from earth stations and adjacent satellites. In an up-link (from earth station to satellite), a loss of signal power due to rain can be overcome by supplying increased power to the transmission. Rain attenuation of the signal is significant only when there are heavy thundershowers. Fortunately such thundershowers usually cover only a small area of about a mile in diameter. One method of overcoming rain attenuation is to transmit data to alter- nate earth stations, carrying it over interconnecting facilities to the appropriate distribution points. A less costly method is to provide more sophisticated transmission technology and satellites. Since 110 TELECOMMUNICATIONS added signal strength is seldom needed in more than one place at a time to overcome rain attenuation, extra power could be provided in the satellite to penetrate heavy rainstorms, along with a mecha- nism for directing this power to the earth stations requiring it. A transmission technique called time division multiple access (TDMA) provides a means for increasing power during rainstorms, although no extra electrical power is used in the transmission. In TDMA, a single radio frequency is shared by multiple stations, with each sta- tion transmitting a burst of digital data during a specific time-slot. New satellite systems will use the higher frequency transmission bands as the lower frequency bands become saturated. In using these higher frequency bands, TDMA transmission techniques will allow effective digital transmission between the satellite and the earth sta- tions, even during atmospheric disturbances. 7.1.2 Satellite Communications Structure A limited number of common carriers offer satellite communications services. Figure 7-2 shows the organizations that are directly re- sponsible for the launch and control of satellite systems under their control, the common carriers using those satellite systems, and the areas of service for each satellite system. Satellite operating organization COMSAT Western Union Satellite system INTELSAT Comstar MARIS