Download Communications Satellites. The Technology of Space by Larry Blonstein PDF

By Larry Blonstein

This article bargains an creation to communications satellites, explaining how they paintings and the way they're used. Written for the neophyte and previous hand alike, the publication assumes no previous wisdom of satellite tv for pc know-how and defines technical phrases as they're encountered. masking the whole variety of applied sciences utilized in area communications, Communications Satellites takes a step by step method of the method of choosing the ideal satellite tv for pc and Earth stations for various purposes

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Extra info for Communications Satellites. The Technology of Space Communications

Sample text

T h e electrical power system is not yet complete. It needs even more equipment because, twice a year, the satellite passes into eclipse. Figure 29 shows why, and when, eclipsing of the satellite occurs in the geosynchronous orbit. T h e Earth's spin axis is inclined at 23° to the plane of the Earth's o w n orbit round the Sun (the ecliptic plane) so the Earth's Equator is inclined at that angle too. And, because it is in line with the Equator, so is the plane of the geosynchronous orbit. Looking first at m i d s u m m e r in the northern hemisphere (to the right of Figure 29) it can be seen that the satellite will be in sunlight around its entire orbit - even when it is 'behind' the Earth, it can see 'over' the top of the Earth's shadow.

T h e fuel and oxidant tanks in fact contain much more fuel than what is needed for station-keeping corrections. As explained in Chapter 3, the satellite must be accelerated to its correct orbital speed of 7000 m p h at geosynchronous orbit from its transfer orbit apogee speed of 3000 mph. Most of the fuel on board is consumed by the liquid apogee engine that provides this acceleration. If a solid-fuelled apogee m o t o r is employed, this extra fuel is not required. However, liquid engines provide more 27 Turning a crate into a satellite thrust per pound than solids, and offer an additional advantage: the amount of fuel put on board for the apogee engine is based on estimates that allow for errors in the positioning and speed of the satellite caused by the launcher; if those errors are less than estimated, less fuel is required, and the unspent fuel can be used to increase the lifetime of the satellite.

In many cases, the horn is mounted on the centre-line of a circular antenna, supported by struts, k n o w n as a centre-fed system (Figure 36). These struts interfere with the antenna beam pattern, and centre-fed systems are being increasingly replaced by offset systems, in which the horn is kept out of the beam by being offset to one side of an elliptically-shaped antenna (Figure 37). In theory, energy emitted from a single point at the focus of a parabolic antenna will result in a beam that is exactly parallel, as implied in Figure 35.

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