Introduction,History of Satellite communications,Satellites. Satellite Link Design,Propagation Effects and their impact. Case DVB S,Conclusions,21 02 2006 2,Introduction I. Satellite communications systems exist because earth is a sphere. Radio waves travel in straight lines at the microwave frequencies used for. wideband communications, repeater is needed to convey signals very long distances. Satellites are important in voice communications video radio. transmission navigation GPS remote sensing maps weather. satellites etc, A majority of communication satellites are in geostationary earth orbit. an altitude of 35 786 km,Satellite in fixed place, typical path length from earth station to to a GEO satellite is 38 500 km. Satellite systems operate in the microwave and millimeter wave. frequency bands using frequencies between 1 and 50 GHz. Above 10 GHz rain causes significant attenuation of the signal. For the first 20 years of satellite communications analog signals were. widely used FM with most links,21 02 2006 3, History of Satellite Communications Some Milestones. Satellite communications began in October 1957 with the launch by the. USSR a small satellite called Sputnik 1 4 10 1957,Beacon transmitter no communications capability. 3 11 1957 Sputnik 2 with Laika,12 4 1961 Vostok 1 with Juri Gagarin. First true communication satellites Telstar I II were launched in July. 1962 May 1963, 10 1964 Syncom 2 First GEO satellite 7 4 1 8 GHz one TV channel or. several 2 way telephone connections, 1987 TVSAT First DBS satellite Direct Broadcast Satellite Television. broadcasts directly to home,21 02 2006 4,Satellite communications Organizations. International Telecommunications Satellite Organization. ITSO previously known by the acronym INTELSAT,global cooperation in satellite communications. Europe The European Space Agency ESA, ESA is responsible for performing R D and developing new. technology for European space industries for the field of. satellite communications,National organizations, National Aeronautics and Space Administration NASA. Japan Aerospace Exploration Agency,China National Space Administration CNSA. 21 02 2006 5,Satellite orbits 5, GEostationary Orbit GEO satellites i e satellites that are stationary with respect. to a fixed point on the earth, good coverage Theoretically only three GEO satellites are sufficient to serve all the earth. the simplest space configuration and simple space control system. no need for tracking system at the earth stations, no variation of propagation delay and elevation angle. negligible Doppler effects, problematic links feasibility due to the long satellite user distance prohibitive power levels. and or too large on board antennas could be required if low power hand held user terminals. are considered, high propagation delays for interactive services and mobile to mobile communications. higher than 400 ms recommended by CCITT in case of double hop communications. low minimum elevation angles at high latitudes i e polar regions cannot be covered. Non GeoStationary Orbit NGSO satellites that are moving with respect to a point. on the earth, excellent links feasibility due to the low orbit altitude. low propagation delays, a satellite constellation is necessary to serve all the earth and the constellation size. increases if the satellite altitude decreases,high system costs. 21 02 2006 6,Satellite Orbits,Highly Elliptical Orbit HEO. high elevation angles 55 60 for European coverage due to the orbital location of the. possibility of tailoring the system to cover specific regions of the earth with a limited. number of satellites, problematic links feasibility even higher than in the GEO case due to the considerable. altitude of the satellites,big on board antennas 6 meters or more required. HEO satellites are not suitable for a global coverage. LOOPUS quasi geostationary Loops in Orbit Occupied Permanently by Unstationary. Satellites is a HEO orbit characterized by an apogee altitude of 39 700 km and a. perigee altitude of 1 250 km with orbital plane inclination of 63 4. 21 02 2006 7,Satellite orbits LEO MEO GEO,LEO Low Earth Orbit satellites. providing mainly mobile data,MEO Medium Earth Orbit. satellites again providing mobile,telephony services. GEO Geostationary Earth Orbit,Satellites major existing. telecommunications and,broadcasting satellites fall into this. Figure 1 Satellite orbits 1,21 02 2006 8,Satellites Satellite subsystems. Attitude and Orbit Control System, rocket motors to move satellite back to the correct orbit. keep antennas point toward to earth,Telemetry tracking command and monitoring. telemetry system monitor satellite health tracking system is located. at the earth station and provides information about elevation and. azimuth angles of the satellite,Power system,electrical power from solar cells. Communication subsystem, major component of communications satellites one or more. antennas and a set of receivers and transmitters transponders. the linear or bent pipe transponders amplifiers the received signal and. retransmits it a different usually lower frequency. baseband processing transporters used with digital signals converts the. received signal to baseband process it and then retransmits a digital. 21 02 2006 9,System Noise Temperature and G T ratio. Noise temperature is a useful concept in communications. receivers since it provides a way of determine how much. thermal noise is generated by active and passive devices in. the receiving system, At microwave frequencies a black body with physical. temperature Tp kelvins generates electrical noise over a. wide bandwidth,The noise power Pn kTp Bn watts,k is Boltzmann s constant 228 6 dBW K Hz. Tp physical temperature of source in kelvin degrees. Bn is the noise Bandwith in herz,Pn is the available noise power in watts. 21 02 2006 10, The sensitivity of a radio telescope is a function of many factors. including antenna gain G and system noise temperature T. Ts is the total system noise temperature in degrees Kelvin and. is equal to the sum of the noise generated in the receiving. system Tr and the noise delivered from the antenna Ta when. the antenna is looking at a region of the sky free of strong. sources Ta includes the galactic background temperature as well. as additional noise picked up by the antenna side lobes viewing. the earth at ambient temperature, The receiving system temperature Tr is related to the system. noise factor FN by,Tr NF 1 T0 1,where the noise factor NF is systems noise figure. NF S N in S N out, and T0 is the reference temperature used to calculate the. standard noise figure usually 290 K,21 02 2006 11,G T ratio II Practical case 6. The principle behind determination of G T is to measure the increase in. noise power which occurs when the antenna is pointed first at a region of. cold sky and then moved to a strong source of known flux density. usually the sun This ratio of received power is known as the Y factor. Y Psun Pcold sky, The following equation shows the relationship between G T the. measured Y factor and the value of solar flux F at the observing. G T Y 1 8 pi k L F Lam 2,Y sun noise rise expressed as a ratio not dB. k Boltzmann s constant 1 38 10 23 joules deg K,L beam size correction factor. Lam wavelength in meters at the operating frequency fo. F solar flux at fo in watts meter 2 Hz, If the dish has a beamwidth larger than 2 or 3 degrees the L can be set. L 1 and forget about next equation,L 1 0 38 Ws Wa 2. Ws diameter of the radio sun in degrees at fo,Wa antenna 3 dB beamwidth at fo. 21 02 2006 12,G T ratio III, The diameter of the radio sun Ws is frequency dependent You can. assume a value of 0 5 degrees for frequencies above 3000 MHz 0 6. degrees for 1420 MHz and 0 7 degrees for 400 MHz, USAF Space Command runs a worldwide solar radio monitoring network. with stations in Massachusetts Hawaii Australia and Italy These. stations measure solar flux density F at 245 410 610 1415 2695. 4995 8800 and 15400 MHz If you are operating near one of these eight. standard frequencies then you can use the reported flux density. When operating between two given frequencies then interpolate between flux. densities at the lower and higher frequencies The best interpolation scheme is. to graph the flux density at several frequencies and use a curve fitting routine. to determine the flux density at your operating frequency. The solar flux density obtained from the USAF must be multiplied by 10 22 in. order to get the units correct for use in equation 4 In other words if the. 1415 MHz solar flux density is 98 10 22 watts meter 2 Hz the operator. may simply state the solar flux at 1415 Mhz is 98,21 02 2006 13. Satellite Link Design, The four factors related to satellite system design. 1 The weight of satellite,2 The choice frequency band. 3 Atmospheric propagation effects,4 Multiple access technique. The major frequency bands are 6 4 GHz 14 11 GHz and 30 20. GHz Uplink Downlink, At geostationary orbit there is already satellites using both 6 4. and 14 11 GHz every 2 minimum space to avoid interference. from uplink earth stations Additional satellites higher BW. Low earth orbit LEO medium earth orbit MEO satellite. systems are closer and produces stronger signals but earth. terminals need omni directional antennas, The design of any satellite communication is based on. meeting of minimum C N ratio for a specific percentage of time. carrying the maximum revenue earning traffic at minimum cost. 21 02 2006 14,Link budgets, C N ratio calculation is simplified by the use of link budgets. evaluation the received power and noise power in radio link. the link budget must be calculated for individual transponder and. for each link, When a bent pipe transponder is used the uplink and down link. C N rations must be combined to give an overall C N.
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