receiving  system.  With  the  path  length  and  direction fixed,   propagation   factors   are   simplified.   This provides  highly  reliable  hf  communications. powered transmitters, lower noise receivers, and more efficient  antennas. Fleet Broadcast Ship-to-Shore High-frequency    atmospheric    communications between   shore   stations   are   relatively   easy   because shore   stations   have   sufficient   space   for   efficient omnidirectional   antennas   or   arrays   that   provide   hf coverage of large areas. Ship-to-shore hf communica- tions are more difficult because the ship is moving and constantly   changing   direction.   This   change   of direction  and  severe  space  limitations  aboard  ships make  the  installation  of  large,  efficient  hf  antennas impractical. To   overcome   these   problems,   ship-to-shore systems  have  two  major  differences  from  point-to- point   systems.   First,   shipboard   antennas   are   omni- directional.  Second,  several  frequencies  are  usually assigned  for  each  circuit.  If  one  frequency  starts  to drop   out,   another   can   be   selected   to   match   the propagation path conditions between the ship and the shore  terminal. Ground-to-Air The   use   of   hf   radio   for   ground-to-air   com- munications   is   similar   to   its   use   for   ship-to-shore communications.   An   additional   problem   is   that   an aircraft  moves  much  more  rapidly  than  a  ship.  This rapid   movement   (plus   additional   space   limitations) requires that all major circuit im-provements be made at the ground stations. Examples of improvements that can  only  be  made  to  the  ground  station  are  higher As   the   name   implies,   this   service   involves broadcast  area  coverage  from  shorebased  transmitters to  ships  at  sea.  To  overcome  propagation  problems, messages  are  sent  on  several  frequencies  at  the  same time  (frequency-diversity).  Space-diversity  with physically   separated   receive   antennas   also   helps overcome  propagation  problems. Now  let’s  look  at  typical  shipboard  high-frequency transmit and receive systems. Shipboard  HF  Transmit The  high-frequency  transmit  signal  can  contain either voice or teletype information. Figure 2-5 shows a  typical  shipboard  high-frequency  transmit  system. The  same  equipment  used  to  receive  teletype messages   on   low   frequencies   (teletype,   DC   Patch Panel   SB-1210/UGQ,   crypto   equipment,   and   DC Patch  Panel  SB-1203/UG)  are  used  to  send  teletype messages on the high-frequency system; but of course, in reverse order. An AN/UCC-1(V) or CV-2460 telegraph terminal converts a dc signal into a tone signal. This signal is fed to the SB-988/SRT transmitter transfer switchboard. A C1004 transmit keying and control/teletype is used to key   the   transmitter   during   tty   operation.   Voice communications also can be connected to the SB-988/ SRT  switchboard.  The  voice  communications  are developed at a handset connected to the C-1138 radio Figure  2-5.—Shipboard  hf  transmit  system. 2-6