无线通信与网络chap9.ppt

Satellite Communications Chapter 9 Satellite-Related Terms Earth Stations – antenna systems on or near earth Uplink – transmission from an earth station to a satellite Downlink – transmission from a satellite to an earth station Transponder – electronics in the satellite that convert uplink signals to downlink signals Ways to CategorizeCommunications Satellites Coverage area Global, regional, national Service type Fixed service satellite (FSS) Broadcast service satellite (BSS) Mobile service satellite (MSS) General usage Commercial, military, amateur, experimental 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) Geometry Terms Elevation angle - the angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite Minimum elevation angle Coverage angle - the measure of the portion of the earths surface visible to the satellite Minimum Elevation Angle Reasons affecting minimum elevation angle of earth station’s antenna (0o) Buildings, trees, and other terrestrial objects block the line of sight Atmospheric attenuation is greater at low elevation angles Electrical noise generated by the earths heat near its surface adversely affects reception GEO Orbit Advantages of the the GEO orbit No problem with frequency changes Tracking of the satellite is simplified High coverage area Disadvantages of the GEO orbit Weak signal after traveling over 35,000 km Polar regions are poorly served Signal sending delay is substantial LEO Satellite Characteristics Circular/slightly elliptical orbit under 2000 km Orbit period ranges from 1.5 to 2 hours Diameter of coverage