CHAPTER 11
Introduction to Satellite Communications
1. A satellite is a physical object that orbits a celestial body.
2. A communications satellite containing electronic equipment acts as a repeater or relay station between two earth stations.
3. The basic component of a communications satellite is a receiver-transmitter combination called a transponder.
4. A satellite stays in orbit because the gravitational pull of the earth, is balanced by the' centripetal force of the revolving satellite.
5. Satellite orbits about the earth are either circular or elliptical.
6. Satellites orbit the earth from heights of 100 to 22,300 mi and travel at speeds of 6800 to 17,500 mi/h.
7. A satellite that orbits directly over the equator 22,300 mi from earth is said to be in a geostationary orbit. It revolves in synchronism with the earth's rotation, so it appears to be stationary when seen from points on the earth.
8. A satellite is stabilized in orbit by spinning it on its axis or building in spinning flywheels for each major axis (roll, pitch, yaw).
9. Attitude adjustments on a satellite are made by firing small jet thrusters to change the satellite's position or speed.
10. The location of a satellite is determined with latitude and longitude measurements that designate a point on the earth subsatellite point (SSP) directly below the satellite.
11. Azimuth and elevation angles determine where to point an earth station so that it intercepts the satellite.
12. Satellites are launched into orbit by rockets that give them vertical as well as forward motion.
13. A geosynchronous satellite is initially put into an elliptical orbit where its apogee is 22,300 mi high. The apogee kick motor is then fired to put the satellite into its final circular geostationary orbit.
14. Many satellites are put into orbit by launching them from NASA's space shuttle.
15. Most satellites operate in the microwave region.
16. Microwave satellites operate on assigned frequency bands designated by a letter. Common communications satellite bands are the C (3.4 to 6.425 GHz) and Ku (10.95 to 14.5 GHz) bands.
17. Satellite bands are typically 500 MHz wide and are divided into 12 segments, each 36 MHz wide. A transponder is used to cover each segment.
18. Frequency reuse is a technique that allows two sets of transponders to operate on the same frequency, thus doubling channel capacity. The two sets of channels use antennas of different polarizations to prevent interference with one another.
19. Spatial isolation is another technique for frequency sharing. It uses highly directional spot-beam antennas to prevent interference between stations on the same frequency.
20. The main subsystems in a satellite are the communications; power; telemetry, tracking; and control (TIC); propulsion; attitude stabilization; and antenna subsystems.
21. A transponder consists of a low-noise amplifier (LNA) that receives and amplifies the up-link signal, a mixer that converts the signal to another (lower) frequency, and a high-power amplifier that retransmits the signal on its new down-link frequency.
22. Double-conversion transponders use two, mixers, one to translate the up-link signal to an IF where it is amplified and filtered, and another to translate the signal to its final down-link frequency.
23. Regenerative transponders demodulate the up-link signal to recover the baseband signals and then use them to remodulate a downlink transmitter. This improves the SIN ratio.
24. In a broadband transponder, a single mixer converts all channels within the 500-MHz bandwidth simultaneously to their downlink frequencies. These are selected by channel bandpass filters and then amplified by individual power amplifiers.
25. In a channelized transponder, each channel has its own LNA, bandpass filters, mixer, and high-power amplifier.
26. The power subsystem consists of solar panels, batteries, dc-to-dc converters, and regulators. The solar panels convert sunlight into de power to operate all satellite electronics and to charge the batteries that take over when sunlight is blocked.
27. The TTC subsystem contains a receiver that picks up commands from a ground station and translates them into control signals that initiate some action on board. The telemetry system monitors physical conditions within the satellites and converts them into electrical signals that are transmitted back to earth.
28. The propulsion system consists of the apogee kick motor that puts the satellite into final orbit and the jet thrusters that are used for positioning and attitude control.
29. The stabilization subsystem for attitude control consists of spin components or three-axis flywheel gyros.
30. The antenna system consists of one or more highly directional horn or parabolic antennas and an omnidirectional TTC antenna.
31. Earth stations consist of transmit, receive, power, antenna, TIC, and ground control equipment (GCE) subsystems.
32. The transmit subsystem takes the baseband voice, video, or computer data signals: multiplexes them; and uses the composite signal to modulate a carrier. An up converter translates the signal to its final up-link frequency before it is amplified and transmitted.
33. The most common forms of modulation used are FM and QPSK.
34. Transistor power amplifiers are used in low-power earth stations; klystrons are used in high-power narrowband stations; and TWTs are used in high-power broadband stations.
35. Earth stations feature large- parabolic dish antennas with high gain and directivity for receiving the weak satellite signal.
36. The receive subsystem in an earth station amplifies the signal with an LNA and then separates the channels with bandpass filters. Down conveners translate the signals to a lower IF where they are demodulated and demultiplexed.
37. The GCE in an earth station interfaces the baseband signals to the transmit and receive subsystems. The receive GCE performs demodulation and demultiplexing. The transmit GCE performs modulation and multiplexing.
38. The most common application for satellites is communications.
39. Another major use of satellites is surveillance and reconnaissance.
40. Film cameras, TV cameras, infrared sensors, and radars are all used to observe a variety of conditions on earth from surveillance satellites.
41. Satellites playa major role in military and defense systems not only for communications but also for surveillance.
42. The 24-satellite Global Positioning System (GPS) makes accurate navigation possible anywhere on earth with a low-cost microwave receiver.
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