Space-based astrometry has a great tradition at ESA. The first space-based astrometric satellite in history, “Hipparcos”, was launched by ESA in 1989 and, in spite of orbital problems, was able to accomplish almost all of its tasks until it was finally shut down in 1993. The results of the Hipparcos mission were published by ESA in 1997 in the form of six CD-ROMs: the Hipparcos Catalogue contains 118,218 entries with median astrometric precision of around 1 milliarcsec, and specific results for double and multiple systems. In practice, Hipparcos drew for the first time the three-dimensional “map” of the spherical region of the Galaxy surrounding the Sun and having a radius of roughly 1,000 light years.
Then, in 1995, ESA launched the study of a new astrometric satellite, named “GAIA” and about a hundred times more powerful than Hipparcos, i.e. with median astrometric precision of around 10 microarcsec. This new satellite is intended to measure the parallaxes of over 50 million stars in the Galaxy, at least for the brightest stars, and this would mean to “draw” the three-dimensional map of the whole Galaxy, reaching out even to the Magellanic Clouds, 180,000 light years away.
The team of European scientists and engineers now designing GAIA, however, is facing hard technological difficulties. One of these is the design and coding of radically new and ultra-powerful mathematical algorithms for the on-board compression of the 50-million-stars data that GAIA will send to Earth from its intended geostationary orbit. Preliminary estimates of the raw data rates from the GAIA focal plane, in fact, are of the order of a few Gigabits per second. To reduce the data stream to the envisaged telemetry link of
1 Megabit per second, on-board data compression with a 1 to 1,000 ratio is the target. Clearly, this is far beyond the capabilities of any lossless compression technique (enabling compression ratios of 1 to some tens), and so some “wise” lossy compression mathematical procedure must be adopted.
In this paper a GAIA-adapted lossy data compression technique is presented, based on the Karhunen-Loève Transform (KLT). The essence of this method was already used by NASA for the Galileo mission when the large antenna got stuck and the mission was rescued by re-programming the on-board computer in terms of the KLT. That transform was officially named ICT — “Integer Cosine Transform” — by the NASA-JPL team led by Dr. Kahr-Ming Cheung. But the KLT here described for GAIA will of course differ from the JPL one in many regards, owing to the advances in computer technology.
Finally, estimates are also given about the possibility of using the KLT for onboard data compression in case GAIA is going to be put into orbit around the Lagrangian point L2 of the Earth-Sun system, and, above all, in case the number of stars to be observed is actually raised from 50 millions to one billion, as ESA currently appears to be likely to pursue. 相似文献
We report here on the science case of a concept for a satellite orbiting at 1 AU from the Sun and using a baffled Fizeau interferometer to look as close as possible to its limb. This configuration, and the need for looking nearby the Sun, is required for the main scientific driver of the mission, namely the measure of the γ parameter of the Parameterized Post-Newtonian formulation to the 10−6–10−7 level at least. This would lead to an accurate test of the General Theory of Relativity against other alternative theories of gravity, and set stringent constraints on some of the most significant issues of Astrophysics like those involving exotic forms of dark matter and dark energy. Exploiting the possibilities offered by the observation strategy, it is also possible to target other interesting scientific goals. One is, again, in the realm of General Relativity and aims at measuring the light deflection nearby the Giant Planets to detect asymmetric effects induced by their quadrupoles, predicted by GR but never measured so far. Others can be found in the observation of selected extrasolar systems where, e.g., the astrometric and photometric capabilities of GAME will help to improve on the knowledge of the brown-dwarf regime and on the search for exo-planets with the transit method, respectively. 相似文献
GPS radio occultation (RO) ionospheric products obtained by Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission during the year of 2014 and the observations from 3 digisonde stations which are located at different latitudes are used to study the influence of different time and space collocation windows on the comparisons of the ionospheric characteristic parameters (ICPs), including the peak density and peak height, derived from the two techniques. The results show that the correlation coefficients (CC) and the standard deviation of the absolute biases (SDAB) between the ICPs derived from the two techniques vary distinctly under different spatial and time collocation windows. Generally, the CC (SDAB) of the ICPs decrease (increase) as the size of the collocation window increases in time dimension or in space dimension. The rate of change of the statistic parameters with the increase in the size of the collocation window in time dimension and space dimension is analyzed for each digisonde station. It is found that within the collocation window of , the influence of the increase of in the space window on the statistical comparison is much more significant than that of the increase of 1?min in the time window, and it is suggested that there can be appropriate relaxation on the time window within the threshold of 60?min to get a balance between the quality of the comparison results and the number of the matched pairs. In addition, it is found that the same variations in the longitude window and in the latitude window may have different influences on the comparison results when the horizontal gradients in electron density are distinctly different along different directions at the digisonde station, and strict space collocation window is preferred when comparing the observations from COSMIC RO with those from the digisonde station in such cases. 相似文献