Estimating the spin axis orientation of the Echostar-2 box-wing geosynchronous satellite

For the first time, the spin axis orientation of an inactive box-wing geosynchronous satellite has been estimated from ground-based optical photometric observations of Echostar-2’s specular reflections. Recent photometric light curves obtained of Echostar-2 over four years suggest that unusually bright and brief specular reflections were occurring twice within an observed spin period. These bright and brief specular reflections suggested two satellite surfaces with surface normals separated by approximately 180°. The geometry between the satellite, the Sun, and the observing location at the time of each of the brightest observed reflections, was used to estimate Echostar-2’s equatorial spin axis orientation coordinates. When considering prograde and retrograde rotation, Echostar-2’s spin axis orientation was estimated to have been located within 30° of either equatorial coordinate pole. Echostar-2’s spin axis was observed to have moved approximately 180° in right ascension, within a time span of six months, suggesting a roughly one year spin axis precession period about the satellite’s angular momentum vector.     

Mapping mean lake surface from satellite altimetry and GPS kinematic surveys

Lake water height is a key variable in water cycle and climate change studies, which is achievable using satellite altimetry constellation. A method based on data processing of altimetry from several satellites has been developed to interpolate mean lake surface (MLS) over a set of 22 big lakes distributed on the Earth. It has been applied on nadir radar altimeters in Low Resolution Mode (LRM: Jason-3, Saral/AltiKa, CryoSat-2) in Synthetic Aperture Radar (SAR) mode (Sentinel-3A), and in SAR interferometric (SARin) mode (CryoSat-2), and on laser altimetry (ICESat). Validation of the method has been performed using a set of kinematic GPS height profiles from 18 field campaigns over the lake Issykkul, by comparison of altimetry’s height at crossover points for the other lakes and using the laser altimetry on ICESat-2 mission. The precision reached ranges from 3 to 7 cm RMS (Root Mean Square) depending on the lakes. Currently, lake water level inferred from satellite altimetry is provided with respect to an ellipsoid. Ellipsoidal heights are converted into orthométric heights using geoid models interpolated along the satellite tracks. These global geoid models were inferred from geodetic satellite missions coupled with absolute and regional anomaly gravity data sets spread over the Earth. However, the spatial resolution of the current geoid models does not allow capturing short wavelength undulations that may reach decimeters in mountaineering regions or for rift lakes (Baikal, Issykkul, Malawi, Tanganika). We interpolate in this work the geoid height anomalies with three recent geoid models, the EGM2008, XGM2016 and EIGEN-6C4d, and compare them with the Mean Surface of 22 lakes calculated using satellite altimetry. Assuming that MLS mimics the local undulations of the geoid, our study shows that over a large set of lakes (in East Africa, Andean mountain and Central Asia), short wavelength undulations of the geoid in poorly sampled areas can be derived using satellite altimetry. The models used in this study present very similar geographical patterns when compared to MLS. The precision of the models largely depends on the location of the lakes and is about 18 cm, in average over the Earth. MLS can serve as a validation dataset for any future geoid model. It will also be useful for validation of the future mission SWOT (Surface Water and Ocean Topography) which will measure and map water heights over the lakes with a high horizontal resolution of 250 by 250 m.     

The Role and Contributions of Energetic Neutral Atom (ENA) Imaging in Magnetospheric Substorm Research

Energetic Neutral Atom (ENA) imaging has contributed substantially to substorm research. This technique has allowed significant advances in areas such as observation and quantification of injected particle drift as a function of energy, observation of dynamics in the tail that are directly related to the effects of imposed (growth phase) and induced (expansion phase) electric fields on the plasma, the prompt extraction of oxygen from the ionosphere during substorms, the relationship between storms and substorms, and the timing of substorm ENA signatures. We present discussion of the advantages and shortcomings of the ENA technique for studying space plasmas. Although the technique is in its infancy, it is yielding results that enrich our understanding of the substorm process and its effects.     

Observations of the Acceleration of Polar Solar Wind Near Solar Maximum

In a previous paper (Tappin et al., 1999) we used cross-correlation analysis of high-cadence observations with the LASCO coronagraphs to trace the acceleration of the solar wind at low latitudes. In this paper we present a similar analysis carried out over the North pole of the Sun. The observations which were made in March 2000 with the C3 coronagraph show low bulk flow speeds (comparable to or lower than those seen at the equator in early 1998). We observe the acceleration continuing to the edge of the C3 field of view at about 30 R _⊙. We also observe, as at low latitude, a high-speed tail but now reaching out well beyond 2000 km s⁻¹. We do not see a clear signature of a fast polar bulk flow. We therefore conclude that at this phase of the solar cycle, any fast bulk flow occupies only a small part of the line of sight and is therefore overwhelmed by the denser slow solar wind in these observations. We also show that the fast component is consistent with observed solar wind speeds at 1 AU. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic turbulence at the magnetopause, a key problem for understanding the solar wind/ magnetosphere exchanges

According to ideal MHD, the magnetopause boundary should split the terrestrial environment in two disconnected domains: outside, the solar wind (including its shocked part, the magnetosheath), and inside, the magnetosphere. This view is at variance with the experimental data, which show that the magnetopause is not tight and that a net transfer of matter exists from the solar wind to the magnetosphere; it implies that the frozen-in condition must break down on the magnetopause, either over the whole boundary or at some points. In the absence of ordinary collisions, only short scale phenomena (temporal and/or spatial) can be invoked to explain this breakdown, and the best candidates in this respect appear to be the ULF magnetic fluctuations which show very strong amplitudes in the vicinity of the magnetopause boundary. It has been shown that these fluctuations are likely to originate in the magnetosheath, probably downstream of the quasi-parallel shock region, and that they can get amplified by a propagation effect when crossing the magnetopause. When studying the propagation across the magnetopause boundary, several effects are to be taken into account simultaneously to get reliable results: the magnetopause density gradient, the temperature effects, and the magnetic field rotation can be introduced while remaining in the framework of ideal MHD. In these conditions, the magnetopause amplification has been interpreted in term of Alfvén and slow resonances occurring in the layer. When, in addition, one takes the ion inertia effects into account, by the way of the Hall-MHD equations, the result appears drastically different: no resonance occurs, but a strong Alfvén wave can be trapped in the boundary between the point where it is converted from the incident wave and the point where it stops propagating back, i.e., the point where k _\|=0, which can exist thanks to the magnetic field rotation. This effect can bring about a new interpretation to the magnetopause transfers, since the Hall effect can allow reconnection near this particular point. The plasma transfer through the magnetopause could then be interpreted in terms of a reconnection mechanism directly driven by the magnetosheath turbulence, which is permanent, rather than due to any local instability of the boundary, for instance of the tearing type, which should be subject to an instability threshold and thus, as far as it exists, more sporadic.     

Simulation experiments of the effect of space environment on bacteriophage and DNA thin films.

The main goal of PUR experiment (phage and uracil response) is to examine and quantify the effect of specific space conditions on nucleic acid models. To achieve this an improved method was elaborated for the preparation of DNA and bacteriophage thin films. The homogeneity of the films was controlled by UV spectroscopy and microscopy. To provide experimental evidence for the hypothesis that interplanetary transfer of the genetic material is possible, phage T7 and isolated T7 DNA thin films have been exposed to selected space conditions: intense UVC radiation (lambda=254 nm) and high vacuum (10(-4) Pa). The effects of DNA hydration, conformation and packing on UV radiation damage were examined. Characteristic changes in the absorption spectrum, in the electrophoretic pattern of DNA and the decrease of the amount of PCR products have been detected indicating the photodamage of isolated and intraphage DNA.     

AFIT neural network research

A brief summary of research done at the Air Force Institute of Technology (AFIT) in the area of neural networks is provided. It has been shown that backpropagation, used for feedforward artificial neural networks, is just a degenerate version of an extended Kalman filter, and that networks can do about as well as the optimum statistical classification technique. A method of finding the importance of features for use by a neural network classifier has been determined. Techniques for using neural networks for image segmentation have been developed. In optical pattern recognition, techniques that allow the processing of real FLIR (forward-looking infrared) images with existing binary spatial light modulators have been devised. An optical direction of arrival detector applicable to laser illumination direction determination has been designed and tested; the design is similar to a fly's eye. Coated mirrors for the optical confocal Fabry-Perot interferometer have been designed, specified, fabricated, and installed. Significant progress has been made in the use of neural networks for processing multiple-feature sets for speech recognition     

Design and implementation of near-field, wideband synthetic aperture beamformers

A coarray-based near-field, wideband synthetic aperture beamformer using stepped-frequency signal synthesis and post-data acquisition processing is presented. While coarray techniques offer significant reduction in the number of array elements for a given angular resolution, the hybrid subarray-stepped frequency realization of wideband systems simplifies implementations and offers flexibility in beamforming. Proof of concept is provided using real data collected in an anechoic chamber for several pulse shapes and array weightings.