□ Space segment equipment of a satellite communication
The space segment is the entire satellite system which is rotating around the earth in its geostationary orbit. And it consists of the satellite which has major two subsystems –
i). The Antenna
ii). Communication Repeater
The main frequency of the antenna is to provide shaped uplink and downlink beams. Repeater consists of–
i). Wideband communication receiver/down converter
ii). Input multiplexer
iii). Channelized TWTA
iv). Output multiplexer
The up-link signal is a filter of about 500 MHz bandwidth and then amplified using Low-noise amplifier. The initial amplifying stage in an earth station receiver designed to generate very little noise LNA with 2 to 4dB noise figure. The amplified signal is down converted to the downlink RF. The input multiplexer is employed to separate 500MHz bandwidth into individual transponder channels. TWTA’s are used to amplify low-level downlink signal for transmission to earth.
□ Ground segment equipment for a satellite communication
The digital signal through the terrestrial network enters the baseband equipment where it is processed (buffered, multiplexed, formatted, etc.). Error correction coding is performed by the encoder which reduces the error rate to an acceptable level. Thereafter it is modulated using the intermediate frequency (IF) carrier of 70MHz (+/- 18MHz), a standard frequency in a satellite communication for a communication channel using 36MHz transponder bandwidth. The modulated IF carrier is further modulated to the satellite uplink radio frequency (RF) carrier and is sent to High-power amplifier. The final amplifier in an uplink transmitter for transmission to the satellite.
One the receiver side, low noise amplifier (LNA) which amplifies it, keeping carrier to noise ratio at an acceptable level to meet the error rate equipment. The down converter translates it to the IF level which is fed to the demodulator where the digital stream of data is extracted.
□ Free space for a satellite communication
Free space is the medium between the satellite and Earth Station which offers certain obstruction for RF in both the up-link and down-link paths.
The received/transmitted signals undergo energy loss as they pass through the free space, termed as free space loss. Assuming transmission under clear sky conditions, the up-link free space loss is given by
|Free space loss (dB)
= 32.4 + 20 log(f) + 20 log(d)
|Where, d = maximum slant range (Km)
f = up-link carrier frequency
For- d, f in meters and kilohertz, respectively, the constant becomes -87.55
For- d, f in meters and megahertz, respectively, the constant becomes -27.55
For- d, f in kilometers and megahertz, respectively, the constant becomes +32.45