Artificial satellite - SpeedyLook encyclopedia

Classification of the satellites according to their orbit

geosynchronous, to which belong the geostationary satellites (Télécommunications, direct Télévision, weather…) who remain fixed with the top of a point of the terrestrial equator;
with run, and in particular satellites in low orbit or LEO (Radio ham, Constellation S of mobile telephony, scientist);
heliosynchronous (polar) used in particular for the Teledetection; its advantage is that the portion of the Earth observed always has the same lighting from one passage to another.

Fields of application

Scientific satellite

A scientific satellite is intended for scientific studies since the space. They are the first applications since Sputnik 1. That can relate to the Géodésie, the Géodynamique, the Médecine, etc the observation of the universe is done with satellite of astronomy.

Satellite of astronomy

These satellites observe space with a scientific aim. It is acted in fact of Télescope S in orbit. There too, the spectrum of observation is broad. Examples: the satellite Hubble, Chandra (observatory in x-rays), ISO, Corot, Herschel, Planck.

Telecommunications satellite

These satellites are used to transmit information of a point to the other of the Ground, in particular the telephone calls, the data transmission (for example Thuraya), the communications satellite and the televised programs. The latter forward mainly by the fleet Eutelsat (Hot-Bird, Atlantic Bird 3, W1,2,3, etc) and that of ITS or European Company of Satellites, Astra 1 and 2. These satellites known as also of direct diffusion emit paying and encrypted ranges of channels, as well as hundreds of TV&Radio chains in light and free, received on a antenna, type Parabole, domestic low-size (< 60 cm) and of weak price, which the strong power of emission of the satellites of diffusion allows.

Satellite of teledetection

The satellites of Télédétection observe the Earth, with a scientific aim (temperature of the sea, snowy coat, dryness,…), economic (natural resources, agriculture, etc) or soldier (important role in the contemporary wars; they are more usually indicated under the name of spy satellites). The spectrum of observation is vast, optical, radar, infra-red, ultra-violet, listening of radioelectric signals… For example, the satellites of the family SPOT observe the terrestrial ground, METEOSAT is a family of Meteorological satellite . The resolution reaches currently less than one meter for certain frequency bands. That Ci depends naturally on technology employed but also on the altitude of the satellite: a good resolution requires a low orbit, but does not allow a monitoring in real-time by the same sensor, this last returning above the same site only after one long time.

The satellites Radar can analyze, by interferometric techniques , variations of a few millimetres of certain structures. They are useful to examine the movements of the continental plates, particularly before or after a Séisme, or the variations thickness of the ice-barrier.

The first satellite with properly weather goal is TIROS-1, launched on April 1st 1960 and followed by long series. In 1971 is launched Eole , satellite French who could analyze the trajectories of balloons probes and help to determine the circulation of the winds in height.

Satellite of positioning

Space station

These satellites are intended to be inhabited by the man, with a scientific aim, technical or industrial, or to be used as relai for more remote missions. After the stations Salyut and Russian Mir and the American station Skylab, the International space station is in orbit since 1998. She is inhabited permanently since 2002.

Space probe

A space probe is intended to observe another Celestial body: planet, asteroid, comet, either at the time of a simple overflight (To travel), or in a way prolonged while putting into orbit itself around the celestial body (Cassini-Huygens, Galileo).

Problems of transmission

Since a satellite Géostationnaire is with approximately 36 000 km of altitude, a radio wave put a little more than 100 ms to reach it, and to be conveyed with its final destination as much, from where the acknowledgment of delivery sets out again in opposite direction, the total advance thus representing 400 ms. Not only this time is shown very awkward at the time of the telephone calls (phenomenon of echo), but it complicates the management of the acknowledgment notably of delivery in the packet transmissions, the work-in-progress amounting then per million. Various techniques, of which historically the first was the method ALOHA try to circumvent the problem.

Example: on a channel ATM current to 622 Mb/s, it is appropriate to notice that the bits in transit in the atmosphere (already parties and not arrived yet) are constantly 124 million, that is to say 15,5 Mo! That means that contents of information corresponding to 10 diskettes are suspended some share in space without material support.

Composition of a satellite

the Payload allowing the satellite to conclude its mission.
the Platform providing the additional functions the mission.

Collect and data processing satellite

The satellite Donnée S must be collected and treated in systems ready to manage information géospatiales.

For example, in the field of the weather data, the world research program on climate (WCRP) involved the creation of the ISCCP (International Satellite Cloud Climatology Project).

The satellite data are collected in processing centers on sectors or special surfaces, then aggregate in a total center of traitemet (ISCCP overview).

Satellites and space pollution

One half-century of space exploration generated an inevitable pollution and several scientists and international agencies seriously pose the problem of the management of tens of million space waste in orbit around the ground.

This waste includes/understands the old satellites become obsolete, the tools, tramp irons, of plastic left on the spot by the astronauts, etc; each launching of satellite generally left in orbit various stages which, while separating, release in their turn of small fragments… (The current trend is to once cause quickly the re-entry of the last stage in the atmosphere the satellite put into orbit). Thus, according to the evaluations, of the million objects turn around the ground without any control, of which more 10 000 exceeding 10 cm (one envisages of them 5 times more within one century).

In 50 years, the number of objects in space exceeded the critical mass right now where the chain reaction seems inevitable. Each collision between remains creates other remains indeed, and so on. Moreover, this waste is a constant threat for the satellites in activity and the space stations current (ISS) or future in which astronauts will remain in a permanent way. Even if they seem negligible by their size, their mean velocity of 7 km a second makes any impact catastrophic and no current shielding resists the objects having a size higher than 1 or 2 cm (estimated at 300 000).

By their displacement in low orbit, the friction of the objects in the atmosphere involves a natural wear over one duration of a few years to several hundred years, but on the higher orbits time amounts in millenia even much more… The ideal would be thus to recover these space objects, but at present no viable technical solution emerges to collect them, which does not prevent companies from proposing the sending of funeral urns in space….

In January 2007, the test of an anti-satellite missile by the China, causing the explosion of one their old meteorological satellites orbits about it, increased at least 1970 remains (census at at the beginning of July 2007) the space dustbin : a significant risk for the International space station and many other satellites which circulate at altitudes encumbered already well.

Today, the place of each new launched satellite (a score on average per year, including three quarters devoted to telecommunications) is carefully studied according to the obstruction of the orbits. A few kilos of fuel are preserved in the tanks so that, at the end of the lifetime, they are directed on an orbit " poubelle". The future of the space activities in orbit depends on it.

Notes and references of the article

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