# Principles of Electronic Communication Systems

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Principles of Electronic Communication Systems
Second Edition Louis Frenzel © 2002 The McGraw-Hill Companies

Principles of Electronic Communication Systems Second Edition
Chapter 16 Satellite Communication ©2003 The McGraw-Hill Companies

Satellite Communication
Satellites are launched and orbited for a variety of purposes. The most common application is communication in which the satellite is used as a repeater.

Topics Covered in Chapter 16
Satellite Orbits Satellite Communication Systems Satellite Subsystems Ground Stations Satellite Applications Global Positioning System

Principles of Satellite Orbits and Positioning
In order for a satellite to go into orbit around the earth, it must have some forward motion. When a satellite is launched, it is given both vertical and forward motion. Forward motion produces inertia, which tends to keep the satellite moving in a straight line Gravity tends to pull the satellite toward the earth. The inertia of the satellite is equalized by the earth’s gravitational pull. The satellite constantly changes its direction from a straight line to a curved line to rotate about the earth.

Principles of Satellite Orbits and Positioning (Continued)
The goal is to give the satellite acceleration and speed that will exactly balance the gravitational pull. Communication satellites are typically about 22,300 miles from the earth. A satellite needs to travel about 6800 mi/hr in order to stay in orbit at that distance. A satellite rotates around the earth in either a circular or elliptical path. A satellite rotates in an orbit that forms a plane that passes through the center of gravity called geocenter.

Satellite Elliptical Orbit

Orbital Plane Passes Through the Geocenter

Satellite Height In a circular orbit, the height is simply the distance of the satellite from the earth. In geometric calculations, the height is the distance between the center of the earth and the satellite. When the satellite is an elliptical orbit, the center of the earth is one of the focal points of the ellipse. The two points of greatest interest are the highest point above the earth (the apogee) and the lowest point (the perigee).

Apogee and Perigee

Satellite Speed Satellite speed varies depending upon the distance of the satellite from the earth. For a circular orbit the speed is constant, but for an elliptical orbit the speed varies depending upon the height. Low earth satellites of about 100 mi in height have a speed of about 17,500 mi/hr. Very high satellites such as communication satellites typically rotate at speeds of about 6800 mi/hr.

Satellite Period The period is the time it takes for a satellite to complete one orbit. This time is also called the sidereal period. One revolution is the period of time that elapses between the successive passes of the satellite over a given meridian of earth longitude. Typical rotational periods range from about 1 ½ hr for a 100-mi height to 24 hr for a 22,300-mi height.

Angle of Inclination The angle of inclination of a satellite orbit is the angle formed between the line that passes through the center of the earth and the north pole and a line that passes through the center of the earth but that is also perpendicular to the orbital plane. It is also defined as the angle between the equatorial plane and the satellite orbital plane as the satellite enters the northern hemisphere. When the satellite has an angle of inclination, the orbit is said to be ascending or descending.

Angle of Inclination

Ascending and Descending Orbits

Angle of Elevation The angle of elevation of a satellite is the angle that appears between the line from the earth station’s antenna to the satellite and the line between the earth’s station’s antenna and the earth’s horizon. Noise in the atmosphere contributes to poor performance. The minimum practical angle of elevation for good satellite performance is generally 5 degrees. The higher the angle of elevation, the better.

Angle of Elevation

By Definition… The satellite location specified by a point known as the subsatellite point (SSP) is on the earth directly below the satellite. Latitude is defined as the angle between the line drawn from a given point on the surface of the earth to the point at the center of the earth called the geocenter and the line between the geocenter and the equator. The prime meridian is used as a reference point for measuring longitude.

Satellite Communication Systems
Communication satellites are not originators of information to be transmitted. Satellites are relay stations for earth sources. The transmitting station sends the information to the satellite, which in turn retransmits it to the receiving station. The satellite in this application is what is generally known as a repeater.

Repeater and Transponder
An earth station transmits information to the satellite. The satellite contains a receiver that picks up the transmitted signal, amplifies it, and translates it on another frequency. The signal on the new frequency is then retransmitted to the receiving stations back on earth. The transmitter-receiver combination in the satellite is known as a transponder.

Frequency Allocations
Most communication satellites operate in the microwave frequency spectrum. The microwave spectrum is divided up into frequency bands that have been allocated to satellites as well as other communication services such as radar. The most widely used satellite communication band is the C band. The C band uplink frequencies are in the to GHz range and the downlink frequencies are in the 3.7- to 4.2-GHz range.

Frequency Bands Used in Satellite Communication

Satellite Subsystems All satellite communication systems consist of two basic parts, the satellite or spacecraft and two or more earth stations. The satellite performs the function of a radio repeater or relay station. Two or more earth stations may communicate with one another through the satellite rather than directly point-to-point on the earth.

Satellite Subsystems (Continued)
The heart of a communication satellite is the communication subsystem. This subsystem is a set of transponders that receive the uplink signals and retransmit them to earth. A transponder is a repeater that implements a wideband communication channel. The communication subsystem consists of multiple transponders.

Communication Subsystems
The main payload on a communication satellite is the communication subsystem that performs the function of a repeater or relay station. An earth station takes the signals to be transmitted, known as baseband signals, and frequency modulates a microwave carrier. The three most common baseband signals are voice, video, or computer data. Most modern satellites contain at least 12 transponders.

Multichannel Configurations
Virtually all modern communication satellites contain multiple transponders. This permits many more signals to be received and transmitted. Each transponder operates on a separate frequency, but its bandwidth is wide enough to carry multiple channels of voice, video, and digital information. The two multichannel architectures used with communication satellites are broadband and fully channelized.

Power Subsystem Today virtually every satellite uses solar panels for its basic power source. Solar panels are large arrays of photocells connected in various series and parallel circuits to create a powerful source of direct current. A key requirement is that the solar panels always be pointed toward the sun. Special DC-to-DC converters are used to translate the DC voltage of the solar panels to a higher DC voltage required by TWT amplifiers for example.

Telemetry, Command, and Control Subsystems
All satellites have a telemetry, command, and control (TC&C) subsystem that allows a ground station to monitor and control conditions in the satellite. The telemetry system is used to report the status of the onboard subsystems to the ground station. A command and control system permits the ground station to control the satellite. Most satellites contain a small digital computer that acts as a central control unit for the entire satellite.

Applications Subsystems
The applications subsystems is made up of the special components that enable the satellite to fulfill its intended purpose. For a communication satellite, this subsystem is made up of the transponders. An observation satellite may use TV cameras or infrared sensors to pick up various conditions on earth and in the atmosphere.

Ground Stations The ground station, or earth station, is the terrestrial base of the system. The ground station communicates with the satellite to carry out the designated mission. The earth station consists of five major subsystems: The antenna subsystem The receive subsystem The transmit subsystem The ground control equipment (GCE) subsystem Power subsystem

Antenna Subsystems All earth stations have a relatively large parabolic dish antenna that is used for sending and receiving signals to and from the satellite. Earth station dishes were 80 to 100 ft or more in diameter, however, with higher power transmission, antennas as small as 18 in in diameter are used. The antenna in an earth station must be steerable. That is, it must be possible to adjust its azimuth and elevation so that the antenna can be properly aligned with the satellite.

Receive Subsystems The downlink is the receive subsystem of the earth station. It usually consists of very low noise preamplifiers that take the small signal received from the satellite and amplify it to a level suitable for further processing. The signal is then demodulated and sent on to other parts of the communication system.

Receiver Circuits The receive subsystem consists of the LNA, down converters, and related components. The purpose of the receive subsystem is to amplify the downlink satellite signal and translate it to a suitable intermediate frequency. The IF signal is then demodulated and demultiplexed as necessary to generate the original baseband signals.

Transmitter Subsystems
The uplink is the transmitting subsystem of the earth station. It consists of all the electronic equipment that takes the signal to be transmitted, amplifies it, and sends it to the antenna. In a communication system, the signals to be sent to the satellite might be TV programs, multiple telephone calls, or digital data from a computer. Signals modulate a carrier, are amplified, and sent to an antenna via waveguides, combiners, and diplexers.

Satellite Applications
Communication satellites act as relay stations in the sky and permit reliable long-distance communication worldwide. From their vantage, surveillance satellites can look at the earth and transmit what they see back to ground stations for a wide variety of purposes. Satellite navigation systems can provide global coverage unavailable with land-based systems satellites.

Global Positioning System
The Global Positioning System (GPS), also known as Navstar, is a satellite-based navigation system that can be used by anyone with an appropriate receiver to pinpoint his or her location on earth. GPS was developed by the US Air Force for the department of Defense as a continuous global radio navigation system. The GPS system consists of three major segments: the space segment, the control segment, and the user segment.

Space Segment The space segment is the constellation of satellites orbiting above the earth that contain transmitters which send highly accurate timing information to GPS receivers on earth.

Control Segment The control segment of the GPS system refers to the various ground stations that monitor the satellites and provide control and update information.

GPS Applications The primary application of the GPS is military navigation and related applications. GPS is used by all services for ships, aircraft, and ground troops. The GPS was first widely used in Operation Desert Storm in 1991. Commercial applications include surveying, mapmaking, and construction. Police, fire, ambulance, and forest services use GPS.