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At least 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Multi-user satellite communications system using an innovative compressive receiver

There is a need for an onboard simultaneous multi-channel demodulation system for a satellite communications system. Studies indicate that Convolve Multiply Convolve (CMC) filtering with surface acoustic wave (SAW) dispersive delay lines will eliminate the necessity of onboard satellite channelized filters of complex fourier transform processors. The reason for choosing the CMC technique is its ability to perform Fourier transformations in a shorter time with less space and power consumption than digital Fourier transform processors. Each ground terminal in this multi-users communications system is remotely located and operates independently; hence, a method of synchronizing the transmission of these users is presented which utilizes the existing Global Positioning System (GPS) system. Each ground user is equipped with a low cost ground terminal that has a synchronization subsystem attached to it. The system design of an onboard Multi-channel Receiver and Demodulator utilizes Quadrature Phase Shift Keying (QPSK) as the modulation technique. This technique provides the best figure of merit, i.e., the lowest transmitter power requirement per communication channel.

Staples, Edward J.↗
NASA NTRS1991

Compression of digital images over local area networks. Appendix 1: Item 3

Differential Pulse Code Modulation (DPCM) has been used with speech for many years. It has not been as successful for images because of poor edge performance. The only corruption in DPC is quantizer error but this corruption becomes quite large in the region of an edge because of the abrupt changes in the statistics of the signal. We introduce two improved DPCM schemes; Edge correcting DPCM and Edge Preservation Differential Coding. These two coding schemes will detect the edges and take action to correct them. In an Edge Correcting scheme, the quantizer error for an edge is encoded using a recursive quantizer with entropy coding and sent to the receiver as side information. In an Edge Preserving scheme, when the quantizer input falls in the overload region, the quantizer error is encoded and sent to the receiver repeatedly until the quantizer input falls in the inner levels. Therefore these coding schemes increase the bit rate in the region of an edge and require variable rate channels. We implement these two variable rate coding schemes on a token wing network. Timed token protocol supports two classes of messages; asynchronous and synchronous. The synchronous class provides a pre-allocated bandwidth and guaranteed response time. The remaining bandwidth is dynamically allocated to the asynchronous class. The Edge Correcting DPCM is simulated by considering the edge information under the asynchronous class. For the simulation of the Edge Preserving scheme, the amount of information sent each time is fixed, but the length of the packet or the bit rate for that packet is chosen depending on the availability capacity. The performance of the network, and the performance of the image coding algorithms, is studied.

Gorjala, Bhargavi↗
NASA NTRS1992

Application of narrow band control to reduce vibrations in magnetic bearing systems

The benefits of narrowband control theory for simple, open-loop stable systems are illustrated, and how the approach changes for magnetic bearing systems, which are open-loop unstable, is discussed. Magnetic bearing systems are good applications for narrowband control theory. Two sources of synchronous forces, the measurement error and the magnetic unbalance, are discussed. Both the measurement error and the magnetic unbalance manifest themselves as synchronous disturbances. It is shown that narrowband control theory for simple, open-loop stable systems provides excellent performance and good stability robustness. Because magnetic bearing systems are open-loop stable, the narrowband control approach becomes more complex. Disturbance accommodating control (DAC) theory is introduced as an effective approach to reduce vibrations in magnetic bearing systems. It is used to develop a control/estimation scheme that enables the rotor to spin about its center of mass in the presence of the measurement error disturbances.

Gaffney, Monique S.↗
NASA NTRS1992

The investigation of bandwidth efficient coding and modulation techniques

The New Mexico State University Center for Space Telemetering and Telecommunications systems has been, and is currently, engaged in the investigation of trellis-coded modulation (TCM) communication systems. In particular, TCM utilizing M-ary phase shift keying is being studied. The study of carrier synchronization in a TCM environment, or in MPSK systems in general, has been one of the two main thrusts of this grant. This study has involved both theoretical modelling and software simulation of the carrier synchronization problem.

Source record↗
NASA NTRS1991

Precise time dissemination and applications development on the Bonneville Power Administration system

The Bonneville Power Administration (BPA) uses IRIG-B transmitted over microwave as its primary system time dissemination. Problems with accuracy and reliability have led to ongoing research into better methods. BPA has also developed and deployed a unique fault locator which uses precise clocks synchronized by a pulse over microwaves. It automatically transmits the data to a central computer for analysis. A proposed system could combine fault location timing and time dissemination into a Global Position System (GPS) timing receiver and close the verification loop through a master station at the Dittmer Control Center. Such a system would have many advantages, including lower cost, higher reliability, and wider industry support. Test results indicate the GPS has sufficient accuracy and reliability for this and other current timing requirements including synchronous phase angle measurements. A phasor measurement system which provides phase angle has recently been tested with excellent results. Phase angle is a key parameter in power system control applications including dynamic braking, DC modulation, remedial action schemes, and system state estimation. Further research is required to determine the applications which can most effectively use real-time phase angle measurements and the best method to apply them.

K. E. Martin↗
NASA NTRS1991

The measurement system of pulse modulated carrier frequency stability and timing jitter

The definition of pulse modulated carrier frequency stability and timing jitter as well as the configuration and synchronous acquisition measurement method of its measurement system. Frequency stability of the pulse modulated carrier is measured with a discrimination technique. The pulse modulated carrier under testing is mixed with a reference frequency synthesizer. A delay line is used to convert the frequency fluctuation mixed IF signal to the voltage fluctuation. The system has the capability to make the phase noise measurement of two port devices on a pulsed carrier using a phase bridge. The noise voltage mentioned above is applied to the data acquisition and processing unit by PC to realize stability measurement. The data acquisition is in the form of pulse synchronization so that the measurement system accuracy is increased. The pulse width is more than 0.3 microseconds. The phase fluctuation variance, sigma, is less than 0.017. The time interval measuring system with high resolution is used to make interpulse timing and pulse width jitters automatic measurement. The pulse width is less than 0.2 ns. The resolution is 0.1 ns. The system is successfully applied to radar measurement.

Li Cheng-Fu↗
NASA NTRS1991

Resonance transition radiation X-ray laser

A free electron laser is proposed using a periodic dielectric and helical magnetic field. Periodic synchronism between the electrons and the optical wave is obtained at the period of the dielectric and not at the period of the helical magnetic field. The synchronism condition and the gain of the new device are derived. The effects on the gain from dephasing and beam expansion due to elastic scattering of the electrons in the periodic medium are included in the gain calculation. Examples of the resonance transition radiation laser and klystron are given. Operation at photon energies between 2.5 and 3.5 keV with net gain up to 12 percent is feasible using high electron-beam energies of 3 and 5 GeV. Moderate (300-MeV) beam energy allows operation between 80 to 110 eV with up to 57 percent net gain using a klystron design. In both cases, rapid foil heating may limit operation to a single electron-beam pulse.

Reid, Max B.↗
NASA NTRS1992

Conservative parallel simulation of priority class queueing networks

A conservative synchronization protocol is described for the parallel simulation of queueing networks having C job priority classes, where a job's class is fixed. This problem has long vexed designers of conservative synchronization protocols because of its seemingly poor ability to compute lookahead: the time of the next departure. For, a job in service having low priority can be preempted at any time by an arrival having higher priority and an arbitrarily small service time. The solution is to skew the event generation activity so that the events for higher priority jobs are generated farther ahead in simulated time than lower priority jobs. Thus, when a lower priority job enters service for the first time, all the higher priority jobs that may preempt it are already known and the job's departure time can be exactly predicted. Finally, the protocol was analyzed and it was demonstrated that good performance can be expected on the simulation of large queueing networks.

Nicol, David↗
NASA NTRS1992

A theoretical and experimental study of the noise behavior of subharmonically injection locked local oscillators

A method for the noise characterization of optically controlled subharmonically injection-locked oscillators is presented. Based on a nonlinear model of synchronized oscillators, this method is used to formulate a general expression for phase noise calculation, so that FM noise degradation of a subharmonically synchronized LO at large-signal levels can be predicted easily and accurately. Experimental results of FM noise measurement of an oscillator confirmed the accuracy of the analysis.

Zhang, Xiangdong↗
NASA NTRS1992

Pituitary oxytocin and vasopressin content of rats flown on Cosmos 2044

Preliminary studies in rats (COSMOS 1887) suggested that levels of posterior pituitary hormones were reduced by exposure to spaceflight. To confirm these preliminary findings, pituitary tissue from rats flown for 14 days on Cosmos 2044 is obtained. Posterior pituitary content of oxytocin (OT) and vasopressin (VP) were measured in these tissues as well as those from ground-based controls. The synchronous control group had feeding and lighting schedules synchronized to those in the spacecraft and were maintained in flight-type cages. Another group was housed in vivarium cages; a third group was tail suspended (T), a method used to simulate microgravity. Flight rats showed an average reduction of 27 in pituitary OT and VP compared with the three control groups. When hormone content was expressed in terms of pituitary protein (microg hormone/mg protein), the average decrease in OT and VP for the flight animals ranged from 20 to 33 percent compared with the various control groups. Reduced levels of pituitary OT and VP were similar to preliminary measurements from the Cosmos 1887 mission and appear to result from exposure to spaceflight. These data suggest that changes in the rate of hormone secretion or synthesis may have occurred during exposure to microgravity.

Keil, L.↗
NASA NTRS1993

Low-Jitter Digital Rate Multiplier

Jitter in digital rate multiplier reduced by improved method involving use of two slightly different minor clock periods. Original application to divide measured period of spin of spacecraft into large number of equal subintervals, by counting cycles of master oscillator running at high frequency. Method also used to reduce jitter in other situations necessary to generate equal subintervals from synchronizing clock signal of arbitrary period. Particularly valuable in situations where synchronizing signals lost temporarily and where drift in analog circuit unacceptable.

Katz, Richard B.↗
NASA NTRS1992

A simple, effective media access protocol system for integrated, high data rate networks

The operation and performance of a dual media access protocol for integrated, gigabit networks are described. Unlike other dual protocols, each protocol supports a different class of traffic. The Carrier Sensed Multiple Access-Ring Network (CSMA/RN) protocol and the Circulating Reservation Packet (CRP) protocol support asynchronous and synchronous traffic, respectively. The two protocols operate with minimal impact upon each other. Performance information demonstrates that they support a complete range of integrated traffic loads, do not require call setup/termination or a special node for synchronous traffic control, and provide effective pre-use and recovery. The CRP also provides guaranteed access and fairness control for the asynchronous system. The paper demonstrates that the CSMA-CRP system fulfills many of the requirements for gigabit LAN-MAN networks most effectively and simply. To accomplish this, CSMA-CRP features are compared against similar ring and bus systems, such as Cambridge Fast Ring, Metaring, Cyclic Reservation Multiple Access, and Distributed Dual Queue Data Bus (DQDB).

Foudriat, E. C.↗
NASA NTRS1992

Approximation methods for stochastic petri nets

Stochastic Marked Graphs are a concurrent decision free formalism provided with a powerful synchronization mechanism generalizing conventional Fork Join Queueing Networks. In some particular cases the analysis of the throughput can be done analytically. Otherwise the analysis suffers from the classical state explosion problem. Embedded in the divide and conquer paradigm, approximation techniques are introduced for the analysis of stochastic marked graphs and Macroplace/Macrotransition-nets (MPMT-nets), a new subclass introduced herein. MPMT-nets are a subclass of Petri nets that allow limited choice, concurrency and sharing of resources. The modeling power of MPMT is much larger than that of marked graphs, e.g., MPMT-nets can model manufacturing flow lines with unreliable machines and dataflow graphs where choice and synchronization occur. The basic idea leads to the notion of a cut to split the original net system into two subnets. The cuts lead to two aggregated net systems where one of the subnets is reduced to a single transition. A further reduction leads to a basic skeleton. The generalization of the idea leads to multiple cuts, where single cuts can be applied recursively leading to a hierarchical decomposition. Based on the decomposition, a response time approximation technique for the performance analysis is introduced. Also, delay equivalence, which has previously been introduced in the context of marked graphs by Woodside et al., Marie's method and flow equivalent aggregation are applied to the aggregated net systems. The experimental results show that response time approximation converges quickly and shows reasonable accuracy in most cases. The convergence of Marie's method and flow equivalent aggregation are applied to the aggregated net systems. The experimental results show that response time approximation converges quickly and shows reasonable accuracy in most cases. The convergence of Marie's is slower, but the accuracy is generally better. Delay equivalence often fails to converge, while flow equivalent aggregation can lead to potentially bad results if a strong dependence of the mean completion time on the interarrival process exists.

Jungnitz, Hauke Joerg↗
NASA NTRS1993

SCATHA mission termination report

The SCATHA (Spacecraft Charging at High Altitudes) satellite was operated from the Consolidated Space Test Center in Sunnyvale, California from February 1979 to May 1991. It was a spin stabilized vehicle in a highly eccentric orbit that collected data on spacecraft charging. The purpose of such data gathering was to predict and/or model the effects of the Earth's magnetic field on synchronous and near synchronous satellites. During the majority of its lifetime, attitude precession maneuvers were done every 10-15 days to maintain solar panel orientation. Maneuver planning was difficult due to the structural characteristics of SCATHA. It is cylindrically shaped and has seven booms ranging in length from 2 to 50 meters. These precession maneuvers induced predictable nutation that damped out after a few days. Eventually fuel began running low due to these frequent maneuvers. Experiments that had required the spin axis be in the orbit plane had already been turned off or had collected all their data. To increase the vehicle lifetime, the spin axis was moved to ecliptic normal. While this stopped the need for frequent attitude maneuvering (only two per year required now), this movement of the spin axis caused nutation that would not damp out for the remainder of the mission. This phase of the mission, with the ecliptic normal orientation, lasted for approximately three years. Although nutation never damped, data gathering was uninterrupted. In late 1990, when SCATHA's transmitter became seriously degraded, the Air Force decided to turn SCATHA off. This would only be done after the satellite was made 'safe'. The most difficult part of making the vehicle safe was quickly purging the fuel. Several plans were considered. The selected plan was to perform a series of 20 degree attitude precession maneuvers (3 days apart to allow for the worst nutation to damp) until the fuel was depleted. Although this sounded simple, the actual execution proved difficult. This was due to a nearly complete lack of available telemetry data, large undamped motion of the long booms, inadequacies in attitude determination software, and an error in the fuel level calculation software. This paper discusses the various proposed termination plans and execution of the selected one. Attitude determination methodologies, nutation from maneuvers, and effects of the flexible booms on the termination mission are presented and analyzed from a satellite analyst point of view.

Stakkestad, Kjell↗
NASA NTRS1992

Digital tracking loops for a programmable digital modem

In this paper, an analysis and hardware emulation of the tracking loops for a very flexible programmable digital modem (PDM) will be presented. The modem is capable of being programmed for 2, 4, 8, 16-PSK, 16-QAM, MSK, and Offset-QPSK modulation schemes over a range of data rates from 2.34 to 300 Mbps with programmable spectral occupancy from 1.2 to 1.8 times the symbol rate; these operational parameters are executable in burst or continuous mode. All of the critical processing in both the modulator and demodulator is done at baseband with very high-speed digital hardware and memory. Quadrature analog front-ends are used for translation between baseband and the IF center frequency. The modulator is based on a table lookup approach, where precomputed samples are stored in memory and clocked out according to the incoming data pattern. The sample values are predistorted to counteract the effects of the other filtering functions in the link as well as any transmission impairments. The demodulator architecture was adapted from a joint estimator-detector (JED) mathematical analysis. Its structure is applicable to most signalling formats that can be represented in a two-dimensional space. The JED realization uses interdependent, mutually aiding tracking loops with post-detection data feedback. To expedite and provide for more reliable synchronization, initial estimates for these loops are computed in a parallel acquisition processor. The cornerstone of the demodulator realization is the pre-averager received data filter which allows operation over a broad range of data rates without any hardware changes and greatly simplifies the implementation complexity. The emulation results confirmed tracking loop operation over the entire range of operational parameters listed above, as well as the capability of achieving and maintaining synchronization at BER's in excess of 10(exp -1). The emulation results also showed very close agreement with the tracking loop analysis, and validated the resolution apportionment of the various hardware elements in the tracking loops.

Poklemba, John J.↗
NASA NTRS1993

The role of global cloud climatologies in validating numerical models

The purpose of this work is to estimate sampling errors of area-time averaged rain rate due to temporal samplings by satellites. In particular, the sampling errors of the proposed low inclination orbit satellite of the Tropical Rainfall Measuring Mission (TRMM) (35 deg inclination and 350 km altitude), one of the sun synchronous polar orbiting satellites of NOAA series (98.89 deg inclination and 833 km altitude), and two simultaneous sun synchronous polar orbiting satellites--assumed to carry a perfect passive microwave sensor for direct rainfall measurements--will be estimated. This estimate is done by performing a study of the satellite orbits and the autocovariance function of the area-averaged rain rate time series. A model based on an exponential fit of the autocovariance function is used for actual calculations. Varying visiting intervals and total coverage of averaging area on each visit by the satellites are taken into account in the model. The data are generated by a General Circulation Model (GCM). The model has a diurnal cycle and parameterized convective processes. A special run of the GCM was made at NASA/GSFC in which the rainfall and precipitable water fields were retained globally for every hour of the run for the whole year.

HARSHVARDHAN↗
NASA NTRS1992

Formation of the stable auroral arc that intensifies at substorm onset

Observational evidence that the auroral arc that intensifies at substorm onset is formed on magnetic field lines that map to within about 1 to 2 Re of synchronous orbit, where plasma pressures are about 1 to 10 nPa during the substorm growth phase, is discussed. It is proposed that the arc is formed by a perpendicular magnetospheric-current divergence that results from a strong dawn-to-dusk directed pressure gradient in the vicinity of magnetic midnight. It is estimated that the current divergence is sufficiently strong that a greater than 1 kV field-aligned potential drop is required to maintain ionospheric-current continuity. It is suggested that the azimuthal pressure gradient results from proton drifts in the vicinity of synchronous orbit that are directed nearly parallel to the cross-tail electric field.

Lyons, L. R.↗
NASA NTRS1992

Thermoregulatory responses of rhesus monkeys during spaceflight

This study examines the activity, axillary temperature (T(ax)), and ankle skin temperature (Tsk) of two male Rhesus monkeys exposed to microgravity in space. The animals were flown on a Soviet biosatellite mission (COSMOS 1514). Measurements on the flight animals, as well as synchronous flight controls, were performed in the Soviet Union. Additional control studies were performed in the United States to examine the possible role of metabolic heat production in the T(ax) response observed during the spaceflight. All monkeys were exposed to a 24-h light-dark cycle (LD 16:8) throughout these studies. During weightlessness, T(ax) in both flight animals was lower than on earth. The largest difference (0.75 degree C) occurred during the night. There was a reduction in mean heart rate and Tsk during flight. This suggests a reduction in both heat loss and metabolic rate during spaceflight. Although the circadian rhythms in all variables were present during flight, some differences were noted. For example, the amplitude of the rhythms in Tsk and activity were attenuated. Furthermore, the T(ax) and activity rhythms did not have precise 24.0 hour periods and may have been externally desynchronized from the 24-h LD cycle. These data suggest a weakening of the coupling between the internal circadian pacemaker and the external LD synchronizer.

Sulzman, F. M.↗