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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Station position results using concentrated C-band tracking of GEOS-3

Station positions for the GEOS 3 C Band tracking network were estimated using C Band and laser data taken during a two-week concentrated tracking period. The C Band stations are located primarily in the continental United States and on Western Atlantic islands. The network, however, included stations in Hawaii, in West Germany, and on Kwajalein atoll. Estimated accuracies for the recovered positions are 2 m for the continental U. S. and Atlantic sites, 5 m for Hawaii, and 10 m for Kwajalein. The dominant contributor to these uncertainties is geopotential model error. Thus, the C Band/laser data set could be used for more accurate center-of-mass positioning of a continental network of stations.

Krabill, W. B.↗
NASA NTRS1977

Robustness of linear quadratic state feedback designs in the presence of system uncertainty

In connection with difficulties concerning an accurate mathematical representation of a linear quadratic state feedback (LQSF) system, it is often necessary to investigate the robustness (stability) of an LQSF design in the presence of system uncertainty and obtain some quantitative measure of the perturbations which such a design can tolerate. A study is conducted concerning the problem of expressing the robustness property of an LQSF design quantitatively in terms of bounds on the perturbations (modeling errors or parameter variations) in the system matrices. Bounds are obtained for the general case of nonlinear, time-varying perturbations. It is pointed out that most of the presented results are readily applicable to practical situations for which a designer has estimates of the bounds on the system parameter perturbations. Relations are provided which help the designer to select appropriate weighting matrices in the quadratic performance index to attain a robust design. The developed results are employed in the design of an autopilot logic for the flare maneuver of the Augmentor Wing Jet STOL Research Aircraft.

Patel, R. V.↗
NASA NTRS1977

Robustness of linear quadratic state feedback designs in the presence of system uncertainty

The paper deals with the problem of expressing the robustness (stability) property of a linear quadratic state feedback (LQSF) design quantitatively in terms of bounds on the perturbations (modeling errors or parameter variations) in the system matrices so that the closed-loop system remains stable. Nonlinear time-varying and linear time-invariant perturbations are considered. The only computation required in obtaining a measure of the robustness of an LQSF design is to determine the eigenvalues of two symmetric matrices determined when solving the algebraic Riccati equation corresponding to the LQSF design problem. Results are applied to a complex dynamic system consisting of the flare control of a STOL aircraft. The design of the flare control is formulated as an LQSF tracking problem.

Patel, R. V.↗
NASA NTRS1977

Onboard magnetic field modeling for Solar Maximum Mission /SMM/

Analysis and simulation results are presented for magnetic field models for use in attitude acquisition onboard Solar Maximum Mission (SMM). A study was made of the degree of the spherical harmonic expansion of the magnetic field required, considering mission requirements, modeling errors, and magnetometer quantization and biases. It is shown that a fifth-degree field is sufficient to provide two-degree roll angle determination accuracy with a residual magnetic bias of 10 milligauss. Also, a spherical harmonic expansion for the McIlwain L-parameter is included for the first time. This parameter will be telemetered to ground with experimental data. The fifth-degree expansion will provide the L-parameter to within two percent of accepted values. The additional onboard computational burden is the storage of 36 coefficients and an increase of about 15% in computation time. Prototype flight code was developed which is anticipated to require about 2000 bytes of core storage and 30 milliseconds of computation time per orbit point on the NSSC-1 computer.

Headrick, R. D.↗
NASA NTRS1978

Robustness and computational aspects of nonlinear stochastic estimators and regulators

Robustness properties of nonlinear extended Kalman filters with constant gains and modeling errors are presented. Sufficient conditions for the nondivergence of state estimates generated by such nonlinear estimators are given. In addition, the overall robustness and stability properties of closed-loop stochastic regulators, based upon the linear-quadratic Gaussian design methodology using linearized dynamics, are presented; the sufficient conditions for closed-loop stability have a 'separation-type' property.

Safonov, M. G.↗
NASA NTRS1978

Attitude control of a flexible spacecraft

An algorithm is presented for designing optimal low order compensators for high order systems and it is applied to the title problem where many vibration modes are excited by the control torque. These low order compensators are compared with the full order optimal compensator and found to be less sensitive to modeling errors and to provide near optimal attitude regulation.

Martin, G. D.↗
NASA NTRS1979

On-board orbit determination for application satellites

A proposal for onboard orbit determination which would rely entirely on reference Doppler beacons located within the Continental United States is presented. Algorithms were conceived which were compatible with small computing machines and sufficiently accurate to maintain an adequate satellite ephemeris. An on board computer equipped with a recursive filter with a fading memory to account for dynamic modeling errors and two reference beacons was found to be necessary for the system to work.

Morduch, G. E.↗
NASA NTRS1979

Utilization of satellite-satellite tracking data for determination of the geocentric gravitational constant (GM)

Range rate tracking of GEOS 3 through the ATS 6 satellite was used, along with ground tracking of GEOS 3, to estimate the geocentric gravitational constant (GM). Using multiple half day arcs, a GM of 398600.52 + or - 0.12 cu km/sq sec was estimated using the GEM 10 gravity model, based on speed of light of 299792.458 km/sec. Tracking station coordinates were simultaneously adjusted, leaving geopotential model error as the dominant error source. Baselines between the adjusted NASA laser sites show better than 15 cm agreement with multiple short arc GEOS 3 solutions.

Martin, C. F.↗
NASA NTRS1978

Navigation capability for an ion drive rendezvous with Halley's Comet

Navigation accuracies are presented for a 1985 rendezvous with Halley's Comet using an ion propulsion system. Individual error sources are examined to determine their relative contributions to final delivery errors. The sensitivity of delivery accuracy to stochastic thrust variations is demonstrated by considering a baseline thrust error model and a more benign alternative model. Also studied are the effects of increased comet ephemeris uncertainties and of operational time delays between the orbit determination process and the guidance implementation.

Thornton, C. L.↗
NASA NTRS1977

Robustness and computational aspects of nonlinear stochastic estimators and regulators

Robustness properties of nonlinear extended Kalman filters with constant gains and modeling errors are presented. Sufficient conditions for the nondivergence of state estimates generated by such nonlinear estimators are given. In addition, the overall robustness and stability properties of closed-loop stochastic regulators, based upon the Linear-Quadratic-Gaussian design methodology using linearized dynamics, are presented; the sufficient conditions for closed-loop stability have 'separation-type' property.

Safonov, M. G.↗
NASA NTRS1979

Application of analytical redundancy management to Shuttle crafts

The study involves the bank of filters approach to analytical redundancy management since this is amenable to microelectronic implementation. Attention is given to a study of the UD factorized filter to determine if it gives more accurate estimates than the standard Kalman filter when data processing word size is reduced. It is reported that, as the word size is reduced, the effect of modeling error dominates the filter performance of the two filters. However, the UD filter is shown to maintain a slight advantage in tracking performance. It is concluded that because of the UD filter's stability in the serial processing mode, it remains the leading candidate for microelectronic implementation.

Montgomery, R. C.↗
NASA NTRS1980

The reduced order model problem in distributed parameter systems adaptive identification and control

The research concerning the reduced order model problem in distributed parameter systems is reported. The adaptive control strategy was chosen for investigation in the annular momentum control device. It is noted, that if there is no observation spill over, and no model errors, an indirect adaptive control strategy can be globally stable. Recent publications concerning adaptive control are included.

Johnson, C. R., Jr.↗
NASA NTRS1980

Large space system control technology overview

The nature of the problems which confront large space structure technology controls are discussed. Emphasis is placed on the following: the basic performance requirements; structural model errors; the effect of certain disturbances (i.e., solar pressure, gravity gradient torques, and aerodynamic drag); and surface or figure control.

Parker, G. L.↗
NASA NTRS1980

Large space system control technology status and accomplishments

In the area of antenna modeling and control, preliminary structural models were defined for two representative parabolic reflectors. A control system design evolved for attitude control of the reflectors. The controller design was based on a lumped control concept where the control hardware (sensor and actuators) was mounted at the base of the antenna. In the area of distributed control, static shape control techniques were worked out to establish a prescribed vehicle shape. The corresponding estimation process for determination of vehicle shape from selected sensor measurements was also developed. A model order reduction study was conducted to find the best preflight dynamical models for on board controller design. The problems caused by truncation of the vehicle dynamics required to minimize on board computations were investigated. Estimator designs were developed for on board detection of large structure model errors.

Rodriguez, G.↗
NASA NTRS1980

A Precision Recursive Estimate for Ephemeris Refinement (PREFER)

A recursive filter/smoother orbit determination program was developed to refine the ephemerides produced by a batch orbit determination program (e.g., CELEST, GEODYN). The program PREFER can handle a variety of ground and satellite to satellite tracking types as well as satellite altimetry. It was tested on simulated data which contained significant modeling errors and the results clearly demonstrate the superiority of the program compared to batch estimation.

Gibbs, B.↗
NASA NTRS1980

Numerical simulation of steady supersonic flow over an ogive-cylinder-boattail body

A recently reported parabolized Navier-Stokes code has been employed to compute the supersonic flow field surrounding an ogive-cylinder-boattail body at incidence. The computations were performed for flow conditions where an extensive series of experimental surface pressure and turbulent boundary-layer profile measurements had been obtained. Comparison between the computational results and experimental measurements for angles of attack up to 6 deg show excellent agreement. At angles greater than 6 deg discrepancies are observed which are tentatively attributed to three-dimensional turbulence modeling errors.

Schiff, L. B.↗
NASA NTRS1980

Comments on stellar boundary cooling and the reality of supermetallicity

The paper discusses the 'super-metal-rich' (SMR) stars and reexamines Peterson's analysis of the SMR prototype mu Leo (1978) with regard to a postulated error in continuum error. Model atmospheres are used to compute theoretical equivalent widths and to explore the sensitivity of these widths to metallicity, temperature, surface gravity, and microturbulence. It is shown that Peterson's results are sensitive to continuum placement, and that her data does not indicate that the temperature gradient is steeper in mu Leo than in normal giants. It is concluded that the SMR stars are very metal rich and are also somewhat boundary cooled, possibly due to high metallicity.

Deming, D.↗
NASA NTRS1980

Failure detection system design methodology

The design of a failure detection and identification system consists of designing a robust residual generation process and a high performance decision making process. The design of these two processes are examined separately. Residual generation is based on analytical redundancy. Redundancy relations that are insensitive to modelling errors and noise effects are important for designing robust residual generation processes. The characterization of the concept of analytical redundancy in terms of a generalized parity space provides a framework in which a systematic approach to the determination of robust redundancy relations are developed. The Bayesian approach is adopted for the design of high performance decision processes. The FDI decision problem is formulated as a Bayes sequential decision problem. Since the optimal decision rule is incomputable, a methodology for designing suboptimal rules is proposed. A numerical algorithm is developed to facilitate the design and performance evaluation of suboptimal rules.

Chow, E. Y.↗