DNA sequence and the organization of chromosomal domains

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• 作者：Tobias Straub ; Peter B Becker
• 关键词：Catalysis ; Electrochemical properties ; Nanocrystals
• 刊名：Current Opinion in Genetics & Development
• 出版年：2008
• 出版时间：April 2008
• 年：2008
• 卷：18
• 期：2
• 页码：175-180
• 全文大小：288 K

文摘

The “parachute” concept presented here is a generic definition for earth observation systems essentially made of a reflector under which a detector associated with a telemetry antenna is suspended [D. Massonnet, (Applicant), Satellite, method and a fleet of satellites for observing a celestial body, Patent 0509-1112, 2006. [1]; D. Massonnet, (Déposant), Satellite, procédé et flotte de satellites d’observation d’un corps céleste, Priorité 04-04327, 2004. [2]]. The concept can be applied to any wavelength (radar, visible and infrared) in passive or active mode (for the latter the suspended part also includes a transmitter). The concept is characterized by its simplicity and performances. We illustrate it in the optical domain by describing a constellation composed of 10–15 kg nanosatellites without propulsion and an attitude control passively obtained by gravity gradient. The elementary satellite does not include a computer nor any storage for energy or data. All satellites transmit their data using the same frequency band, gathered on the ground by stations within the visibility circle. The system works by saturation: with 100 satellites (total constellation mass of 1500 kg) each with 20-km swath width and capable of a resolution on the order of the metre, the probability to reach any point on the equator in less than 24 h is 53 % . At the latitude of Paris, the probability reaches 68 % . With 200 similar satellites, these probabilities are 78 % and 90 % , respectively. Without propulsion and taking into account the possible roll of the suspended part, the position of the acquired imagery is precisely known only after reception. This leads to new architectures for the ground segment: orders are placed within an archive very frequently updated. In addition, several satellites may be simultaneously visible from a given station, which should ideally have several antennas. Among other features, the system is extremely robust (very simple satellites in great number) and in practice invulnerable. The uplink is drastically reduced: the satellites turn on as soon as they are sufficiently illuminated by the sun; the reflector is easily made wide enough to achieve metre resolution from low altitude, whereas the length of the “suspending rods” allows a large focal length that in turn forgives length errors. Without propulsion the choice of orbital altitude is limited to less than 600 km or more than 2000 km in order to observe space debris regulations. In folded position, the satellite is essentially no more bulky than its reflector that has the solar array fixed on the non-reflecting side. Even a small launcher is capable of injecting a few tens of these satellites into orbit.