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Satellite Microwave

Satellite Microwave. Agenda. Propagation in Soft Channels Satellite Microwave Orbits Classes Bands Uses Limitations. Kilo. Mega. Giga. Tera. Peta. Frequency (Hz). 10 2. 10 3. 10 4. 10 5. 10 6. 10 7. 10 8. 10 9. 10 10. 10 11. 10 12. 10 13. 10 14. 10 15. Power &

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Satellite Microwave

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  1. Satellite Microwave

  2. Agenda • Propagation in Soft Channels • Satellite Microwave • Orbits • Classes • Bands • Uses • Limitations

  3. Kilo Mega Giga Tera Peta Frequency (Hz) 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 Power & Telephone Radio Micro-wave Infrared VisibleLight Propagation in Soft ChannelsLine-of-Sight (LOS) • Signals occupy spectrum above 30MHz • EM waves in this frequency band or spectrum are not reflected back to the Earth and easily pass through the atmosphere. LOS to satellite. LOS is blocked.

  4. Satellite Microwave • After WWII, a British engineer named Arthur C. Clarke thought of putting microwave repeaters in space. • Satellites are microwave repeaters that receive transmissions on one frequency band (uplink), amplify or repeat the signal, and transmit them on another frequency (downlink). • A single satellite operates on several frequency bands, called transponderchannels or transponders.

  5. Satellite Microwave Footprint: the area the satellite can “cover”. This can be very large. Downlink is broadcast. Uplink is “directed” using antenna.

  6. Satellite Microwave • If a satellite orbited in the same plane as the equator and if its velocity took it around the Earth every 24 hours then… • The satellite would appear “stationary” from the point of view of an observer on the Earth.

  7. Satellite Orbits • A Geostationary Orbit seen from above one of the poles. Observer on Earth makes one circuit per day. Earth Satellite makes one circuit per day. Relative E/W positions of observer and satellite never changes.

  8. Satellite Orbits • Orbits don’t have to be circular! An elliptical orbit. Earth Moving slowestat apogee. Moving fastestat perigee. Satellite is constantly changing distance from the observer and speed.

  9. Satellite Orbits • A Geostationary Orbit seen from above the equator Angle up/down in the sky of observer and satellite never changes. N The orbit is “equatorial”. S From the POV of an Earth-bound observer, what happens to position in the sky of the satellite as the observer moves North?

  10. Satellite Orbits • Orbits can also be “inclined”. A “polar” orbithighly inclined An inclined orbit N Plane of the equator Some satellites have orbits that are both elliptical and inclined. S

  11. Satellite Orbits • Three satellites, orbiting in the same plane, 35,579 Km from the Earth, 120o apart can fully “cover” the planet’s surface. Each satellite’s footprint covers 1/3 of the Earth’s surface. Earth

  12. Satellite Orbits • Geosynchronous: A satellite with an orbital period equal to the earth's rotation period and approximately 35,579 Km (22, 237 miles) above the earth's surface. • Geostationary: Geosychnronous with an orbital plane in the earth's equatorial plane (i.e. zero inclination). Satellites in Geostationary orbit are always moving with the rotation of the earth. • Number of satellite “slots” in an orbital plane is limited by 4o angular displacement requirement.

  13. Satellite Classes • Geosynchronous or Geostationary (GEOs) • Low Earth Orbit (LEOs) • Motorola's 66-satellite Iridium system • Globalstar 48-satellite system from Loral/Qualcomm • Medium Earth Orbit (MEOs) • Highly Elliptical Orbit (HEOs) • a.k.a Molnya Orbit

  14. GEOs • Orbit is about 35,579 Km above the equator and orbitalperiod equals the Earth's rotation period. • Therefore satellite is stationary with respect to observers on the Earth. • Finding and tracking is relatively easy. • No problem with frequency changes as satellites move closer/farther from the observer. • Long propagation delays. • Very large footprints - wasteful of spectrum.

  15. LEOs • Orbit at 500-2,200 Km. • Each orbits the Earth every 1-2 hours. Not geosynchronous or geostationary! Therefore receivers must be “passed-off” from satellite to satellite. • A large constellation of satellites is required to guarantee continuous coverage. • Much stronger signals are received for a given power of transmission than from a GEOs. • Can use hand-held receivers instead of dish antennas. • Smaller footprint than GEOs.

  16. MEOs • Orbit at 5,000-15,000 Km. • Longer orbital period than LEOs. Not geosynchronous or geostationary. However, receivers need not be passed-off as often. • Fewer satellites are required for complete coverage than for a constellation of LEOs. • Signals received will be weaker than for LEOs but stronger than for GEOs. • Can use hand-held receivers instead of dish antennas.

  17. HEOs • Near the poles, GEOs appear near the horizon in the sky. • Signals have farther in the atmosphere to travel and can encounter obstacles. • Some Russian satellites are in highly inclined (65o above the equator), highly elliptical orbits. • Each satellite orbits in about 12 hours, spending most of its time above the northern hemisphere. • Greater distance/attenuation than GEOs.

  18. Satellite Microwave • As the frequencies used get higher… • Attenuation increases so more amplification is necessary. • This means more powerful transmitters are required. • Wavelength gets shorter. • The dish antennas used can become smaller for the same amount of amplification

  19. Satellite BandsC Band - 4/6 GHz • Uplink frequencies: 5.925-6.425 GHz • Downlink frequencies: 4.2-4.7 GHz • First choice for satellites because it was also used for terrestrial microwave. Equipment and expertise were available. • Interference with terrestrial microwave • Used for video delivery, VSAT,news gathering, telephony • Band is saturated

  20. Satellite BandsKu Band - 12/14 GHz • Uplink frequencies: 14-14.5 GHz • Downlink frequencies: 11.7-12.2 GHz • Up/downlink freqencies differ for US and Europe. • Smaller/cheaper earth stations used than for C Band. • Rain produces attenuation problems so more powerful transmitters are needed. • Used for video delivery, VSAT, news gathering, telephony, direct-to-home video/audio, internet access, voice, video, data • Band will become saturated.

  21. Some Other Bands • L Band 1.530-2.700 GHz • Voice/low speed data to mobile terminals. • S Band 2.7-3.5 GHz • Cellular telephony, data, paging • Ka Band 18-31 GHz • Internet access, voice, video, data, videoconferencing • Very powerful transmitters to deal with attenuation

  22. Satellite Uses • Used for television (both network and DSB), long-distance telephone, and private business networks. • Particularly suitable for creating networks where cabling is not practical. • Low subscriber densities • Mobile stations

  23. Satellite Limitations • Limited bandwidth, even with dish antennas directionality is limited. • Satellites cannot be placed close to each other unless they are processing the same signals. • Long propagation delays due to distance. • Distracting for voice communications. • Data communications requires special protocols.

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