NEETS Module 17 — Radio-Frequency Communications Principles
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INTRODUCTION TO SATELLITE COMMUNICATIONS
Upon completion of this chapter you will be able to:
1. Describe the basic operation of the
two types of satellites.
2. Describe the basic components of an operational satellite system.
3. Describe the function of earth terminal equipment.
4. Describe the basic
signal flow of a typical shipboard receive-only system.
5. Describe the basic signal flow of a
typical shipboard transceiver system.
6. Describe the advantages of satellite communications
in terms of capacity, reliability, vulnerability, and flexibility.
7. Describe the limitations
of satellites in terms of power, receiver sensitivity, and availability.
HISTORY OF SATELLITE COMMUNICATIONS
The first artificial satellite was placed in orbit by the Russians in 1957. That satellite, called Sputnik,
signaled the beginning of an era.
The United States, who was behind the Russians, made an all-out effort
to catch up, and launched Score in 1958. That was the first satellite with the primary purpose of communications.
The first regular satellite communications service was used by the Navy in 1960. The moon was used to bounce
teletypewriter signals between Hawaii and Washington, D.C. During the early 1960s, the Navy used the moon as a
medium for passing messages between ships at sea and shore stations. This method of communications proved reliable
when other methods failed.
Military satellite communications technology was at a low level until 1965. At
that time high quality voice transmissions were conducted between a satellite and two earth stations. That was the
stepping stone to the Initial Defense Communications Satellite Program (IDCSP), which will be covered later in
Experience with satellite communications has demonstrated that satellite systems can satisfy many military
requirements. They are reliable, survivable, secure, and a cost effective method of telecommunications. You can
easily see that satellites are the ideal, if not often the only, solution to problems of communicating with highly
mobile forces. Satellites, if properly used, provide much needed options to large, fixed-ground installations.
For the past fifty years, the Navy has used high-frequency (HF) transmissions as the principal method of
sending messages. In the 1970s, the HF spectrum was overcrowded and "free" frequencies were at a
premium. HF jamming and electronic countermeasures (ECM) techniques became highly sophisticated during
that period. As a result the need for new and advanced long-range transmission methods became apparent.
Communications via satellite is a natural outgrowth of modern technology and of the continuing demand for greater
capacity and higher quality in communications.
In the past, the various military branches have had the
resources to support their communications needs. Predicted usage indicates that large-scale improvements will have
to be made to satisfy future needs of the Department of Defense. These needs will require greater capacity for
long-haul communications to previously inaccessible areas. Satellite communications has the most promise for
satisfying these future requirements.
DEFENSE COMMUNICATIONS SATELLITE PROGRAM (DCSP)
The Defense Communications Satellite Program (DCSP) was initiated by the Secretary of Defense in 1962. Phase
I of the program was given the title Initial Defense Communications Satellite Program (IDCSP). The first satellite
launch occurred in June 1966 when seven experimental satellites were placed into orbit. The final launch of this
program consisted of eight satellites and occurred in June 1968.
DEFENSE SATELLITE COMMUNICATIONS
SYSTEM (DSCS) PHASE II
The Phase II Defense Satellite Communications System (DSCP Phase II) has
changed from an all- analog communications system to an all-digital communications system. The performance
capability provided by the Phase II DSCS is limited by equipment availability. Extensive digital traffic
capability has become common. You can credit this to the availability of digital modems (modulator/demodulator)
and broadband equipment. Overall performance of the Phase II DSCS is a great improvement over the capabilities
provided by Phase I DSCS. The Phase II satellites provide a great increase in effective radiated power and rf
bandwidths. You will find these satellite configurations use wide coverage and narrow beam antennas. They provide
an extensive range of communications services and capabilities. (This will be further discussed later, in this
FUNDAMENTAL SATELLITE COMMUNICATIONS SYSTEM
A satellite communications
system uses satellites to relay radio transmissions between earth terminals. The two types of communications
satellites you will study are ACTIVE and PASSIVE. A passive satellite only reflects received radio signals back to
earth. An active satellite acts as a REPEATER; it amplifies signals received and then retransmits them back to
earth. This increases signal strength at the receiving terminal to a higher level than would be available from a
A typical operational link involves an active satellite and two or more earth
terminals. One station transmits to the satellite on a frequency called the UP-LINK frequency. The satellite then
amplifies the signal, converts it to the DOWN-LINK frequency, and transmits it back to earth. The signal is next
picked up by the receiving terminal. Figure 4-1 shows a satellite handling several combinations of links
Figure 4-1.—Satellite communications system.
DESCRIPTION OF COMMUNICATIONS SATELLITE SYSTEM
The basic design of a satellite
communications system depends to a great degree upon the characteristics of the orbit of the satellite. In general
terms, an orbit is either elliptical or circular in shape. A special type of orbit is a SYNCHRONOUS ORBIT. In this
type you will find the period (time required for one revolution) of the orbit the same as that of the earth. An
orbit that is not synchronous is called ASYNCHRONOUS. A period of orbit that approaches that of the earth is
called NEAR SYNCHRONOUS (subsynchronous). Orbits are discussed in more detail later in this chapter.
addition to the fundamental components shown in figure 4-1, the design of the overall system determines the
complexity of the various components and the manner in which the system operates. Current satellites are capable
of handling many teletypewriter (TTY) and voice circuits at the same time.
Orbits generally are described according to the physical shape of
the orbit and the angle of inclination of the plane of the orbit. These terms aye discussed in the following
PHYSICAL SHAPE.—All satellites orbit the earth in elliptical orbits. (A circle is a special case
of an ellipse.) The shape of the orbit is determined by the initial launch parameters and the later deployment
PERIGEE and APOGEE are two, of the three parameters used to describe orbital data of a
satellite. These are shown on figure 4-2. Perigee is the point in the orbit nearest to the center of the earth.
Apogee is the point in the orbit the greatest distance from the center of the earth. Both distances are expressed
in nautical miles.
Figure 4-2.—Elliptical satellite orbit.
ANGLE OF INCLINATION.—The ANGLE OF INCLINATION (angle between the equatorial plane of the
earth and the orbital plane of the satellite) is the third parameter used to describe the orbit data of a
satellite. Figure 4-3 depicts the angle of inclination between the equatorial plane and the orbital plane. Most
satellites orbit the earth in orbital planes that do not coincide with the equatorial plane of the earth. A
satellite orbiting in any plane not identical with the equatorial plane is in an INCLINED ORBIT.
Figure 4-3.—Inclined satellite orbit.
The inclination of the orbit determines the area covered by the path of the satellite. As shown in figure 4-4,
the greater the inclination, the greater the amount of surface area covered by the satellite.
Figure 4-4.—Effect of orbit plane inclination on satellite coverage.
SPECIAL TYPES OF INCLINED ORBITS.—A satellite orbiting in a plane that coincides with the
equatorial plane of the earth is in an EQUATORIAL ORBIT. A satellite orbiting in an inclined orbit with an angle
of inclination of 90 degrees or near 90 degrees is in a POLAR ORBIT.
SPECIAL TYPES OF CIRCULAR
ORBITS.—We stated previously that a circular orbit is a special type of elliptical orbit. You should
realize a circular orbit is one in which the major and minor axis distances are equal or approximately equal. Mean
height above earth, instead of perigee and apogee, is used in describing a circular orbit. While we are discussing
circular orbits, you should look at some of the terms mentioned earlier in this chapter. A satellite in a circular
orbit at a height of approximately 19,300 nautical miles above the earth is in a synchronous orbit. At this
altitude the period of rotation of the satellite is 24 hours, the same as the rotation period of the earth. In
other words, the orbit of the satellite is in sync with the rotational motion of the earth. Although inclined and
polar synchronous orbits are possible, the term synchronous usually refers to a synchronous equatorial orbit. In
this type of orbit, satellites appear to hover motionlessly in the sky. Figure 4-5 shows how one of these
satellites can provide coverage to almost half the surface of the earth.
Figure 4-5.—Illumination from a synchronous satellite.
Three of these satellites can provide coverage over most of the earth (except for the extreme north and
south polar regions). A polar projection of the global coverage of a three-satellite system is shown in figure
Figure 4-6.—Worldwide synchronous satellite system viewed from above the North Pole.
A satellite in a circular orbit at other than 19,300 nautical miles above the earth is in a near- synchronous
orbit. If the orbit is lower than 19,300 nautical miles, the period of orbit of the satellite is less than the
period of orbit of the earth. The satellite then appears to be moving slowly around the earth from west to east.
(This type of orbit is also called subsynchronous.) If the orbit is higher than 19,300 nautical miles, the period
of orbit of the satellite is greater than the period of orbit of the earth. The satellite then appears to be
moving slowly around the earth from east to west. Although inclined and polar near-synchronous orbits are
possible, near synchronous implies an equatorial orbit.
A satellite in a circular orbit from approximately
2,000 miles to 12,000 miles above the earth is considered to be in a MEDIUM ALTITUDE ORBIT. The period of a medium
altitude satellite is considerably less than that of the earth. When you look at this altitude satellite, it
appears to move rather quickly across the sky from west to east.
Q1. What are the two types of
Q2. A typical satellite communications operational link consists of a satellite and what two other
Q3. A satellite in a synchronous orbit can cover how much of the surface of the earth?
What areas of the earth are not normally covered by satellites?
Early communications satellites were limited in size to the diameter of the final stage of the rocket that was
used for launching. Weight was determined by the thrust of the rocket motors and the maximum weight the rocket
could lift into orbit.
As early as June 1960, two satellites were successfully placed in orbit by the same launch vehicle.
With the development of multilaunch capability, added flexibility became available. We then had choices as to the
size, weight, and number of satellites to be included in each launch.
Using our multilaunch capabilities,
the Defense Satellite Communications System (DSCS) has placed larger and heavier satellites in synchronous
equatorial orbits. Figure 4-7 is a drawing of a DSCS satellite. It shows each pair of transmit and receive dish
antennas. As you can see, a large area of the earth can be covered using only one satellite.
Figure 4-7.—DSCS satellite.
Satellite Power Sources
Early communications satellites were severely limited by the
lack of suitable power sources. This severely limited the output power of the satellite transmitter. The only
source of power available within early weight restrictions was a very inefficient panel of solar cells without
battery backup. A major disadvantage of this type of power source is that the satellite has no power when it is in
ECLIPSE (not in view of the sun). For continuous communications, this outage is unacceptable.
combination of solar cells and storage batteries is a better prime power source. This is a practical choice, even
though the result is far from an ideal power source. About ten percent of the energy of the sunlight that strikes
the solar cells is converted to electrical power. This low rate is sometimes decreased even further. You find this
when the solar cells are bombarded by high-energy particles that are sometimes found in space.
satellites had over 8,500 solar cells mounted on the surface of the satellite, which supplied about 42 watts of
power. No battery backup was provided in these satellites.
Newer communications satellites have about
32,000 solar cells mounted on the surface of the satellite, and they supply about 520 watts. A nickel cadmium
battery is used for backup power during eclipses.
Nuclear power sources have been used in space for
special purposes, but their use stops there. Technology has not progressed sufficiently for nuclear power sources
to be used as a power source.
Satellite orientation in space is important for
continuous solar cell and antenna orientation. Since the primary source of power in most satellites is from solar
cells, a maximum number of the solar cells must be exposed to the sun at all times. The satellite antenna must
also be pointed at the appropriate earth terminals. Our communications satellites use what is termed spin
stabilization to meet these important requirements.
Spin stabilization operates on the principle that
direction of the spin axis of a rotating body tends to remain fixed in space. An example of spin stabilization is
the effect of the rotation of the earth in keeping its axis fixed in space. A satellite that has a spin axis
parallel to the axis of the earth will maintain this position since both axes are fixed in space. Figure 4-8
illustrates the use of this principle. It depicts an equatorial orbit satellite used to keep a doughnut-shaped
antenna pattern pointing toward the earth.
Figure 4-8.—Spin-stabilized satellite antenna pattern.
Once the system is in motion, spin stabilization requires virtually no additional energy. A spin- stabilized
satellite is usually constructed like a flywheel. Its heavier equipment is mounted in the same plane and as close
to the outside surface as possible.
After reaching its orbit, the radial jets are pulsed to start the
satellite spinning. The satellite spin axis is orientated to the axis of the earth by means of small axial jets.
Velocity jets are used to place the satellite in orbit position and provide velocity correction. Figure 4-9 is an
example of spin stabilization.
Figure 4-9.—Spin-stabilized satellite controls.
Solar cells are installed around the outside surface of a spin-stabilized satellite. This gives you a
large number of solar cells exposed to the sun at all times (except when the satellite is in eclipse). The use of
omnidirectional antennas causes a small part of the total radiated energy to be directed toward the earth at all
Omnidirectional antennas radiate only a small amount of energy toward the earth. Many techniques have been tried
to achieve an earth-oriented antenna system. One system developed uses spin stabilization for orientation of the
satellite. It uses a stationary inner platform for mounting remote controlled antennas. The satellite is
constructed in two parts with both parts having radial jets. The inner platform contains the communications
antennas and the communications package. After the satellite is stabilized in space, inner radial jets spin the
inner platform. The inner platform is stationary with respect to earth and is oriented to such a position that the
communications antennas point continuously toward the earth. This arrangement allows the use of high-gain
directional antennas that concentrate the majority of the radiated energy in the direction of the earth.
The latest versions of communications satellites use a stationary platform with four high-gain antennas. Two
steerable narrow beam antennas are used for communications between and within regions of high traffic density. Two
horn antennas provide for earth communications between facilities outside the narrow beam coverage. The antenna
arrangement for these types of communications satellites is shown in figure 4-7.
Q5. What was
the major operational limitation of early communications satellites?
Q6. Satellite orientation in space is important for what two reasons?
Communications satellite earth terminals are usually located in areas remote from
the actual users of these communications. This is necessary to minimize RF interference to the satellite.
Locating the terminals in these remote locations requires interconnecting communications links. Links permit
communications flow to and from the users of the satellite systems. Interconnect links are usually connected via
telephone cables or microwave radio links with normal terminal equipment.
Earth satellite communications
terminals generally have a single, large antenna; a highly sensitive receiver; a powerful transmitter; multiplex
equipment; modulating-demodulating equipment; and telemetry equipment. Each of these elements will be discussed
later in this chapter.
Earth terminal antennas are highly directional, high-gain antennas capable of
transmitting and receiving signals simultaneously. Generally, large, high-gain, parabolic antennas are used.
Generally speaking, three sizes of parabolic-type antennas are currently in use at earth terminal sites. These are
a parabolic antenna sixty feet in diameter, a parabolic antenna forty feet in diameter, and a cluster of four
parabolic antennas, each ten feet in diameter. These four in combination are equal to a parabolic antenna eighteen
feet in diameter. They are shown in figures 4-10, 4-11, and 4-12, respectively.
Figure 4-10.—Typical satellite earth terminal with sixty-foot antenna.
Figure 4-11.—Forty-foot antenna and pedestal.
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