some of the various earth terminals and how they interface. The end use or purpose determines the system’s complexity and how the system is used. ROLE OF SATELLITE COMMUNICATIONS SATCOM links, one of several kinds of long- distance communications links, interconnect com- munications centers located strategically throughout the world. These SATCOM links are part of the Defense Satellite Communications System (DSCS) and Fleet Satellite Communications. Satellite communications systems are very important to the worldwide military com- munications network for two primary reasons. First, they continue to operate under conditions that cause problems for other methods of communication. Second, they provide reliable and secure com- munications to previously inaccessible areas. In many cases, these communications requirements can o n ly be satisfied by sophisticated satellitecommunications systems. By satisfying such needs, SATCOM makes a significant contribution to the improved reliability of naval communications. ADVANTAGES OF SATELLITE COMMUNICATIONS Some of the unique advantages SATCOM has over conventional long-distance communications are as follows: SATCOM links are unaffected by the propagation problems associated with hf radio communications. SATCOM links are free from the high-attenuation problems of facilities that use wire or cable for routing communications. SATCOM links span long distances. The numerous repeater stations required for line-of-sight and troposcatter systems are not needed. As you can see, satellite links provide the required flexibility and reliability needed to support military operations. In the following paragraphs, we will look at SATCOM capacity, reliability, vulnerability, flexibility, and limitations. Capacity Currently, military SATCOM systems can provide communications between backpack, shore, airborne, and shipboard terminals. These SATCOM systems can handle thousands of communications channels at the same time. Reliability SATCOM frequencies are only slightly affected by atmospheric phenomena and do not depend onreflection or refraction. Reliability is based on the skill of operators and maintenance personnel and the condition of the satellite communications equipment. Vulnerability Communications satellites are relatively safe from threats of harm. Because these satellites are in such high orbits, any attempt to disable or destroy them from the Earth would be difficult and expensive. However, Earth terminals are a different story. They offer a more attractive target for destruction by conventional methods. But these terminals can be protected by the same methods taken to protect other vital installations. So overall, the satellite system is nearly free from harm by an enemy. Operationally, highly directional earth terminalantennas provide a high degree of freedom from jamming. The wideband system can use antijamming techniques, which also reduces vulnerability. Flexibility Mobile military satellite earth terminals with trained crews can be deployed and put into operation anywhere in the world within hours. Limitations The technical characteristics of the satellite and its orbital parameters are the main limitations to a satellite communications system. Two additional limiting factors for active satellites are transmitter power andreceiver sensitivity. Energy for electricity is limited to whatever can be produced by the solar cells, which limits the satellite’s output power. This problem is made worse by users who increase their output power to the satellite, causing the satellite to try to retransmit at the new power level, at the expense of reducing signals to other users. 3 -2
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