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At least 829 records · Page 46
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NASA NTRS1992

The 10 kW power electronics for hydrogen arcjets

A combination of emerging mission considerations such as 'launch on schedule', resource limitations, and the development of higher power spacecraft busses has resulted in renewed interest in high power hydrogen arcjet systems with specific impulses greater than 1000 s for Earth-space orbit transfer and maneuver applications. Solar electric propulsion systems with about 10 kW of power appear to offer payload benefits at acceptable trip times. This work outlines the design and development of 10 kW hydrogen arcjet power electronics and results of arcjet integration testing. The power electronics incorporated a full bridge switching topology similar to that employed in state of the art 5 kW power electronics, and the output filter included an output current averaging inductor with an integral pulse generation winding for arcjet ignition. Phase shifted, pulse width modulation with current mode control was used to regulate the current delivered to arcjet, and a low inductance power stage minimized switching transients. Hybrid power Metal Oxide Semiconductor Field Effect Transistors were used to minimize conduction losses. Switching losses were minimized using a fast response, optically isolated, totem-pole gate drive circuit. The input bus voltage for the unit was 150 V, with a maximum output voltage of 225 V. The switching frequency of 20 kHz was a compromise between mass savings and higher efficiency. Power conversion efficiencies in excess of 0.94 were demonstrated, along with steady state load current regulation of 1 percent. The power electronics were successfully integrated with a 10 kW laboratory hydrogen arcjet, and reliable, nondestructive starts and transitions to steady state operation were demonstrated. The estimated specific mass for a flight packaged unit was 2 kg/kW.

Hamley, John A.↗
NASA NTRS1992

PARAS program: Phased array radio astronomy from space

An orbiting radio telescope is proposed which, when operated in a Very Long Baseline Interferometry (VLBLI) scheme, would allow higher (than currently available) angular resolution and dynamic range in the maps, and the ability of observing rapidly changing astronomical sources. Using a passive phases array technology, the proposed design consists of 656 hexagonal modules forming a 150 meter diameter dish. Each observatory module is largely autonomous, having its own photovoltaic power supply and low-noise receiver and processor for phase shifting. The signals received by the modules are channeled via fiber optics to the central control computer in the central bus module. After processing and multiplexing, the data is transmitted to telemetry stations on the ground. The truss frame supporting each observatory pane is a hybrid structure consisting of a bottom graphite/epoxy tubular triangle and rigidized inflatable Kevlar tubes connecting the top observatory panel and bottom triangle. Attitude control and stationkeeping functions are performed by a system of momentum wheels in the bus and four propulsion modules located at the compass points on the periphery of the observatory dish. Each propulsion module has four monopropellant thrusters and six hydrazine arcjets, the latter supported by a nuclear reactor. The total mass of the spacecraft is 22,060 kg.

Jakubowski, Antoni K.↗
NASA NTRS1993

Optical communications and a comparison of optical technologies for a high data rate return link from Mars

The important principles of direct- and heterodyne-detection optical free-space communications are reviewed. Signal-to-noise-ratio (SNR) and bit-error-rate (BER) expressions are derived for both the direct-detection and heterodyne-detection optical receivers. For the heterodyne system, performance degradation resulting from received-signal and local oscillator-beam misalignment and laser phase noise is analyzed. Determination of interfering background power from local and extended background sources is discussed. The BER performance of direct- and heterodyne-detection optical links in the presence of Rayleigh-distributed random pointing and tracking errors is described. Finally, several optical systems employing Nd:YAG, GaAs, and CO2 laser sources are evaluated and compared to assess their feasibility in providing high-data-rate (10- to 1000-Mbps) Mars-to-Earth communications. It is shown that the root mean square (rms) pointing and tracking accuracy is a critical factor in defining the system transmitting laser-power requirements and telescope size and that, for a given rms error, there is an optimum telescope aperture size that minimizes the required power. The results of the analysis conducted indicate that, barring the achievement of extremely small rms pointing and tracking errors (less than 0.2 microrad), the two most promising types of optical systems are those that use an Nd:YAG laser (lambda = 1.064 microns) and high-order pulse position modulator (PPM) and direct detection, and those that use a CO2 laser (lambda = 10.6 microns) and phase shifting keying homodyne modulation and coherent detection. For example, for a PPM order of M = 64 and an rms pointing accuracy of 0.4 microrad, an Nd:YAG system can be used to implement a 100-Mbps Mars link with a 40-cm transmitting telescope, a 20-W laser, and a 10-m receiving photon bucket. Under the same conditions, a CO2 system would require 3-m transmitting and receiving telescopes and a 32-W laser to implement such a link. Other types of optical systems, such as a semiconductor laser systems, are impractical in the presence of large rms pointing errors because of the high power requirements of the 100-Mbps Mars link, even when optimal-size telescopes are used.

Spence, Rodney L.↗
NASA NTRS1992

Multiple symbol partially coherent detection of MPSK

It is shown that by using the known (or estimated) value of carrier tracking loop signal to noise ratio (SNR) in the decision metric, it is possible to improve the error probability performance of a partially coherent multiple phase-shift-keying (MPSK) system relative to that corresponding to the commonly used ideal coherent decision rule. Using a maximum-likeihood approach, an optimum decision metric is derived and shown to take the form of a weighted sum of the ideal coherent decision metric (i.e., correlation) and the noncoherent decision metric which is optimum for differential detection of MPSK. The performance of a receiver based on this optimum decision rule is derived and shown to provide continued improvement with increasing length of observation interval (data symbol sequence length). Unfortunately, increasing the observation length does not eliminate the error floor associated with the finite loop SNR. Nevertheless, in the limit of infinite observation length, the average error probability performance approaches the algebraic sum of the error floor and the performance of ideal coherent detection, i.e., at any error probability above the error floor, there is no degradation due to the partial coherence. It is shown that this limiting behavior is virtually achievable with practical size observation lengths. Furthermore, the performance is quite insensitive to mismatch between the estimate of loop SNR (e.g., obtained from measurement) fed to the decision metric and its true value. These results may be of use in low-cost Earth-orbiting or deep-space missions employing coded modulations.

Simon, M. K.↗
NASA NTRS1993

Low-frequency source parameters of twelve large earthquakes

A global survey of the low-frequency (1-21 mHz) source characteristics of large events are studied. We are particularly interested in events unusually enriched in low-frequency and in events with a short-term precursor. We model the source time function of 12 large earthquakes using teleseismic data at low frequency. For each event we retrieve the source amplitude spectrum in the frequency range between 1 and 21 mHz with the Silver and Jordan method and the phase-shift spectrum in the frequency range between 1 and 11 mHz with the Riedesel and Jordan method. We then model the source time function by fitting the two spectra. Two of these events, the 1980 Irpinia, Italy, and the 1983 Akita-Oki, Japan, are shallow-depth complex events that took place on multiple faults. In both cases the source time function has a length of about 100 seconds. By comparison Westaway and Jackson find 45 seconds for the Irpinia event and Houston and Kanamori about 50 seconds for the Akita-Oki earthquake. The three deep events and four of the seven intermediate-depth events are fast rupturing earthquakes. A single pulse is sufficient to model the source spectra in the frequency range of our interest. Two other intermediate-depth events have slower rupturing processes, characterized by a continuous energy release lasting for about 40 seconds. The last event is the intermediate-depth 1983 Peru-Ecuador earthquake. It was first recognized as a precursive event by Jordan. We model it with a smooth rupturing process starting about 2 minutes before the high frequency origin time superimposed to an impulsive source.

Harabaglia, Paolo↗
NASA NTRS1993

Planar microstrip YAGI antenna array

A directional microstrip antenna includes a driven patch surrounded by an isolated reflector and one or more coplanar directors, all separated from a ground plane on the order of 0.1 wavelength or less to provide end fire beam directivity without requiring power dividers or phase shifters. The antenna may be driven at a feed point a distance from the center of the driven patch in accordance with conventional microstrip antenna design practices for H-plane coupled or horizontally polarized signals. The feed point for E-plane coupled or vertically polarized signals is at a greater distance from the center than the first distance. This feed point is also used for one of the feed signals for circularly polarized signals. The phase shift between signals applied to feed points for circularly polarized signals must be greater than the conventionally required 90 degrees and depends upon the antenna configuration.

Huang, John↗
NASA NTRS1992

Acquisition performance of various QPSK carrier tracking loops

The frequency and phase acquisition performance of three quadrature phase shift keying (QPSK) carrier tracking loops, the MAP estimation loop, the Costas crossover loop, and the generalized Costas loop, is described. Acquisition time and probability of acquisition as a function of both loop signal-to-noise ratio and frequency offset to loop bandwidth ratio are obtained via computer simulations for type II and III loops. It is shown that the MAP loop, which results in the smallest squaring loss for all signal-to-noise ratios, is sometimes outperformed by the other two loops in terms of acquisition time and acquisition probability.

Hinedi, S.↗
NASA NTRS1992

Density waves in Saturn's rings probed by radio and optical occultation - Observational tests of theory

A parallel examination is conducted of Voyager radio and photopolarimeter occultation observations of the Saturn A ring's density waves. The radio instrument waves exhibit an average -90 deg offset from the dynamical phase. A warping height of about 100-m amplitude can qualtitatively reproduce this phase shift, while preserving the overall model wave shape. These results may be profoundly relevant for satellite-ring torque calculations in Saturn's rings, given the deposition of all of the net torque of the standard model in the first wavelength.

Brophy, Thomas G.↗
NASA NTRS1991

An electro-optic resonant modulator for coherent optical communication

A resonant cavity electro-optic phase modulator has been designed and implemented to operate at a data rate of 10 Mbps. The modulator consists of an electro-optic crystal located in a highly resonant cavity. The cavity is electro-optically switched on and off resonance, and the phase dispersion near the cavity resonance provides the output phase modulation. The performance of the modulator was measured by first heterodyne detecting the signal to an intermediate frequency, and measuring the spectral characteristics using an RF spectrum analyzer. The measured phase shift is shown to be in good agreement with the theoretical predictions. Further theoretical analysis shows that the design of the modulator can be scaled to operate at 100 Mbps.

Robinson, D. L.↗
NASA NTRS1991

Sleep and circadian rhythms

Three interacting processes are involved in the preservation of circadian rhythms: (1) endogenous rhythm generation mechanisms, (2) entrainment mechanisms to keep these rhythms 'on track', and (3) exogenous masking processes stemming from changes in environment and bahavior. These processes, particularly the latter two, can be dramatically affected in individuals of advanced age and in space travelers, with a consequent disruption in sleep and daytime functioning. This paper presents results of a phase-shift experiment investigating the age-related effects of the exogeneous component of circadian rhythms in various physiological and psychological functions by comparing these functions in middle aged and old subjects. Dramatic differences were found between the two age groups in measures of sleep, mood, activation, and performance efficiency.

Monk, Timothy H.↗
NASA NTRS1992

Strain sensing technology for high temperature applications

This review paper discusses the status of strain sensing technology for high temperature applications. Technologies covered are those supported by NASA such as required for applications in hypersonic vehicles and engines, advanced subsonic engines, as well as material and structure development. The applications may be at temperatures of 540 C (1000 F) to temperatures in excess of 1400 C (2500 F). The most promising technologies at present are the resistance strain gage and remote sensing schemes. Resistance strain gages discussed include the BCL gage, the LaRC compensated gage, and the PdCr gage. Remote sensing schemes such as laser based speckle strain measurement, phase-shifting interferometry and X-ray extensometry will be discussed. Present status and limitations of these technologies are presented.

Williams, W. D.↗
NASA NTRS1991

Perceiving the coherent movements of spatially-separated features

When a partially-occluded object is represented in an image, it is defined by a set of spatially-separated features that may be registered at different spatial scales. To understand the image, human vision must organize these fragmented optical features into common and distinct object surfaces. Although the common fate of moving features often is considered a primary source of reliable information for image segmentation, little is known of the visual system's capacity to discriminate the coherent relative movements of spatially-separated features. In a series of experiments, observers viewed elements whose movements were correlated (direction and magnitude) or were uncorrelated. Our results indicate that observers can discriminate the two types of movement about as well as they can detect any movement at all. Moreover, the ability to perceive coherent movements is maintained under a variety of conditions including differences in the elements' spatial frequency content, spatial position and contrast, and temporal phase shifts between the spatially-correlated displacements. These results suggest that coherent relative motion may be a fundamental source of information exploited by vision, despite considerable variability in the spatial and temporal characteristics of the individual features.

Mowafy, Lyn↗
NASA NTRS1991

Phase shifter technology assessment - Prospects and applications

Capabilities and limitations of MMIC phase shifter technology at microwave and millimeter wave frequencies are reviewed. MMIC-based phase arrays make it possible to integrate active elements at the array face, i.e., to incorporate transmit power amplifiers and/or low noise amplifiers at each antenna element. Active elements make it possible to increase power efficiency and reliability and provide graceful degradation. Monolithic integration of the various transmit/receive functions including phase shifting is considered to be feasible through at least the lower millimeter-wave frequency range (about 30-100 GHz). MMIC integration also allows more flexibility in array design including those that are intended for airborne conformal applications.

Sokolov, Vladimir↗
NASA NTRS1993

Collision cross sections and diffusion parameters for H and D in atomic oxygen

Modeling the behavior of H and D in planetary exospheres requires detailed knowledge of the differential scattering cross sections for all of the important neutral-neutral and ion-neutral collision processes affecting these species over their entire ranges of interaction energies. In the upper atmospheres of Earth, Venus, and other planets as well, the interactions of H and D with atomic oxygen determine the rates of diffusion of escaping hydrogen isotopes through the thermosphere, the velocity distributions of exospheric atoms that encounter the upper thermosphere, the lifetimes of exospheric orbiters with periapsides near the exobase, and the transfer of momentum in collisions with hot O. The nature of H-O and D-O collisions and the derivation of a data base consisting of phase shifts and the differential, total, and momentum transfer cross sections for these interactions in the energy range 0.001 - 10 eV are discussed. Coefficients of mutual diffusion and thermal diffusion factors are calculated for temperatures of planetary interest.

Hodges, R. R., Jr.↗
NASA NTRS1993

The discovery of pulsed iron line emission from Centaurus X-3

We present the first discovery of pulsed iron line emission from an X-ray binary, namely Cen X-3. Compared with the continuum pulsations, the iron line pulsations are shallow (50 percent change in amplitude), smeared (the profile is a single-peaked sinusoid) and phase-shifted (by about half a cycle). We also discuss the constraints on the origin of the line imposed by this discovery and by other observations.

Day, C. S. R.↗
NASA NTRS1993

10-Mbps electro-optic resonant phase modulator

A resonant cavity electro-optic phase modulator has been designed and implemented to operate at a data rate of 10 Mbps. The modulator consists of an electro-optic crystal located in a highly resonant cavity. The cavity is electro-optically switched on and off resonance, and the phase dispersion near the cavity resonance provides the output phase modulation. The performance of the modulator was measured by first heterodyne-detecting the signal to an intermediate frequency and then measuring the spectral characteristics using an rf spectrum analyzer. The measured phase shift is shown to be in good agreement with the theoretical predictions. Further theoretical analysis shows that the design of the modulator can be scaled to operate at 100 Mbps.

Robinson, D. L.↗
NASA NTRS1993

Digital data over video channels - Techniques for Spacelab

A study was made of various techniques for transmitting digital data over a composite video channel. Transmitting data from experiments on the Space Shuttle Orbiter over a video channel is attractive since extra channel capacity is needed. Broadcast television telecommunications schemes and other techniques described in the literature were reviewed and compared. PSK (Phase Shift Keying), MPSK (m-ary PSK), or PAM (Pulse Amplitude Modulation) can be transmitted on visible lines of a frame, unassigned lines of the VBI (Vertical Blanking Interval), or during the HBI (Horizontal Blanking Interval). All three modulation techniques can be attractive under the proper conditions. However, PAM on visible lines or during the VBI should be relatively easy to implement, provide adequate average data rates, and give acceptable BERs (Bit Error Rates).

Smith, Dean L.↗
NASA NTRS1991

Optical figure testing of prototype mirrors for JPL's Precision Segmented Reflector (PSR) program

JPL's PSR program is developing enabling technologies for large space telescopes employing segmented optics, and in particular, lightweight, thermally stable mirrors for telescopes operating at sub-mm wavelengths. JPL's vacuum cryointerferometric optical test facility includes a computer-controlled phase shifting interferometer operating at 10.59 microns and laser metrology for measurement of piston and absolute radius of curvature. The optical metrology has been integrated with a large vacuum chamber including a liquid nitrogen shroud capable of radiatively cooling 1.5 meter aperture mirrors down to 150 K. The facility is now being used to characterize prototype mirrors in both their room-temperature 'as-replicated' condition, and under orbital temperature conditions. The performance and limitations of the optical metrology hardware and software components are noted. Representative test results on prototype one meter-class composite mirrors being developed for PSR and related ground-based programs are also presented.

Hochberg, Eric B.↗