Principles of Electronic Communication Systems

1. Principles of ElectronicCommunication Systems Third Edition Louis E. Frenzel, Jr.

2. Chapter 17 Satellite Communication

3. Topics Covered in Chapter 17 • 17-1: Satellite Orbits • 17-2: Satellite Communication Systems • 17-3: Satellite Subsystems • 17-4: Ground Stations • 17-5: Satellite Applications • 17-6: Global Positioning System

4. 17-1: Satellite Orbits • 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.

5. 17-1: Satellite Orbits Principles of Satellite Orbits and Positioning • The ability to launch a satellite and keep it in orbit depends upon following well-known physical and mathematical laws called orbital dynamics. • 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.

6. 17-1: Satellite Orbits Principles of Satellite Orbits and Positioning • 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.

7. 17-1: Satellite Orbits Principles of Satellite Orbits and Positioning • 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 of the earth called geocenter.

8. 17-1: Satellite Orbits Figure 17-1: Satellite orbits. (a) Circular orbit. (b) Elliptical orbit.

9. 17-1: Satellite Orbits Figure 17-2: The orbital plane passes through the geocenter.

10. 17-1: Satellite Orbits Principles of Satellite Orbits and Positioning: Satellite Height • In a circular orbit, the height is 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).

11. 17-1: Satellite Orbits Figure 17-4: Elliptical orbit showing apogee and perigee.

12. 17-1: Satellite Orbits Principles of Satellite Orbits and Positioning: 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 travel at speeds of about 6800 mi/hr.

13. 17-1: Satellite Orbits Principles of Satellite Orbits and Positioning: 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 h for a 100-mi height to 24 h for a 22,300-mi height.

14. 17-1: Satellite Orbits Principles of Satellite Orbits and Positioning: 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.

15. 17-1: Satellite Orbits Figure 17-5: (a) Angle of inclination. (b) Ascending and descending orbits. Figure 17-5: (a) Angle of inclination. (b) Ascending and descending orbits.

16. 17-1: Satellite Orbits Principles of Satellite Orbits and Positioning: 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 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 5°. • The higher the angle of elevation, the better.

17. 17-1: Satellite Orbits Figure 17-6: Angle of elevation.

18. 17-1: Satellite Orbits Principles of Satellite Orbits and Positioning: Geosynchronous Orbits • To use a satellite for communication relay or repeater purposes, the ground station antenna must be able to follow or track the satellite as it passes overhead. • Depending upon the height and speed of the satellite, the earth station is able to use it only for communication purposes for that short period when it is visible. • The best solution to this problem is to launch a synchronous or geostationary satellite.

19. 17-1: Satellite Orbits Principles of Satellite Orbits and Positioning: Geosynchronous Orbits • In a geosynchronous earth orbit (GEO), the satellite rotates about the earth in exactly 24 h. • It appears to be fixed or stationary. The antenna is pointed at the satellite and remains in a fixed position, making continuous communication possible. • Most communication satellites in use today are of the geosynchronous variety.

20. 17-1: Satellite Orbits Principles of Satellite Orbits and Positioning: Position Coordinates in Latitude and Longitude • The satellite location is specified by a point on the earth directly below the satellite known as the subsatellite point (SSP). • 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.

21. 17-2: 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.

22. 17-2: Satellite Communication Systems • Repeaters and Transponders • 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.

23. 17-2: Satellite Communication Systems • Repeaters and Transponders • The original signal being transmitted from the earth station to the satellite is called the uplink. • The retransmitted signal from the satellite to the receiving stations is called the downlink. • The transmitter-receiver combination in the satellite is known as a transponder.

24. 17-2: Satellite Communication Systems Figure 17-10: Using a satellite as a microwave relay link.

25. 17-2: Satellite Communication Systems • Repeaters and Transponders: 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 5.925- to 6.425-GHz range and the downlink frequencies are in the 3.7- to 4.2-GHz range.

26. 17-2: Satellite Communication Systems • Frequency bands used in satellite communication:

27. 17-3: 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.

28. 17-3: Satellite Subsystems • 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 capable of carrying many simultaneous communication transmissions.

29. 17-3: Satellite Subsystems Figure 17-14: General block diagram of a communication satellite.

30. 17-3: Satellite Subsystems 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 modulates a microwave carrier. • The three most common baseband signals are voice, video, or computer data. • Most modern satellites contain at least 12 transponders.

31. 17-3: Satellite Subsystems Communication Subsystems: 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.

32. 17-3: Satellite Subsystems 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. • Solar panels generate a direct current that is used to operate the various components of the satellite and to charge secondary batteries that act as a buffer.

33. 17-3: Satellite Subsystems 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.

34. 17-3: Satellite Subsystems Applications Subsystems • The applications subsystem 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. This information is then transmitted back to earth by a special transmitter designed for this purpose.

35. 17-4: 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

36. 17-4: Ground Stations 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.

37. 17-4: Ground Stations 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.

38. 17-4: Ground Stations Receive Subsystems: 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.

39. 17-4: Ground Stations Figure 17-18: Dual-conversion down converters. (a) RF tuning. (b) IF tuning.

40. 17-4: Ground Stations Receiver Ground Control Equipment • The receiver ground control equipment (GCE) consists of one or more racks of equipment used for demodulating and demultiplexing the received signals. • The down converters provide initial channelization by transponder, and the demodulators and demultiplexing equipment process the 70-MHz IF signal into the original baseband signals. • Other intermediate signals may be developed as required by the application.

41. 17-4: Ground Stations 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.

42. 17-4: Ground Stations Transmit Ground Control Equipment • The transmit subsystem begins with the baseband signals, which are first fed to a multiplexer, if multiple signals are to be carried by a single transponder. • The multiplexer output is then fed to a modulator. • In analog systems, a wideband frequency modulator is normally used. • In digital systems, analog signals are first digitized with PCM converters. The resulting serial digital output is then used to modulate a QPSK modulator.

43. 17-4: Ground Stations Power Subsystems • Most earth stations receive their power from the normal ac mains. Standard power supplies convert the ac power to the dc voltages required to operate all subsystems. • Most earth stations have backup power systems that take over if an ac power failure occurs. • The backup power system may consist of a diesel engine driving an ac generator, which automatically starts when ac power fails. • Smaller systems may use uninterruptible power supplies (UPS), which derive their main power from batteries.

44. 17-4: Ground Stations Telemetry and Control Subsystems • The telemetry equipment consists of a receiver and the recorders and indicators that display the telemetry signals. • The signal may be received by the main antenna or a separate telemetry antenna. • A separate receiver on a frequency different from that of the communication channels is used for telemetry purposes.

45. 17-4: Ground Stations Telemetry and Control Subsystems • In some satellite systems where communication is not the main function, some instrumentation may be a part of the ground station. • Instrumentation is a general term for all the electronic equipment used to deal with the information transmitted back to the earth station.

46. 17-4: Ground Stations Very Small-Aperture Terminal • A very small-aperture terminal (VSAT)is a miniature low-cost satellite ground station. • These units are extremely small and mount on the top or side of a building and in some versions even fit into a suitcase. • Costs range from a few thousand dollars to no more than about $6000 today. • They can be installed very quickly by plugging them in and pointing the antenna.

47. 17-4: Ground Stations Very Small-Aperture Terminal • The most common application of VSATs today is in connecting remote company or organization sites to a main computer system. • Gas stations and retail stores use VSATs as point-of-sale (PoS) terminals to transmit sales transaction information to the home office, check customer credit cards, and relay inventory data. • Tollbooths using SpeedPass and other radio-frequency identification (RFID) of vehicles for tolls use VSATs. • The settop box receiver used by consumers for Direct Broadcast Satellite (DBS) TV reception is a receive-only (RO) VSAT.

48. 17-5: Satellite Applications • Communication: The main application for satellites today is in communication. Communication satellites act as relay stations in the sky and permit reliable long-distance communication worldwide. • Direct Broadcast Satellite (DBS) service:This is a TV signal distribution system designed to distribute signals directly to consumers.

49. 17-5: Satellite Applications • Satellite Cell Phones. Satellite-based cellular telephone service is under development. The proposed new systems use low-earth-orbit satellites to perform the relay services to the main telephone system or to make connection directly between any two cellular telephones using the system.

50. 17-5: Satellite Applications • Digital Satellite Radio: One of the newest satellite applications is in digital satellite radioor the digital audio radio service (DARS). • This service provides hundreds of channels of music, news, sports, and talk radio to car portable and home radios. • It provides full continuous coverage of the station you select wherever you are in the United States. • Its digital transmission techniques ensure high-quality stereo sound that is immune to noise. • The satellites transmit other information such as song title and artist, type of music, and other data, which are displayed on a LCD screen.