Satellite Microwave

Satellite Microwave

Satellite microwave systems relay transmissions through communication satellites that operate in geosynchronous orbits 22,300 miles above the earth. Satellites orbiting at this distance remain located above a fixed point on earth.

Earth stations use parabolic antennas (satellite dishes) to communicate with satellites. These satellites then can retransmit signals in broad or narrow beams, depending on the locations set to receive the signals. When the destination is on the opposite side of the earth, for example, the first satellite cannot transmit directly to the receiver and thus must relay the signal through another satellite.

Because no cables are required, satellite microwave communication is possible with most remote sites and with mobile devices, which enables transmission with ships at sea and motor vehicles.

The distances involved in satellite communication result in an interesting phenomenon: Because all signals must travel 22,300 miles to the satellite and 22,300 miles when returning to a re-ceiver, the time required to transmit a signal is independent of distance. It takes as long to transmit a signal to a receiver in the same state as it does to a receiver a third of the way around the world. The time required for a signal to arrive at its destination is called propagation delay. The delays encountered with satellite transmissions range from 0.5 to 5 seconds.

Unfortunately, satellite communication is extremely expensive. Building and launching a satellite can cost easily in excess of a billion dollars. In most cases, organizations share these costs or purchase services from a commercial provider. AT&T, Hughes Network Services, and Scientific-Atlanta are among the firms that sell satellite-based communication services.

Satellite links operate in the low-gigahertz range, typically at 11-14 GHz. Costs are extremely high and usually are distributed across many users by selling communication services. Bandwidth is related to cost, and firms can purchase almost any required bandwidth. Typical data rates are 1-10 Mbps. Attenuation characteristics depend on frequency, power, and atmospheric conditions. Properly designed systems also take attenuation into account—rain and atmospheric conditions might attenuate higher frequencies. Microwave signals also are sensitive to EMI and electronic eavesdropping, so signals transmitted through microwave frequently are encrypted.

Earth stations can be installed by numerous commercial pro viders. Transmitters operate on licensed frequencies and require an FCC license.