Your RF Cafe
1-1 shows the radio-frequency spectrum broken down into nine bands used by the military. Propagation of radio waves varies widely at different frequencies. Frequencies and equipment are chosen to meet the communications application desired. We will discuss the radio-frequency spectrum in the following paragraphs.
Table 1-1.—Radio-Frequency Spectrum
Extremely Low-Frequency Communications
The purpose of the EXTREMELY LOW-FREQUENCY (elf) communications system is to send short "phonetic letter spelled out" (PLSO) messages from operating authorities in the continental United States (CONUS) to submarines operating at normal mission speeds and depths. Elf has the ability to penetrate ocean depths to several hundred feet with little signal loss. This ability allows submarines to be operated well below the immediate surface and enhances submarine survivability by making detection more difficult.
This is a one-way communications system from the operating authority to submarines at sea. The large size of elf transmitters and antennas makes elf transmission from submarines impractical.
The communications commitments of the Navy now cover the face of the earth. New sea frontiers to the north have opened a four-million-square-mile, ice-covered ocean of strategic importance. Our Navy must maintain control of the operating forces in an ever expanding coverage area. This additional area requires changes in communications capacity, range, and reliability. Additional needs have been particularly great in the North Atlantic and the newly opened Arctic Ocean. High-frequency circuits are too unreliable in these areas because of local atmospheric disturbances.
VERY-LOW-FREQUENCY (VLF) transmissions provide a highly reliable path for communications in these northern latitudes as well as over and under all oceans and seas of the world. At present, practically all Navy VLF transmitters are used for fleet communications or navigation. The VLF transmission is normally considered a broadcast, that is, one-way transmission, no reply required. The VLF transmitter normally transmits single-channel RTTY.
VLF is currently used for communications to large numbers of satellites and as a backup to shortwave communications blacked out by nuclear activity. Our Navy depends on VLF for crucial communications during hostilities.
Secondary applications of the VLF range include worldwide transmission of standard frequency and time signals. Standard frequency and time signals with high accuracy over long distances have become increasingly important in many fields of science. It is essential for tracking space vehicles, worldwide clock synchronization and oscillator calibration, international comparisons of atomic frequency standards, radio navigational aids, astronomy, national standardizing laboratories, and communications systems.
A VLF broadcast of standard time and frequency signals has more than adequate precision for the operation of synchronous cryptographic devices, decoding devices, and single-sideband transmissions.
The LOW-FREQUENCY (lf) band occupies only a very small part of the radio-frequency spectrum. This small band of frequencies has been used for communications since the advent of radio.
Low-frequency transmitting installations are characterized by their large physical size and by their high construction and maintenance costs. Another disadvantage is low-frequency signal reception being seriously hampered by atmospheric noise, particularly at low geographical latitudes. Over the years, propagation factors peculiar to the low-frequency band have resulted in their continued use for radio communications. Low-frequency waves are not so seriously affected during periods of ionospheric disturbance when communications at the high frequencies are disrupted. Because of this, the Navy has a particular interest in the application of low frequencies at northern latitudes.
The Navy's requirement to provide the best possible communications to the fleet requires operation on all frequency bands. Constant research is being done to improve existing capabilities and to use new systems and developments as they become operationally reliable.
In the past, the fleet broadcast system provided ships at sea with low-frequency communications via CW telegraph transmissions. As technology advanced, the system was converted to single-channel radio teletypewriter transmission. Today If communications is used to provide eight channels of frequency- division multiplex RTTY traffic on each transmission of the fleet multichannel broadcast system.
The MEDIUM-FREQUENCY (mf) band of the radio-frequency spectrum includes the international distress frequencies (500 kilohertz and approximately 484 kilohertz). Some ships have mf equipment. If desired the distress frequencies may be monitored. When this is done the transmitter usually is kept in the standby position. Ashore, the mf receiver and transmitter equipment configuration is usually affiliated with search and rescue organizations, which are generally located near the coast.
Only the upper and lower ends of the mf band have naval use because of the commercial broadcast band (AM) extending from 535 to 1,605 kilohertz. Frequencies in the lower portion of the mf band (300 to 500 kilohertz) are used primarily for ground-wave transmission for moderately long distances over
water and for moderate to short distances over land. Transmission in the upper mf band is generally limited to short-haul communications (400 miles or less).
SHIP-TO-SHORE.—This application of the HF band is more difficult than point-to-point since the ship is moving and constantly changing its position. In ship-to-shore the path length and direction are variable. Aboard ship, limited space and other restrictions prohibit installation of large, efficient HF antennas. Because of the mobility of ships, shipboard antennas are designed to be as nearly omnidirectional as possible.
Our problems are not as severe at the shore terminal where we have sufficient space for efficient omnidirectional antennas or arrays designed for coverage of large areas of the earth. At shore stations, rotatable, high-gain antennas or fixed, point-to-point antennas are used. For example, a rhombic antenna ashore may work well for long-haul, ship-to-shore communications when the ship is within range of the antenna.
Several frequencies are usually assigned for each circuit. Therefore, a frequency can be selected that best matches the propagation path conditions between the shore terminal and the ship.
GROUND-TO-AIR.—The use of HF radio for ground-to-air communications is similar to ship-to- shore. The only exception is an aircraft moves more rapidly than a ship. All major circuit improvements must be made at the ground station. For example, higher powered transmitters, lower noise receivers, and more efficient antennas must be used on the ground.
FLEET BROADCASTS.—As the name implies, this service involves broadcast area coverage from shore-based transmitters to ships at sea. Messages to be sent to ships are delivered by various means to the proper broadcast station. They are then broadcast for shipboard reception. To overcome propagation problems, naval communicators send the messages on several frequencies at once. This is known as frequency-diversity transmission. This type of transmission allows the ship to choose the best frequency for reception. Space-diversity with physically separated receive antennas also helps to overcome this problem.
Very-High-Frequency and Above Communications
Frequencies above 30 megahertz are not normally refracted by the atmosphere and ground-wave range is minimal. This normally limits our use of this frequency spectrum to line of sight. The exception to this is increased range through the use of tropospheric scatter techniques. Some communications using VHF and above frequencies use a technique called forward propagation by tropospheric scatter (fpts). This method will be discussed in more detail in chapter 5.
Certain atmospheric and ionospheric conditions can also cause the normal line-of-sight range to be extended. Frequencies at the lower end of this band are capable of overcoming the shielding effects of hills and structures to some degree; but as the frequency is increased, the problem becomes more pronounced. Reception is notably free from atmospheric and man-made static. (The VERY-HIGH- FREQUENCY (VHF) and ULTRAHIGH-FREQUENCY (UHF) bands are known as line-of-sight transmission bands.) Because this is line-of-sight communications, the transmitting antenna is in a direct line with the receiving antenna and not over the horizon. The line-of-sight characteristic makes the VHF band ideal for amphibious operations (beach landing from sea craft) and the UHF well suited for tactical voice transmissions (maneuvering of ships traveling together). The SUPERHIGH-FREQUENCY (SHF) band is used for radar and satellite communications, whereas the EXTREMELY HIGH-FREQUENCY (EHF) band is used only in the experimental stage.
Q9. he majority of VLF transmitters are used for what purpose?
Q10. Today the Navy uses lf communications as a segment of what operational system?
Q11. Why does the Navy only use the upper and lower ends of the mf band?
Q12. What are the four general types of communications services in the HF band?
Q13. A message transmitted on several frequencies at the same time is an example of what type of transmission?
Q14. Physically separating receive antennas is an example of what technique?
Q15. When using frequencies above 30 megahertz, you are normally limited to using what range?
Now that we have learned the Navy's fundamental use of the various frequency bands, we should look at the types of communications links and their modes of operation. The Navy uses many modes of operation; the type used is based upon the function of the circuit or network. These modes (or functions) are combined to form a communications link. We will also discuss some of the actual networks the Navy uses on a daily basis.
A complex of links forms a major communications system. The naval communications system is broken down into strategic and tactical groups.
Strategic communications are generally world-wide in nature. They are operated on a common-user (Navy, Army, Department of Defense, and so on) or special-purpose basis. A strategic system may be confined within a specified area or limited to a specific type of traffic, but the configuration is designed so that combined operations with other strategic systems are possible. As an example, we will look at the automatic voice network, automatic digital network, and the defense special security communications system later in this chapter.
Tactical communications are usually limited to a specific area of operations and are used to direct or report the movement of specific forces. Some tactical networks are used only for operational traffic; others may be used for operational and administrative traffic. For instance, the task force, task-group, and air-control networks are ordinarily used for operational traffic. Ship-to-shore networks and broadcast networks serve both types of traffic.
Modes of Operation
Communications links have numerous modes of operation. In our discussion, a mode of operation is identified as a link or path between two or more points that is capable of providing one or more channels for the transmission of intelligence. Let's take a look at the five most common modes of operation.
SIMPLEX.—The simplex (SPLX) mode uses a single channel or frequency to exchange information between two or more terminals. Communications is in one direction only.
HALF DUPLEX.—The half-duplex (HDX) mode has one-way flow of information between terminals. Technical arrangements often permit transmission in either direction, but not simultaneously. This term must be qualified to show s/o (send only), r/o (receive only), or s/r (send or receive).
SEMIDUPLEX.—The semiduplex (SDX) uses an arrangement of equipment where one terminal is simplex configured and the other uses two channels or frequencies in full duplex. A clarifying example is
a ship in a simplex mode terminated full duplex with a shore station. The ship may send or receive but not do both at the same time.
FULL DUPLEX.—The full-duplex (FDX) mode is a method of operation in which telecommunications between stations takes place simultaneously in both directions using two separate frequencies. In other words, a ship may send and receive different messages at the same time. The term "full duplex" is synonymous with "duplex."
BROADCAST.—Broadcast (BC) is the type of operation in which one station transmits information on one or more channels directed to more than one station and/or unit. The broadcast system has no provision for receipt or reply; however, special arrangements may require the receiving station to reply or receipt for the message at a later time by other means. Broadcasts are the primary means of delivering messages to the fleet. Since Navy units copying broadcasts are not required to receipt for messages received, they can maintain radio silence while still receiving essential messages.
Message traffic is normally sent to the fleet by three methods: broadcast, intercept, and receipt. The first two are "do not answer" methods; the third, as its name implies, requires a receipt from the addressee (addee) for each message. Broadcast and intercept methods allow the fleet to preserve radio silence, which is a great advantage from the standpoint of security. By the intercept method, a shore radio station transmits messages to another shore station that repeats them back. Ships intercept and copy all of this message traffic.
Broadcast is preferable to intercept chiefly because it is faster. It is the method by which nearly all fleet traffic is handled. It uses radiotelegraph, radiotelephone, radio teletypewriter, and facsimile.
There is some similarity between civilian and naval broadcasts. Just as commercial stations in the broadcast band transmit programs to radio receivers in the homes in their communities, Navy communications stations broadcast messages to fleet units in their particular geographic areas. The resemblance between Navy and commercial stations ceases there. Information broadcast by naval communications stations is contained in chronologically numbered messages addressed to the ships. The messages are copied by the fleet units, which check the serial numbers to ensure they have a complete file. This checks and balances system ensures the ship has not missed any of the broadcast message traffic.
Fleet broadcasts follow regular schedules. Messages are placed on the schedules in order of precedence. If a message of higher precedence is given to a transmitter station while a lower precedence message is being transmitted, the latter message may be interrupted to transmit the message of higher precedence. All ships copy all messages appearing on the broadcast schedule they are guarding.
Messages are normally transmitted on several frequencies to make sure they are received. This gives the receiving station the choice of frequency selection when considering time of day and atmospheric conditions for best reception.
Q16. The naval communications system is made up of what two groups of communications?
Q17. What are the five most prominent communications modes of operation?
The defense communications system (DCS) is composed of all worldwide, long-haul, government- owned and leased point-to-point circuits, trunks, terminals, switching centers, control facilities, and tributaries of military departments and other defense activities. In essence the DCS combines into a single
system all the elements that make up the naval communications system and the Army and Air Force equivalent.
The switched networks discussed in this section, automatic voice network, automatic secure voice communications, automatic digital network, and the defense special security communications system, are part of the DCS and are managed by the Defense Communications Agency (DCA). You should not confuse these DCS networks with the HICOM (high-command communications network) and NORATS (Navy operational radio and telephone switchboard) networks. We will discuss both of these Navy-only networks later in this chapter.
Automatic Voice Network (AUTOVON)
The DCS AUTOVON offers rapid, direct interconnection of DOD and certain other government installations through worldwide telephone exchanges. AUTOVON is a worldwide, general-purpose direct dialing telephone system. The goal of the AUTOVON system is to complete connections between two points anywhere in the world in about two seconds and to complete regular connections with push-button speed.
The AUTOVON system is made up of several installations comparable in function to commercial telephone exchanges. An installation is referred to as an AUTOVON switch, or simply switch. Within individual areas we have local command, control, and administrative voice communications systems. These systems connect into the worldwide AUTOVON through manually operated telephone switchboards or automatic dial exchanges by using direct in and out dialing.
Normal AUTOVON service allows your station to call other stations on a worldwide basis for day- to-day communications by using the telephone.
Automatic Secure Voice Communications (AUTOSEVOCOM)
Another close relative to the AUTOVON system is the AUTOSEVOCOM a worldwide, switched telephone network. It provides authorized users with a means for exchanging classified information over communications security (COMSEC) circuitry or over other approved circuitry. The system consists of both manual and automated networks within a single system.
For subscribers to the AUTOSEVOCOM network, telephone directories containing subscriber listings, general instructions for placing calls, and trouble-reporting procedures are provided.
Automatic Digital Network (AUTODIN)
The DCS AUTODIN is a fully automatic, digital system. The system converts word messages to digital form for transmission.
AUTODIN is used to furnish instantaneous, error-free, and secure communications around the world to several thousand directly connected subscriber terminals. Daily capacity of the system is about five- million average-length messages.
AUTODIN switching centers are interconnected through a network of high-frequency radio channels, submarine cables, microwave and tropospheric channels, and a variety of wire lines.
The whole concept of AUTODIN is to reduce manual handling of messages to a minimum by the use of automated equipment. This system has reduced message delivery times and delay anywhere in the world to a matter of seconds rather than minutes or hours.
Defense Special Security Communications System (DSSCS)
The defense special security communications system (DSSCS) was established for the purpose of integrating the critical intelligence communications (CRITICOMM) and the special intelligence communications (SPINTCOMM) networks into a single automated communications network. In effect, the integration of DSSCS subscribers into AUTODIN provides two separate systems within AUTODIN- one system for special intelligence (SI) message traffic and the other for the AUTODIN regular message traffic.
Some networks are used by the Navy only. As mentioned previously, these are the high command communications network (HICOM) and the Navy operational radio and telephone switchboard (NORATS) networks. Let's look at some of their functions and purposes.
High Command Communications Network (HICOM)
The HICOM network provides a voice link between the Chief of Naval Operations (CNO) and all subordinate commands ashore, afloat, and airborne. CNO is the master control station and each fleet commander in chief has an area network control station. All naval communications stations are members.
In cases where a fleet unit is suffering communications difficulties with normal channels, HICOM is used on a not-to-interfere basis to restore communications. All naval communications stations are required to guard HICOM for their respective area networks and use this system.
Navy Operational Radio and Telephone Switchboard (NORATS)
The NORATS meets our need for a connection between Navy tactical voice systems of the operating forces and the various fixed telephone services ashore. This system extends tactical voice to shore-based operational commands. NORATS provides a connecting point in the fleet center of each communication station. This point allows us to connect or patch all ship-to-shore voice circuits and designated local shore telephone systems and extensions. A combined HICOM/NORATS console exists at many naval communications stations.
Q18. What four switched networks are part of the defense communications system?
Q19. What two elements support only designated Navy requirements?
Now that you have completed this chapter, a short review of what you have learned is in order. The following
summary will refresh your memory of radio-frequency communications terms.
TELECOMMUNICATIONS refers to transmission, emission, or reception of signs, signals, writings, images, or sounds. This is done by visual, oral, wire, radio, or other electromagnetic means.
RADIO COMMUNICATIONS is the term describing teletypewriter, voice, telegraphic, and facsimile communications.
SYSTEM is a combination of sets, units, assemblies, subassemblies, and parts joined together to form a specific operational function or several functions.
SET is a unit or units and the assemblies, subassemblies, and parts connected or associated together to perform a specific function.
GROUP is a collection of units, assemblies, subassemblies, and parts. It is a subdivision of a set or system but is not capable of performing a complete operational function.
UNIT is an assembly or any combination of parts, subassemblies, and assemblies mounted together. Normally capable of independent operation.
ASSEMBLY is a number of parts or subassemblies, or any combination thereof, joined together to perform a specific function.
SUBASSEMBLY consists of two or more parts that form a portion of an assembly or a unit.
PART is one component or two or more components joined together. It is not normally subject to disassembly without destruction.
EXTREMELY LOW FREQUENCY is the band of frequencies up to 300 hertz.
VERY LOW FREQUENCY is the band of frequencies from 3 kilohertz to 30 kilohertz.
LOW FREQUENCY is the band of frequencies from 30 kilohertz to 300 kilohertz.
MEDIUM FREQUENCY is the band of frequencies from 300 kilohertz to 3 megahertz.
HIGH FREQUENCY is the band of frequencies from 3 megahertz to 30 megahertz.
VERY HIGH FREQUENCY is the band of frequencies from 30 megahertz to 300 megahertz.
ULTRAHIGH FREQUENCY is the band of frequencies from 300 megahertz to 3 gigahertz.
SUPERHIGH FREQUENCY is the band of frequencies from 3 gigahertz to 30 gigahertz.
EXTREMELY HIGH FREQUENCY is the band of frequencies from 30 gigahertz to 300 gigahertz.
ANSWERS TO QUESTIONS Q1. THROUGH Q19.
A1. Radio and wire.
A3. It is direct, convenient and easy to use.
A4. Static, enemy interference or a high local noise level.
A5. High speed automatic communications across ocean areas.
A6. The process used to transmit photographs, charts and other graphic information electronically.
A7. Set, group, unit, assembly, subassembly, and part.
A9. Fleet communications or navigation.
A10. Fleet Multichannel Broadcast System.
A11. Due to the commercial broadcast (AM) band.
A12. Point-to-point, ship-to-shore, ground-to-air, and fleet broadcast.
A15. Line of sight.
A16. Strategic and tactical.
A17. Simplex, half-duplex, semiduplex, duplex, and broadcast.
A18. AUTOVON, AUTOSEVOCOM, AUTODIN, and DSSCS.
A19. HICOM and NORATS.