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   satellite communications   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  on reflection 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   terminal antennas   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  and receiver 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