Electronics Material Officer Course

MODULE NUMBER FOUR

LESSON TOPIC ONE

COMMUNICATIONS SYSTEMS/EQUIPMENT

LESSON TOPIC OVERVIEW

LESSON TOPIC ONE

COMMUNICATIONS SYSTEMS/EQUIPMENT

This lesson topic presents information common systems and equipment used in naval telecommunications.

The LEARNING OBJECTIVES of this LESSON TOPIC are as follows:

The student should review the "LIST OF STUDY RESOURCES" and read the Lesson Topic LEARNING OBJECTIVES before beginning the lesson topic.

LIST OF STUDY RESOURCES

COMMUNICATIONS SYSTEMS/EQUIPMENT

To learn the material in this LESSON TOPIC, you will use the following study resources:

Written Lesson Topic presentations in the Module Booklet:

Additional Materials:

References:

LESSON TOPIC SUMMARY

COMMUNICATIONS SYSTEMS/EQUIPMENT

This lesson topic will introduce you to a variety of communications systems and equipment that you may be materially responsible for as EMO. The lesson begins with a introduction to the Defense Communications System and communications/propagation fundamentals. It covers basic electronic equipment and systems, satellite communications, quality monitoring, and teletype/facsimile. The lesson also provides some technical background to assist you in understanding electronic equipment operation and describes system interrelationships and the future of U.S. Navy communications. The lesson narrative is organized as follows:

NARRATIVE FORM

OF

COMMUNICATIONS SYSTEMS/EQUIPMENT

LESSON TOPIC 4.1

The primary means of communicating between ships and stations is known as telecommuni-cations. Telecommunications refers to communications over a distance and includes the transmission and reception of intelligence by wire, radio, and other electromagnetic systems and equipment.

The DCS is composed of worldwide, long-haul, government owned and leased point-to-point circuits, trunks, terminals, switching centers, and control facilities of military departments and defense activities. The DCS combines into a single system all the elements that make up U.S. Navy, Army, and Air Force communications systems. The DCS is managed by the Defense Information System Agency (DISA), formerly known as the Defense Communications Agency (DCA). DISA's mission is to plan, develop, test, engineer, implement, operate, and maintain joint information systems in support of command, control, and communications. The DISA organization consists of the DISA Director, White House Communications Agency, Defense Communications System, and Naval Computer Telecommunication System. The DISA Director is a general or flag-rank officer responsible for coordinating the combined communications elements of the military departments. The White House Communications Agency provides telecommunications and related support to the President and elements related to the President. DCS subsystems provides communications between all levels of command through the use of four switched systems and a space component. The NTCS provides, operates, and maintains U.S Navy ashore communications and non-tactical information resources.

The DSN/AUTOVON provides rapid, direct interconnection of DOD and other government installations through telephone exchanges. The DSN is a worldwide, general-purpose direct dialing telephone system. Eventually DSN switches will replace all existing AUTOVON switches. The difference between the two is that AUTOVON is an analog switched network and DSN is a digital switched network.

AUTOSEVOCOM is a worldwide, switched telephone network that provides authorized users with the means to exchange classified information over communications security (COMSEC) equipment.

The DCS AUTODIN is a modular computer-controlled, automatic, and secure data communications system that converts word messages to digital format for transmission, providing instantaneous secure communications between subscriber terminals around the world. AUTODIN is a defense message switching, store, and forward system. Its switching centers are interconnected through a network of high frequency radio channels, submarine cables, microwave and tropospheric channels, and wire lines. The purpose of AUTODIN is to reduce message handling and delivery times through automation. General message traffic connectivity from DOD communications networks to fleet units is provided by the shore-based Naval Communications Processing and Routing System (NAVCOMPARS) via transmission on the Common User Digital Information Exchange Subsystem (CUDIXS) radio frequency link, Fleet Broadcast transmission, and the Manual Relay Center Modernization Program (MARCEMP) high frequency (HF) communications link to afloat units. The NAVCOMPARS is interfaced with AUTODIN switching centers to provide automatic connectivity with DOD communications networks.

Shorebased CUDIXS suites provide high speed digital data exchange using U.S. Navy SATCOM systems for ship-to-shore communications, acting as an interface between NAVCOMPARS and the shipboard Naval Modular Automated Communication Subsystem (NAVMACS), a shipboard message handling system. CUDIXS provides a communications path for sixty subscribers. NAVCOMPARS provides on-line communications interface between operational fleet, selected shore command, and other local subscribers and DISA AUTODIN switching centers for diverse connectivity. NAVCOMPARS provides the Manual Relay Center Modernization Program (MARCEMP) for automated HF message relay operations with fleet units using UNISYS Desktop III computers for tapeless communications. NAVCOMPARS also provides Personal Computer Message Terminal (PCMT) interface. PCMT is a message processing package that allows message transmission and reception using 3½" and 5¼" floppy diskettes and MTF Editor software.

The DDN is a packaged digital switching network that provides the same function as AUTODIN. The difference between DDN and AUTODIN is that DDN processes data in packages of known quantities. DDN converts messages that have indiscriminate amounts of data into a finite size for more efficient message handling at faster data rates. DDN started as a research and development data transfer system and was adopted by the U.S. Navy to create three separate secure Military Networks (MILNET) for handling classified shore message traffic (secret, top secret, and special intelligence). AUTODIN was originally established in 1963 with a ten year life cycle and still serves as the primary backbone system for defense message processing. The Defense Message System (DMS) is planned to create a gateway between a merged DDN and AUTODIN. DDN is planned to eventually replace AUTODIN as DMS.

DSCS provides satellite connectivity between the National Command Authority (NCA) and the Worldwide Military Command and Control System (WWMCCS) and fleet commanders. WWMCCS provides operational direction and control of all U.S. military forces. The DSCS provides secure inter/intra-battle group communications.

DSSCS was established to integrate critical and special intelligence communications into a single automated communications network, using AUTODIN switching networks. Integration of this

system into AUTODIN provides two separate systems within AUTODIN, one for special intelligence (SI) message traffic and the other for regular message traffic.

Secure voice communications from ships are interfaced to public switched telephone networks, AUTOSEVOCOM, and/or DSN by a telephone secure switchboard operator via radio wireline interface or satellite radio wireline interface. Connectivity from shipboard phones (STU-III, Red Phones, and ANDVT) is made using DSCS superhigh frequency (SHF), International Maritime Satellite (INMARSAT), ultrahigh frequency (UHF) satellite, and HF communications links.

NCTS provides, operates, and maintains U.S. Navy shore communications and non-tactical information resources and those elements of the Defense Information System assigned to the U.S. Navy. NCTS is a worldwide network of voice and data communications systems that support U.S. Navy surface, submarine, aviation, and special force users at all levels. Commander, Naval Computer and Telecommunications Command (COMNAVTELCOM) reports directly to the CNO as type commander for all NCTS activities and is responsible for their administration, maintenance, and readiness. The NCTC Naval Computer and Telecommunications Command (NCTC) is responsible for an AUTODIN switching center, Naval Computer Telecommunication Stations (NAVCOMTELSTA), Naval Computer Telecommunications Area Master Stations (NCTAM), Naval Telecommunications Centers (NTCC), and the Defense Communications Security Material System (CMS).

NCTAMSs provide regional operational management of COMNAVTELCOM sites to execute Fleet Commanders in Chief's (CINC) communications requirements. NCTAMSs provide a major interface between shorebased communications and afloat units. Area master stations provide operational control of communications assets based on fleet needs, as determined by Fleet CINCs. While all NCTAMSs have similar operational capabilities, no two have identical assets. NCTAMS responsibilities include:

The Copernicus architecture involves a major restructuring of U.S. Navy C⁴I (command, control, communications, computers, and intelligence) to place the operator at the center of the command and control universe. Rather than "push" data to the battle group/battle commander, data is collected, correlated, and fused to efficiently disseminate it (only once) when it is required. Copernicus uses extant communications systems and equipment and a Communication Support System (CSS) to integrate U.S. Navy communications assets. CSS is the communications subarchitecture that provides multimedia access and media sharing, permitting users to share total network capacity on a priority demand basis in accordance with the tactical commander's communications plan. Communications pathways are automatically selected and not dedicated, making the transmission medium is invisible to the user.

A complex of links forms a major communications system. The naval communications system consists of a strategic group and tactical group. Strategic communications are generally worldwide and operated on a common user (Navy, Army, DOD, etc.) or special-purpose basis. A strategic system may be limited to a specified area or specific type of traffic, but its configuration is designed to permit combined operations with other strategic systems. An example is the automatic voice network and automatic digital network. Tactical communications are usually limited to a specific area of operations and are used to direct or report the movement of forces. Tactical networks may be used for operational and/or administrative traffic, e.g., task group and broadcast networks. Communication links have four basic modes of operation:

Figure 4.1-1 shows the international electromagnetic frequency spectrum as defined by the International Telegraph Union.

Figure 4.1-1 Frequency Spectrum

Rapid growth in the quantity and complexity of communications equipment and increased worldwide international requirements for radio frequencies placed large demands on the RF spectrum. These demands include military and civilian applications, such as communications, location and ranging, identification, standard time and frequency transmission, and industrial, medical, and other scientific uses. The military frequency spectrum as shown in Table 4.1-1. Note that the U.S. government plans to sell portions of the military frequency spectrum as of this writing.

Table 4.1-1 Military Frequency Spectrum

The allocation, assignment, and protection of all frequencies used by U.S. Navy components is the responsibility of Commander, Naval Telecommunications Command (COMNAVTELCOM). Frequencies and equipment are chosen to meet the communications application desired. The following paragraphs discuss the characteristics and use of the RF spectrum.

An ELF communications system is a one-way system, used to send short phonetic letter spelled out (PLSO) messages from operating authorities in CONUS to submarines. ELF has the ability to penetrate ocean depths to several hundred feet with little signal loss. This ability allows submarines to operate well below the immediate surface, making detection more difficult. The ELF system transmits only from shore to ship. The large size of ELF transmitters and antennas makes ELF transmission from submarines impractical.

The location of operating forces affects the capacity, range, and reliability of communications, particularly in the North Atlantic and Arctic Oceans. HF circuits are unreliable in these areas because of local atmospheric disturbances. VLF transmissions provide highly reliable communications in these northern latitudes. VLF transmission is normally considered a broadcast; that is, a one-way transmission, no reply required. VLF is used for communications to satellites and as a backup to shortwave communications blacked out by nuclear activity. Other applications include transmission of standard frequency and time signals, tracking space vehicles, oscillator calibration, international comparisons of atomic frequency standards, radio navigation aids, astronomy, national standardizing laboratories, and communications systems. At present, practically all Navy VLF transmitters are used for fleet communications or navigation. A VLF broadcast of standard time and frequency is sufficiently accurate for the operation of cryptographic equipment, decoding devices, and single sideband transmissions.

The LF band has been used for communications since the advent of radio. LF transmitting installations are characterized by a large physical size and high construction and maintenance costs. Over the years, propagation factors peculiar to the LF band have resulted in their continued use for radio communications. LF waves are not seriously affected during periods of

ionospheric disturbance when communications at higher frequencies are disrupted. The use of low frequencies at latitudes near the equator is sometime limited by atmospheric noise. Because of this, the navy has a particular interest in the application of low frequencies in northern latitudes. In the past, the fleet broadcast system provided ships at sea with LF communications via CW (continuous wave) telegraph transmissions. As technology advanced, the system was converted to single-channel radio teletypewriter (RTTY) transmission. Today, LF communications are used to provide eight channels of frequency-division multiplex RTTY traffic on each transmission of the fleet multichannel broadcast system.

The MF band of the RF spectrum includes the international distress frequencies. Some ships have MF equipment and can monitor these distress frequencies. Ashore, the MF receiver and transmitter equipment is usually affiliated with coastal SAR organizations. Only the upper and lower ends of the MF band have naval use because the commercial broadcast band (AM) extends from 535 to 1605 kHz. Frequencies in the lower portion of the MF band (300 to 500 kHz) are

used primarily for ground wave transmission for moderately long distances over land. Transmission in the upper MF band is generally limited to short haul communications (400 miles or less).

The U.S. Navy began using HF for radio communications during World War I. A few communications systems were operated on frequencies near 3 MHz and used for very short range communications of a few miles. The general belief at the time was that frequencies above 1.5 MHz were useless for long range communications. Today HF is used extensively for long range communications. The military HF band has been expanded to include frequencies from 2 MHz through 32 MHz. Navy HF use currently includes Fleet Broadcast (as a satellite backup), teletype circuits, single channel voice, and Link 11. One of the prominent features of HF long distance communications is a change in signal strength due to changes in the propagation medium. HF radio waves are transmitted over long distances by being reflected by the ionosphere. The ground distance a radio wave travels and the strength it has when it gets to the receiver depend on how dense the ionosphere is and how far it is from the earth. High, dense ionosphere layers tend to produce longer and stronger signals than lower and less dense ionosphere layers.

The height and density of the ionosphere are determined primarily by ultraviolet radiation from the sun. Height and density vary significantly with time of day, season, and sunspot activity. As the height and density of the ionosphere change, the strength of the radio signal will change, sometimes fading or disappearing completely. Therefore, you must generally use more than a single frequency, sometimes up to four or five, to maintain HF communications. HF

communicators have had difficulty communicating at ranges between 20 and 300 miles, that tactically important region called the skip zone - too far for ground wave coverage and not far enough for the reflected sky wave. Conventional wisdom holds that HF has no role to play in the skip zone, but with the right antenna configuration and proper frequency selection, the skip zone can be covered with a clear, strong HF signal. See "Tactical HF Enters the Skip Zone" by LCDR Toher, Proceedings, April 1993.

In spite of the difficulties encountered with HF propagation, the economic and technical advantages of using HF led to a rapid expansion in the use of the band, resulting in near saturation. The HF spectrum is shared by many domestic and foreign users, and only portions scattered throughout the band are allocated to the military. Navy requirements for HF also grew, with the capacity of the Navy's assigned portion of the spectrum becoming severely taxed. The use of single sideband (SSB) equipment and the application of independent sideband techniques increased the capacity, but not enough to catch up with demand. Newer technologies, namely satellite communications and UHF relays and repeaters have overshadowed HF operations. Regardless, the HF spectrum most likely will continue to be in high demand for some time, due to HF system's versatility, survivability, and cost. Generally, less than optimum HF performance of shipboard equipment is partially offset by powerful transmitters and sensitive receiving systems at shore terminals. Naval communications in the HF band can be grouped as point-to-point, ship-to-shore, ground-to-air, and broadcast. All but fleet broadcast are normally operated as two way communications.

Frequencies above 30 MHz are not normally refracted by the atmosphere and groundwave range is minimal. This normally limits our use of this frequency spectrum to line of sight (LOS). The exception is increasing 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). Certain atmospheric and ionospheric conditions can also cause the normal LOS range to be extended. Frequencies at the lower end of this band can overcome 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 manmade static. (VHF and UHF bands are known as LOS transmission bands.) Because this is LOS communications, the transmitting antenna is in a direct line with the receiving antenna and not over the horizon (OTH). The LOS characteristic makes the VHF band ideal for amphibious operations and the UHF band well suited for tactical voice transmissions.

A large portion of the lower end of the VHF band is assigned to commercial television and FM radio industry and used by the Navy in amphibious operations. The upper portion of the VHF band (225 MHz to 300 MHz) and the lower portion of the UHF band (300 MHz to 400 MHz) are used extensively by the Navy for short range, aircraft, and satellite communications. The SHF band is used for missile illuminators, navigation/surface search radars, missile acquisition radars, Lamps III Data Link and satellite communications. The EHF band is used for experimental purposes and satellite communications.

PROPAGATION REVIEW

RF currents flowing through a transmitting antenna produce electromagnetic waves that radiate from the antenna in a manner similar to the way waves radiate on the surface of a pond when a rock is thrown into the water. However, electromagnetic waves travel either parallel to the surface of the earth (horizontally polarized) or perpendicular to the surface of the earth (vertically polarized). These transmitted waves travel toward receiving antennas by a means known as propagation. Figure 4.1-2 identifies the divisions of the transmitted electromagnetic wave according to propagation characteristics: the groundwave, skywave, and spacewave.

The groundwave (Figure 4.1-3) is the portion of the radiated wave that moves along the surface of the earth. The field strength of the groundwave diminishes with distance much more rapidly than the waves that move through free space. Absorption by the earth increases with an increase in frequency, so long distance communications by groundwaves are limited to low frequencies at very high power. Daytime reception of the Standard Broadcast Band (AM) is an example. The type of soil near the antenna site is also a factor in attenuation of the groundwave. Clay or loam will attenuate the signal less than sand or rock. However, salt water will propagate the signal

Figure 4.1-2 Divisions of the Transmitted Electromagnetic Wave

Figure 4.1-3 Groundwave Component of the Transmitted Electromagnetic Wave

better than either type of soil. The horizontally polarized wave is short circuited by the earth and is attenuated much more rapidly than the vertically polarized wave. Thus, to gain maximum advantage, the groundwave must be transmitted and received using vertically polarized antennas. Despite these limiting factors, the groundwave remains the most reliable means of radio communications because most of the restricting factors do not vary with time of day or weather conditions.

The skywave is the portion of the electromagnetic signal radiated upward that may or may not be refracted back to earth by the ionosphere (the upper atmosphere beginning 40 to 50 miles above the earth). Figure 4-1.4 shows the skywave component during daylight hours, as refracted by the layers of the ionosphere. Skywave propagation is not as reliable as groundwave propagation however, much greater distances may be covered by skywave because the radiated electromagnetic field directed toward the ionosphere is refracted (bent and reflected) back by the denser ionosphere at distances of hundreds or even thousands of miles. The angle at which the energy is refracted depends on many variables, the major ones being the frequency and angle of the radiation, and the height and density of the ionospheric layers. There is an angle at which the electromagnetic waves that enter the ionosphere will be refracted, but not enough to return to

earth. This angle is called the critical angle. Any transmitted energy entering the ionosphere beyond this angle continues into free space.

The ionosphere differs from other atmospheric layers because it contains a much higher number of positive and negative ions. The negative ions are believed to have energy levels that have been increased greatly by solar bombardment of ultraviolet and particle radiation. Extending from about 30 miles to 250 miles in the ionosphere are four layers of ionization: D, E, F1, and F2. Although ionization appears in distinguishable layers, the intensity and height of ionized layers in any given region depend on many factors including season, sunspot cycle, and most readily apparent, the time of day. The D layer is present only during daylight and has little effect on refraction, but it is a factor in absorbing energy from the electromagnetic fields that pass through it. The E layer is much stronger during the day than at night and can refract

frequencies up to approximately 20 MHz during the daylight hours. The F layers have the most effect on the refraction of electromagnetic energy. During the night the D layer fades, the E

layer becomes much weaker, and the F1 and F2 layers combine into a single F layer. The

Figure 4.1-4 Skywave Component of the Transmitted Electromagnetic Wave

reduction in absorption losses because of the fading of the D and E layers can cause the electromagnetic energy to cover greater distances at night. Additionally the combined F1 and F2 layers refract higher energy level signals at greater angles. Figure 4.1-5 depicts a comparison of day and night ionospheric layers.

Figure 4.1-5 Layers of the Ionosphere

Long-distance radio communications may be conducted by using multi-hop transmissions. During these transmissions, a sequence of refractions in the ionosphere and reflections from the earth occur, causing the electromagnetic energy to "bounce" several times over the distance covered. The complete effects of all the variables on skywave propagation are not fully understood. Researchers are continuously searching for means to improve the reliability of long distance skywave communications.

The spacewave (Figure 4.1-6), also referred to as the direct ground wave, is the part of the total wavefront that travels directly, or is reflected by the earth from the transmitting antenna, to the receiving antenna. The spacewave is limited to LOS distances plus the additional small distance created as atmospheric diffraction bends the wave a slight amount around the curvature of the earth. Naturally, LOS distance can be increased by increasing either one or both of the heights of the transmitting and receiving antennas and by using a repeater.

Figure 4.1-6 Spacewave Component of the Transmitted Electromagnetic Wave

may cause the reflected wave to be out of phase with the LOS wave at the point of reception. In other words, the two waves may have a tendency to cancel each other at the receiving antenna.

Variations in the earth's atmosphere have a direct effect on wave propagation. The factor that has the greatest effect on radio communications is absorption. Absorption decreases the energy of a radio wave and has a pronounced effect on both the strength of received signals and the ability to communicate over long distances.

The most troublesome and frustrating problem in receiving radio signals is variations in signal strength, most commonly known as fading. There are several conditions that can produce fading.

When a radio wave is refracted by the ionosphere or reflected from the earth's surface, random changes in the wave's polarization may occur. This change in polarization may adversely affect reception, depending on the polarization of the receiving antenna. An antenna, depending on how it is designed and mounted, will receive either vertically polarized or horizontally polarized signals at full strength, but not both. If an antenna meant to receive vertically polarized signals receives a horizontally polarized signal, or vice versa, it will pass only a small portion of the signal to the receiver. This decrease in signal passed to the receiver will be indicated by a decrease in receiver output. At the lower frequencies, wave polarization will remain fairly constant as it travels through space. At higher frequencies polarization usually varies, sometimes

quite rapidly, because the wavefront splits into several components which follow different paths. Fading also results from absorption of RF energy in the ionosphere. Absorption fading occurs for a longer period of time than other types of fading since absorption takes place slowly. For the most part however, fading on ionospheric circuits is mainly a result of multipath propagation.

Multipath fading is the propagation of identical signals via separate paths to a particular location. Multipath fading results from the multiple paths a radio wave may follow between a transmitter and receiver. Propagation paths include the ground wave, ionospheric refraction, re-radiation by the ionospheric layers, and reflection from the earth's surface or from more than one ionospheric layer. Multipath fading may be minimized by practices called space diversity and frequency diversity. In space diversity, two or more receiving antennas are spaced some distance apart. Fading does not occur at the same time at both antennas; therefore, sufficient output is almost

always available from one of the antennas to provide a useful signal. In frequency diversity, two transmitters and two receivers are used, each pair tuned to a different frequency, with the same information being transmitted at the same time over both frequencies. One of the two receivers will almost always provide a useful signal.

Fading resulting from multipath propagation varies with frequency since signals of different frequencies arrive at the receiving point via a different radio paths. When a wide band of frequencies is transmitted at the same time, each frequency will vary in the amount of fading. This variation is called selective fading. When selective fading occurs, all frequencies of the

transmitted signal do not retain their original phases and relative amplitudes. This fading causes severe distortion of the signal and limits the total signal transmitted.

All radio waves propagated over ionospheric paths undergo energy losses on their way to the receiving site. As discussed earlier, absorption in the ionosphere and lower atmospheric levels account for a large part of these energy losses. There are two other types of losses that also significantly affect the ionospheric propagation of radio waves. These losses are known as ground reflection loss and freespace loss. The combined effects of absorption, ground reflection loss, and freespace loss account for almost all the energy losses of radio transmissions propagated by the ionosphere.

Because the existence of the ionosphere is directly related to radiations from the sun, the earth's orbit around the sun, solar flares and sunspots create variations in the ionosphere. These variations are of two general types: those that are more or less regular and occur in cycles and those that are irregular. Regular variations can be predicted and planned for, whereas irregular variations cannot. Both significantly effect propagation.

The regular variations that affect the extent of ionization in the ionosphere can be divided into four main classes: daily, seasonal, 11-year, and 27-day variations.

Irregular variations in ionospheric conditions also effect propagation. Because these variations are irregular and unpredictable, they can drastically affect communication capabilities without

warning. The more common irregular variations are sporadic E, sudden ionospheric disturbances, and ionospheric storms.

within the auroral zones. The effects are most pronounced between the hours of ionospheric sunset and sunrise (or approximately 2200 to 0600 local time on the earth's surface). Records indicate scintillation is most severe near the vertical and autumnal equinoxes and at a minimum near the summer and winter solstices. The effect of scintillation on RF signals decreases as the frequency increases. Consequently, while scintillation may disrupt UHF satellite communications in a specific area, the effect on SHF satellite communications is negligible. Scintillation is caused by the same factors that make stars appear to twinkle, i.e., irregularities in the electron density of the ionosphere. During daylight hours, solar energy maintains a reasonably level degree of ionization (i.e., a stable level of Total Electron Count or TEC). At ionospheric sunset, removal of the sun's energy results in a rapid decrease in the TEC. Ions and electrons recombine, causing release of excess energy. This excess energy is absorbed and ionization occurs. This process continues until energy from the sun again establishes a stable TEC, around ionospheric sunrise.

Weather is an additional factor that affects the propagation of radio waves. In this section, we explain how and to what extent various weather phenomena affect wave propagation. Wind, air temperature, and water content of the atmosphere can combine in many ways. Certain combinations can cause signals to be heard hundreds of miles beyond the ordinary range of radio communications. Conversely, a different combination can attenuate the signal to the extent that it may not be heard even over a normally satisfactory path. Unfortunately, there are no hard and fast rules concerning the effects of weather on radio transmissions since weather is extremely

complex and subject to frequent change. Therefore, the following discussion of the effects of weather on radio waves is limited to general terms.

Calculating the effect of weather on radio wave propagation would be comparatively simple if

there were neither water nor water vapor in the atmosphere. However, water in vapor, liquid, or

solid form is always present and must be considered in all calculations. Before we discuss the specific effects that several forms of precipitation (rain, snow, fog) have on radio waves, you

should note that the attenuation effects of precipitation are generally proportionate to the frequency and wavelength of the radio wave. For example, rain has a pronounced effect on microwave frequencies. You can assume then, that as the wavelength becomes smaller with increases in frequency, precipitation causes a greater attenuation of the radio waves. Conversely, you can assume that as the frequency is reduced and the wavelength increased, precipitation has little effect on radio waves in the HF range and below.

Attenuation caused by rain is greater than attenuation due to other forms of precipitation. Attenuation may be caused by absorption and scattering. A raindrop, acting as a poor dielectric, absorbs power from the radio wave and dissipates the power in the form of heat loss. At frequencies above 100 MHz raindrops cause greater attenuation by scattering than by absorption. At frequencies above 6 GHz, attenuation by raindrop scatter is even greater.

The amount of attenuation is determined by the quantity of water per unit volume and by the size of the droplets. Attenuation due to fog is of minor importance at frequencies lower than 2 GHz. However, above 2 GHz fog can cause serious attenuation by absorption.

Scattering caused by snow is difficult to compute because of the irregular sizes and shapes of snowflakes. Scientists assume that attenuation from snow is less than that from rain falling at an equal rate because the density of the rain is eight times that of snow. In other words, rain falling one inch per hour would have more water per cubic inch than snow falling at the same rate.

Attenuation by hail is determined by the size of the stones and their density. Attenuation of radio waves by scattering because of hailstones is considerably less than by rain.

We have discussed uncontrollable factors that effect propagation. Selection of transmitting and

receiving antennas and operating frequency are controllable factors. For successful communications between any two specified locations at any given time of the day, there is a maximum frequency, lowest frequency, and optimum frequency that can be used.

As discussed earlier, higher frequency radio waves are refracted (and reflected) less by ionized layers of the atmosphere than are lower frequency radio waves. Therefore, for a given angle of

incidence and time of day, there is a maximum frequency that can be used for communications between given locations. This frequency is known as the maximum usable frequency (MUF). Frequencies above MUF are normally refracted so slowly that they return to earth beyond the desired location, or pass on through the ionosphere and are lost. You should understand, however, that use of an established MUF certainly does not guarantee successful communications. Variations in the ionosphere may occur at any time and consequently raise or lower the predetermined MUF. This is particularly true for radio waves being refracted by the highly variable F2 layer. The MUF is highest around noon when ultraviolet light waves from the sun are the most intense. It then drops rather sharply as recombination of ionospheric layers begins to take place.

The minimum operating frequency that can be used for communications between two points is referred to as the lowest usable frequency (LUF). As the frequency of a radio wave is lowered,

the rate of refraction increases. Consequently, a wave whose frequency is below the established LUF is refracted back to earth at a shorter distance than desired. The transmission path that results from the rate of refraction is not the only factor that determines the LUF. As a frequency is lowered, absorption of the radio wave increases. A frequency that is too low is absorbed to such an extent that it is too weak for reception. Likewise, atmospheric noise is greater at lower frequencies; thus, a low frequency radio wave may have an unacceptable signal-to-noise ratio. In other words, the amplitude of noise may obscure the signal, thereby decreasing receiver sensitivity, i.e., the receiver's ability to reproduce weak signals. For a given angle of incidence

and set of ionospheric conditions, the LUF for successful communications depends on the refraction properties of the ionosphere, absorption considerations and the amount of atmospheric noise present.

Neither the MUF nor the LUF is a practical operating frequency. While the LUF can be refracted back to earth at the desired location, the signal-to-noise ratio is still much lower than at the higher frequencies and the probability of multipath propagation is much greater. Operating at or near the MUF can result in frequent signal fading and dropouts when ionospheric variations alter the length of the transmission path. A practical operating frequency is one that is high enough to avoid the problems of multipath, absorption and noise encountered at the lower frequencies; but not so high as to experience the adverse effects of rapid changes in the

ionosphere. A frequency that meets the above criteria is known as the FOT from the French "frequency optimum de travail". The FOT is roughly 85% of the MUF and varies considerably.

Communications equipment can be divided into three broad categories: transmitting equipment, receiving equipment, and terminal equipment. Transmitting equipment generates, amplifies, and modulates a signal. Receiving equipment receives the signal, then amplifies and demodulates it to extract the original intelligence. Terminal equipment is used primarily to convert audio signals

or data into the original intelligence. A basic radio communication system consists of a transmitter and a receiver connected by electromagnetic waves (Figure 4.1-7).

Figure 4.1-7 Basic Radio Communication System

The transmitting equipment creates a radio frequency (RF) carrier and modulates it with audio intelligence to produce an RF signal. This RF signal is amplified and fed to the transmitting antenna, which converts it to electromagnetic energy for propagation. The receiving antenna

converts the transmitted electromagnetic energy into alternating RF currents. The receiver then

converts this signal back into audio intelligence.

Although there are many communications antenna designs used in U.S. Navy applications, all of these antennas have common characteristics. In this section, we will discuss antenna types and terminology.

Whenever RF current flows through a transmitting antenna, electromagnetic (radio) waves are radiated from the antenna in all directions. These waves travel at approximately the speed of light. The frequency of the radio wave that is radiated by the antenna will be the same as the frequency of the RF current flowing through the antenna. The velocity of a radio wave remains the same regardless of frequency. This is important to remember in computations that concern antenna length. When referring to the length of an antenna, the term wavelength is used. You will hear antennas referred to as halfwave, quarterwave, and fullwave. These terms describe the relative length, electrical or physical, of an antenna.

Simply stated, wavelength is defined as "the distance traveled by the radio wave in the time required for one cycle." This means that wavelength is inversely proportional to frequency. The electrical length of an antenna is not necessarily the same as its physical length. Radio frequency energy travels at the speed of light in free space however, it travels at a much slower speed in an antenna. The difference in velocity results in a difference between electrical and physical lengths. Thus, an antenna may be called a halfwave antenna because its electrical length is a halfwave, but its physical length may be much shorter. Antennas are tuned by electrically lengthening or shortening their wavelength to achieve resonance at a particular frequency.

The orientation of an antenna in space determines the polarization of the emitted radio wave. An antenna that is oriented vertically with respect to the earth radiates a vertically polarized radio wave, while a horizontally oriented antenna radiates a horizontally polarized wave. Polarization of a radio wave is a major consideration in efficient transmission and reception of radio signals. For example, if a single-wire antenna is used to extract energy from a passing radio wave, it will extract the most energy (maximum signal) when it lies physically in the same direction as the electric field component. In theory, a vertical antenna should be used for efficient reception of vertically polarized waves. A horizontal antenna should be used for reception of horizontally polarized waves. For this reason, shipboard antennas are installed with correct polarization in mind. If antennas are relocated, ensure that correct polarization is maintained.

In general, we use three terms to describe the direction or directions in which an antenna can transmit or receive: omnidirectional, bidirectional, and unidirectional. Omnidirectional antennas

radiate and receive equally well in all directions, except from their ends. Bidirectional antennas radiate or receive efficiently in only two directions: for example north/south or east/west. Unidirectional antennas radiate or receive efficiently in one direction only. Most antennas are either omnidirectional or unidirectional. Bidirectional antennas are rarely used in naval communications. Examples of an omnidirectional antenna are the antennas used to transmit fleet broadcasts or most medium-to-high frequency antennas used aboard ship. An example of a unidirectional antenna is a parabolic antenna or "dish".

A wire antenna consists of a wire rope suspended either vertically or horizontally from a yardarm or mast to outriggers, another mast, or to the superstructure. A simplified diagram of a shipboard wire antenna is shown in Figure 4.1-8. Single wire antennas are not used aboard ship as extensively now as they were in the past. They have, to a large extent, been replaced by whip, dipole, and other antenna assemblies. In some installations, wire antennas are used only in emergencies. Because of the frequency range in which these antennas are used, the portion of the ship's structure used to support the wire and other nearby structures are an electrically

Figure 4.1-8 Wire Rope Antenna

integral part of the wire antenna. Therefore, wire antennas are usually designed for a particular ship or installation. A transmitting and receiving wire rope antenna will have a vinyl insulating jacket (as will transceiving wire antennas) to reduce interference from precipitation static (static interference due to the discharge of large charges built up by rain, sleet, snow, or electrically charged clouds).

Whip antennas are essentially self-supporting. They can be installed in locations where space is at a premium or locations that are unsuitable for other antenna types. They may be deck mounted or mounted on brackets on the stacks or superstructure. Whip antennas that are used for receive only are mounted as far away from the transmitting antennas as possible to minimize the amount of energy they can pick up from a local transmitter. You can distinguish receiving whip antennas from transmitting antennas by the color of the base. Receive antennas have blue bases. Transmit antennas have red bases. A common whip antenna is constructed with 7-foot sections of aluminum rod. The lower rod is three inches in diameter and the whip tapers to a diameter of

one inch at the upper section. (Fiberglass whips are replacing the aluminum whips in some

installations.) Some whips may be trussed with wire rope (which increases the frequency bandwidth), resulting in better performance (Figure 4-1.9).

Figure 4.1-9 Trussed Whip Antenna

Whip antennas over 35 feet long are mounted on a plate supported by three or four insulators for greater strength. Small whip antennas have been mounted horizontally on yardarms or masts in some installations for use as low frequency probe antennas. Such antennas usually come supplied with a line termination box, which is normally mounted to the ship's structure. Some applications use two whips connected as a single antenna for better electrical performance. If the

antennas are less than 25 feet apart, they are usually connected with a crossbar (Figure 4.1-10), which has the feedpoint at its center. If the antennas are a considerable distance apart, or for some other reason, a direct connection is not practical, transmission line termination is used. Wire rope is used in place of the whips in some installations. On aircraft carriers and missile ships, antennas are tilted on a trunnion when installed along the edges of the flight deck or in the missile firing zone. The tilting mounts may be mechanically or hydraulically operated. Mechanically operated mounts have a counterweight at the base of the antenna heavy enough to balance the antenna in almost any position. The antenna may be locked in either a vertical or horizontal position by positive locking devices in both the operating and stowed positions.

Figure 4.1-10 Twin Whip Antennas with Crossbar Terminations

Broadband antennas for use in the HF and UHF bands have been developed for use with antenna multicouplers. To be used with a multicoupler, the antenna must be capable of handling simultaneous transmissions from several transmitters without excessive loss of power in the multicoupler equipment. The antenna must, therefore, function satisfactorily over a relatively

wide band of frequencies. The effectiveness of a given antenna depends largely on impedance matching. If a good impedance match exists between the transmission line and the antenna

throughout the operating band of frequencies, efficiency and power transfer are improved. One type of broadband antenna, called a fan, is shown in Figure 4.1-11.

Figure 4.1-11 Five-Wire Vertical Fan Antenna

A large variety of UHF antennas have been developed for shipboard use. Two of these antennas, the AT-150/SRC and AS-390/SRC, are shown in Figure 4.1-12. They are used for transmitting or receiving vertically polarized waves in the 220 MHz and 400 MHz range.

Figure 4.1-12 UHF Antennas

An antenna matching network consists of one or more parts (such as coils, capacitors, and lengths of transmission line) connected to the transmission line at its feed point to make the antenna resonate at the applied frequency. Matching networks ensure maximum transfer of RF energy with minimum power loss. Matching networks are usually adjusted when they are installed and require no further adjustment for proper operation. Matching networks can also be built with variable components enabling one antenna to transmit a range or band of frequencies. These networks are antenna tuners or couplers. These are adjusted automatically or manually each time the operating frequency is changed.

Antenna tuning is accomplished by using matching networks in tuners, antenna couplers, and multicouplers. Antenna couplers and tuners are used to match a single transmitter or receiver to one antenna. Antenna multicouplers are used to match more than one transmitter or receiver to one antenna for simultaneous operation. There is one antenna that is perfect for radiating at every discrete frequency. At a specific frequency, all of the power being transmitted from the transmitter to the antenna will be radiated into space. Unfortunately, this is the ideal and not the rule. Normally, some power is lost between the transmitter and the antenna. This power loss is the result of the antenna not having the perfect dimensions and size to radiate perfectly all of the power delivered to it from the transmitter. Naturally, it would be unrealistic to carry a separate antenna for every frequency that a communications center is capable of radiating; a ship would require many antennas. To overcome this problem, we use antenna tuners/couplers to electrically lengthen and shorten antennas to match the frequency we want to transmit. The coupler is electrically connected to the antenna and is used to adjust the apparent physical length of the antenna by electrical means. This simply means that the antenna does not physically change length; instead, the antenna is electrically adapted, using matching networks, to the output frequency of the transmitter and "appears" to change its physical length.

Antenna Coupler Group AN/URA-38 is an automatic antenna tuning system intended primarily for use with the AN/URT-23(V) HF transmitter. The equipment also includes provisions for manual and semiautomatic tuning, making the system readily adaptable for use with other transmitters. The manual tuning feature is useful when a failure occurs in the automatic tuning circuits. Tuning can also be done without the use of RF power (silent tuning). This method is useful in installations where radio silence must be maintained except for brief transmission periods. The antenna coupler matches the impedance of a 15, 25, 28, or 35-foot whip antenna to the transmission line, in the 2 to 30 MHz range. When the coupler is used with the

AN/URT-23(V), control signals from the associated antenna coupler control unit automatically

tune the coupler's matching network in less than five seconds. During manual and silent

operation, the operator uses the controls mounted on the antenna coupler control unit to tune the antenna. A low power (not to exceed 250 watts) CW signal is required for tuning. Once tuned, the CU-938A/URA-38 is capable of handling 1,000 watts of peak power.

Antenna Coupler Groups AN/SRA-56, 57, and 58 are designed primarily for shipboard use. Each coupler group permits several transmitters to operate simultaneously into a single, associated, broadband antenna, reducing the number of antennas required. These antenna coupler

groups provide a coupling path between each transmitter and associated antenna. They also

provide isolation between transmitters, matching networks, and tunable bandpass filters that allow only the desired band or range of frequencies to pass through the coupler. The three antenna coupler groups (AN/SRA-56, 57, 58) are similar in appearance and function, but they differ in frequency ranges. Antenna Coupler Group AN/SRA-56 operates in the frequency range from 2 to 6 MHz. The AN/SRA-57 operates from 4 to 12 MHz. The AN/SRA-58 operates in the 10 to 30 MHz range. When more than one coupler is used in the same frequency range, a 15% frequency separation must be maintained to avoid any interference.

Antenna coupler group AN/SRA-33 operates in the UHF (225-400 MHz) frequency range. It provides isolation between as many as four transmitter and receiver combinations operating simultaneously into a common UHF antenna without degrading operation. The AN/SRA-33 is designed for operation with shipboard radio sets AN/SRC-20, AN/SRC-21, and AN/WSC-3.

The OA-9123/SRC multicoupler enables up to four UHF transceivers, transmitters, or receivers to operate on a common antenna. The multicoupler is compatible with the channel select control signals from AN/WSC-3(V) radio sets (except (V)1). It is also compatible with AN/SRC-20/21 radio sets (only in LOCAL mode). The unit is self-contained and is configured to fit into a standard 19-inch open equipment rack. The OA-9123/SRC consists of a cabinet assembly, control power supply assembly and four identical filter assemblies. This multicoupler is a replacement for the AN/SRA-33 and requires about one half the space.

The AN/SRA-12 filter assembly multicoupler provides seven radio frequency channels in the frequency range from 14 kHz to 32 MHz. Any of these channels may be used independently of the other channels, or they may operate simultaneously. Connections to the receiver are made by use of coaxial patch cords, which are short lengths of cable with a plug attached to each end.

The AN/SRA-38, AN/SRA-39, AN/SRA-40, AN/SRA-49, AN/SRA-49A, and AN/SRA-50 coupler groups are designed to connect up to 20 MF and HF receivers to a single antenna, with a highly selective degree of frequency isolation. Each of the six coupler groups consists of 14 to 20 individual antenna couplers and a single-power supply module, all slide-mounted in an equipment rack. Dummy loads take the place of antennas for the purpose of testing and troubleshooting without radiating. Additionally, there are provisions for patching the outputs from the various antenna couplers to external receivers.

Receiving antenna distribution systems operate at low power levels and are designed to prevent multiple signals from being received. The basic distribution system has several antenna transmission lines and several receivers. The system includes two basic patch panels, one that terminates the antenna transmission lines and the other that terminates the lines leading to the receivers. Thus any antenna can be patched to any receiver via patch cords. Some distribution systems will be more complex i.e., four antennas can be patched to four receivers, or one antenna can be patched to more than one receiver via the multicouplers.

Your voice, viewed electronically, is a waveform in the audio frequency range. The audio frequency range in radiotelephone circuits is quite low, approximately 250 - 3000 Hz. It is not practical to transmit audio frequencies into space because circuit components and antennas would be too large. Consequently, audio signals are impressed on an RF signal in a process called modulation. This RF signal is called a carrier. The audio signal is called the modulating signal. The amplitude or frequency of the carrier signal is varied by the modulating signal. When the

demodulating and carrier signals are combined they produce a complex wave. The complex wave is transmitted and received by the use of antennas. When the complex wave is received by an antenna it must be demodulated to reproduce the component carrier and modulating signal. Therefore, demodulation permits recovery of the audio signal.

A simple analogy of this process is a conversation between two people at a distance. If a person in Newport wants to talk to someone in Washington, DC, simply going outside and yelling while facing south will not work. The voice in Newport must be amplified and carried in some way to Washington. In radio communications voice is carried by a radio signal through the process of

modulation. When this radio signal is received in Washington, it cannot be heard by the human

ear until the voice has been extracted from the carrier by the process of demodulation.

Amplitude modulation (AM) is the process of combining audio frequency and radio frequency signals in a manner that causes the amplitude of the radio frequency waves to vary at an audio frequency rate.

Frequency modulation (FM) is the process of combining audio and carrier signals in a manner that causes the frequency of the carrier wave to vary at an audio rate, while the amplitude of the carrier wave remains essentially constant. The carrier frequency can be varied a small amount on

either side of its average or assigned frequency by means of the audio frequency (AF) modulating signal. The amplitude of the audio modulating signal determines the amount of change (increase) in the frequency of the FM signal. The greater the audio signal amplitude (i.e., the louder the sound in voice modulation), the greater the increase in frequency of the FM signal).

The number of communications networks in operation at any given time is constantly increasing. As a result, all areas of the RF spectrum have become highly congested. To a great extent, the maximum permissible number of intelligible transmissions taking place in the radio spectrum per unit of time is being increased through the use of multiplexing. Multiplexing involves the simultaneous transmission of a number of intelligible signals using only a single transmitting path. Two methods of multiplexing, time division and frequency division multiplexing, are widely used in U.S. Naval communications.

A transmitter may be a simple, low power (milliwatts) unit for sending voice messages a short distance, or it may be a highly sophisticated unit using thousands of watts of power, for sending many channels of data (voice, teletype, TV, telemetry, etc.) simultaneously over long distances. Basic transmitters are discussed in the following section.

The CW transmitter is turned on and off (keyed) to produce long or short radio frequency (RF) pulses that correspond to the dots and dashes of the Morse code characters. The transmitter (Figure 4.1-13) has four essential components:

CW is one of the oldest and least complicated forms of radio communications. Two advantages of CW transmission are a narrow bandwidth, which requires less power out, and a degree of

intelligibility that is high even under severe noise conditions. A major disadvantage is that the

CW transmitter must be manually turned on and off at specified intervals to produce Morse code

keying. This method of transmitting intelligence is very slow and inefficient by current standards. Therefore, the navy relies on modulation of the carrier frequency (RF output of the transmitter) for communications.

Figure 4.1-13 Continuous Wave Transmitter Block Diagram

Figure 4.1-14, a block diagram of an AM transmitter, gives you an idea of what a simple AM transmitter looks like. The oscillator produces an RF carrier that is amplified and modulated with an audio signal for transmission. A microphone converts the audio frequency input (voice)

into electrical energy. The driver amplifies the audio, and the modulator further amplifies the audio signal to the amplitude necessary to modulate the carrier. The output of the modulator is applied to the power amplifier (PA). The PA combines the RF carrier and the modulating signal

to produce the amplitude modulated signal output for transmission. In the absence of a modulating signal, a continuous RF carrier is radiated by the antenna. The waveform at the antenna contains three major frequencies: the carrier frequency, the carrier frequency plus the audio frequency (sum frequency), and the carrier frequency minus the audio frequency (difference frequency). The sum frequency is called the upper sideband; the difference frequency, the lower sideband. The sideband frequencies are always related to the carrier frequency by the sum and difference of the modulation frequency. During modulation, the peak voltages and currents on the RF power amplifier stage are greater than those that occur when the stage is not modulated. To prevent damage to the equipment, a transmitter, designed to transmit both CW and radiotelephone signals, is provided with controls that reduce the power output for radiotelephone operation.

Figure 4.1-14 AM Radiotelephone Transmitter Block Diagram

Figure 4.1-15 is a block diagram of a frequency modulated transmitter. With no modulation, the oscillator generates a steady center frequency. With modulation applied, the output of the modulator is varied around the center frequency according to the modulating signal. The amount of variation from the carrier frequency depends on the magnitude of the modulating signal and the rate of variation in carrier frequency depends on the frequency of the modulating signal. The oscillator output is then fed to a frequency multiplier to increase the frequency and then to a power amplifier to increase the amplitude to the desired level for transmission.

Frequency modulation and phase modulation are essentially the same. The primary difference is in the physical method of making the frequency shift in the transmitter. Both FM and PM can be received on FM receivers, and both are commonly referred to as FM. In PM, frequency modulation is achieved by a phase shift system. The transmitter oscillator is maintained at a constant frequency by a quartz crystal. The constant oscillator output, or carrier, is amplified and modulated in a carrier phase shift network in a manner that causes the frequency of the carrier to shift according to the variations of the audio signal. The FM output of the phase shift network is fed into a series of frequency multipliers that raise the signal to the desired output frequency. Then the signal is amplified in the power amplifier and coupled to the antenna for radiation.

Figure 4.1-15 FM Transmitter Block Diagram

A carrier that has been modulated by voice or music is accompanied by two identical sidebands, each carrying the same intelligence. In AM transmitters, the carrier and both sidebands are transmitted. In a single sideband (SSB) transmitter, only one of the sidebands, the upper or lower, is transmitted, while the remaining sideband and the carrier are suppressed, eliminating the undesired portions of the signal.

There are several advantages to SSB communications. When the carrier and one sideband are eliminated, less power is required to transmit the signal. Also, an SSB signal occupies a smaller portion of the frequency spectrum. This results in two advantages, narrower receiver bandpass and the ability to place more signals in a small portion of the frequency spectrum. SSB communication systems have some disadvantages. The process of producing an SSB signal is somewhat more complicated than simple amplitude modulation, and frequency stability is much more critical in SSB communication. While there is not the annoyance of heterodyning from adjacent signals, a weak SSB signal may be completely masked or hidden from the receiving station by a stronger signal. Also, a carrier of proper frequency and amplitude must be reinserted at the receiver because of the direct relationship between the carrier and the sidebands.

Figure 4.1-16 is a block diagram of a SSB transmitter. The audio amplifier increases the amplitude of the incoming signal to a level adequate to operate the SSB generator. The SSB generator (modulator) combines its audio input and its carrier input to produce the two sidebands. The two sidebands are then fed to a filter that selects the desired sideband and suppresses the

other one. In most cases SSB generators operate at very low frequencies compared to the normally transmitted frequencies. For that reason, we must convert (or translate) the filter output to the desired frequency. This is the purpose of the mixer stage. A second output is obtained from the frequency generator and fed to a frequency multiplier to obtain a higher carrier frequency for the mixer stage. The output from the mixer is fed to a linear power amplifier to increase the level of the signal for transmission.

Figure 4.1-16 SSB Transmitter Block Diagram

In SSB the carrier is suppressed (or eliminated) at the transmitter and the sideband frequencies produced by the carrier are reduced to a minimum. You will probably find this reduction (or elimination) is the most difficult aspect in the understanding of SSB. In a SSB suppressed

carrier, no carrier is present in the transmitted signal. It is eliminated after the signal is modulated and is reinserted at the receiver during the demodulation process. All RF energy appearing at the transmitter output is concentrated in one sideband. After the carrier is eliminated, the upper and lower sidebands remain. If one of the two sidebands is filtered out before it reaches the power amplifier stage of the transmitter, the same intelligence can be transmitted on the remaining sideband. All power is then transmitted in one sideband, instead of being divided between the carrier and both sidebands, as it is in conventional AM.

A receiver processes modulated signals received by its antenna and reproduces the original signal that modulated the RF carrier at the transmitter. The signal can then be applied to a terminal

device such as a loudspeaker or a teletypewriter.

Whatever its degree of sophistication, a receiver must perform the following basic functions: reception, selection, detection, and reproduction. Reception occurs when a transmitted electromagnetic wave passes through the receiver antenna and induces a voltage in the antenna. Selection is the ability to select a particular station's frequency from all other station frequencies appearing at the receiver's antenna. Detection is the action of separating the low (audio) frequency intelligence from the high (radio) frequency carrier and is done in a detector circuit.

Reproduction is the action of converting the electrical signals to sound waves that can then be interpreted by the ear as speech.

Receiver characteristics are useful in determining operational conditions and for comparing one receiver to another. Important receiver characteristics are sensitivity, noise, selectivity, and fidelity.

Sensitivity is a measure of a receiver's ability to reproduce very weak signals. The weaker the signal that can be applied to a receiver and still produce a certain value of signal output, the better that receiver's sensitivity. Receiver sensitivity is measured under standardized conditions and expressed in terms of signal voltage, usually in the microvolts that must be applied to the antenna input terminals to give a particular output level. The output may be an AC or DC voltage measured at the detector output or a power measurement at the loudspeaker or headphones.

All receivers generate a certain amount of noise that must be taken into account. Noise is a limiting factor on the minimum usable signal that the receiver can process and still deliver a

usable output. Therefore, the measurement is made by determining the amplitude of the signal at the receiver input required to give a signal plus noise output at a predetermined ratio above the static noise output of the receiver (signal-to-noise ratio).

Selectivity is the degree of distinction made by the receiver between the desired signal and unwanted signals. The better the receiver's ability to exclude unwanted signals, the better its selectivity. The degree of selectivity is determined by the sharpness of resonance to which the frequency determining circuits have been engineered and tuned. Measurement of selectivity is usually by a series of sensitivity readings in which the input signal is stepped along a band of frequencies above and below resonance of the receiver's circuit (e.g., 100 kHz below to 100 kHz

above tuned frequency). As the frequency to which the receiver is tuned is approached, the input level required to maintain a given output level will fall. As the tuned frequency is passed, the

required input level will rise. Input voltage levels are then plotted against frequency. The steepness of the curve at the tuned frequency indicates the selectivity of the receiver.

The fidelity of a receiver is its ability to accurately reproduce the signal that appears at its input. In general, the broader the band passed by frequency selection circuits, the greater the fidelity. Fidelity may be measured by modulating an input frequency with a series of audio frequencies, then plotting the output measurements at each step against the audio input frequencies. The resulting curve will show the limits of reproduction. Good selectivity requires that a receiver pass a narrow frequency band. Good fidelity, on the other hand, requires that the receiver pass a broader band to amplify the outermost frequencies of the sidebands. Therefore, receiver design is generally a compromise between good selectivity and high fidelity.

Figure 4.1-17 shows a block diagram with waveforms of a typical AM superheterodyne receiver. The RF carrier comes in from the antenna and is applied to the RF amplifier. The output of the amplifier is sent to the mixer. The mixer also receives an input from the local oscillator. These two signals are mixed to obtain an intermediate frequency or IF carrier through a process called heterodyning. This is done to recover the audio signal that was impressed on the RF carrier. The IF carrier is applied to an IF amplifier. The amplified IF carrier is then sent to a detector.

Figure 4.1-17 AM Superheterodyne Receiver

The output of the detector is the audio component of the input signal. This audio component is amplified and sent to a speaker or headphones for reproduction. A superheterodyne receiver may have more than one frequency converting stage and as many amplifiers as needed to obtain the desired power output.

The intermediate frequency is developed by heterodyning in the mixer stage, which is sometimes called a converter or first detector. The local oscillator tracks with the tuning of the incoming signal so that it produces a frequency higher or lower than the frequency of the incoming signal by the exact amount of the fixed IF frequency. Four frequencies appear at the mixer output. They are the incoming RF signal, the local oscillator signal, and the sum and difference of the incoming RF signal and local oscillator signal. The IF amplifier will be tuned to the difference, or intermediate, frequency. A typical IF for AM communication receivers is 455 kHz.

Once the IF stages have amplified the IF to a sufficient level, it is fed to the detector (or second detector, if referring to the mixer as first detector) to extract the audio signal. The detector stage consists of a rectifier and filter, which respond only to the amplitude variations of the IF signal in order to develop an output voltage varying at an audio frequency rate. The audio output from the detector is amplified in the audio amplifier to drive a speaker or headphones.

The function of a frequency modulated (FM) superheterodyne receiver is essentially the same as that of an AM superheterodyne receiver; however, there are circuit differences due to the change in modulating technique. A comparison of block diagrams (Figures 4.1-17 and 4.1-18) shows that in both AM and FM receivers the RF stage amplifies the incoming signal. The mixer combines the RF with a local oscillator RF frequency to produce the IF. Notice that the IF in the FM receiver is amplified by wideband IF amplifiers. The bandwidth for any type of modulation must be wide enough to pass all the frequency components of a modulated signal. The IF amplifier in an FM receiver has a broader bandpass than in an AM receiver because an FM signal inherently occupies a wider band than AM. Beyond the IF stage there is a marked difference between the two receivers. While the AM detector detects variations in amplitude in order to extract audio, an FM discriminator detects variations in frequency. The discriminator is preceded by a limiter that limits amplitude to minimize noise interference. We can do this because the amplitude variations do not contain intelligence. The detected audio signal is then amplified, as in the AM receiver. Electrically, there are only two fundamental sections of the FM receiver that are different from the AM receiver, the discriminator and limiter.

Advantages of FM

In normal reception, FM signals are free of static, while AM signals are subject to crackling noises and whistles. FM also has a greater number of frequencies available. FM signals provide a much more realistic reproduction of sound because of an increased number of sidebands.

Figure 4.1-18 FM Receiver Block Diagram

Disadvantages of FM

The disadvantage of FM is the wide bandpass required to transmit the FM signals. Each station must be assigned a wide band in the frequency spectrum.

Figure 4.1-19 illustrates the block diagram of a basic SSB receiver. It is not significantly different from a conventional superheterodyne AM receiver. However, a special type of detector and a carrier reinsertion oscillator must be used. The carrier reinsertion oscillator must furnish a carrier to the detector circuit at a frequency that corresponds almost exactly to the position of the carrier in producing the original signal. The filters used in the RF amplifier section of the SSB receivers serve several purposes. As previously stated, many SSB signals may exist in a small portion of the frequency spectrum. Therefore, filters used supply the selectivity necessary to adequately receive only one of the many signals that may be present. They may also select upper sideband or lower sideband operation when desired, as well as reject noise and other interference. SSB receivers may use additional circuits that enhance frequency stability, improve image rejection, or provide automatic gain control (AGC) however, the circuits contained in the basic receiver of Figure 4.1-19 will be found in all single sideband receivers. In SSB receivers extreme frequency stability is necessary because a small deviation from the correct value in local oscillator frequency will cause the IF produced by the mixer to be displaced from its correct value. In AM reception this is not too damaging, since the carrier and sidebands are all present and will all be displaced an equal amount. The relative positions of carrier and sidebands will be

Figure 4.1-19 Basic SSB Receiver Block Diagram

retained. However, in SSB reception there is no carrier, and only one sideband is present in the incoming signal. It is very important that the local oscillator and carrier reinsertion oscillator be extremely stable, otherwise the audio signal will be unintelligible.

A communications system is a collection of equipment configured to accomplish a specific communications requirement. For a pictorial view of a typical communications system containing the necessary components for transmission and reception of voice, CW, and teletype signals, refer to Figure 4.1-20. In the following section common LF, HF, VHF, and UHF communication equipment configurations are discussed.

The LF band is used for long range direction finding, medium, and long range communications,

and aeronautical radio navigation.

The LF transmitter is used to transmit a high-powered signal over very long distances. The AN/FRT-72 transmitter is designed for this purpose. The transmitter produces 50 kW peak envelope power (25 kW average power) and covers a frequency range of 30 to 150 kHz. LF

transmitters are normally used only on shore stations or for special applications.

The LF receiver is designed to receive LF broadcast signals and reproduce the intelligence that was transmitted. The LF signal received by antennas is sent to a multicoupler and patch panel.

Figure 4.1-20 Communications System

The multicoupler and patch panel (AN/SRA-17, AN/SRA-49) allow the operator to

select different antennas and connect them to different receivers, selecting the correct combination for a particular requirement. Examples of LF receivers are the AN/SRR-19A or R-2368A/URR. The AN/SRR-19A operates in the frequency range of 30 to 300 kHz, while the R-2368A/URR operates from 14 kHz to 30 MHz. The output of the receiver (audio) is fed to the SB-973/SRR receiver transfer switchboard. The switchboard can connect the receiver output to numerous pieces of equipment. Receiver output is connected to an AN/URA-17 or a CV-2460 converter comparator set. The set converts the received signal to DC for use by teletype (TTY) equipment. The DC output is fed to a DC patch panel, such as the SB-1203/UG. The DC patch panel permits the signal to be patched to any crypto equipment desired. The crypto equipment decrypts the signal and sends it to a red DC patch panel such as the SB-1210/UGQ. The SB-1210/UGQ can be patched to a selected teletype printer that prints the signal in plain text, or to a reperforator, where a paper tape is punched and stored, for printing at a later date. An LF receive system is shown in Figure 4.1-21.

Figure 4.1-21 LF Receive System

The HF band is used primarily by mobile and maritime units. The military uses this band for long range voice and teletype communications. This band is also used as a backup for the satellite communications system.

The HF transmit signal can contain either voice or teletype information. Figure 4.1-22 shows a typical HF transmit system used aboard ship.

Figure 4.1-22 HF Transmit System

The same equipment used to receive teletype messages on low frequencies (the teletype, DC Patch Panel SB-1210/UGQ, crypto equipment, and DC Patch Panel SB-1203/UG) is used to transmit teletype messages on the HF frequency system, in reverse order. An AN/UCC-1(V) or CV-2460 telegraph terminal converts DC signals into tone signals. The output of the AN/UCC-1(V) is connected to the SB-988/SRT transmitter transfer switchboard. A C-1004 control/teletype keys the transmitter during TTY operation. Voice signals from remote handsets connected to C-1138 radio set controls are also connected to this switchboard. The AN/URT-23 transmitter modulates an RF signal with the input signal. The modulated RF signal is connected to the AN/SRA-34, 56, 57, 58, or AN/URA-38 antenna coupler. The antenna radiates the signal into the atmosphere.

A typical HF receive system is shown in Figure 4.1-23. A transmitted signal, similar to the one just described in the previous section, is received by the antenna and converted to electrical energy. The signal is distributed to any number of receivers by an antenna patch panel. Receivers R-1051/URR, R-2368/URR, or R-1903/URR convert the RF signal into a teletype (FSK) or voice signal. The receiver output is routed through an SB-973/SRR receiver transfer switchboard to C-1138 or AM-3729 remote speaker amplifier.

Figure 4.1-23 HF Receive System

The VHF band, 30 to 300 MHz, is used for aeronautical radio navigation and communications, radar, amateur radio, and mobile communications. The navy uses this band for mobile communications, such as for boat crews and landing parties, and bridge-to-bridge

communications. A basic block diagram of a VHF transmit and receive system is shown in

Figure 4.1-24. The AN/URC-80 and AN/VRC-46 are common VHF transceivers.

Figure 4.1-24 VHF Transmit and Receive System

The transmitter and receiver used in the UHF system form one piece of equipment and share common circuitry (a transceiver); however, the transmit and receive systems are discussed separately in the following paragraphs. Although this description pertains specifically to voice communication, UHF equipment also processes teletype data.

A basic block diagram of a UHF transmit system is shown in Figure 4.1-25. On the transmit side of the secure voice system, the operator at a remote location talks into the secure voice Remote Phone Unit (RPU). The RPU is connected to the secure voice matrix. The matrix is the tie point for the connection of more than one remote phone unit. The output of the matrix is connected to secure voice equipment, which encrypts the signal received. The output of the secure voice equipment in connected to an SB-988/SRT transmitter transfer switchboard.

The transmitter transfer switchboard is used to connect numerous remote phone units to any number of transmitters. The output of the patch panel is connected to the transmitter side of the AN/SRC-20, AN/SRC-21, or AN/WSC-3, which in turn is connected via an AN/SRA-33 or OA-9123 antenna coupler to the antenna.

A basic block diagram of a UHF receive system is shown in Figure 4.1-26. The received signal is picked up by the antenna and sent to the receiver side of the AN/SRC-20/21 or AN/WSC-3

through an AN/SRA-33 or OA-9123/SRC antenna coupler. The output of the receiver is

Figure 4.1-25 UHF Transmit System

connected to an SB-973/SRR receiver transfer switchboard, where it can be connected to either the nonsecure or secure voice system, depending on the mode of transmission. When a

nonsecure signal is received, the output of the receive transfer switchboard is connected to either

the C-1138 radio set control or the AM-3729 speaker amplifier, or both, depending on user preference.

Figure 4.1-26 UHF Receive System

If a secure voice transmission is received, the output of the receiver transfer switchboard is connected to secure voice equipment, where it is decrypted. The output of the secure voice equipment is connected to the secure voice matrix, which performs the same function as the matrix on the transmit system. The output of the secure voice matrix is connected to the secure remote phone unit, where the signal is converted back to its original form.

Because portable and pack radio sets must be lightweight, compact, and self-contained, they usually are powered by a battery or hand generator, have low output power, and are either transceivers or transmitter-receivers. Navy ships carry these radio sets for emergency and amphibious communications. The numbers and types of portable equipment vary according to the individual ship.

The AN/CRT-3A radio transmitter, known as the "Gibson Girl," is a rugged emergency transmitter carried aboard ships and aircraft for use in lifeboats and liferafts. It is shown in Figure 4.1-27. No receive equipment is included.

Figure 4.1-27 AN/CRT-3A

The transmitter operates on the international distress frequency (500 kHz) and the survival craft communication frequency (8,364 kHz). It automatically repeats CW distress signals, shifting between 500 and 8,364 kHz every 50 seconds. The complete radio transmitter, including the power supply, is contained in an airtight and waterproof aluminum cabinet. The cabinet is shaped to fit between the operator's legs and has a strap for securing it in the operating position. The only operating controls are a three position selector switch and a pushbutton telegraph key. A handcrank screws into a socket in the top of the cabinet. The generator, automatic keying and automatic frequency changing are all operated by turning the handcrank. While the handcrank is being turned, the set automatically transmits the distress signal "SOS" in Morse code. The transmitter can be keyed manually, on 500 kHz only, by means of the pushbutton telegraph key.

Additional items (not shown) packaged with the transmitter include the antenna, a box kite and balloons for supporting the antenna, hydrogen-generating chemicals for inflating the balloons and a signal lamp that can be powered by the handcrank generator. The equipment floats and is painted brilliant orange-yellow to provide greatest visibility against dark backgrounds.

The AN/PRC-96 radio set (Figure 4.1-28) is a dual-channel, battery powered, portable transceiver that provides homing beacon signal and two-way AM voice communications between life rafts and by search and rescue ships and aircraft. It is a microminiature, solid state, hand-held radio that operates on the 121.5 MHz and 243 MHz guard channels. The AN/PRC-96 antenna is a 7.75-inch, rubber covered, omnidirectional, flexible whip antenna. The batteries supplied with the radio set are lithium D cells. Each cell is fused to protect against damage from external short circuits. Two cells are installed in the transceiver and four are packaged as spares.

Figure 4.1-28 AN/PRC-96

The AN/PRC-41 radio set (Figure 4.1-29) is a watertight, lightweight, portable UHF transceiver that may be operated on any of 1,750 channels spaced 100 kHz apart in the 225 to 400 MHz range. Its only mode of operation is AM voice, at an average output power of 3 watts. Although designed principally for manpack operation, the set may also be used for fixed station and vehicular operation when complemented by certain accessories. When not in use, the equipment is disassembled and stowed in a compartmentalized aluminum case similar to an ordinary suitcase.

Figure 4.1-29 AN/PRC-41

The Single Audio System (SAS) was developed to fulfill the requirement for an integrated secure or nonsecure shipboard voice communications system. It consists of telephone sets, voice signal switching devices, various control devices, and field changes to existing equipments in conjunction with other existing elements of the radio communications system. The SAS is essentially a baseband (HF, VHF, or UHF) audio subset of the shipboard exterior communications system. It incorporates voice communications circuits, user control of the operating mode (secure and nonsecure) and various degrees of user control over circuit selection. There are two versions of SAS, automated (ASAS) and manual (MSAS). The principal differences between ASAS and MSAS are in their voice switching equipments and in the means provided for user control over circuit selection. (An example of user control is the choice of

desired communication channel.) Information in this section applies to both the ASAS and the MSAS unless otherwise specified. There is no specific list of equipment that constitutes an SAS installation. Equipment types and quantities are dictated by communications requirements.

The SAS provides the capabilities for configuring and operating voice communications circuits. A SAS installation permits secure or nonsecure communications at the discretion of the operator, from a single telephone or NTDS device. Interface with crypto or plain subsystems in the ship's communications system is the essence of the SAS. The SAS has the following capabilities:

In addition to the capabilities listed above for the SAS, the ASAS version has the following features:

User station equipment is located in various operations centers throughout the ship, such as the bridge, CIC, flag plot, secondary conn, and other stations where exterior voice communication is required by the ship's mission. This equipment consists of telephone sets, audio amplifiers, loudspeakers, headsets, recorders, audio jackboxes, NTDS consoles and intercom units, and local switching devices for system configuration flexibility.

The voice switching equipment is a major component of the SAS. It is the interface and primary switch between the user's equipment and all crypto and plain subsystems. It is designed for very high interchannel isolation, a TEMPEST requirement for all equipments that handle both secure and nonsecure signals simultaneously. (The ASAS and MSAS use different switches for this function.)

Crypto and plain subsystems are located in the main communications spaces. Cryptographic devices and other "red" equipment are located in a secure area within these spaces. The crypto and plain subsystems used within the SAS are VINSON (UHF Secure Voice), Fleet Satellite Communications (FLTSATCOM) Secure Voice, and PLAIN ONLY. Additional classified information on these subsystems is available on a need-to-know basis.

This equipment is a part of the overall exterior communications switching system and is located in the main communications spaces, generally in the vicinity of the technical control working area. The switchboard equipment group interconnects crypto and plain subsystem equipment with the appropriate radio equipments. These switchboards are also the interconnecting points for other subsystems within the overall exterior communications system and are not unique to the SAS.

Radio equipment may be located in both the main communications spaces and in separate rooms located in various parts of the ship. This equipment group consists of the transmitters, transceivers, and receivers used for voice nets. The more common transceivers that you will encounter are the AN/SRC-20 series, AN/VRC-46, AN/WSC-3(V)3 and (V)7, AN/URC-93, and AN/WSC-6. Common transmitters include the AN/URT-23, AN/URT-24, T-1322/SRC, and AN/GRT-21. Common receivers are the R-1051 series, R-1903, and AN/GRR-23. For additional information on individual equipment, refer to the applicable technical manual.

The radio equipment group of the SAS interfaces with the usual RF components of the ship's exterior communications system. While these components must operate satisfactorily for the SAS to operate successfully, they have no direct application to the single audio concept and therefore are not discussed in this section. If you require additional information on RF components such as couplers, RF switches, and antennas, consult your ship's documentation.

In the 1970s, when the HF spectrum was becoming overcrowded, free frequencies were at a premium, and HF jamming techniques were becoming highly sophisticated, the need for new and advanced long range transmission methods became apparent. Although HF communications will continue to provide limited opportunities for traffic delivery, the inherent uncertainties of HF propagation seriously impair the ability of commands to deliver high precedence command and control traffic. HF, while suitable for narrative traffic and intra-battle group communications, is not suited in terms of affordability for data rates above 1200 baud and, in comparison to SATCOM, is manpower intensive to operate. HF systems are susceptible to atmospheric absorption and HFDF and are the systems most susceptible to jamming. HF is considered more vulnerable to nuclear blackout than other frequencies, though ground waves are minimally affected. UHF SATCOM, SHF SATCOM, and modernized HF currently have the greatest potential for near-term contribution. Satellite systems provide reliable, high speed, high volume communications capabilities. The use of discrete UHF/SHF frequencies on satellite channels minimizes the effects of traditional jamming and interception. EHF systems are being developed and fielded. Note that affordability may constrain extensive HF modernization.

A satellite communication system uses satellites to relay radio transmissions between earth terminals. Communications systems use active satellites that amplify and retransmit RF signals. A signal is transmitted to the satellite on a frequency called the uplink frequency. The satellite transponder amplifies and translates this signal to the downlink frequency, and transmits it back to earth. This concept is illustrated in Figure 4.1-30, showing a variety of earth terminals.

Figure 4.1-30 Satellite Communication System

The design of a satellite communication system depends to a great degree on the satellite's orbit. An orbit is generally either elliptical or circular, with an inclined, polar, or equatorial orientation. A special type of orbit, called a synchronous orbit, has a period the same as the earth's rotation. Communication satellites are in equatorial synchronous orbits and therefore

appear stationary to an earth terminal. The area of the earth that a particular satellite covers is called the footprint, depicted in Figure 4.1-31.

Figure 4.1-31 Satellite Footprint

The following sections are divided into EHF, SHF, and UHF SATCOM and provide an introduction to SATCOM systems, subsystems, and equipment.

EHF SATCOM is the next generation of the military SATCOM systems, providing the services with interoperable and survivable SATCOM. The EHF portion of the frequency spectrum extends from 30 to 300 GHz. The extensive bandwidth available at EHF frequencies can be used for either high data rate transmission or for extremely robust anti-jam (AJ) communications. The currently funded EHF SATCOM programs take the latter approach, using the Satellite Data Link Standard (SDLS), limiting data rate to 2.4 kbps (bits per second). SDLS EHF SATCOM is primarily oriented toward intra-battle group tactical data information exchange. Note that a medium data rate (MDR) capability is being developed that provides some AJ performance at user data rates up to 1.544 Mbps. The disadvantages of EHF SATCOM are: the EHF portion of the spectrum is highly sensitive to atmospheric attenuation, the narrow beam makes interception difficult, and EHF is less vulnerable to the effects of nuclear blackout and scintillation. The EHF medium is suitable for record services, database exchange, voice, graphic data exchange, facsimile, imagery transmission, and video teleconferencing. EHF provides survivable wartime C³ for designated commanders and their assigned forces and AJ, low probability of intercept (LPI) minimum essential communications in a stressed environment for shore, ship and submarine platforms. The EHF satellite system consists of the space segment, control segment, and terminal segment.

The UFO/E SATCOM package provides an EHF uplink EC antenna, an SHF downlink EC antenna, and an EHF uplink/SHF downlink spot beam antenna. The uplink and downlink EC antennas will typically be used to support shore site terminals and the spot beam antenna will typically be used to support disadvantaged terminals, such as battle group terminals. These satellites will also replace aging UHF satellites. Milstar may feature satellites in both geostationary and highly inclined orbits in order to achieve virtually full earth coverage. The system features three type of arrays: an EC beam, three steerable spot beams providing a concentrated beam of energy within a bounded geographical area, and agile beams which are an array of fixed beams covering the view of the earth from the satellite to provide the best link beam coverage through electronic switching from beam to beam. The Milstar constellation is illustrated in Figure 4.1-32.

Figure 4.1-32 Milstar Configuration

All Milstar uplink antennas are at EHF except one, the UHF Air Force Satellite Communications antenna. Five downlink antennas are at SHF and two at UHF. Downlink capabilities are divided between spot beams, agile beams, and EC antennas.

The terminal segment consists of the terminals developed by the U.S. Navy, U.S. Army, and U.S. Air Force and the baseband systems that interface with all terminals. The three U.S. Navy EHF Satellite Communications Program (NESP) terminals are designed for use in surface ships, submarines, and at shore stations. The AN/USC-38(V)2 is the surface ship terminal. The AN/USC-38 is a general purpose communications terminal that uses an EHF uplink and SHF downlink. System design accommodates secure voice, teletype, data, and fleet broadcast systems. An AN/USC-38 ship terminal block diagram is shown in Figure 4.1-33. The AN/USC-38(V) is comprised of three major components: the CEG (Communication Equipment Group), HPA (High Power Amplifier), and Antenna Group.

Figure 4.1-33 AN/USC-38(V)1 Ship Terminal Block Diagram

SHF SATCOM terminals are installed in selected surface vessels to provide high capacity, AJ, real time command and control communications between naval force commanders and National Command Authorities (NCA). SHF SATCOM terminals are also installed in all Military Sealift Command Oceanographic Surveillance Ships (T-AGOS) to provide a path for acoustic data to the

two Naval Ocean Processing Facilities. Marine Expeditionary Forces also use SHF SATCOM terminals for long-haul connectivity during extended operations ashore. NCTAMS and selected NCTSs are the main termination points for U.S Navy SHF SATCOM. Increased connectivity with other DSCS and DCS users is planned involving the increased use of U.S. Army and U. S. Air Force facilities for access to DSCS. DSCS ground entry stations are shown in Figure 4.1-34.

DSCS SHF carrier signals permit a large bandwidth that can be used for AJ signal processing at low and high user data rates. The navy uses both services and plans to extend the latter, seeking up to 1.544 Mbps non-AJ service per ship. The navy plans to install SHF SATCOM on most combatants, with initial focus on aircraft carriers, amphibious flagships, and flag-capable cruisers. Other considerations concerning SHF SATCOM are:

SHF SATCOM is required on flagships to satisfy minimum tactical C², intelligence, and warfighting communications requirements and improve Joint and Allied communications interoperability. SHF SATCOM is designated as the primary spectrum for Joint and

Allied/NATO interoperability. With a satellite constellation and world-wide shore terminals in

place, SHF SATCOM is capable of providing communication pipelines for all kinds of

information exchange. SHF terminals operating with DSCS will support wider bandwidth for greater throughput for digital data, voice, and imagery transmissions to meet tactical C⁴I and warfighting requirements including:

Figure 4.1-34 DSCS Ground Entry Stations

Figure 4.1-35 DSCS Satellite Coverage

SHF terminals will support joint communications interface with shore GMF (Ground Mobile Force) multichannel networks via DSCS-GMF gateway stations to extend the range of terrestrial

communications and provide:

╔═══════════════╤═════════════════╤══════════════╤═══════════════════╤════════╗

║TERMINAL │ANTENNA │SHF MODEM │BASEBAND │AJ/ ║

║TYPE │CONFIGURATION │ │MULTIPLEXER │NON-AJ ║

╠═══════════════╪═════════════════╪══════════════╪═══════════════════╪════════╣

║AN/WSC-6(V)1 │Single │MD-1030A(V) │N/A │Non-AJ ║

║(SURTASS) │ │ │ │ ║

╟───────────────┼─────────────────┼──────────────┼───────────────────┼────────╢

║AN/WSC-6(V)2 │Single or Dual │OM-55(V)/USC │TDM-1389(V) LRM │AJ ║

║(SATURN) │ │ │ │ ║

╟───────────────┼─────────────────┼──────────────┼───────────────────┼────────╢

║AN/TSC-93B │Single or Dual │MD-1030A(V) │AN/FCC-100(V)1/2 │Non-AJ ║

║(QUICKSAT) │ │ │LSTDM │ ║

╚═══════════════╧═════════════════╧══════════════╧═══════════════════╧════════╝

Although the commercial INMARSAT system falls outside of the U.S. Navy portion of the UHF spectrum, it can be used to provide support to surface units at sea. The INMARSAT system is a multi-country controlled SATCOM network that links an INMARSAT terminal into existing national or international telephone networks. INMARSAT service is available to commands with an authorized installed INMARSAT terminal. U.S. Navy ships equipped with INMARSAT terminals are authorized to establish direct communications with shore commands or other USN/USNS INMARSAT equipped ships via INMARSAT earth stations operated by COMSAT General. Interface via NCTC facilities is not required. Use of NSA-approved crypto systems is mandatory. INMARSAT satellites provide worldwide coverage between 76°N and 76°S.

The U.S. Navy Fleet SATCOM system allows two-way ship-to-shore, shore-to-ship, and ship-to-ship communications using satellites in geostationary orbit over the Atlantic, Indian, and Pacific Oceans. The following satellites provide worldwide communications between 70° north and 70° south: FLTSAT, LEASAT, and MARISAT. The portion of MARISAT leased by the navy, GAPFILLER, augments FLTSAT and LEASAT until a sufficient number of next generation satellites, UFO assets, are available. Control of the MARISAT system was transferred to INMARSAT (International Maritime Satellite) in 1982. These satellite are essentially UHF, but also have SHF uplink capability. They have 23 separate RF channels: ten 25 kHz narrowband channels, twelve 5 kHz narrowband channels, and one 500 kHz wideband channel. The use of DAMA (Demand Assigned Multiple Access) multiplexers allows several baseband subsystems to share a single satellite channel, increasing the capacity of each satellite channel to up to four circuits. FLTSATs can be used with DAMA and non-DAMA configurations. Figure 4.1-38 depicts UHF fleet satellite coverage. The earth segment of UHF SATCOM consists of earth terminals that are located at NCTAMSs, NAVCOMMSTAs, and NAVCOMTELSTAs. The earth segment includes antennas, transmitters, receivers, baseband equipment, and subsystems ashore and afloat.

The TACTINTEL II+ Subsystem will be designed with sufficient flexibility to incorporate future SI communications requirements. TACTINTEL II+ will have the capability to receive, route, and transmit messages automatically on a priority basis. The system will be capable of communications via any required transmission media either net or point-to-point configurations. Communications across any mix of these media will be possible: UHF LOS, UHF satellite, SHF satellite, EHF satellite, HF, or landline. The design of TACINTEL II+ software will permit the automatic switching of data by CSS to different media to provide the most efficient use of communications assets.