Satellite Ground Station Basics
The stations mainly designed to interface with the orbiting satellite for various applications are referred as satellite ground stations or earth stations or Hub stations.
Satellite Ground station play a critical role in a satellite network as it need to take care of recovering the informaion from received signal which is very weak and noisy after travelling path from the satellite to the earth.
Ground station antennas usually will have antenna size based on the application. For INTELSAT application for example it will be about 30 meter in diameter. For other satellites it will be about 7.5 or 11.2 meter in diameter. To receive DBS TV signals it will be very small and will have 0.7 meter of diameter and is referred as dish antenna.
Figure depicts modules at the typical earth station. As mentioned just after that antenna there is a diplexer which separates uplink and downlink frequencies and pass them to appropriate modules in the system. The downlink signals are passed to LNA(Low noise amplifier) and uplink frequencies from the PA(Power amplifier) are trasmitted to the antenna without any leakage to the LNA module.
Let us understand the receiving process at the satellite earth station.
• received satellite signals are passed through LNA which amplifies the signal and adds less noise to it.
• The LNA amplified signal is passed to the power divider which is the RF power divider, the output is connected to the RF down converters(DC).
• The down converter converts RF frequency(RF IN) to the IF frequency(IF OUT).
This IF frequencies are connected to demodulators(IF IN port of the modem). • Demodulator converts modulated IF signal and retrieve information which is in the form of multiplexed signal, hence need to be passed to demultiplexer.
• the demultiplexer separated signals and based on information type passed to EPABX if it is voice and to the PC/LAN network if it is data.
• If the destination of the information is not that ground station then it need to be relayed either using terrestrial network, point to point microwave link or fiber optics or back to the satellite in the case of mesh networks.
Let us understand the transmission process at the satellite earth station.
• Various information signals either voice or data channels originated from exchange or voice lines or computers are multiplexed using Multiplexer.
• This multiplexed signals are modulated using modem. One modem is required for one VSAT station as per network configuration.
• This modulated IF signal is converted to modulated RF signal using RF Up converter(UC).
• All the up converted RF signals from multiple up converters are combined using RF combiner.
• The output of power combiner is given to power amplifier(PA) before the signal is fed to the antenna via diplexer.
The carrier to noise power ratio at the ground station depends on G/T of the ground station antenna.
Here G is the antenna gain and T is the system noise temperature.
G/T= G(dB) - 10 Log (T), Where T is the temperature in kelvin
C/N = ((Pr G)/([k Ts B] G)) = (Pr/K Ts B)
Typical Satellite Earth Station Specifications
Uplink frequency - 14.437 Ghz
Downlink frequency - 11.637 Ghz
Main earth station:
Diameter of Antenna - 7.5 meter
G/T Ratio - 25.7 dB/K
Remote earth station:
Diameter of Antenna - 4.5 Mtr
G/T Ratio - 19.7 dB/K
SATELLITE RELATED LINKS
Refer VSAT System Overview.
Refer our page on RF Link Budget which covers basics of RF link budget and friis equation.
Example calculation of VSAT-1(2.4 meter) to Ku band satellite to VSAT-2(2.4 meter) star system RF Link Budget, Download Excel sheet
Refer our Page on LNB basics and manufacturers.
Refer our page on RF LNA manufacturers.
Refer our page on VSAT subsystem equipment Providers.
Antenna diversity types SCPC MCPC Set Top Box Receiver sensitivity versus receiver selectivity VSAT NMS protocol