SEARCH · Search NASA
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Scheme for mobile communication reduces multipath fading and interference between adjacent channels. Proposed communication system lends itself to almost completely digital implementation, eliminating costly and bulky crystal filters. Scheme suitable for satellite-aided or terrestrial mobile communication, including cellular mobile telephony, at frequencies in 150-to-900-MHz range.
New methods and approaches are being tried and evaluated with the goal of increasing productivity and quality. The underlying concept in all of these approaches, methods or processes is that people require interactive communication to maximize the organization's strengths and minimize impediments to productivity improvement. This paper examines Bendix Field Engineering Corporation's organizational structure and experiences with employee involvement programs. The paper focuses on methods Bendix developed and implemented to open lines of communication throughout the organization. The Bendix approach to productivity and quality enhancement shows that interactive communication is critical to the successful implementation of any productivity improvement program. The paper concludes with an examination of the Bendix methodologies which can be adopted by any corporation in any industry.
The efficient implementation of algorithms on multiprocessor machines requires that the effects of communication delays be minimized. The effects of these delays on the performance of a model problem on a hypercube multiprocessor architecture is investigated and methods are developed for increasing algorithm efficiency. The model problem under investigation is the solution by red-black Successive Over Relaxation YOUN71 of the heat equation; most of the techniques described here also apply equally well to the solution of elliptic partial differential equations by red-black or multicolor SOR methods. Methods for reducing communication traffic and overhead on a multiprocessor are identified and results of testing these methods on the Intel iPSC Hypercube reported. Methods for partitioning a problem's domain across processors, for reducing communication traffic during a global convergence check, for reducing the number of global convergence checks employed during an iteration, and for concurrently iterating on multiple time-steps in a time-dependent problem. Empirical results show that use of these models can markedly reduce a numewrical problem's execution time.
During the planning Stages of the NUSAT satellite, an obvious issue to be discussed was the method of communications to be used. The frequencies would have to be high enough to pass through the atmosphere relatively unattenuated but low enough that antennas and transmission lines would not be so critical in length and properties that unexperienced students would have difficulty with handling them. In conversations with the Federal Communications Commission (FCC) in Washington D.C., the frequencies of 450.000 MHz and 137.900 MHz were decided upon and applied for licensing. The 450.000 MHt up-link is used on a "non-interference" basis to control the satellite while the 137.900 MHz frequency down-links data from the satellite. A reasonable amount of time was spent discussing the mods of transmission to be used in communication with the satellite. Representatives of the amateur radio satellite organization, AMSAT, were contacted for ides. This organization seems to favor AM types of emissions such as CW to control their OSCAR series of satellites and also the current Phase 111 unit. NUSAT personnel felt, however, that there would be merit in the improved signal to noise ratio usually obtained in an FM mode. Doppler shift of the transmitted information on the NUSAT also had to be considered. The final decision WBS to use Audio Frequency Shift Keying (AFSK) modulated on an FM carrier. In this mode the audio tones used would not shift frequency with Doppler - only the carrier would shift in frequency. If necessary, the receivers could be fitted with a form of AFC control or simply have enough bandwidth to handle the Doppler shift with an acceptable increase in noise.
NASA plans to launch a permanent manned space station in the early 1990's. The station will be used to support a wide variety of activities involving Earth and space observation, satellite maintenance, scientific experimentation, and commercial manufacturing. The control and monitoring of many of these activities will require extensive computer and communications system support. In order to identify an appropriate computer and communication system for supporting the space station, an attempt to characterize the space station's data communications subsystem workload is currently underway. Some of the special aspects of the workload characterization problem are discussed in connection with the space station, and some possible approaches are presented.
This research project was set up to study various kinds of coding techniques for error control in satellite and space communications for NASA Goddard Space Flight Center. During the project period, researchers investigated the following areas: (1) decoding of Reed-Solomon codes in terms of dual basis; (2) concatenated and cascaded error control coding schemes for satellite and space communications; (3) use of hybrid coding schemes (error correction and detection incorporated with retransmission) to improve system reliability and throughput in satellite communications; (4) good codes for simultaneous error correction and error detection, and (5) error control techniques for ring and star networks.
High-power, single-mode, double-heterojunction AlGaAs diode lasers are being developed to meet source requirements for both fiber optic local area network and free space communications systems. An individual device, based on the channeled-substrate-planar (CSP) structure, has yielded single spatial and longitudinal mode outputs of up to 90 mW CW, and has maintained a single spatial mode to 150 mW CW. Phase-locked arrays of closely spaced index-guided lasers have been designed and fabricated with the aim of multiplying the outputs of the individual devices to even higher power levels in a stable, single-lobe, anastigmatic beam. The optical modes of the lasers in such arrays can couple together in such a way that they appear to be emanating from a single source, and can therefore be efficiently coupled into optical communications systems. This paper will review the state of high-power laser technology and discuss the communication system implications of these devices.
Communication technology for deep-space scientific missions is surveyed, with a focus on systems being developed for Galileo, a NASA orbiter/probe mission to Jupiter. The problems faced in designing the links from the probe (descending into the Jovian atmosphere) to the orbiter and from the orbiter to earth are examined; the technological solutions adopted for Galileo are discussed; the expansion of the Deep Space Network for the mission is described; the advantages and limitations of cm-band communication systems being considered to meet the data-rate requirements of future deep-space missions are outlined; and the potential of optical communication for the 21st century is indicated. Also included are tables listing past and planned deep-space missions and a brief description of the use of spacecraft radio signals by radio and planetary scientists to probe solar-system plasmas and gravitational fields.
An analytical cost-performance model for a ground-based optical communications receiving telescope is presented. The model considers costs of existing telescopes as a function of diameter and field of view. This, coupled with communication performance as a function of receiver diameter and field of view, yields the appropriate telescope cost versus communication performance curve.
A financial model was developed which described quantitatively the economics of the space segment of communication satellite systems. The model describes the economics of the space system throughout the lifetime of the satellite. The expected state-of-the-art status of communications satellite systems and operations beginning service in 1995 were assessed and described. New or enhanced space-based activities and associated satellite system designs that have the potential to achieve future communications satellite operations in geostationary orbit with improved economic performance were postulated and defined. Three scenarios using combinations of space-based activities were analyzed: a spin stabilized satellite, a three axis satellite, and assembly at the Space Station and GEO servicing. Functional and technical requirements placed on the Space Station by the scenarios were detailed. Requirements on the satellite were also listed.
The part of the geostationary (GEO) orbital arc used for United States domestic fixed, communications service is rapidly becoming filled with satellites. One of the factors currently limiting its utilization is that communications satellites must be designed to have sufficient capacity to handle peak traffic leads, and thus are under utilized most of the time. A solution is to use satellites in suitable non-geostationary orbits to unload the traffic peaks. Three different designs for a non-geostationary orbit communications satellite system are presented for the 1995 time frame. The economic performance is analyzed and compared with geostationary satellites for two classes of service, trunking and customer premise service. The result is that the larger payload of the non-geostationary satellite offsets the burdens of increased complexity and worse radiation environment to give improved economic performance. Depending on ground terminal configuration, the improved economic performance of the space segment may be offset by increased ground terminal expenses.
A simplified model of the communications network for the Communications and Tracking Data Processing System (CTDP) was developed. It was simulated by use of programs running on several on-site computers. These programs communicate with one another by means of both local area networks and direct serial connections. The domain of the model and its simulation is from Orbital Replaceable Unit (ORU) interface to Data Management Systems (DMS). The simulation was designed to allow status queries from remote entities across the DMS networks to be propagated through the model to several simulated ORU's. The ORU response is then propagated back to the remote entity which originated the request. Response times at the various levels were investigated in a multi-tasking, multi-user operating system environment. Results indicate that the effective bandwidth of the system may be too low to support expected data volume requirements under conventional operating systems. Instead, some form of embedded process control program may be required on the node computers.
The mathematical nature of the orbital and frequency assignment problem for communications satellites is explored, and it is shown that choosing the correct permutations of the orbit locations and frequency assignments is an important step in arriving at values which satisfy the signal-quality requirements. Two methods are proposed to achieve better spectrum/orbit utilization. The first, called the delta S concept, leads to orbital assignment solutions via either mixed-integer or restricted basis entry linear programming techniques; the method guarantees good single-entry carrier-to-interference ratio results. In the second, a basis for specifying service areas is proposed for the Fixed Satellite Service. It is suggested that service areas should be specified according to the communications-demand density in conjunction with the delta S concept in order to enable the system planner to specify more satellites and provide more communications supply.
The present conference considers topics encompassing the fields of satellite communications technology, optical subsystems, transmitters and receivers, subsystems for pointing and tracking, onboard processing- and component-related technologies, fiber-optic distribution networks, and reliability-related considerations. Attention is given to lightwave technology in microwave systems, the status of CO2 laser technology and homodyne receiver concepts for communication satellite optical links, laser Doppler measurement techniques for spacecraft, fiber-optic gyros for space applications, integrated acoustooptic device modules for communication, signal processing and computing, radiation-hardened optoelectronic components, and radiation effects on fiber-optics.
Summarized and compared are the major results of two NASA sponsored studies that defined potential communication payload concepts to meet the satellite traffic forecast for the turn of the century for the continental US and Region 2 of the International Telecommunications Union. The studies were performed by the Ford Aerospace and Communications Corporation and RCA Astro-Electronics (now GE-RCA Astro-Space Division). Future scenarios of aggregations of communications services are presented. Payload concepts are developed and defined in detail for nine of the scenarios. Payload costs and critical technologies per payload are also presented. Finally the payload concepts are compared and the findings of the reports are discussed.
As Mars exploration grows in complexity with time, the corresponding communication needs will grow in variety and complexity also. From initial Earth/Mars links, further needs will arise for complete surface connectivity for the provision of navigation, position location, and voice, data, and video communications services among multiple Mars bases and remote exploration sites. This paper addresses the likely required communication functions over the first few decades of Martian exploration and postulates systems for providing these services. Required technologies are identified and development requirements indicated.
The communications system for the space station will require a space based multiple access component to provide communications between the space based program elements and the station. A study was undertaken to investigate two of the concerns of this multiple access system, namely, the issues related to the frequency spectrum utilization and the possibilities for higher order (than QPSK) modulation schemes for use in possible modulators and demodulators (modems). As a result of the investigation, many key questions about the frequency spectrum utilization were raised. At this point, frequency spectrum utilization is seen as an area requiring further work. Simulations were conducted using a computer aided communications system design package to provide a straw man modem structure to be used for both QPSK and 8-PSK channels.
A study of the requirements and options for interconnecting the development systems and demonstration test beds at NASA centers supporting the Systems Autonomy Demonstration Project (SADP) is presented. The communications requirements of SADP development and demonstration environments are described, potential communications protocols are examined, and expected communications error rates and system availability are described.