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At least 973 records · Page 54

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NASA NTRS1991

Dynamic modelling and analysis of a magnetically suspended flexible rotor

A 12-state lumped-element model is presented for a flexible rotor supported by two attractive force electromagnetic journal bearings. The rotor is modeled as a rigid disk with radial mass unbalance mounted on a flexible, massless shaft with internal damping (Jeffcott rotor). The disk is offset axially from the midspan of the shaft. Bearing dynamics in each radial direction are modeled as a parallel combination of a negative (unstable) spring and a linear current-to-force actuator. The model includes translation and rotation of the rigid mass and the first and second bending models of the flexible shaft, and it simultaneously includes internal shaft damping, gyroscopic effects, and the unstable nature of the attractive force magnetic bearings. The model is used to analyze the dependence of the system transmission zeros and open-loop poles on system parameters. The dominant open-loop poles occur in stable/unstable pairs with bandwidth dependent on the ratios of bearing (unstable) stiffnesses to rotor mass and damping dependent on the shaft spin rate. The zeros occur in complex conjugate pairs with bandwidth dependent on the ratios of shaft stiffness to rotor mass and damping dependent on the shaft spin rate. Some of the transmission zeros are non-minimum phase when the spin rate exceeds the shaft critical speed. The transmission zeros and open-loop poles impact the design of magnetic bearing control systems. The minimum loop cross-over frequency of the closed-loop system is the speed of the unstable open-loop poles. For the supercritical shaft spin rates, the presence of non-minimum phase zeros limits the distribution rejection achievable at frequencies near or above the shaft critical speed. Since non-minimum phase transmission zeros can only be changed by changing the system inputs and/or outputs, closed-loop performance is limited for supercritical spin rates unless additional force or torque actuators are added.

Mccallum, Duncan C.↗

NASA NTRS1991

Digital CODEC for real-time processing of broadcast quality video signals at 1.8 bits/pixel

Advances in very large scale integration and recent work in the field of bandwidth efficient digital modulation techniques have combined to make digital video processing technically feasible an potentially cost competitive for broadcast quality television transmission. A hardware implementation was developed for DPCM (differential pulse code midulation)-based digital television bandwidth compression algorithm which processes standard NTSC composite color television signals and produces broadcast quality video in real time at an average of 1.8 bits/pixel. The data compression algorithm and the hardware implementation of the codec are described, and performance results are provided.

Shalkhauser, Mary JO↗

NASA NTRS1991

Symbol lock detection in the ARX 2 and Block 5 receivers

The performances of 5 symbol-lock detectors are compared. These detectors are the square-law detector with overlapping (SQOD) and nonoverlapping integrators, the absolute-value detectors with overlapping and nonoverlapping integrators, and the signal-power estimator detector (SPED). The analysis considers various scenarios in which the observation interval is much larger than or equal to the symbol-synchronizer loop bandwidth, and which were not considered in previous analyses. Also, the case of threshold setting in the absence of signal is considered. It is shown, that the SQOD outperforms all others when the threshold is set in the presence of a signal, independent of the relationship between loop bandwidth and observation period. On the other hand, the SPED outperforms all others when the threshold is set in the presence of noise only.

Shihabi, M.↗

NASA NTRS1991

Correlation between some current parameters and optical radiation generated by 280 mm long laboratory sparks

Discussed here is the optical radiation generated by 280 mm long laboratory discharges and its relationship to the current flowing in the discharge channel. In an experiment, the optical radiation generated by the discharges was measured at wavelengths 777nm (bandwidth 10 nm) due to O I(1), and 500 nm (bandwidth 5 nm) due to N II(19), and the broadband optical radiation between the wavelengths 400 to 1100 nm. The shape of the current waveform, which had a rise time of 0.1 microsecond and a decay time of 5 microsecond, remained the same with increasing peak value. The experiment was conducted with peak current amplitudes in the range of 1-4 kA. In order to test the effect of current rise time on the optical radiation, researchers also conducted studies with a current waveform having a 5 microsecond rise time. It was observed that the peak amplitude of the optical radiation pulse at the wavelengths mentioned above is proportional to the peak amplitude of the current flowing through the discharge channel. The rise time of the optical radiation pulse at a given wavelength does not depend significantly on the peak amplitude of the current waveform. The rise time of the optical radiation pulse decreases with decreasing wavelength. A slight increase in the decay time of the optical pulse, at a given wavelength, is observed with increasing peak amplitude of the current waveform.The results show that the rise time of the optical radiation pulse increases with the increasing rise time of the current waveform. The relationship between peak current, peak optical power, and peak electrical power is studied. Also, the relationship between peak current, optical energy (400-1100 and 777 nm), and electrical energy is discussed.

Windmar, Dan↗

NASA NTRS1991

Beta measurements

The second year's results of the BETA project research are presented. The program is divided into two areas, aerosol modification and climatology in the trade wind region and the climatology of BETA (CO2) on remote mountain top locations. Limited data is available on the aerosol climatology of the marine free troposphere (MFT) in the trade wind region. In order to study the effects of cumulus convection on the MFT values of BETA, a cloud model was developed to simulate the evolution of a typical Pacific trade wind cumulus cloud. The stages involved in this development are outlined. The assembly of the major optical components of the lidar was made. Tests were run of the spectral bandwidth of the Synrad laser when a portion of the beam is mixed with a component which has traveled 450 meters corresponding to a delay of 1.5 microsecs. The bandwidth of the beat signal was measured to be 3 KHz. The data processing system based on a parallel processing filter bank analyzer using true time squaring detectors at each filter was completed.

Schotland, R. M.↗

NASA NTRS1991

Lightning testing at the subsystem level

Testing at the subsystem or black box level for lightning hardness is required if system hardness is to be assured at the system level. The often applied philosophy of lighting testing only at the system level leads to extensive end of the line design changes which result in excessive costs and time delays. In order to perform testing at the subsystem level two important factors must be defined to make the testing simulation meaningful. The first factor is the definition of the test stimulus appropriate to the subsystem level. Application of system level stimulations to the subsystem level usually leads to significant overdesign of the subsystem which is not necessary and may impair normal subsystem performance. The second factor is the availability of test equipment needed to provide the subsystem level lightning stimulation. Equipment for testing at this level should be portable or at least movable to enable efficient testing in a design laboratory environment. Large fixed test installations for system level tests are not readily available for use by the design engineers at the subsystem level and usually require special operating skills. The two factors, stimulation level and test equipment availability, must be evaluated together in order to produce a practical, workable test standard. The neglect or subordination of either factor will guarantee failure in generating the standard. It is not unusual to hear that test standards or specifications are waived because a specified stimulation level cannot be accomplished by in-house or independent test facilities. Determination of subsystem lightning simulation level requires a knowledge and evaluation of field coupling modes, peak and median levels of voltages and currents, bandwidths, and repetition rates. Practical limitations on test systems may require tradeoffs in lightning stimulation parameters in order to build practical test equipment. Peak power levels that can be generated at specified bandwidths with standard electrical components must be considered in the design and costing of the test system. Stimulation tests equipment and test methods are closely related and must be considered a test system for lightning simulation. A non-perfect specification that can be reliably and repeatedly applied at the subsystem test level is more desirable than a perfect specification that cannot be applied at all.

Luteran, Frank↗

NASA NTRS1989

Optical distributed sensing and computation for a proposed flexible beam experiment

A new technology is being developed that supports distributed sensing and computation wherein crystal mediums are used with real-time holography to gather images that contain vibration and instantaneous geometry data for a spacecraft. Optical computation techniques are employed to process the image data and to compute commands to actuators located on the spacecraft. This paper presents an integrated sensor/processor design for an experiment to demonstrate this technology. A performance estimate for the design is presented in terms of the optical spatial bandwidth of the imaging system, as opposed to the temporal bandwidth. The advantages of an integrated optical sensor/processor relative to control of large spacecraft are addressed.

Welch, S. S.↗

NASA NTRS1991

Precision pointing control for an orbital earth observing system

The design concept developed for the pointing system of the high-resolution imaging spectrometer (HIRIS) which will be flown on one of NASA's earth observing system platforms is presented. The instrument is an F/5.4-aperture spectrometer with a focal length of 1222 mm, and it uses a precision two-axis gimballed mirror pointing system to image and track targets. Pointing accuracy of better than 585 arcsec (peak-to-peak), and pointing jitter of less than 2.65 arcsec in 33 ms are ensured through the use of gimbal position and basebody rate sensors. A state-space controller implemented with a digital computer is used to provide a position loop bandwidth of 1 Hz and a rate loop bandwidth of 7 Hz. An overview of the system design and flight hardware is given, the development of the controller architecture is addressed, and a simulation assessment of the pointing system that takes into consideration issues such as nonlinear effects, sensor noise, and noncollocated sensors and actuators in a flexible structure is discussed.

Robeck, Linda S.↗

NASA NTRS1990

Future X-ray observations of 'normal' galaxies

The study of the X-ray properties of galaxies is discussed in terms of existing observational results and the capacity for interpreting the data. The NASA Einstein Observatory data is first examined to describe the X-ray properties of spiral and irregular galaxies as well as elliptical and SO galaxies. Issues that remain to be addressed can be explored by studying the X-ray properties of very luminous 'super-Eddington' sources, starburst galaxies, and different phases of the interstellar medium. The specific requirements are set forth for the observatories conducting the research; particular conditions are given for detecting galaxies, high-resolution studies of individual galaxies, bandwidth, spectral studies, and time-variability studies. An ideal X-ray telescope is described based on these conditions, the primary features of which are a collecting area greater than 100 sq m, a large bandwidth, and the capacity for both photometric and spectroscopic investigation.

Fabbiano, Giuseppina↗

NASA NTRS1991

Error control techniques for satellite and space communications

Research activities related to error control techniques for satellite and space communication are reported. Specific areas of research include: coding gains for bandwidth efficient codes, hardware implementation of a bandwidth efficient coding scheme for the Hubble Space Telescope, construction of long trellis codes for use with sequential decoding, performance analysis of multilevel trellis codes, and M-algorithm decoding of trellis codes. Each topic is discussed in a corresponding paper that appears in the appendices.

Costello, Daniel J., Jr.↗

NASA NTRS1991

Baseband pulse shaping techniques for nonlinearly amplified pi/4-QPSK and QAM systems

A new generation of multi-stage pi/4-shifted QPSK and of superposed quadrature-amplitude-modulated (SQAM) modulators-coherent demodulators (modems) and of continuous phase modulated (CPM)-gaussian premodulation filtered minimum-shift-keying (MGMSK) systems is proposed and studied. These modems will lead to bandwidth and power efficient satellite communications systems designs. As an illustrative application, a baseband processing technique pi/4-controlled transition PSK (pi/4-CTPSK) is described. To develop a cost and power efficient design strategy, we assume that nonlinear, fully saturated high power amplifiers (HPA) are utilized in the satellite earth station transmitter and in the satellite transponder. Modem structures which could lead to application specific integrated circuit (ASIC) satellite on-board processing universal modem applications are also considered. Multistate GMSK (i.e., MGMSK) signal generation methods by means of two or more RF combined nonlinearly amplified SQAM modems and by one multistate (in-phase and quadrature-baseband premodulation filtered-superposed) SQAM architecture and one RF nonlinear amplifier are studied. During the SQAM modem development phase we investigate the potential system advantages of the pi/4-shifted logic. The bandwidth efficiency of the proposed multistate GMSK and baseband filtered PAM-FM modulator (a new class in the CPM family) will be significantly higher than that of conventional G-MSK systems. To optimize the practical P(sub e) = f((E sub b)/(N sub o)) performance we consider improved coherent demodulation MGMSK structures such as deviated-frequency locking coherent demodulators. For relative low bit rate SATCOM applications, e.g., bit rates less than 300 kb/s, phase noise tracking cancellation (for fixed site earth station) and phase noise cancellation as well as Doppler compensation (for satellite to mobile earth station) applications may be required. We study digital channel sounding methods which could cancel the phase noise-caused degradations of CPM and GMSK modems.

Feher, Kamilo↗

NASA NTRS1992

An investigation of networking techniques for the ASRM facility

This report is based on the early design concepts for a communications network for the Advanced Solid Rocket Motor (ASRM) facility being built at Yellow Creek near Iuka, MS. The investigators have participated in the early design concepts and in the evaluation of the initial concepts. The continuing system design effort and any modification of the plan will require a careful evaluation of the required bandwidth of the network, the capabilities of the protocol, and the requirements of the controllers and computers on the network. The overall network, which is heterogeneous in protocol and bandwidth, is being modeled, analyzed, simulated, and tested to obtain some degree of confidence in its performance capabilities and in its performance under nominal and heavy loads. The results of the proposed work should have an impact on the design and operation of the ASRM facility.

Moorhead, Robert J., II↗

NASA NTRS1992

Real-time demonstration hardware for enhanced DPCM video compression algorithm

The lack of available wideband digital links as well as the complexity of implementation of bandwidth efficient digital video CODECs (encoder/decoder) has worked to keep the cost of digital television transmission too high to compete with analog methods. Terrestrial and satellite video service providers, however, are now recognizing the potential gains that digital video compression offers and are proposing to incorporate compression systems to increase the number of available program channels. NASA is similarly recognizing the benefits of and trend toward digital video compression techniques for transmission of high quality video from space and therefore, has developed a digital television bandwidth compression algorithm to process standard National Television Systems Committee (NTSC) composite color television signals. The algorithm is based on differential pulse code modulation (DPCM), but additionally utilizes a non-adaptive predictor, non-uniform quantizer and multilevel Huffman coder to reduce the data rate substantially below that achievable with straight DPCM. The non-adaptive predictor and multilevel Huffman coder combine to set this technique apart from other DPCM encoding algorithms. All processing is done on a intra-field basis to prevent motion degradation and minimize hardware complexity. Computer simulations have shown the algorithm will produce broadcast quality reconstructed video at an average transmission rate of 1.8 bits/pixel. Hardware implementation of the DPCM circuit, non-adaptive predictor and non-uniform quantizer has been completed, providing realtime demonstration of the image quality at full video rates. Video sampling/reconstruction circuits have also been constructed to accomplish the analog video processing necessary for the real-time demonstration. Performance results for the completed hardware compare favorably with simulation results. Hardware implementation of the multilevel Huffman encoder/decoder is currently under development along with implementation of a buffer control algorithm to accommodate the variable data rate output of the multilevel Huffman encoder. A video CODEC of this type could be used to compress NTSC color television signals where high quality reconstruction is desirable (e.g., Space Station video transmission, transmission direct-to-the-home via direct broadcast satellite systems or cable television distribution to system headends and direct-to-the-home).

Bizon, Thomas P.↗

NASA NTRS1991

Finite difference time domain implementation of surface impedance boundary conditions

Surface impedance boundary conditions are employed to reduce the solution volume during the analysis of scattering from lossy dielectric objects. In a finite difference solution, they also can be utilized to avoid using small cells, made necessary by shorter wavelengths in conducting media throughout the solution volume. The standard approach is to approximate the surface impedance over a very small bandwidth by its value at the center frequency, and then use that result in the boundary condition. Two implementations of the surface impedance boundary condition are presented. One implementation is a constant surface impedance boundary condition and the other is a dispersive surface impedance boundary condition that is applicable over a very large frequency bandwidth and over a large range of conductivities. Frequency domain results are presented in one dimension for two conductivity values and are compared with exact results. Scattering width results from an infinite square cylinder are presented as a 2-D demonstration. Extensions to 3-D should be straightforward.

Beggs, John H.↗

NASA NTRS1991

Finite difference time domain implementation of surface impedance boundary conditions

Surface impedance boundary conditions are employed to reduce the solution volume during the analysis of scattering from lossy dielectric objects. In the finite difference solution, they also can be utilized to avoid using small cells, made necessary by shorter wavelengths in conducting media throughout the solution volume. The standard approach is to approximate the surface impedance over a very small bandwidth by its value at the center frequency, and then use that result in the boundary condition. Here, two implementations of the surface impedance boundary condition are presented. One implementation is a constant surface impedance boundary condition and the other is a dispersive surface impedance boundary condition that is applicable over a very large frequency bandwidth and over a large range of conductivities. Frequency domain results are presented in one dimension for two conductivity values and are compared with exact results. Scattering width results from an infinite square cylinder are presented as a two dimensional demonstration. Extensions to three dimensions should be straightforward.

Beggs, John H.↗

NASA NTRS1992

Rate 3/4 coded 16-QAM for uplink applications

First phase development of an advanced modulation technology which synergistically combines coding and modulation to achieve 2 bits per second per Hertz bandwidth efficiency in satellite demodulators is nearing completion. A proof-of-concept model is being developed to demonstrate technology feasibility, establish practical bandwidth efficiency limitations, and provide a data base for the design and development of engineering model satellite demodulators. The basic considerations leading to the choice of 4 x 4 quadrature amplitude modulation (16-QAM) and its associated coding format are discussed, along with the basic implementation of the carrier and clock recovery, automatic gain control, and decoding process. Preliminary performance results are presented. Spectra for the modulated signal shows the effects of the square root Nyquist filters in the modulation. Bit error rate (BER) results for the encoder/decoder subsystem show near ideal results, although power consumption is high and baseband BER performance of the Nyquist filter set is poor. Recommendations regarding the present system to improve BER performance and acquisition speed are given.

Mrozek, Eric M.↗

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 NTRS1991

An adjustable RF tuning element for microwave, millimeter wave, and submillimeter wave integrated circuits

Planar RF circuits are used in a wide range of applications from 1 GHz to 300 GHz, including radar, communications, commercial RF test instruments, and remote sensing radiometers. These circuits, however, provide only fixed tuning elements. This lack of adjustability puts severe demands on circuit design procedures and materials parameters. We have developed a novel tuning element which can be incorporated into the design of a planar circuit in order to allow active, post-fabrication tuning by varying the electrical length of a coplanar strip transmission line. It consists of a series of thin plates which can slide in unison along the transmission line, and the size and spacing of the plates are designed to provide a large reflection of RF power over a useful frequency bandwidth. Tests of this structure at 1 GHz to 3 Ghz showed that it produced a reflection coefficient greater than 0.90 over a 20 percent bandwidth. A 2 GHz circuit incorporating this tuning element was also tested to demonstrate practical tuning ranges. This structure can be fabricated for frequencies as high as 1000 GHz using existing micromachining techniques. Many commercial applications can benefit from this micromechanical RF tuning element, as it will aid in extending microwave integrated circuit technology into the high millimeter wave and submillimeter wave bands by easing constraints on circuit technology.

Lubecke, Victor M.↗