SATELLITE COMMUNICATION UNIT I OVERVIEW OF SATELLITE SYSTEMS, ORBITS AND LAUNCHING METHODS Introduction – Frequency allocations for satellite services – Intelsat – U.S domsats – Polar orbiting satellites – Problems – Kepler’s first law – Kepler’s second law – Kepler’s third law – Definitions of terms for earth – Orbiting satellites – Orbital elements – Apogee and perigee heights – Orbital perturbations – Effects of a non-spherical earth – Atmospheric drag – Inclined orbits – Calendars – Universal time – Julian dates – Sidereal time – The orbital plane – The geocentric – Equatorial coordinate system – Earth station referred to the IJK frame – The topcentric – Horizon co-ordinate system – The subsatellite point – Predicting satellite position. PART A 1. What is Satellite? Mention the types. An artificial body that is projected from earth to orbit either earth (or) another body of solar systems. Types: Information satellites and Communication Satellites 2. State Kepler’s first law. It states that the path followed by the satellite around the primary will be an ellipse. An ellipse has two focal points F1 and F2. The center of mass of the two body system, termed the barycenter is always centered on one of the foci. e = [square root of ( a2– b2) ] / a 3. State Kepler’s second law. It states that for equal time intervals, the satellite will sweep out equal areas in its orbital plane, focused at the barycenter. 4. State Kepler’s third law. It states that the square of the periodic time of orbit is perpendicular to the cube of the mean distance between the two bodies.
a3= 3 / n2 Where, n = Mean motion of the satellite in rad/sec. 3 = Earth’s geocentric gravitational constant. With the n in radians per sec. the orbital period in second is given by,P = 27 / n 5. Define Inclination. The angle between the orbital plane and the earth’s equatorial plane. It is measured at the ascending node from the equator to the orbit going from east to north. 6. Define mean anomaly and true anomaly. Mean anomaly gives an average bvalue of the angular position of the satellite with reference to the perigee. True anomaly is the angle from perigee to the satellite position, measured at the earth’s center 7. Mention the apogee and perigee height. r a = a(1+e) r p = a(1+e) ha=ra–Rp hp=rp–Rp 8. What is meant by azimuth angle? It is defined as the angle produced by intersection of local horizontal plane and the plane passing through the earth station, the satellite and center of earth. 9. Give the 3 different types of applications with respect to satellite systems. • The largest international system (Intelsat) • The domestic satellite system (Dom sat) in U.S. • U.S. National oceanographic and atmospheric administrations (NOAA) 10. Mention the 3 regions to allocate the frequency for satellite services. • Region1: It covers Europe, Africa and Mangolia
• Region2: It covers North & South Ameriaca and Greenland. • Region3: It covers Asia, Australia and South West Pacific. 11. Give the types of satellite services. • Fixed satellite service • Broadcasting satellite service • Mobile satellite service • Navigational satellite services • Meteorological satellite services 12. What is mean by Dom sat? Domestic Satellites. These are used for voice, data and video transmissions within the country. These are launched by GSLV vehicles. They are designed in a manner to continuously monitor the region. 13. What is mean by INTELSAT? International Telecommunication Satellite. It’s a constellation of 17 satellites from U.S and European union. It serves as basis for GPS coordinates all over the world. 14. What is mean by SARSAT? Search and rescue satellite. They are kind of remote sensing satellites, are useful to find the particular location during catastrophe periods. 15. Define polar-orbiting satellites. Polar orbiting satellites orbit the earth in such a way as to cover the north and south polar regions. 16. Give the advantage of geostationary orbit. There is no necessity for tracking antennas to find the satellite positions. They are able to monitor the particular place continuously without the necessity in change of coordinates. 17. Define look angles.
The azimuth and elevation angles of the ground station antenna are termed as look angles. 18. Write short notes on station keeping. It is the process of maintenance of satellite’s attitude against different factors that can cause drift with time. Satellites need to have their orbits adjusted from time to time, because the satellite is initially placed in the correct orbit, natural forces induce a progressive drift. 19. What are the geostationary satellites? The satellites present in the geostationary orbit are called geostationary satellite. The geostationary orbit is one in which the satellite appears stationary relative to the earth. It lies in equatorial plane and inclination is ‘0’. The satellite must orbit the earth in the same direction as the earth spin. The orbit is circular. 20. What is sun transit outage. The sun transit is nothing but the sun comes within the beam width of the earth station antenna. During this period the sun behaves like an extremely noisy source and it blanks out all the signal from the satellite. This effect is termed as sun transit outage.