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At least 721 records · Page 40

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NASA NTRS1992

Pilot-in-the-loop analysis of propulsive-only flight control systems

Longitudinal control system architectures which directly couple flight stick motions to throttle commands for a multi-engine aircraft are presented. This coupling enables positive attitude control with complete failure of the flight control system. The architectures chosen vary from simple feedback gains to classical lead-lag compensators with and without prefilters. Each architecture is reviewed for its appropriateness for piloted flight. The control systems are then analyzed with pilot-in-the-loop metrics related to bandwidth required for landing. Results indicate that current and proposed bandwidth requirements should be modified for throttles-only flight control. Pilot ratings consistently showed better ratings than predicted by analysis. Recommendations are made for more robust design and implementation. The use of quantitative feedback theory for compensator design is discussed. Although simple and effective augmented control can be achieved in a wide variety of failed configurations, a few configuration characteristics are dominant for pilot-in-the-loop control.

Chou, Hwei-Lan↗

NASA NTRS1991

Control of a small working robot on a large flexible manipulator for suppressing vibrations

The short term objective of this research is the completion of experimental configuration of the Small Articulated Robot (SAM) and the derivations of the actuator dynamics of the Robotic Arm, Large and Flexible (RALF). In order to control vibrations SAM should have larger bandwidth than that of the vibrations. The bandwidth of SAM consist of 3 parts; structural rigidity, processing speed of controller, and motor speed. The structural rigidity was increased to a reasonably high value by attaching aluminum angles at weak points and replacing thin side plates by thicker ones. The high processing speed of the controller was achieved by using parallel processors (three 68000 process, three interface board, and one main processor (IBM-XT)). Maximum joint speed and acceleration of SAM is known as about 4 rad/s and 15 rad/sq s. Hence SAM can move only .04 rad at 3 Hz which is the natural frequency of RALF. This will be checked by experiment.

Lee, Soo Han↗

NASA NTRS1994

Dual Overlapping, Weighted Preaveragers

Improved technique incorporated into digital processing of received radio signals that bear digital data reduces bandwidth of incoming noise when receiver operated over broad range of potential signal bandwidths. Technique in question is updated version of older preaveraging technique; two modifications decrease vulnerability to interference from adjacent channels.

Poklemba, John J.↗

NASA NTRS1993

PTTI-aided ephemeris calculation and rapid data link acquisition for manned space flight

Complexity of future manned space flight mission control can be significantly reduced by integrating GPS, the PTTI source, into telemetry, tracking, and command (TT&C). Future telecommunications, space tracking electronic intelligence, metrology, navigation, and data acquisition will thereby be served, including: on-board ephemeris determination, reduced synchronization time for time division multiple access (TDMA) links, and in-flight clock calibration, increasing on-board autonomy and reducing ground support costs. Manned space transportation through the first quarter of the 21st century will probably depend on a mix of vehicles, including the Advanced Manned Launch System (AMLS), the Personnel Launch System (PLS), and continued use of the Shuttle Fleet. Precise Ephemeris is important on-board for mission success, status monitoring, also for rendezvous and docking. Use of GPS can eliminate ground based tracking/processing, enhancing autonomy and reducing communications bandwidth. GPS time can simplify complicated functions used in bandwidth efficient time division multiple access (TDMA) communications, such as: precise and realtime synchronization of receive reference timing, transmit-timing and acquisition control, unique synchronization word (UW) detection, and elastic buffering. High clock accuracy provides increased signal-to-noise (S/N) ratio during acquisition, permitting narrower acquisition frequency and time windows. Spaceborne systems requirements to provide capabilities such as: refinement of the GEM-72 gravity model based on satellite tracking observations from ATS-6 to GEOS-3, relativistic clock experiments, NASA crustal dynamics program for developing space geodetic techniques to study the earth's crust, its gravity field, and earthquake mechanisms, and multi-disciplinary space geodetic tracking for studying global climatic changes are also reviewed.

Anderman, Alfred↗

NASA NTRS1993

High speed all-optical networks

An inherent problem of conventional point-to-point WAN architectures is that they cannot translate optical transmission bandwidth into comparable user available throughput due to the limiting electronic processing speed of the switching nodes. This report presents the first solution to WDM based WAN networks that overcomes this limitation. The proposed Lightnet architecture takes into account the idiosyncrasies of WDM switching/transmission leading to an efficient and pragmatic solution. The Lightnet architecture trades the ample WDM bandwidth for a reduction in the number of processing stages and a simplification of each switching stage, leading to drastically increased effective network throughputs.

Chlamtac, Imrich↗

NASA NTRS1993

Insertion loss and noise-temperature contribution of high-temperature superconducting bandpass filters centered at 2.3 and 8.45 GHz

Two superconducting Tl-Ca-Cu-Ba-O bandpass filters were fabricated for JPL by Superconductor Technologies Incorporated, Santa Barbara, California. The filters were designed to operate at 2.3 GHz (S-band) with a 0.5-dB bandwidth of 60 MHz and at 8.45 GHz (X-band) with a 0.5-dB bandwidth of 150 MHz. The structure selected for both filters incorporates half-wavelength thin-film resonators in a stripline configuration. The S-band filter uses an edge-coupled interdigital design and the X-band filter uses an end-coupled design. The insertion loss and the noise-temperature contribution were measured at 12 K for both filters.

Prater, J. L.↗

NASA NTRS1993

Recent activities in printed Antennas at LeRC

This paper will report two recent R&D efforts in printed antennas at NASA Lewis Research Center. These efforts are: (1) to enhance the current antenna performance in gain, bandwidth and pattern characteristics, and (2) to develop coplanar waveguide/aperture coupled feeding technique for dual excitation of a patch antenna. Research in area (1) has led to the development of a nonplanar linearly tapered slot antenna (LTSA) which has exhibited over 10 dB gain with broad bandwidth and excellent radiation patterns. This endfire antenna element is most suitable for use in MMIC arrays of 'brick' construction. A space power amplifier composed of active LTSA has been demonstrated and shown to have a gain of 30 dB at 20 GHz. In each of the antenna elements, a GaAs monolithic microwave integrated circuit (MMIC) three-stage power amplifier is integrated with two LTSA's. A single active LTSA has also been demonstrated and exhibited a power gain of 6.7 dB with the MMIC amplifier turned on. The aperture coupled feeding technique with coplanar waveguide feeds has demonstrated high coupling efficiency on both LTSA and patch antennas. Recent efforts have been focused on applying this technique for dual excitation (dual frequency and/or dual polarization) of a patch antenna. Preliminary results confirm the feasibility of this approach. Further development is required to improve the coupling efficiency and antenna radiation characteristics.

Lee, Richard Q.↗

NASA NTRS1993

Low-power millimeter-wave radar observations of the atmosphere

Historically, cloud structures, dynamics, and precipitation processes have been observed and measured with sensors from two different spatial resolutions. Laser-based sensors have volume resolutions on the order of 10(exp -4) to 10(exp -2) cubic meters for a 1-s sample. Radar systems operating at wavelengths between 1 and 10 cm have resolutions on the order of 10(exp 4) to 10(exp 7) cubic meters. The resolutions of micro-wave systems depend primarily on the system RF bandwidth and antenna bandwidth. Both resolution regimes were useful in the study of cloud structures and processes - the former for determining resolution on the individual cloud particle scale and the latter for studying the coarse characteristics of cloud dynamics and structure. There are, however, cloud processes and structures that occur on scales that lie between these two regimes: the process of entrainment, where outside air is brought within the cloud boundaries; the mixing of in-cloud particles with different histories; cloud particle coalescence; and ice formation. The use of mm-wave (30- to 300-GHz) radars offers an opportunity to observe cloud processes at these scales and determine their influence on precipitation development, cloud albedos, cloud lifetimes, chemical cycling of tract substances, aircraft icing, and other meteorological phenomena. To determine the usefulness of a 35-GHz radar for observing these precipitation and cloud processes, a research program was initiated. The objectives of this program are to develop a 35-GHz radar, to measure scattering from precipitation and clouds, and to develop a model to compute scattering from clouds using the finite-difference time-domain (FDTD) technique.

Ronnau, James F.↗

NASA NTRS1992

Efficient transfer of images over networks

Effective remote observing requires sending large images over long distances. The usual approach to the transfer problem is to require high bandwidth transmission links, which are expensive to install and operate. An alternative approach is to use existing low-bandwidth connections, such as phone lines or the Internet, in a highly efficient manner by compressing the images. The combined use of existing low-cost infrastructure and standard networking software means that remote observing can be made practical even for small observatories with limited network resources. The authors have implemented such a scheme based on the H-transform compression method developed for astronomical images, which are often resistant to compression because they are noisy. The H-transform can be used for either lossy or lossless compression, and compression factors of at least 10 can be achieved with no noticeable losses in the astrometric or photometric properties of the compressed images. The H-transform allows us to organize the information in an image so that the 'useful' information can be sent first, followed by the noise, which makes up the bulk of the transmission. The receiver can invert a partially received set of H-coefficients, creating an image that improves with time. The H-transform is particularly well-suited to this style of incremental reconstruction, because the spatially localized nature of the basis functions of the H-transorm prevents the appearance of artifacts such as ringing around point sources and edges. The authors' implementation uses the WIYN Telescope Control System's TCP-based communications protocol. An 800x800 16-bit astronomical image was sent over a 2400 baud connection, which would normally take about 71 minutes; after only 60 seconds, the partially received H-transform produced an image that did not differ appreciably from the original. This poster presents a quantification of the efficiencies, as well as examples of images reconstructed from partial data.

Percival, J. W.↗

NASA NTRS1993

ACTS broadband aeronautical experiment

In the last decade, the demand for reliable data, voice, and video satellite communication links between aircraft and ground to improve air traffic control, airline management, and to meet the growing demand for passenger communications has increased significantly. It is expected that in the near future, the spectrum required for aeronautical communication services will grow significantly beyond that currently available at L-band. In anticipation of this, JPL is developing an experimental broadband aeronautical satellite communications system that will utilize NASA's Advanced Communications Technology Satellite (ACTS) as a satellite of opportunity and the technology developed under JPL's ACTS Mobile Terminal (AMT) Task to evaluate the feasibility of using K/Ka-band for these applications. The application of K/Ka-band for aeronautical satellite communications at cruise altitudes is particularly promising for several reasons: (1) the minimal amount of signal attenuation due to rain; (2) the reduced drag due to the smaller K/Ka-band antennas (as compared to the current L-band systems); and (3) the large amount of available bandwidth. The increased bandwidth available at these frequencies is expected to lead to significantly improved passenger communications - including full-duplex compressed video and multiple channel voice. A description of the proposed broadband experimental system will be presented including: (1) applications of K/Ka-band aeronautical satellite technology to U.S. industry; (2) the experiment objectives; (3) the experiment set-up; (4) experimental equipment description; and (5) industrial participation in the experiment and the benefits.

Abbe, Brian S.↗

NASA NTRS1993

Asynchronous timing and Doppler recovery in DSP based DPSK modems for fixed and mobile satellite applications

While conventional analog modems employ some kind of clock wave regenerator circuit for synchronous timing recovery, in sampled modem receivers the timing is recovered asynchronously to the incoming data stream, with no adjustment being made to the input sampling rate. All timing corrections are accomplished by digital operations on the sampled data stream, and timing recovery is asynchronous with the uncontrolled, input A/D system. A good timing error measurement algorithm is a zero crossing tracker proposed by Gardner. Digital, speech rate (2400 - 4800 bps) M-PSK modem receivers employing Gardner's zero crossing tracker were implemented and tested and found to achieve BER performance very close to theoretical values on the AWGN channel. Nyguist pulse shaped modem systems with excess bandwidth factors ranging from 100 to 60 percent were considered. We can show that for any symmetric M-PSK signal set Gardner's NDA algorithm is free of pattern jitter for any carrier phase offset for rectangular pulses and for Nyquist pulses having 100 percent excess bandwidth. Also, the Nyquist pulse shaped system is studied on the mobile satellite channel, where Doppler shifts and multipath fading degrade the pi/4-DQPSK signal. Two simple modifications to Gardner's zero crossing tracker enable it to remain useful in the presence of multipath fading.

Koblents, B.↗

NASA NTRS1993

Experimental modification and identification of the DSS-13 antenna control system model

This work is the first step toward increasing the bandwidth of the DSS-13 antenna position loop controller. A wider bandwidth of the controller allows for faster tracking rates and better pointing performance under windy conditions. To achieve this goal, the antenna control system model has to be improved, such that it will accurately reflect the dynamic properties of the antenna. The existing analytical model, due to its many uncertainties, could not be used in the design of the controller. However, by using experimental data, the analytical model is modified and improved, and a new model is obtained through system identification techniques.

Racho, C. S.↗

NASA NTRS1993

VENI, video, VICI: The merging of computer and video technologies

The topics covered include the following: High Definition Television (HDTV) milestones; visual information bandwidth; television frequency allocation and bandwidth; horizontal scanning; workstation RGB color domain; NTSC color domain; American HDTV time-table; HDTV image size; digital HDTV hierarchy; task force on digital image architecture; open architecture model; future displays; and the ULTIMATE imaging system.

Horowitz, Jay G.↗

NASA NTRS1994

Integrated active and passive control design methodology for the LaRC CSI evolutionary model

A general design methodology to integrate active control with passive damping was demonstrated on the NASA LaRC CSI Evolutionary Model (CEM), a ground testbed for future large, flexible spacecraft. Vibration suppression controllers designed for Line-of Sight (LOS) minimization were successfully implemented on the CEM. A frequency-shaped H2 methodology was developed, allowing the designer to specify the roll-off of the MIMO compensator. A closed loop bandwidth of 4 Hz, including the six rigid body modes and the first three dominant elastic modes of the CEM was achieved. Good agreement was demonstrated between experimental data and analytical predictions for the closed loop frequency response and random tests. Using the Modal Strain Energy (MSE) method, a passive damping treatment consisting of 60 viscoelastically damped struts was designed, fabricated and implemented on the CEM. Damping levels for the targeted modes were more than an order of magnitude larger than for the undamped structure. Using measured loss and stiffness data for the individual damped struts, analytical predictions of the damping levels were very close to the experimental values in the (1-10) Hz frequency range where the open loop model matched the experimental data. An integrated active/passive controller was successfully implemented on the CEM and was evaluated against an active-only controller. A two-fold increase in the effective control bandwidth and further reductions of 30 percent to 50 percent in the LOS RMS outputs were achieved compared to an active-only controller. Superior performance was also obtained compared to a High-Authority/Low-Authority (HAC/LAC) controller.

Voth, Christopher T.↗

NASA NTRS1994

Lasercom system architecture with reduced complexity

Spatial acquisition and precision beam pointing functions are critical to spaceborne laser communication systems. In the present invention, a single high bandwidth CCD detector is used to perform both spatial acquisition and tracking functions. Compared to previous lasercom hardware design, the array tracking concept offers reduced system complexity by reducing the number of optical elements in the design. Specifically, the design requires only one detector and one beam steering mechanism. It also provides the means to optically close the point-ahead control loop. The technology required for high bandwidth array tracking was examined and shown to be consistent with current state of the art. The single detector design can lead to a significantly reduced system complexity and a lower system cost.

Lesh, James R.↗

NASA NTRS1991

Orbital debris radar instrumentation

In order to increase the usefulness of Goldstone orbital debris observations, several improvements are planned to the experimental Goldstone radar. The first improvement is to add a ranging capability of 1-km accuracy to the radar. This would provide much more detailed information about whatever debris is observed. The second improvement is to widen the bandwidth of the system from 10 kHz to 85 kHz so that particles in more elliptical orbits can be monitored. The two antennas could then also be pointed away from the zenith, say south, where orbits of lesser inclination might also be observed. In any case, the addition of ranging requires a substantial increase of system bandwidth. This article describes the instrumentation necessary to achieve this, together with required signal-processing modifications.

Goldstein, Richard M.↗

NASA NTRS1993

A gimbaled low noise momentum wheel

The bus actuators are the heart and at the same time the Achilles' heel of accurate spacecraft stabilization systems, because both their performance and their perturbations can have a deciding influence on the achievable pointing accuracy of the mission. The main task of the attitude actuators, which are mostly wheels, is the generation of useful torques with sufficiently high bandwidth, resolution and accuracy. This is because the bandwidth of the whole attitude control loop and its disturbance rejection capability is dependent upon these factors. These useful torques shall be provided, without - as far as possible - parasitic noise like unbalance forces and torques and harmonics. This is because such variable frequency perturbations excite structural resonances which in turn disturb the operation of sensors and scientific instruments. High accuracy spacecraft will further require bus actuators for the three linear degrees of freedom (DOF) to damp structural oscillations excited by various sources. These actuators have to cover the dynamic range of these disturbances. Another interesting feature, which is not necessarily related to low noise performance, is a gimballing capability which enables, in a certain angular range, a three axis attitude control with only one wheel. The herein presented Teldix MWX, a five degree of freedom Magnetic Bearing Momentum Wheel, incorporates all the above required features. It is ideally suited to support, as a gyroscopic actuator in the attitude control system, all High Pointing Accuracy and Vibration Sensitive space missions.

Bichler, U.↗

NASA NTRS1994

A comparison of full-spectrum and complex-symbol combining techniques for the Galileo S-band mission

Full-spectrum combining (FSC) and complex-symbol combining (CSC) are two antenna-arraying techniques being considered for the Galileo spacecraft's upcoming encounter with Jupiter. This article describes the performance of these techniques in terms of symbol signal-to-noise ratio (SNR) degradation and symbol SNR loss. It is shown that both degradation and loss are approximately equal at low values of symbol SNR but diverge at high SNR values. For the Galileo S-band (2.2 to 2.3 GHz) mission, degradation provides a good estimate of performance as the symbol SNR is typically below -5 dB. For the following arrays - two 70-m antennas, one 70-m and one 34-m antenna, one 70-m and two 34-m antennas, and one 70-m and three 34-m antennas - it is shown that FSC has less degradation than CSC when the subcarrier and symbol window-loop bandwidth products are above 3.0, 10.0, 8.5, and 8.2 mHz at the symbol rate of 200 sym/sec, and above 1.2, 4.5, 4.0, and 3.5 mHz at a symbol rate of 400 sym/sec, respectively. Moreover, for an array of four 34-m antennas, FSC has less degradation than CSC when the subcarrier and symbol window-loop bandwidth products are above 0.32 mHz at the symbol rate of 50 sym/sec and above 0.8 mHz at the symbol rate of 25 sym/sec.

Million, S.↗