Effects of uncertainties and flexible dynamic contributions on the control of a spacecraft full-coupled model

One of the most important problems for performing a good design of the spacecraft attitude control law is connected to its robustness when some uncertainty parameters are present on the inertial and/or on the elastic characteristics of a satellite. These uncertainties are generally intrinsic on the modeling of complex structures and in the case of large flexible structures they can be also attributed to secondary effects associated to the elasticity. One of the most interesting issues in modeling large flexible space structures is associated to the evaluation of the inertia tensor which in general depends not only on the geometric ‘fixed’ characteristic of the satellite but also on its elastic displacements which of course in turn modify the ‘shape’ of the satellite. Usually these terms can be considered of a second order of magnitude if compared with the ones associated to the rigid part of a structure. However the increasing demand on the dimension of satellites due to the presence for instance of very large solar arrays (necessary to generate power) and/or large antennas has the necessity to investigate their effects on their global dynamic behavior in more details as a consequence. In the present paper a methodology based on classical Lagrangian approach coupled with a standard Finite Element tool has been used to derive the full dynamic equations of an orbiting flexible satellite under the actions of gravity, gravity gradient forces and attitude control. A particular attention has been paid to the study of the effects of flexibility on the inertial terms of the spacecraft which, as well known, influence its attitude dynamic behavior. Furthermore the effects of the attitude control authority and its robustness to the uncertainties on inertial and elastic parameters has been investigated and discussed.     

Connection between the discrete auroral structures drifting equatorward during the substorm expansion phase and the energetic particle injections at the geostationary orbit

The formation of an auroral bulge with a bright dynamical arc at its polar boundary is one of the main manifestations of the magnetospheric substorm expansion phase at the ionospheric level. At the same time, the region of discrete aurora broadens not only polewards but equatorwards as well. The discrete forms of auroras moving equatorwards form a dynamical equatorial boundary of the auroral bulge shifting together with them. The paper presents a spatial-time comparison of the drifting discrete auroras to the injection of energetic particles at the geostationary orbit. It is shown that bursts in the fluxes of energetic particles at the LANL geostationary satellites located in the same sector of MLT correspond to the majority of drifting discrete auroral structures observed by the all-sky camera. In the cases when the bursts in the fluxes are absent, the minimum latitude reached by the auroral structures at the equatorward drift is higher than the ionospheric projection of the geostationary orbit. A possible relation of the drifting discrete auroras to the plasma stream jets in the plasma sheet is discussed.     

System of works on numerical modeling of the dynamics of the structure of the Space Radiotelescope in the RadioAstron project

The results of modeling the dynamics of the Space Radiotelescope are presented. The results of ground-based vibration-dynamic tests are used to validate the calculation models and estimate the damping levels. The dynamic distortions of the reflecting surface caused by the operation of the pointing mechanism of the high-gain antenna are estimated.     

Categorical Bias in Line Angle Judgments: Sex Differences and the Use of Multiple Categories

According to the Category Adjustment (CA) model, spatial estimates (e.g., of location) involve Bayesian combination across multiple, hierarchical pieces information, each weighted by its relative certainty. Recent work, though, has shown that men and women differ in terms of their certainty regarding fine-grained and categorical information in location memory. Here we demonstrate that this reflects a more general sex difference in visuospatial processing by examining bias patterns in a line angle judgment task (JLAP-15). In addition, the data suggest that multiple, hierarchical levels of categorical information influenced spatial judgments. These results imply that the hierarchical combination outlined by the CA model extends beyond two levels, and that men and women apply differential weighting to these representations.     

ESA's Biopan 1--"Vitamin" experiment preliminary results

The purpose of “Vitamin” experiment is to study the efficiency of protective substances on three biological acellular systems aqueous solutions exposed to cosmic radiation in space. The first system “LDL”is a low density lipoprotein. The second is “E2-TeBG complexe” in which estradiol (E2) is bound to its plasmatic carrier protein, testosterone-estradiol binding globulin (TeBG). The third is “pBR 322”, a plasmid. “Vitamin” experiment was accomodated in the Biopan which had been mounted on the outer surface of a Foton retrievable satellite. The experiment was exposed to space environment during 15 days. A stable temperature of about 20 °C was maintained throughout the flight. “Vitamin” experiment preliminary results are presented and discussed.     

Leg venous hemodynamics and leg volumes during a 42 day -6 degrees head-down bedrest

Seven healthy subjects were submitted to a 42-day head down bedrest, where leg venous compliance (venous distensibity index VDI) and leg volumes were assessed by mercury strain gauge plethysmography with venous occlusion and optoelectronic plethysmography, respectively. Plethysmographic and volometric measurements were made, before, during (at days 1, 4, 7, 14, 21, 26, 34 and 41), and after bedrest (days 1, 4, 7, 11 and 30 of the recovery period). Results showed a continuous decrease in leg volumes throughout bedrest, when VDI increased until day 26 of bedrest, and then decreased afterwards. The recovery period was characterized by a rapid return of VDI to prebedrest levels while leg volumes progressively normalised. These results showed that leg venous compliance changes are not always dependant upon skeletal muscle changes, and that factors other than size of muscle compartment are able to determine increases in leg venous compliance during long-term bedrest.     

Highly mobile space suit material optimization

This paper discusses the factors that control the flexibility of fabric space suit elements by examining a bending model of a pressurized fabric tube. Results from the model are used to evaluate the current direction in highly mobile EVA glove research and suggest that changes are necessary in the suit and glove fabric selection methodology.     

Educating Europe - finding the best way

The subject of space education is attracting increasing attention, but there are diverging views as to how it should be approached, as can be seen from the following two reports of Education Remote Sensing '92, a conference held in Cardiff, Wales, 28–30 June 1992.     

Localization performance of arrays subject to phase errors

The passive localisation of radiating sources using an array subject to random perturbations in sensor phases is presented. All source signals as well as additive noises observed at the sensors are assumed to be independent identically distributed zero-mean Gaussian random processes. Cramer-Rao bounds are derived for source bearings and ranges for the phase errors at each sensor. It is shown that accurate phase calibration can be achieved when the number of sources exceeds a certain minimum. The locations of the calibrating sources need not be known a priori and need only satisfy mild regularity conditions. A calibration procedure is proposed which uses maximum-likelihood techniques     

The autocorrelation of m-sequences over nonprime finitefields

Nonbinary m-sequences (maximal length sequences) for spread-spectrum communication systems that have a two-level autocorrelation are presented. The autocorrelation function of an m -sequence over the Galois field of q elements GF(q), where q=p^k, for p a prime and k an integer greater than 1, is developed and shown to be bilevel when the elements of GF(q) are expressed as elements of a vector space over the pth roots of unity