SATELLITE COMMUNICATION SYSTEMS Girish K.P. www.edutalks.org Communication satellites bring the world to you anywhere and any time….. www.edutalks.org What exactly is a satellite? • The word satellite originated from the Latin word “Satellit”- meaning an attendant, one who is constantly hovering around & attending to a “master” or big man. • For our own purposes however a satellite is simply any body that moves around another (usually much larger) one in a mathematically predictable path called an orbit. • A communication satellite is a microwave repeater staion in space that is used for tele communcation , radio and television signals. • The first man made satellite with radio transmitter was in 1957. There are about 750 satellite in the space, most of them are used for communication. www.edutalks.org How do satellite work? www.edutalks.org How do Satellites Work? * Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means. The two stations can use a satellite as a relay station for their communication. * One Earth Station transmits the signals to the satellite. Up link frequency is the frequency at which Ground Station is communicating with Satellite. * The satellite Transponder converts the signal and sends it down to the second earth station. This frequency is called a Downlink. www.edutalks.org Consider the light bulb example: www.edutalks.org Components of a satellite www.edutalks.org Advantages of satellite over terrestrial communication : * The coverage area of a satellite greatly exceeds that of a terrestrial system. * Transmission cost of a satellite is independent of the distance from the center of the coverage area. * Satellite to Satellite communication is very precise. * Higher Bandwidths are available for use. Disadvantages of satellites: * Launching satellites into orbit is costly. * Satellite bandwidth is gradually becoming used up. * There is a larger propagation delay in satellite communication than in terrestrial communication. www.edutalks.org How does a satellite stay in it’s orbit? www.edutalks.org How do we escape gravity & place an object in orbit? • If an object is fired fast enough it should escape the earths pull. • This is done through the use of Rocket Launchers www.edutalks.org Multi-stage Rockets • Stage 1: Raises the payload e.g. a satellite to an elevation of about 50 miles. • Stage 2: Satellite 100 miles and the third stage places it into the transfer orbit. • Stage 3: The satellite is placed in its final geo- synchronous orbital slot by the AKM, a type of rocket used to move the satellite. www.edutalks.org Applications www.edutalks.org Major problems for satellites • Positioning in orbit • Stability • Power • Communications • Harsh environment www.edutalks.org Positioning • This can be achieved by several methods • One method is to use small rocket motors • These use fuel - over half of the weight of most satellites is made up of fuel • Often it is the fuel availability which determines the lifetime of a satellite • Commercial life of a satellite typically 10- 15 years www.edutalks.org Stability • It is vital that satellites are stabilised - to ensure that solar panels are aligned properly, communication antennae are aligned properly • Early satellites used spin stabilisation - either this requires an inefficient omni-directional aerial Or antennae were precisely counter-rotated in order to provide stable communications. * Modern satellites use reaction wheel stabilisation - a form of gyroscopic stabilisation. www.edutalks.org Power • Modern satellites use a variety of power means •Solar panels are now quite efficient, so solar power is used to generate electricity • Batteries are needed as sometimes the satellites are behind the earth - this happens about half the time for a LEO satellite • Nuclear power has been used - but not recommended www.edutalks.org Satellite - satellite communication • It is also possible for satellites to communicate with other satellites • Communication can be by microwave or by optical laser 2. 2. 2. 1. 1. 1. Point-Point System Crosslink System Hybrid System www.edutalks.org Harsh Environment • Satellite components need to be specially “hardened” • Circuits which work on the ground will fail very rapidly in space • Temperature is also a problem - so satellites use electric heaters to keep circuits and other vital parts warmed up - they also need to control the temperature carefully www.edutalks.org Early satellites • Telstar – Allowed live transmission across the Atlantic • Syncom 2 – First Geosynchronous satellite TELSTAR SYNCOM 2 www.edutalks.org Satellite orbits Classification of orbits: www.edutalks.org * Circular orbits are simplest * Inclined orbits are useful for coverage of equatorial regions * Elliptical orbits can be used to give quasi stationary behavior viewed from earth using 3 or 4 satellites * Orbit changes can be used to extend the life of satellites www.edutalks.org Classification of orbits: Satellite orbits are also classified based on their heights above the earth: – GEO – LEO – MEO – Molniya Orbit – HAPs www.edutalks.org Satellite orbit altitudes www.edutalks.org Geostationary Earth Orbit (GEO) • These satellites are in orbit 35,786 km above the earth’s surface along the equator. • Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates. This means GEO satellites remain in the same position relative to the surface of earth. www.edutalks.org GEO contd. • Advantages –A GEO satellite’s distance from earth gives it a large coverage area, almost a fourth of the earth’s surface. – GEO satellites have a 24 hour view of a particular area. – These factors make it ideal for satellite broadcast and other multipoint applications – Minimal doppler shift • Disadvantages – A GEO satellite’s distance also cause it to have both a comparatively weak signal and a time delay in the signal, which is bad for point to point communication. – GEO satellites, centered above the equator, have difficulty for broadcasting signals to near polar regions – Launching of satellites to orbit are complex and expensive. www.edutalks.org Low Earth Orbit (LEO) • LEO satellites are much closer to the earth than GEO satellites, ranging from 500 to 1,500 km above the surface. • LEO satellites don’t stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass. • A network of LEO satellites is necessary for LEO satellites to be useful www.edutalks.org The Iridium system has 66 satellites in six LEO orbits, each at an altitude of 750 km. Iridium is designed to provide direct worldwide voice and data communication using handheld terminals, a service similar to cellular telephony but on a global scale www.edutalks.org LEO Contd. • Advantages A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication. A LEO satellite’s smaller area of coverage is less of a waste of bandwidth. • Disadvantages A network of LEO satellites is needed, which can be costly LEO satellites have to compensate for Doppler shifts cause by their relative movement. Atmospheric drag effects LEO satellites, causing gradual orbital deterioration. www.edutalks.org Medium Earth Orbit (MEO) • A MEO satellite is in orbit somewhere between 8,000 km and 18,000 km above the earth’s surface. • MEO satellites are similar to LEO satellites in functionality. • MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 to 8 hours. • MEO satellites have a larger coverage area than LEO satellites. www.edutalks.org MEO contd. • Advantage A MEO satellite’s longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than a LEO network. • Disadvantage A MEO satellite’s distance gives it a longer time delay and weaker signal than a LEO satellite, though not as bad as a GEO satellite. www.edutalks.org MEO satellites The GPS constellation calls for 24 satellites to be distributed equally among six circular orbital planes Glonass (Russian) www.edutalks.org Molniya Orbit Used by Russia for decades. Molniya Orbit is an elliptical orbit. The satellite remains in a nearly fixed position relative to earth for eight hours. A series of three Molniya satellites can act like a GEO satellite. Useful in near polar regions. www.edutalks.org High Altitude Platform (HAP) One of the newest ideas in satellite communication. A blimp or plane around 20 km above the earth’s surface is used as a satellite. HAPs would have very small coverage area, but would have a comparatively strong signal. Cheaper to put in position, but would require a lot of them in a network. www.edutalks.org HAP www.edutalks.org Satellite frequency band Downlink, Bandwidth, Band Uplink, GHz GHz MHz L 1.5 1.6 15 S 1.9 2.2 70 C 4 6 500 Ku 11 14 500 Ka 20 30 3500 www.edutalks.org Solar day and Sidereal day • A day is defined as the time that it takes the Earth to rotate on its axis. • However, there is more than one way to define a day: – A sidereal day is the time that it takes for the Earth to rotate with respect to the distant stars. – A solar day is the time that it takes to rotate with respect to the Sun. www.edutalks.org The Length of the Day • A solar day is slightly longer than a sidereal day. – A sidereal day is 23h 56m 4.091s. • We set our watches according to the solar day. • Astronomers use sidereal time because we are mostly interested in distant celestial objects. www.edutalks.org Solar day and Sidereal day • A solar day is measured using the passage of the Sun across the sky—it lasts 24 hours • A sidereal day (from the Latin word meaning star) is measured with respect to fixed stars—it lasts a little less than 24 hours.