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EC 723 Satellite Communication Systems

EC 723 Satellite Communication Systems. Mohamed Khedr http://webmail.aast.edu/~khedr. Grades. Textbook and website. Textbook: non specific Website: http://webmail.aast.edu/~khedr. Syllabus. Tentatively. Satellite Components. Satellite Subsystems Telemetry, Tracking, and Control

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EC 723 Satellite Communication Systems

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  1. EC 723Satellite Communication Systems Mohamed Khedr http://webmail.aast.edu/~khedr

  2. Grades

  3. Textbook and website • Textbook: non specific • Website: http://webmail.aast.edu/~khedr

  4. Syllabus • Tentatively

  5. Satellite Components • Satellite Subsystems • Telemetry, Tracking, and Control • Electrical Power and Thermal Control • Attitude Control • Communication Subsystems • Link Budget • Modulation Techniques • Coding and Error Correction • Networking (service provisioning, multimedia constraints and QoS) • Multiple Access and On-board Processing • Applications (Internet, Mobile computing)

  6. Classification of Satellite Orbits • Circular or elliptical orbit • Circular with center at earth’s center • Elliptical with one foci at earth’s center • Orbit around earth in different planes • Equatorial orbit above earth’s equator • Polar orbit passes over both poles • Other orbits referred to as inclined orbits • Altitude of satellites • Geostationary orbit (GEO) • Medium earth orbit (MEO) • Low earth orbit (LEO)

  7. Satellite Orbits • Equatorial • Inclined • Polar

  8. Here’s the Math… • Gravity depends on the mass of the earth, the mass of the satellite, and the distance between the center of the earth and the satellite • For a satellite traveling in a circle, the speed of the satellite and the radius of the circle determine the force (of gravity) needed to maintain the orbit • The radius of the orbit is also the distance from the center of the earth. • For each orbit the amount of gravity available is therefore fixed • That in turn means that the speed at which the satellite travels is determined by the orbit

  9. Let’s look in a Physics Book… • From what we have deduced so far, there has to be an equation that relates the orbit and the speed of the satellite: R^3=mu/n^2 N=2pi/T T is the time for one full revolution around the orbit, in seconds r is the radius of the orbit, in meters, including the radius of the earth (6.38x106m).

  10. The Most Common Example • “Height” of the orbit = 22,300 mile • That is 36,000km = 3.6x107m • The radius of the orbit is 3.6x107m + 6.38x106m = 4.2x107m • Put that into the formula and …

  11. The Geosynchronous Orbit • The answer is T = 86,000 sec (rounded) • 86,000 sec = 1,433 min = 24hours (rounded) • The satellite needs 1 day to complete an orbit • Since the earth turns once per day, the satellite moves with the surface of the earth.

  12. Assignment • How long does a Low Earth Orbit Satellite need for one orbit at a height of 200miles = 322km = 3.22x105m • Do this: • Add the radius of the earth, 6.38x106m • Compute T from the formula • Change T to minutes or hours

  13. Classical satellite systems Inter Satellite Link (ISL) Mobile User Link (MUL) MUL Gateway Link (GWL) GWL small cells (spotbeams) base station or gateway footprint GSM ISDN PSTN User data PSTN: Public Switched Telephone Network

  14. Basics • Satellites in circular orbits • attractive force Fg = m g (R/r)² • centrifugal force Fc = m r ² • m: mass of the satellite • R: radius of the earth (R = 6370 km) • r: distance to the center of the earth • g: acceleration of gravity (g = 9.81 m/s²) • : angular velocity ( = 2  f, f: rotation frequency) • Stable orbit • Fg = Fc

  15. Satellite period and orbits Velocity Km/sec satellite period [h] 12 24 velocity [ x1000 km/h] 10 20 8 16 6 12 4 8 2 4 synchronous distance 35,786 km 10 20 30 40 x106 m radius

  16. Basics • elliptical or circular orbits • complete rotation time depends on distance satellite-earth • inclination: angle between orbit and equator • elevation: angle between satellite and horizon • LOS (Line of Sight) to the satellite necessary for connection  high elevation needed, less absorption due to e.g. buildings • Uplink: connection base station - satellite • Downlink: connection satellite - base station • typically separated frequencies for uplink and downlink • transponder used for sending/receiving and shifting of frequencies • transparent transponder: only shift of frequencies • regenerative transponder: additionally signal regeneration

  17. Inclination plane of satellite orbit satellite orbit perigee d inclination d equatorial plane

  18. Elevation Elevation: angle e between center of satellite beam and surface minimal elevation: elevation needed at least to communicate with the satellite e footprint

  19. Orbits I • Four different types of satellite orbits can be identified depending on the shape and diameter of the orbit: • GEO: geostationary orbit, ca. 36000 km above earth surface • LEO (Low Earth Orbit): ca. 500 - 1500 km • MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit): ca. 6000 - 20000 km • HEO (Highly Elliptical Orbit) elliptical orbits

  20. Orbits II GEO (Inmarsat) HEO MEO (ICO) LEO (Globalstar,Irdium) inner and outer Van Allen belts earth Van-Allen-Belts: ionized particles 2000 - 6000 km and 15000 - 30000 km above earth surface 1000 10000 35768 km

  21. Geostationary satellites • Orbit 35,786 km distance to earth surface, orbit in equatorial plane (inclination 0°) •  complete rotation exactly one day, satellite is synchronous to earth rotation • fix antenna positions, no adjusting necessary • satellites typically have a large footprint (up to 34% of earth surface!), therefore difficult to reuse frequencies • bad elevations in areas with latitude above 60° due to fixed position above the equator • high transmit power needed • high latency due to long distance (ca. 275 ms) •  not useful for global coverage for small mobile phones and data transmission, typically used for radio and TV transmission

  22. LEO systems • Orbit ca. 500 - 1500 km above earth surface • visibility of a satellite ca. 10 - 40 minutes • global radio coverage possible • latency comparable with terrestrial long distance connections, ca. 5 - 10 ms • smaller footprints, better frequency reuse • but now handover necessary from one satellite to another • many satellites necessary for global coverage • more complex systems due to moving satellites • Examples: • Iridium (start 1998, 66 satellites) • Bankruptcy in 2000, deal with US DoD (free use, saving from “deorbiting”) • Globalstar (start 1999, 48 satellites) • Not many customers (2001: 44000), low stand-by times for mobiles

  23. MEO systems • Orbit ca. 5000 - 12000 km above earth surface • comparison with LEO systems: • slower moving satellites • less satellites needed • simpler system design • for many connections no hand-over needed • higher latency, ca. 70 - 80 ms • higher sending power needed • special antennas for small footprints needed • Example: • ICO (Intermediate Circular Orbit, Inmarsat) start ca. 2000 • Bankruptcy, planned joint ventures with Teledesic, Ellipso – cancelled again, start planned for 2003

  24. Routing • One solution: inter satellite links (ISL) • reduced number of gateways needed • forward connections or data packets within the satellite network as long as possible • only one uplink and one downlink per direction needed for the connection of two mobile phones • Problems: • more complex focusing of antennas between satellites • high system complexity due to moving routers • higher fuel consumption • thus shorter lifetime • Iridium and Teledesic planned with ISL • Other systems use gateways and additionally terrestrial networks

  25. Localization of mobile stations • Mechanisms similar to GSM • Gateways maintain registers with user data • HLR (Home Location Register): static user data • VLR (Visitor Location Register): (last known) location of the mobile station • SUMR (Satellite User Mapping Register): • satellite assigned to a mobile station • positions of all satellites • Registration of mobile stations • Localization of the mobile station via the satellite’s position • requesting user data from HLR • updating VLR and SUMR • Calling a mobile station • localization using HLR/VLR similar to GSM • connection setup using the appropriate satellite

  26. Handover in satellite systems • Several additional situations for handover in satellite systems compared to cellular terrestrial mobile phone networks caused by the movement of the satellites • Intra satellite handover • handover from one spot beam to another • mobile station still in the footprint of the satellite, but in another cell • Inter satellite handover • handover from one satellite to another satellite • mobile station leaves the footprint of one satellite • Gateway handover • Handover from one gateway to another • mobile station still in the footprint of a satellite, but gateway leaves the footprint • Inter system handover • Handover from the satellite network to a terrestrial cellular network • mobile station can reach a terrestrial network again which might be cheaper, has a lower latency etc.

  27. Overview of LEO/MEO systems

  28. Kepler’s First Law The path followed by a satellite around the primary will be an ellipse. An ellipse has two focal points shown as F1 and F2. The center of mass of the two-body system, termed the barycenter, is always centered on one of the foci. In our specific case, because of the enormous difference between the masses of the earth and the satellite, the center of mass coincides with the center of the earth, which is therefore always at one of the foci. The semimajor axis of the ellipse is denoted by a, and the semiminor axis, by b. The eccentricity e is given by

  29. Kepler’s Second Law For equal time intervals, a satellite will sweep out equal areas in its orbital plane, focused at the barycenter.

  30. Kepler’s Third Law The square of the periodic time of orbit is proportional to the cube of the mean distance between the two bodies. The mean distance is equal to the semimajor axis a. For the satellites orbiting the earth, Kepler’s third law can be written in the form where n is the mean motion of the satellite in radians per second and is the earth’s geocentric gravitational constant. With a in meters, its value is

  31. Definition of terms for earth-orbiting satellite ApogeeThe point farthest from earth. Apogee height is shown as ha in Fig PerigeeThe point of closest approach to earth. The perigee height is shown as hp Line of apsidesThe line joining the perigee and apogee through the center of the earth. Ascending nodeThe point where the orbit crosses the equatorial plane going from south to north. Descending nodeThe point where the orbit crosses the equatorial plane going from north to south. Line of nodesThe line joining the ascending and descending nodes through the center of the earth. InclinationThe 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. The inclination is shown as i in Fig. Mean anomaly M gives an average value 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. This gives the true angular position of the satellite in the orbit as a function of time.

  32. Prograde orbitAn orbit in which the satellite moves in the same direction as the earth’s rotation. The inclination of a prograde orbit always lies between 0 and 90°. Retrograde orbitAn orbit in which the satellite moves in a direction counter to the earth’s rotation. The inclination of a retrograde orbit always lies between 90 and 180°. Argument of perigeeThe angle from ascending node to perigee, measured in the orbital plane at the earth’s center, in the direction of satellite motion. Right ascension of the ascending nodeTo define completely the position of the orbit in space, the position of the ascending node is specified. However, because the earth spins, while the orbital plane remains stationary the longitude of the ascending node is not fixed, and it cannot be used as an absolute reference. For the practical determination of an orbit, the longitude and time of crossing of the ascending node are frequently used. However, for an absolute measurement, a fixed reference in space is required. The reference chosen is the first point of Aries, otherwise known as the vernal, or spring, equinox. The vernal equinox occurs when the sun crosses the equator going from south to north, and an imaginary line drawn from this equatorial crossing through the center of the sun points to the first point of Aries (symbol ). This is the line of Aries. Definition of terms for earth-orbiting satellite

  33. Six Orbital Elements • Earth-orbiting artificial satellites are defined by six orbital elements referred to as the keplerian element set. • The semimajor axis a. • The eccentricity e • give the shape of the ellipse. • A third, the mean anomaly M, gives the position of the satellite in its orbit at a reference time known as the epoch. • A fourth, the argument of perigee  , gives the rotation of the orbit’s perigee point relative to the orbit’s line of nodes in the earth’s equatorial plane. • The inclination I • The right ascension of the ascending node  • Relate the orbital plane’s position to the earth.

  34. NASA

  35. Gravitational force is inversely proportional to the square of the distance between the centers of gravity of the satellite and the planet the satellite is orbiting, in this case the earth. The gravitational force inward (FIN, the centripetal force) is directed toward the center of gravity of the earth. The kinetic energy of the satellite (FOUT, the centrifugal force) is directed opposite to the gravitational force. Kinetic energy is proportional to the square of the velocity of the satellite. When these inward and outward forces are balanced, the satellite moves around the earth in a “free fall” trajectory: the satellite’s orbit. Forces acting on a satellite in a stable orbit around the earth.

  36. The initial coordinate system that could be used to describe the relationship between the earth and a satellite. A Cartesian coordinate system with the geographical axes of the earth as the principal axis is the simplest coordinate system to set up. The rotational axis of the earth is about the axis cz, where c is the center of the earth and cz passes through the geographic north pole. Axes cx, cy, and cz are mutually orthogonal axes, with cx and cy passing through the earth’s geographic equator. The vector r locates the moving satellite with respect to the center of the earth. Cartesian coordinate system

  37. In this coordinate system, the orbital plane of the satellite is used as the reference plane. The orthogonal axes, x0 and y0 lie in the orbital plane. The third axis, z0, is perpendicular to the orbital plane. The geographical z-axis of the earth (which passes through the true North Pole and the center of the earth, c) does not lie in the same direction as the z0 axis except for satellite orbits that are exactly in the plane of the geographical equator. The orbital plane coordinate system.

  38. The plane of the orbit coincides with the plane of the paper. The axis z0 is straight out of the paper from the center of the earth, and is normal to the plane of the satellite’s orbit. The satellite’s position is described in terms of the radius from the center of the earth r0 and the angle this radius makes with the x0 axis, Φo. Polar coordinate system in the plane of the satellite’s orbit.

  39. A satellite is in orbit about the planet earth, E. The orbit is an ellipse with a relatively high eccentricity, that is, it is far from being circular. Two shaded portions of the elliptical plane in which the orbit moves, one is close to the earth and encloses the perigee while the other is far from the earth and encloses the apogee. The perigee is the point of closest approach to the earth while the apogee is the point in the orbit that is furthest from the earth. While close to perigee, the satellite moves in the orbit between times t1 and t2 and sweeps out an area denoted by A12. While close to apogee, the satellite moves in the orbit between times t3 and sweeps out an area denoted by A34. If t1– t2 = t3 – t4, then A12 = A34. Kepler’s second law of planetary motion.

  40. The orbit as it appears in the orbital plane.The point O is the center of the earth and the point C is the center of the ellipse. The two centers do not coincide unless the eccentricity, e, of the ellipse is zero (i.e., the ellipse becomes a circle and a = b). The dimensions a and b are the semimajor and semiminor axes of the orbital ellipse, respectively.

  41. Point O is the center of the earth and point C is both the center of the orbital ellipse and the center of the circumscribed circle. The satellite location in the orbital plane coordinate system is specified by (x0, y0). A vertical line through the satellite intersects the circumscribed circle at point A. The eccentric anomaly E is the angle from the x0 axis to the line joining C and A. The circumscribed circle and the eccentric anomaly E.

  42. This geocentric system differs from that shown in Figure 2.1 only in that the xi axis points to the first point of Aries. The first point of Aries is the direction of a line from the center of the earth through the center of the sun at the vernal equinox (about March 21 in the Northern Hemisphere), the instant when the subsolar point crosses the equator from south to north. In the above system, an object may be located by its right ascension RA and its declination . The geocentric equatorial system.

  43. Locating the orbit in the geocentric equatorial system. The satellite penetrates the equatorial plane (while moving in the positive z direction) at the ascending node. The right ascension of the ascending node is  and the inclination i is the angle between the equatorial plane and the orbital plane. Angle , measured in the orbital plane, locates the perigee with respect to the equatorial plane.

  44. The elevation angle is measured upward from the local horizontal at the earth station and the azimuth angle is measured from the true north in an eastward direction to the projection of the satellite path onto the local horizontal plane. The definition of elevation (EI) and azimuth (Az).

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