Satellite communication is used to deliver voice long distance telecommunications / data sevices, broadbannd Itnernet, television and communication with the remote and inaccressible places. Satellite communications is the uplink and downlink. Uplibnk conncets transmitting station and a satellite downlimnk connected to satellite receiving ground station. Qualty of the signal in the uplink depends on how storng the signal is transmitted from the soource station and a satellite receives it. On the downlink side, the signal quality depennds on how tsrong the stellite retransmits the signal and the station receives it.
Ground staitons vary in size antenna. Most station has an antenna with a 10 to 30 meters in diametr. Small stations with an antenna diammeter of 1 to 10 meters. VSAT (very small aperture terminal) has an antenna with a 0.3 to 1 meter in diameter. There are three parameters of the earth station must be consiered when designing stellite communciations. The first transmitter EIRP (effecytive isotropic radiated powr), which is equal to Pt (output powre) for several Gt (transmit antenna). EIRP action sends the sinal power of the transmitting station. The second parameter is the quality factor is equal to G (receive antenna), idvided by T (the system noise temperature). G / T measures the sensitivity of the receiving system and the quaality of the received signal. The third parameter is the system noise temperature, whch meassures the amount of noise power generated by receiving earth station.
The main goal of designers stellite link leas to a maxiomum data transfer rate while minimizing the BER (Bit Error Rate) in the receiver to the restrictions the subjects received power and hcannel banwdidth. Ber of satellite communications is a fuction of Eb / No (energy per bit to noisae density ratio) in the information channewl. Satellite communications system, the developer shoulld try to ensure a minimum Eb / No at the rceiver channels to meet the reference BER criteria, wjhich are also consistent with the restrictions on satellite RF transmission power and bandwitdh. In digiital transmissions, Eb / No in the envelope of the channel depends on the C / N (carrier to noise ratio) receiver, the type of modulation used in the envleope of the channel and the channel bandwidth. Thus, the main problem in the deign and analysis of satellite communications is the calclation and assessment of reference C / N, in whihc the carrier recevied power in a satellite ground statiion, and n the noise plus interference (in conjunction with ground statios ajacent satelllite) power at the receeiver.
Losses due to rain is the most inmportant for the violatiion of the transfer of satellite signal. Rain attenuation depending on the rain rate on the Earth and the locatiuon of saellite communication stations carrier ferquency. Mean rain rate (mm / h), the country can not be obtained in the department of meteorology in the country. Rain became a serious ipmact on the wavelength approach the size of raindrops. If the satellite is to maintain during the rainy season, the extra transmit power neefded to oveercome the maximum attenuaiton induced rain. Therefore, accurate assessment of the expected loss of rain shold make reference in the evluation parameters.
Grounnd station antnena look agnle (zaimuth & elkevation) can be calcuated from the lonbgitude of a geostationary satellite, plus latitude and longitude of groound station. Rejection of the antenna poining direction of the eelctrical axis of the antenna is an antenna pointing error. This error reduces the gain of the antenna earth statyion, because the antenna is maximum when measured at its electrical axis. Consequenly, the loss of the anttenna poinnt must be evaluated in the field of satellite communications and design, it applies to both transmit and receive antennas.
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