Determination of aerodynamic forces on satellites by theory and wind tunnel experiments

Theoretical and experimental aspects of satellite aerodynamics in free molecular flow have been studied. A FORTRAN-program package has been developed to calculate free molecular aerodynamic force and moment coefficients of general and special convex shapes, using an approximation of the surfaces by plane triangular elements.As a special case for concave bodies, the drag of a satellite of TD1A-similar shape has been investigated by an approximate theory and by wind tunnel experiments. Comparisons are made between experimental results, the approximate theory and exact theory.     

The Extreme Ultraviolet Imager Investigation for the IMAGE Mission

The Extreme Ultraviolet Imager (EUV) of the IMAGE Mission will study the distribution of He⁺ in Earth's plasmasphere by detecting its resonantly-scattered emission at 30.4 nm. It will record the structure and dynamics of the cold plasma in Earth's plasmasphere on a global scale. The 30.4-nm feature is relatively easy to measure because it is the brightest ion emission from the plasmasphere, it is spectrally isolated, and the background at that wavelength is negligible. Measurements are easy to interpret because the plasmaspheric He⁺ emission is optically thin, so its brightness is directly proportional to the He⁺ column abundance. Effective imaging of the plasmaspheric He⁺ requires global `snapshots in which the high apogee and the wide field of view of EUV provide in a single exposure a map of the entire plasmasphere. EUV consists of three identical sensor heads, each having a field of view 30° in diameter. These sensors are tilted relative to one another to cover a fan-shaped field of 84°×30°, which is swept across the plasmasphere by the spin of the satellite. EUVs spatial resolution is 0.6° or 0.1 R _E in the equatorial plane seen from apogee. The sensitivity is 1.9 count s^–1 Rayleigh^–1, sufficient to map the position of the plasmapause with a time resolution of 10 min.     

Spacecraft survivability in the natural debris environment near the stable Earth-Moon Lagrange points

The theoretical analysis of the motion of natural space debris near the stable Earth-Moon Lagrange Points, $L_4$ and $L_5$, is presented with a focus on the potential debris risks to spacecraft operating near these points. Specifically, the research formulates a debris propagation model using four-body dynamics, then applies candidate probabilistic survivability models to a notional spacecraft operating at the $L_4$ and $L_5$ Lagrange points to quantify the collision risks to the spacecraft from natural debris particles. Of the survivability models implemented, the natural debris collision risks to spacecraft survivability are found to be incredibly low, but mitigation strategies to reduce the risk further are identified in this study. Overall, research into stable Lagrange point natural debris propagation improves understanding of the collision risks posed by the naturally occurring Kordylewski clouds and enhances operational planning for Lagrange point space missions.     

The effect of small vibrations on Marangoni convection and the free surface of a liquid bridge

The effects of small vibrations on Marangoni convection were investigated experimentally using a liquid bridge of 5 cSt silicone oil with a disk diameter of 7.0 mm, and an aspect ratio close to 0.5. Experiments were performed to determine the critical temperature difference data for no vibration case and with small vibrations applied. The experimental results have shown that the effect of small vibrations on the onset of oscillatory flow is small since the critical temperature difference data for different aspect ratios were not affected by the vibrations. To clarify the surface oscillation phenomena induced by external vibrations, a 3-D numerical simulation model was also developed using a level set algorithm to predict the surface oscillations of isothermal silicone oil bridges. By subjecting the liquid bridge to small vibrations, the surface oscillation characteristics were predicted numerically, and the numerical results compared well with the predictions of an analytical model proposed previously. Furthermore, the effect of small vibrations on the surface vibration amplitude of the liquid bridge is also discussed.     

The Rosat mission

The primary scientific objective of the ROSAT mission is to perform the first all sky survey with an imaging X-ray telescope leading to an improvement in sensitivity by several orders of magnitude compared with previous surveys. Consequently a large number of new sources (> 10⁵) will be discovered and located with an accuracy of 1 arcmin. After completion of the survey which will take about half a year the instrument will be used for detailed observations of selected targets.The X-ray telescope consists of a fourfold nested Wolter type I mirror system with 80 cm aperture and 240 cm focal length, and three focal plane detectors. In the baseline version these will be imaging proportional counters (0.1 – 2 keV) providing a field of view of 2⁰ × 2⁰.     

Gravity and perceptual stability during translational head movement on earth and in microgravity

We measured the amount of visual movement judged consistent with translational head movement under normal and microgravity conditions. Subjects wore a virtual reality helmet in which the ratio of the movement of the world to the movement of the head (visual gain) was variable. Using the method of adjustment under normal gravity 10 subjects adjusted the visual gain until the visual world appeared stable during head movements that were either parallel or orthogonal to gravity. Using the method of constant stimuli under normal gravity, seven subjects moved their heads and judged whether the virtual world appeared to move “with” or “against” their movement for several visual gains. One subject repeated the constant stimuli judgements in microgravity during parabolic flight. The accuracy of judgements appeared unaffected by the direction or absence of gravity. Only the variability appeared affected by the absence of gravity. These results are discussed in relation to discomfort during head movements in microgravity.     

Space–Time Structure of Energetic Electron Precipitations according to the Data of Balloon Observations and Polar Satellite Measurements on February 1–6, 2015

Cosmic Research - The results of an analysis of the space–time characteristics and dynamics of precipitations of magnetospheric electrons with energies in the range from 0.1 to 0.7 MeV are...     

Analysis of the ionosphere/plasmasphere electron content variability during strong geomagnetic storm

The ionosphere/plasmasphere electron content (PEC) variations during strong geomagnetic storms in November 2004 were estimated by combining of mid-latitude Kharkov incoherent scatter radar observations and GPS TEC data derived from global TEC maps. The comparison between two independent measurements was performed by analysis of the height-temporal distribution for specific location corresponding to the mid-latitudes of Europe. The percentage contribution of PEC to GPS TEC indicated the clear dependence from the time with maximal values (more than 70%) during night-time. During day-time the lesser values (30–45%) were observed for quiet geomagnetic conditions and rather high values of the PEC contribution to GPS TEC (up to 90%) were observed during strong negative storm. These changes can be explained by the competing effects of electric fields and winds, which tend to raise the layer to the region with lower loss rate and movement of the ionospheric plasma to the plasmasphere.     

Simple adaptive control for aircraft control surface failures

This work extends the so-called simple adaptive control approach to direct model reference adaptive control of multi-input multi-output systems to include loss of control effectiveness failures. It is proven that all signals are bounded for loss of control effectiveness failures during a bounded input disturbance. A state space approach is introduced for computing the feedforward compensator that is required by the stability result. The adaptive algorithm is applied to a three input model of the linearized lateral dynamics of the F/A-18 aircraft. Simulation results are obtained with single, double, and triple control effectiveness failures of 88% during the occurrence of a lateral gust. These results show that the adaptive controller exhibits improved model following as compared with a fixed gain eigenstructure assignment controller.     

UWB radar detection of targets in foliage using alpha-stableclutter models

We address the problem of detection of targets obscured by a forest canopy using an ultrawideband (UWB) radar. The forest clutter observed in the radar imagery is a highly impulsive random process that is more accurately modeled with the recently proposed class of alpha-stable processes as compared with Gaussian, Weibull, and K-distribution models. With this more accurate model, segmentation is performed on the imagery into forest and clear regions. Further, a region-adaptive symmetric alpha stable (SαS) constant false-alarm rate (CFAR) detector is introduced and its performance is compared with the Weibull and Gaussian CFAR detectors. The results on real data show that the SαS CFAR performs better than the Weibull and Gaussian CFAR detectors in detecting obscured targets