FAX systems are not intended as replacements for other standard communications methods. They are a useful supplemental system for rapid communications. Fleet Broadcasts Radio traffic is sent to the fleet by two methods: broadcast and receipt. The first is a “do not answer” method; the second, as its name implies, requires a receipt from addressees for each message. The broadcast method allows the fleet to preserve radio silence, which is a great advantage from the standpoint of security. Civilian and naval broadcasts have some similarity. Commercial stations in the broadcast band transmit programs to radio receivers in the homes in their communities. Likewise, Navy com- munications stations broadcast messages to fleet units in their particular geographic areas. The term broadcast, in fact, originated in naval communications. The resemblance between Navy commercial stations ceases here, however. Information broadcast by naval communications stations is contained in chronologically numbered messages assigned to the ships. Fleet units copy the messages and check the numbers to ensure they have a complete file of all messages they should have received. Automated systems now key fleet broadcasts. Messages are broadcast in their order of precedence. If the automated system receives a higher-precedence message while transmitting a lower-precedence message, it may interrupt the latter to transmit the higher-precedence message. All ships copy all messages addressed to them that appear on the broadcast schedule they are guarding. Fleet broadcasts use satellites as their primary transmission media, High-frequency (hf) radio transmission provides broadcast services to ships that are unable to copy the satellite systems. Satellite Communications A satellite communications (SATCOM) system is one that uses earth-orbiting vehicles or satellites to relay radio transmissions between earth terminals. A typical operational link involves a satellite and two earth terminals. One station transmits to the satellite on a frequency called the up-link frequency. The satellite amplifies the signal, translates it to the down-link frequency, and then transmits it back to earth where the signal is picked up by the receiving terminal. The Commander, Naval Telecommunications Command (COMNAVTELCOM), is designated the communications manager for Navy-assigned satellite systems. The responsibilities of the communications manager include operating the earth terminals and publishing Satellite Com- munications Operating Procedures (NTP-2). Commander, Naval Space Command (COM- NAVSPACECOM), is the operational manager for Navy satellites. The operational manager plans the location of spacecraft and fixed earth terminals and allocates satellite capacity, power, bandwidth, and operating frequencies. The Navy uses two primary SATCOM systems: • Long-haul (long-distance) communications takes place via the defense satellite communica- tions system (DSCS), which is managed by the Defense Communications Agency (DCA). This high-capacity global system uses satellites equally spaced around the world operating on superhigh frequencies (shf). Ships and stations located anywhere on the earth from 70 degrees north latitude to 70 degrees south latitude have access to one of these satellites. • The fleet satellite communications (FLTSATCOM) system operates at ultrahigh fre- quency (uhf), making possible the use of relatively low-cost terminals and simple antennas. Leased satellites (LEASAT) are part of this system. • FLTSATCOM provides the primary means of Navy tactical satellite ship-shore-ship communications over the officer in tactical com- mand information exchange subsystem (OTCIXS) and the tactical data exchange subsystem (TADIXS). The common user digital information exchange system (CUDIXS) and the naval modular automated communications system (NAVMACS) combine to form a general-service message traffic network. 12-19