Link Design For Satellites

VSAT Satellite Antenna

VSAT Satellite Antenna

Satellites are designed to deliver voice/data services, broadband net solutions, television broadcast and communications to distant areas. Satellite link technology includes both a downlink and also an uplink to facilitate the transfer of data. The satellite Uplink will transfer from an earth terminal to the satellite and Downlink connects the satellite to the receiving earth terminal.

The signal quality in the uplink connection relies on just how solid the signal is transferred from the source terminal as well as exactly how the satellite receives it. On the downlink side, a high signal quality depends upon how strong the satellite retransmits the signal and exactly how the earth station can receive it.

Earth satellite antennas are most often classified into three categories.

  • VSAT which stands for Very Small Aperture Terminal, will have a smaller antenna size, generally with a diameter from 1/3 meter to 1 meter.
  • Small earth stations will have an antenna diameter of somewhere between 1 and 10 meters in size.
  • Large stations will usually have a antenna dish diameter larger than 10 meters.  Most are smaller than 30 meters, however larger antenna do exist.

There are three earth terminal system specifications that should be taken into consideration when making satellite telecommunication links.

  1. Transmitter EIRP (Effective Isotropic Radiated Power) gauges the signal power sent out from the transfer earth station.
  2. Figure of Merit is the measure of the sensitivity of the receiving station and the signal quality received there.
  3. System Noise Temperature determines the quantity of noise power produced by the receive earth station.

A good satellite link design will maximize the link data throughput while minimizing the error rate of the transmitted signal (called the BER or bit error rate). The design must meet requirements for BER within the constraints of transmission power and RF bandwidth.

The major concern in design as well as evaluation of a satellite link is to determine C/N in which C is the obtained provider power in the earth station/satellite and N is the sound plus noise (caused by nearby earth station antennas) power in the receiver.

ATM Microwave Pyramidal Horn AntennasHorn Antennas and Pyramidal Microwave Horn Antennas are used quite often in microwave RF satellite link designs and applications. Horn antennas like those shown at right are available from http://www.atmmicrowave.com

Loss because of rainfall is the most crucial problems to the transmission of a satellite signal. Rainfall attenuation is a function of rainfall rates at the earth station as well as satellite link carrier frequency. Typical rainfall rates in a given area can be obtained from the division of Meteorology of that nation. Rain effects end up being extreme depending on the size of the raindrops. Added transfer power is needed to conquer the maximum attenuation generated by the rain if the satellite link is to preserved throughout a rainstorm without interruption. Precise assessment of expected rain loss needs to be made when evaluating satellite link criteria.

Satellite Earth Station Antenna Maintenance

The earth antenna look angle (azimuth & elevation) can be determined utilizing the longitude of the geosynchronous satellite plus the latitude as well as longitude of the planet station. Antenna loss has to be reviewed carefully in satellite link designs. It must be considered carefully since it affects both transfer and receive antennas.

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