Satellite Communication Fig.No. LIST OF FIGURES PAGE Fig.1 Geostationary orbit 6 Fig.2 Low Earth Orbit 7 Fig.3 Transponders 10 Fig.4 Power subsystems 11 Fig.5 Basic operation 12 Dept of ECE Page 1 Satellite Communication 1. INTRODUCTION A satellite is a moon, planet or machine that orbits a planet or star. For example, Earth is a satellite because it orbits the sun. Likewise, the moon is a satellite because it orbits Earth. Usually, the word "satellite" refers to a machine that is launched into space and moves around Earth or another body in space. Earth and the moon are examples of natural satellites. Thousands of artificial, or man-made, satellites orbit Earth. Some take pictures of the planet that help meteorologists predict weather and track hurricanes. Some take pictures of other planets, the sun, black holes, dark matter or faraway galaxies. These pictures help scientists better understand the solar system and universe. Still other satellites are used mainly for communications, such as beaming TV signals and phone calls around the world. 1.1 IMPORTANCE The bird's-eye view that satellites have allows them to see large areas of Earth at one time. This ability means satellites can collect more data, more quickly, than instruments on the ground. Satellites also can see into space better than telescopes at Earth's surface. That's because satellites fly above the clouds, dust and molecules in the atmosphere that can block the view from ground level. Dept of ECE Page 2 Satellite Communication 1.2 TYPES OF SATELLITE Natural satellite: Earth and the moon are examples of natural satellites. Artificial satellite: An artificial body placed by human endeavor into an orbit around the earth or another planet in order to collect information or for communication. Dept of ECE Page 3 Satellite Communication 2. HISTORY The first artificial satellite was the Soviet Sputnik 1, launched on October 4, 1957 and equipped with an onboard radio- transmitter that worked on two frequencies: 20.005 and 40.002 MHz. The first American satellite to relay communications was Project SCORE in 1958, which used a tape recorder to store and forward voice messages. It was used to send a Christmas greeting to the world from U.S. President Dwight D. Eisenhower. NASA launched an Echo satellite in 1960; the 100-foot (30 m) aluminized PET film balloon served as a passive reflector for radio communications. Courier 1B, built by Philco, also launched in 1960, was the world's first active repeater satellite. Telstar was the first active, direct relay communications satellite. Belonging to AT&T as part of a multi-national agreement between AT&T, Bell Telephone Laboratories, NASA, the British General Post Office, and the French National PTT (Post Office) to develop satellite communications, it was launched by NASA from Cape Canaveral on July 10, 1962, the first privately sponsored space launch. Relay 1 was launched on December 13, 1962, and became the first satellite to broadcast across the Pacific on November 22, 1963.[1] An immediate antecedent of the geostationary satellites was Hughes' Syncom 2, launched on July 26, 1963. Syncom 2 revolved around the earth once per day at constant speed, but because it still had north-south motion, special equipment was needed to track it.The first artificial satellite was the Soviet Sputnik 1, launched on October 4, 1957 and equipped with an onboard radio-transmitter that worked on two frequencies: 20.005 and 40.002 MHz. The first American satellite to relay communications was Project SCORE in 1958, which used a tape recorder to store and forward voice messages. Dept of ECE Page 4 Satellite Communication 2.1 INDIAN CONTRIBUTION: India has launched 60 satellites (as of 26 April 2012) of many types since its first attempt in 1975. Satellites have been launched from various vehicles, including American, Russian, European satellite- launch rockets, and the U.S. Space Shuttle. The organization responsible for Indian satellites is the Indian Space Research Organization (ISRO). Aryabhata was India’s 1st satellite launched on 19th April 1975 which provided technological experience in building and operating a satellite system. Latest communication satellite is GSAT-10 launched on 29th September 2012. It is India’s advanced communication satellite; a high power satellite inducted into the INSAT system and it weighs 3400 kg at lift-off. Recently launched satellite is SARAL on 25th February 2013 is a joint Indo- French satellite mission for oceanographic studies. Dept of ECE Page 5 Satellite Communication Non-military satellite services There are three basic categories of non-military satellite services: Fixed satellite services Fixed satellite services handle hundreds of billions of voice, data, and video transmission tasks across all countries and continents between certain points on the Earth's surface. Mobile satellite systems Mobile satellite systems help connect remote regions, vehicles, ships, people and aircraft to other parts of the world and/or other mobile or stationary communications units, in addition to serving as navigation systems. Scientific research satellites (commercial and noncommercial) Scientific research satellites provide us with meteorological information, land survey data (e.g remote sensing), Amateur (HAM) Radio, and other different scientific research applications such as earth science, marine science, and atmospheric research. Dept of ECE Page 6 Satellite Communication 3. ORBITS An orbit is a regular, repeating path that an object in space takes around another one. An object in an orbit is called a satellite. Orbits are the result of a perfect balance between the forward motion of a body in space, such as a planet or moon, and the pull of gravity on it from another body in space, such as a large planet or star. An object with a lot of mass goes forward and wants to keep going forward; however, the gravity of another body in space pulls it in. There is a continuous tug-of-war between the one object wanting to go forward and away and the other wanting to pull it in. These forces of inertia and gravity have to be perfectly balanced for an orbit to happen. If the forward movement (inertia) of one object is too strong, the object will speed past the other one and not enter orbit. If inertia or momentum is much weaker than the pull of gravity, the object will be pulled into the other one completely and crash. There is only one main force acting on a satellite when it is in orbit, and that is the gravitational force exerted on the satellite by the Earth. This force is constantly pulling the satellite towards the centre of the Earth. A satellite doesn't fall straight down to the Earth because of its velocity. Throughout a satellites orbit there is a perfect balance between the gravitational force due to the Earth, and the centripetal force necessary to maintain the orbit of the satellite. The formula for centripetal force is: F = (mv2)/r The formula for the gravitational force between two bodies of mass M and m is (GMm)/r2 The most common type of satellite orbit is the geostationary orbit. This is described in more detail below, but is a type of orbit where the satellite is over the same point of Earth always. It moves around the Earth at the same angular speed that the Earth rotates on its axis. We can use our formulae above to work out characteristics of the orbit. (mv2/r) = (GMm)/r2 Dept of ECE Page 7 Satellite Communication => v2/r = (GM)/r2 Now, v = (2πr)/T. => (((2πr)/T)2)/r = (GM)/r2 => (4π2r)/T2 = (GM)/r2 => r3 = (GMT2)/4π2 T period of revolution M is the mass of the Earth, which is 6 x 1024 kg or mass of the object the satellite is revolving around. Lastly, G (Newton's Gravitational Constant) is 6.67 x 10-11 m3/kg.s2 3.1 TYPES OF ORBITS Geostationary Orbit: The most common orbit used for satellite communications is the geostationary orbit (GEO). This is the orbit described above – the rotational period is equal to that of the Earth. The orbit has zero inclination so is an equatorial orbit (located directly above the equator). The satellite and the Earth move together so a GEO satellite appears as a fixed point in the sky from the Earth. The advantages of such an orbit are that no tracking is required from the ground station since the satellite appears at a fixed position in the sky. The satellite can also provide continuous operation in the area of visibility of the satellite. Many communications satellites travel in geostationary orbits, including those that relay TV signals into our homes. However, due to their distance from Earth GEO satellites have a signal delay of around 0.24 seconds for the complete send and receive path. This can be a problem with telephony or data transmission. Also, since they are in an equatorial orbit, the angle of elevation decreases as the latitude or longitude difference increases between the satellite and earth station. Dept of ECE Page 8 Satellite Communication Low elevation angles can be a particular problem to mobile communications. Low Earth Orbit/Medium Earth Orbit: A low earth orbit (LEO), or medium earth orbit (MEO) describes a satellite which circles close to the Earth. Generally, LEOs have altitudes of around 300 – 1000 km with low inclination angles, and MEOs have altitudes of around 10,000 km. A special type of LEO is the Polar Orbit. This is a LEO with a high inclination angle (close to 90degrees). This means the satellite travels over the poles. Satellites that observe our planet such as remote sensing and weather satellites often travel in a highly inclined LEO so they can capture detailed images of the Earth’s surface due to their closeness to Earth.