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NASA NTRS1992

Singularity perturbed zero dynamics of nonlinear systems

Stability properties of zero dynamics are among the crucial input-output properties of both linear and nonlinear systems. Unstable, or 'nonminimum phase', zero dynamics are a major obstacle to input-output linearization and high-gain designs. An analysis of the effects of regular perturbations in system equations on zero dynamics shows that whenever a perturbation decreases the system's relative degree, it manifests itself as a singular perturbation of zero dynamics. Conditions are given under which the zero dynamics evolve in two timescales characteristic of a standard singular perturbation form that allows a separate analysis of slow and fast parts of the zero dynamics.

Isidori, A.↗

NASA NTRS1992

Influence of animation on dynamical judgments

The motions of objects in the environment reflect underlying dynamical constraints and regularities. The conditions under which people are sensitive to natural dynamics are considered. In particular, the article considers what determines whether observers can distinguish canonical and anomalous dynamics when viewing ongoing events. The extent to which such perceptual appreciations are integrated with and influence common-sense reasoning about mechanical events is examined. It is concluded that animation evokes accurate dynamical intuitions when there is only 1 dimension of information that is of dynamical relevance. This advantage is lost when the observed motion reflects higher dimension dynamics or when the kinematic information is removed or degraded.

Kaiser, Mary K.↗

NASA NTRS1992

Sub-domain decomposition methods and computational controls for multibody dynamical systems

This paper presents a concurrent methodology to simulate the dynamics of flexible multibody systems with a large number of degrees of freedom. A general class of open-loop structures is treated and a redundant coordinate formulation is adopted. A range space method is used in which the constraint forces are calculated using a preconditioned conjugate gradient method. By using a preconditioner motivated by the regular ordering of the directed graph of the structures, it is shown that the method is order N in the total number of coordinates of the system. The overall formulation has the advantage that it permits fine parallelization and does not rely on system topology to induce concurrency. It can be efficiently implemented on the present generation of parallel computers with a large number of processors. Validation of the method is presented via numerical simulations of space structures incorporating large number of flexible degrees of freedom.

Menon, R. G.↗

NASA NTRS1992

A scenario for solar wind penetration of earth's magnetic tail based on ion composition data from the ISEE 1 spacecraft

Based on He(2+) and H(-) ion composition data from the Plasma Composition Experiment on ISEE 1, a scenario is proposed for the solar wind penetration of the earth's magnetic tail, which does not require that the solar wind plasma be magnetized. While this study does not take issue with the notion that earth's magnetic field merges with the solar wind magnetic field on a regular basis, it focuses on certain aspects of interaction between the solar wind particles and the earth's field, e.g, the fact that the geomagnetic tail always has a plasma sheet, even during times when the physical signs of magnetic merging are weak or absent. It is argued that the solar plasma enters along slots between the tail lobes and the plasma sheet, even quite close to earth, convected inward along the plasma sheet boundary layer or adjacent to it, by the electric fringe field of the ever present low-latitude magnetopause boundary layer (LLBL). The required E x B drifts are produced by closing LLBL equipotential surfaces through the plasma sheet.

Lennartsson, W.↗

NASA NTRS1993

The computation of the post-stall behavior of a circulation controlled airfoil

The physics of the circulation controlled airfoil is complex and poorly understood, particularly with regards to jet stall, which is the eventual breakdown of lift augmentation by the jet at some sufficiently high blowing rate. The present paper describes the numerical simulation of stalled and unstalled flows over a two-dimensional circulation controlled airfoil using a fully implicit Navier-Stokes code, and the comparison with experimental results. Mach numbers of 0.3 and 0.5 and jet total to freestream pressure ratios of 1.4 and 1.8 are investigated. The Baldwin-Lomax and k-epsilon turbulence models are used, each modified to include the effect of strong streamline curvature. The numerical solutions of the post-stall circulation controlled airfoil show a highly regular unsteady periodic flowfield. This is the result of an alternation between adverse pressure gradient and shock induced separation of the boundary layer on the airfoil trailing edge.

Linton, Samuel W.↗

NASA NTRS1991

On the radiometric stability of the shuttle borne solar backscatter ultraviolet spectrometer

The Shuttle Solar Backscatter Ultraviolet (SSBUV) spectrometer instrument provides regular in-orbit calibration checks on the SBUV/2 ozone/solar irradiance monitoring instruments which are being flown routinely on NOAA operational satellites. The goal of the long-term ozone monitoring program is to detect possible changes in stratospheic ozone with a two sigma accuracy of approximately 1 percent over the course of a decade. This translates into a requirement that the SSBUV instrument be calibrated to a one sigma precision of 1 percent at the wavelengths used for ozone monitoring. We have previously shown that the precision of the SSBUV calibrations is such that we can meet this requirement. Here we discuss SSBUV radiometric sensitivity changes occurring as a result of the first two Space Shuttle flights. Finally, we present and compare SSBUV solar irradiance measurements taken during these flights.

Cebula, Richard P.↗

NASA NTRS1993

Rapid oscillations in cataclysmic variables. IX - BG Canis Minoris (=3A 0729+103)

Results of a 10-yr photometric program on BG Canis Minoris, a novalike variable distinguished by the presence of a strictly coherent 913-s wave in its light curve, are presented. Long-term ephemerides for the orbital clock, which manifests itself through a 3.2-h light modulation, and the 913-s clock, are presented. The light curve exhibits a persistent 0.3 mag modulation at a period consistent with the 3.2-h spectroscopic period. Well-distributed timings of the orbital minima prove that the period is 0.1347486 day. It is inferred from the shape of the waveform and from the existence of X-ray dips with a sharp energy dependence that the modulation is caused by absorption. Power spectra of the nightly light curves show variable secondary features, in addition to the regular 913-s pulse. The 913-s period, presumably the first harmonic of the true white-dwarf spin period, is decreasing by 0.0022 s/yr.

Patterson, Joseph↗

NASA NTRS1991

Measurement of atmospheric composition by the ATMOS instrument from Table Mountain Observatory

Following its first flight on board the Space Shuttle 'Challenger' as part of the Spacelab 3 payload, the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument has been operated at the Jet Propulsion Laboratory's Table Mountain Observatory (TMO; 34.4 deg N, 117.7 deg W, 2.23 km altitude) in the San Gabriel Mountains of Southern California. With the delay in the resumption of regular Shuttle flights, ATMOS has acquired a large number of high-quality, high-resolution infrared solar absorption spectra, spanning a period between late-1985 and mid-1990. These spectra are being analyzed to derive the column abundances of several atmospheric species including O3, HCl, HF, and HNO3. Although limited in temporal coverage, the preliminary results for these gases are discussed here in the context of the requirement and contribution to be made by similar instruments in detecting long term changes in stratospheric composition.

Gunson, Michael R.↗

NASA NTRS1992

Vibronic spectra of Cu(2+) in ZnTe

Infrared-absorption spectra of substitutional Cu(2+) ions in ZnTe have been measured at 4.6 K. Several distinct absorption peaks are observed between 800 and 2000/cm. Absorption peaks at 1002 and 1069/cm are identified as zero-phonon lines arising from 2T2-2E transitions. Between 1069 and 2000/cm, several sets of sharp absorption lines are seen to recur regularly at an interval of 210/cm, corresponding to the LO phonon energy. Within each set distinct vibronic sidebands that cannot be identified with critical-point energies of TA, LA, TO or LO phonon modes are observed. A dynamic Jahn-Teller effect, involving coupling between a single-phonon mode and the electronic states of the 2E level, is proposed to account for the observed spectra.

Volz, M. P.↗

NASA NTRS1993

Pseudobreakup and substorm growth phase in the ionosphere and magnetosphere

Observations made during the growth phase and the onset of a substorm on August 31, 1986 are presented. About 20 min after the epsilon parameter at the magnetopause had exceeded 10 exp 11 W, magnetic field dipolarization with an increase of energetic particle fluxes was observed by the AMPTE Charge Composition Explorer (CCE) spacecraft at the geocentric distance of 8.7 R(E) close to magnetic midnight. The event exhibited local signatures of a substorm onset at AMPTE CCE and a weak wedgelike current system in the midnight sector ionosphere, but did not lead to a full-scale substorm expansion; neither did it produce large particle injections at GEO. Only after another 20 min of continued growth phase could the entire magnetosphere-ionosphere system allow the onset of a regular substorm expansion. The initial activation is interpreted as a 'pseudobreakup'. We examine the physical conditions in the near-Earth plasma sheet and analyze the development in the ionosphere using ground-based magnetometers and electric field observations from the STARE radar.

Koskinen, H. E. J.↗

NASA NTRS1992

Coronal Alfven waves detected by radio sounding during the solar occultations of the Helios spacecraft

The two Helios spacecraft underwent regular solar occultations during their extended missions from Dec 1974-Feb 1986 (Helios 1) and Jan 1976-Mar 1980 (Helios 2) thereby providing many opportunities for radio propagation experiments in the solar corona. On certain rare occasions over the course of these investigations, Faraday rotation measurements of the linearly polarized Helios signals could be recorded simultaneously at two widely-spaced ground stations. Many of these two-station measurement intervals display clear evidence of wave-like structures with quasi-periods of the order of a few minutes to a few hours. These structures are attributed to coronal Alfven waves. The radial propagation direction and velocity of these waves are estimated from a cross-correlation analysis of the data between the two stations. The majority of the waves appear to propagate away from the Sun, but about 30 percent of the cases indicate a propagation direction toward the Sun.

Bird, M. K.↗

NASA NTRS1992

Ions with low charges in the solar wind as measured by SWICS on board Ulysses

We present new data on rare ions in the solar wind. Using the Ulysses-SWICS instrument with its very low background we have searched for low-charge ions during a 6-d period of low-speed solar wind and established sensitive upper limits for many species. In the solar wind, we found He(1+)/He(2+) of less than 5 x 10 exp -4. This result and the charge state distributions of heavier elements indicate that all components of the investigated ion population went through a regular coronal expansion and experienced the typical electron temperatures of 1 to 2 million Kelvin. We argue that the virtual absence of low-charge ions demonstrates a very low level of nonsolar contamination in the source region of the solar wind sample we studied. Since this sample showed the FlP effect typical for low-speed solar wind, i.e., an enhancement in the abundances of elements with low first ionization potential, we conclude that this enhancement was caused by an ion-atom separation mechanism operating near the solar surface and not by foreign material in the corona.

Geiss, J.↗

NASA NTRS1993

Construction and parameterization of all static and dynamic H2-optimal state feedback solutions, optimal fixed modes, and fixed decoupling zeros

This paper considers an H2 optimization problem via state feedback. The class of problems dealt with here are general singular type which have a left invertible transfer matrix function from the control input to the controlled output. This class subsumes the regular H2 optimization problems. The paper constructs and parameterizes all the static and dynamic H2 optimal state feedback solutions. Moreover, all the eigenvalues of an optimal closed-loop system are characterized. All optimal closed-loop systems share a set of eigenvalues which are termed here as the optimal fixed modes. Every H2 optimal controller must assign among the closed-loop eigenvalues the set of optimal fixed modes. This set of optimal fixed modes includes a set of optimal fixed decoupling zeros which shows the minimum absolutely necessary number and locations of pole-zero cancellations present in any H2 optimal design. It is shown that both the sets of optimal fixed modes and optimal fixed decoupling zeros do not vary depending upon whether the static or the dynamic controllers are used.

Chen, Ben M.↗

NASA NTRS1993

Photometric and spectroscopic observations of 5145 Pholus

Light curve observations carried out in January and February 1992 are presented. We find an amplitude of 0.2 magnitude and a period of 0.4157 day. Fine structures of the light curve and their variability with time are discussed in terms of the changing phase angle and deviations of the object's shape from a regular ellipsoid. The present orbital and physical properties alone for 5145 Pholus do not provide a clue for either a cometary or an asteroidal origin. It seems to have properties in common with both and thus could form a bridge between these two groups.

Hoffmann, Martin↗

NASA NTRS1993

The Galileo solar redshift experiment

From the October 1989 launch to the first December 1990 earth gravity assist, we regularly obtained frequency measurements of the spacecraft clock - an ultrastable crystal oscillator (USO) supplied by Frequency Electronics, Inc. The solar gravitational redshift in frequency was readily detectable, and because of the unique variations in heliocentric distance we could separate the general relativistic effects from the USO's intrinsic frequency variations. We have verified the total frequency shift predicted by general relativity to 0.5 percent accuracy, and the solar gravitational redshift to 1 percent accuracy.

Krisher, Timothy P.↗

NASA NTRS1993

Regional studies of the land surface using space-based means

Regional ecological studies are considered in the context of the global change problem. The Kursk-91 international experiment is used to illustrate applications of remote sensing data and data bases of field experiments for assessment of parameters of the state of the soil and vegetative cover and subsequent study of biospheric stability on the basis of regular satellite observations.

Kozoderov, V. V.↗

NASA NTRS1993

Origins of outer-planet satellites

Aspects of satellite origin and evolution which bear on the formation of the solar system are addressed, with emphasis on the chemical evidence contained in satellites concerning the modification of the interstellar material which went to form the solar system. New results on the tidal evolution of outer-planet satellite systems are presented. The gross physical and chemical properties of the satellite systems of the outer planets are outlined. Models for regular satellite formation are reviewed. Attention is given to the evolution of the satellites, with emphasis on tidal evolution, including a discussion of the dynamical origin of Triton. The volatile budgets of Titan, Triton, and Pluto/Charon are discussed, and models are presented for the origin of these budgets which tie them to the larger set of processes associated with solar system formation.

Lunine, Jonathan I.↗

NASA NTRS1992

Computation of ephemerides for long-period variable stars for the Hipparcos mission

In order to predict the luminosities of LPVs observed by Hipparcos, many visual light curves, characterized by irregularly spaced data, were analyzed using original and complementary methods. The obtained power spectra, including amplitudes and phases, are utilized in the computation of the ephemerides. The predictions are regularly checked with observations. In this paper we present the methods and suggest some physical implications that are evoked.

Mennessier, M. O.↗