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Although such high frequency signals are especially susceptible to attenuation in the atmosphere, they can propagate (travel) infinite distances in the vacuum of space with no signal loss. Attenuation can, however, be problematic within the few miles of atmosphere on the uplink and downlink segments. The uplink and downlink generally utilize different frequencies for transmission, so as to avoid interference between incoming and outgoing signals. As suggested in Table 3.5, the higher of the two frequencies is used for the uplink, as that signal is stronger and can better deal with atmospheric distortion. The lower frequency, used for the downlink, can better penetrate the earths atmosphere and electromagnetic field, which can act to bend the incoming signal much as light bends when entering a pool of water. Table 3.4 provides a set of example frequencies and spacecraft serving various applications, with LEO referring to Low-Earth Orbiting satellites.
Frequency Range | Band Designation | Example Spacecraft |
---|---|---|
136137 & 148 MHz | VHF | NOAA (LEO Weather) |
400 MHz | UHF | Orbcomm |
16101625.5 MHz | ||
2483.52500 MHz | Lband | Big LEOs |
23102360 MHz | Sband | Civil Defense Radio |
37004200 MHz | ||
59256425 MHz | Cband | Galaxy, Satcom, Telstar, Intelsat, etc. |
4 GHz6 GHz | Cband | Intelsat, Comsat, etc. |
11.712.2 GHz | ||
14.014.5 GHz | Kuband | Globalstar, etc. |
20 GHz & 30 GHz | Kaband | ACTS |
Source: International Telecommunications Union (ITU); Comsat |
In order to maximize the strength of such a high frequency signal, as well as to direct the uplink transmission to a specific satellite, the uplink radio beams are highly focused. The transmit antenna is centered in a concave, reflective dish which serves to focus the radio beam, with maximum effect, on the receiving satellite antenna. The receiving antenna, similarly, is centered in a concave metal dish, which serves to collect the maximum amount of incoming signal. Table 3.5 provides example uplink and downlink frequencies, with mention of standard applications.
Frequency Band | Uplink/Downlink Frequency Range | Example Application |
---|---|---|
Cband | 6 GHz/4 GHz | TV, Voice, Videoconferencing |
Kuband | 14 GHz/11 GHz | TV, DBS/DSS |
Kaband | 30 GHz/20 GHz | Mobile Voice |
Similarly, the downlink transmission is focused on a particular footprint, or area of coverage. Although a satellite can see roughly one-third of the earths surface from its vantage point, the signal would weaken so as to be unusable at the fringes of such a footprint, were the signal not tightly focused. Spot beams, even more tightly focused downlinks, serve specific applications over smaller regions.
The wide footprint of a satellite radio system allows a signal to be broadcast over a wide area. Thereby any number (theoretically an infinite number) of terrestrial antennae can receive the signal, more or less simultaneously. In this manner, satellites can serve a point-to-multipoint network requirement through a single uplink station and multiple downlink stations.
Recently, satellites have been developed which can serve a mesh network requirement, whereby each terrestrial site can communicate directly with any other site. Previously, all such communications were required to travel through a centralized site, known as a head end. Such a mesh network, of course, imposes an additional level of difficulty on the network in terms of management of the flow and direction of traffic.
Satellite radio systems consist of antennae and reflective dishes, much as does terrestrial microwave. The dish serves to focus the signal from a transmitting antenna to a receiving antenna. The send/receive dishes which make up the earth segment are of varying sizes, depending on power levels and frequency bands. They generally are mounted on a tripod or other type of brace, which is anchored to the earth, pad or roof, or attached to a structure such as building. Cables connect the antennae to the actual transmit/receive equipment. The terrestrial antennae support a single frequency band (e.g., Cband, Kuband or Kaband). A hybrid satellite may support a number of bands for various applications, such as radio, TV, paging, voice, and data. The higher the frequency band, the smaller the possible size of the dish, as the higher frequency dishes can be designed for greater signal gain at a smaller size. Therefore, while Cband TV dishes tend to be rather large, Kuband DBS (Direct Broadcast Satellite) TV dishes tend to be very small. Additionally, flat, passive, phased array dishes are being built in very small sizes and at very low cost for DBS TV and other applications. As a point of reference, the Intelsat I (1968) dishes were 30 meters in diameter.
The space segment dishes are mounted on a satellite, of course. The satellite can support multiple transmit/receive dishes, depending on the various frequencies which it employs to support various applications, and depending on whether it covers an entire footprint or divides the footprint into smaller areas of coverage through the use of more tightly focused spot beams. Satellite repeaters are in the form of transponders. The transponders accept the weak incoming signals, boost them, shift from the uplink to the downlink frequencies, and transmit the information to the earth stations. Contemporary satellites, such as Intelsat VI, commonly support as many as 46 transponders.
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