Consortium (AMSC). AMSC will build a three-satellite MSS system that will offer superior performance, reliability and cost effectiveness for organizations requiring mobile communications across the U.S. The development and operation of MSS in North America is being coordinated with Telesat Canada and Mexico. AMSC expects NASA to provide launch services in exchange for capacity on the first AMSC satellite for MSAT-X activities and for government demonstrations.

Agnew, Carson E.↗
NASA NTRS1988

Distributed cooperating processes in a mobile robot control system

A mobile inspection robot has been proposed for the NASA Space Station. It will be a free flying autonomous vehicle that will leave a berthing unit to accomplish a variety of inspection tasks around the Space Station, and then return to its berth to recharge, refuel, and transfer information. The Flying Eye robot will receive voice communication to change its attitude, move at a constant velocity, and move to a predefined location along a self generated path. This mobile robot control system requires integration of traditional command and control techniques with a number of AI technologies. Speech recognition, natural language understanding, task and path planning, sensory abstraction and pattern recognition are all required for successful implementation. The interface between the traditional numeric control techniques and the symbolic processing to the AI technologies must be developed, and a distributed computing approach will be needed to meet the real time computing requirements. To study the integration of the elements of this project, a novel mobile robot control architecture and simulation based on the blackboard architecture was developed. The control system operation and structure is discussed.

Skillman, Thomas L., Jr.↗
NASA NTRS1988

Fade measurements at L-band and UHF in mountainous terrain for land mobile satellite systems

Fading results related to land mobile satellite communications at L-band (1502 MHz) and UHF (870 MHz) are described. These results were derived from an experiment performed in a series of canyon passes in the Boulder, Colorado region of the US. The experimental configuration involved a helicopter as the source platform, which maintained a relatively fixed geometry with a mobile van containing the receiver and data-acquisition system. An unobstructed line of sight between the radiating sources and the receiving van was, for the most part, also maintained. In this configuration, the dominant mechanism causing signal fading (or enhancement) is a result of multipath. The resulting fade distributions demonstrated that at the 1 percent and 5 percent levels, 5.5- and 2.6-dB fades were on the average exceeded at L-band and 4.8- and 2.4-dB at UHF, respectively, for a path elevation angle of 45 deg. The canyon results as compared with previous roadside-tree-shadowing results demonstrate that the deciding factor dictating fade margin for future land mobile satellite systems is tree shadowing rather than fades caused by multipath.

Vogel, Wolfhard J.↗
NASA NTRS1987

Multiple trellis coded modulation (MTCM) performance on a fading mobile satellite channel

The author recently introduced the notion of multiple trellis coding, in which more than one channel symbol per trellis branch is transmitted. He showed that on the ideal additive white Gaussian noise (AWGN) channel, the combination of multiple trellis coding with M-ary modulation yields a performance gain with symmetric signal sets comparable to and in some cases better than that previously achieved only with signal constellation asymmetry. The combination of conventional trellis coding with multiple phase-shift-keyed (MPSK) signaling has recently been shown by the author to be a well-suited modulation/coding scheme for transmission over the fading mobile satellite channel. In particular, a rate 2/3 coded 8-PSK scheme operating at 4800 b/s is currently under development for use in NASA's Mobile Satellite Experiment (MSAT-X). The author applies the multiple trellis-coded modulation technique in the same fading mobile satellite environment, extending the analysis results previously found for its performance over the AWGN channel to the MSAT-X channel.

Simon, Marvin K.↗
NASA NTRS1989

Controlled method of reducing electrophoretic mobility of various substances

A method of reducing electrophoretic mobility of macromolecules, particles, cells, and the like is provided. The method comprises interacting the particles or cells with a polymer-linked affinity compound composed of: a hydrophilic neutral polymer such as polyethylene glycol, and an affinity component consisting of a hydrophobic compound such as a fatty acid ester, an immunocompound such as an antibody or active fragment thereof or simular macromolecule, or other ligands. The reduction of electrophoretic mobility achieved is directly proportional to the concentration of the polymer-linked affinity compound employed, and the mobility reduction obtainable is up to 100 percent for particular particles and cells. The present invention is advantageous in that analytical electrophoretic separation can not be achieved for macromolecules, particles, and cells whose native surface charge structure had prevented them from being separated by normal electrophoretic means. Depending on the affinity component utilized, separation can be achieved on the basis of specific/irreversible, specific/reversible, semi-specific/reversible, relatively nonspecific/reversible, or relatively nonspecific/irreversible ligand-substance interactions. The present method is also advantageous in that it can be used in a variety of standard laboratory electrophoresis equipment.

Vanalstine, James M.↗
NASA NTRS1988

Progress in utilization of a mobile laboratory for making storm electricity measurements

A mobile atmospheric science laboratory has been used to intercept and track storms on the Great Plains region of the U.S., with the intention of combining the data obtained with those from Doppler and conventional radars, NASA U-2 aircraft overflights, balloon soundings, and fixed-base storm electricity measurements. The mobile lab has proven to be valuable in the gathering of ground truth verifications for the two commercially operated lightning ground-strike locating systems. Data acquisition has recently been expanded by means of mobile ballooning before and during storms.

Rust, W. David↗
NASA NTRS1988

Current developments in NASA's mobile satellite program

The technical background of the NASA mobile satellite program is described, with attention given to the principal system parameters considered by NASA and JPL over the past decade. Recent system concepts and technology developments are discussed with emphasis on the mobile user terminal. Based on L-band channel studies, ground and space segment analyses, and field testing of developed mobile equipment, a first-generation LMSS configuration is proposed. In addition, a future system configuration with potential for significantly increased capacity is examined.

Rafferty, W.↗
NASA NTRS1989

A CCIR aeronautical mobile satellite report

Propagation effects in the aeronautical mobile-satellite service differ from those in the fixed-satellite service and other mobile-satellite services because: small antennas are used on aircraft, and the aircraft body may affect the performance of the antenna; high aircraft speeds cause large Doppler spreads; aircraft terminals must accommodate a large dynamic range in transmission and reception; and due to their high speeds, banking maneuvers, and three-dimensional operation, aircraft routinely require exceptionally high integrity of communications, making even short-term propagation effects very important. Data and models specifically required to characterize the path impairments are discussed, which include: tropospheric effects, including gaseous attenuation, cloud and rain attenuation, fog attenuation, refraction and scintillation; surface reflection (multipath) effects; ionospheric effects such as scintillation; and environmental effects (aircraft motion, sea state, land surface type). Aeronautical mobile-satellite systems may operate on a worldwide basis, including propagation paths at low elevation angles. Several measurements of multipath parameters over land and sea were conducted. In some cases, laboratory simulations are used to compare measured data and verify model parameters. The received signals is considered in terms of its possible components: a direct wave subject to atmospheric effects, and a reflected wave, which generally contains mostly a diffuse component.

Davarian, Faramaz↗
NASA NTRS1989

The development of the Canadian Mobile Servicing System Kinematic Simulation Facility

Canada will develop a Mobile Servicing System (MSS) as its contribution to the U.S./International Space Station Freedom. Components of the MSS will include a remote manipulator (SSRMS), a Special Purpose Dexterous Manipulator (SPDM), and a mobile base (MRS). In order to support requirements analysis and the evaluation of operational concepts related to the use of the MSS, a graphics based kinematic simulation/human-computer interface facility has been created. The facility consists of the following elements: (1) A two-dimensional graphics editor allowing the rapid development of virtual control stations; (2) Kinematic simulations of the space station remote manipulators (SSRMS and SPDM), and mobile base; and (3) A three-dimensional graphics model of the space station, MSS, orbiter, and payloads. These software elements combined with state of the art computer graphics hardware provide the capability to prototype MSS workstations, evaluate MSS operational capabilities, and investigate the human-computer interface in an interactive simulation environment. The graphics technology involved in the development and use of this facility is described.

Beyer, G.↗
NASA NTRS1990

Teleautonomous guidance for mobile robots

Teleautonomous guidance (TG), a technique for the remote guidance of fast mobile robots, has been developed and implemented. With TG, the mobile robot follows the general direction prescribed by an operator. However, if the robot encounters an obstacle, it autonomously avoids collision with that obstacle while trying to match the prescribed direction as closely as possible. This type of shared control is completely transparent and transfers control between teleoperation and autonomous obstacle avoidance gradually. TG allows the operator to steer vehicles and robots at high speeds and in cluttered environments, even without visual contact. TG is based on the virtual force field (VFF) method, which was developed earlier for autonomous obstacle avoidance. The VFF method is especially suited to the accommodation of inaccurate sensor data (such as that produced by ultrasonic sensors) and sensor fusion, and allows the mobile robot to travel quickly without stopping for obstacles.

Borenstein, J.↗