Satellite Communications

Transcription

1 Satellite Communications Department of Electrical Engineering Faculty of Engineering Chiangmai University Origin of Satellite Communications Arthur C. Clark (1945) British Science fiction writer propose the idea of a synchronous satellite communication 3 satellites in an equatorial orbit, radius of about 42,242 km. spaced 120 apart from each other could cover the whole world to transmit and receive signals that relayed between the satellites Brief History 1957 SPUTNIKS USSR 1958 SCORE USA 1965 Early Bird or USA INTELSAT 1 The first commercial satellite Satellite Communications System is growing so rapidly Difficult to summarize the list of satellites in space Satellite positions are assigned by ITU (International Telecommunication Union) which is part of UN -1-

2 Components of Satellite Communications Frequency Band Designations Frequency range, GHz Band Designation VHF UHF L S C X Ku K Ka mm Satellite Orbit -2-

3 Kepler s First Law radius of orbit r 0 p = semilatus rectum = h 2 / e = eccentricity = h 2 C / h = magnitude of angular momentum vector(constant) = Kepler s constant = 3.98x10 5 km 3 / s 2 C = Boundary condition(constant) For e < 1, the orbit is an ellipse e = 0, the orbit is circle Kepler s Second Law The differential area swept out by vector r 0 da = 0.5 h dt The radius vector to the Satellite sweeps out equal areas in equal times The orbital period T Kepler s Third Law The square of the period of revolution is proportional to the cube of the semimajor axis -3-

4 Types of Orbits Polar Orbit Circular movement from N to S, LEO or MEO Navigation, Weather Forecasting, Resources Surveying Inclined Orbit Elliptical inclined orbit, LEO or MEO Navigation, Weather Forecasting, Resources Surveying Equatorial Orbit Lies in equatorial plane and appear stationary relative to the earth Geosynchronous orbit or geostationary orbit Most widely used and more larger Orbital Perturbation Effects of a nonspherical Earth The earth s gravitational potential is not uniform drift toward and circulate around the nearer of longitudes of 105 W and 75 E called graveyards (they collect old satellites) Effects of the Sun and Moon Gravitational attractions change the orbital inclination, the Moon > the Sun Orbital Effects in Communication Doppler Shift The change in transmitting and receiving frequency due to the different of their velocities negligible for geosynchronous satellites quite effected for low earth orbit satellites compensated by tracking in narrow receiver -4-

5 Orbital Effects in Communication Eclipse earth prevents sunlight from reaching satellite no power from its solar cell use batteries stop using some transponders Orbital Effects in Communication Sun Transit Outage satellite stay between the sun and the earth noise rise up and interfere normal operation using other satellites to help prevent from losing itself due to no control signal Satellite Launching Methods 1. ELV (Expendable Launch Vehicles) Delta (USA) Ariane (France) # put satellite on transfer orbit from 300 km to their real orbit 38,600 km by its AKM (apogee kick motor) -5-

6 Satellite Launching Methods 2. STS (Space Transportation System) The Space Shuttle by NASA 296 km circular orbit put satellite to transfer orbit using PAM (Payload Assist Module) using AKM for geosynchronous orbit The Steps in Launching 1. Spinner Cylindrical satellite spin around the axis that is parallel to the N S axis rev/min decrease disturbance torques Types of Satellites -6-

7 Types of Satellites 2. The three axis stabilization stabilizing for each of the three axis body stabilization 3 flywheels react disturbance torques from each axis Controlling the orbit and attitude Telemetry Tracking and Command (TT&C) Telemetry Collect data from many sensors and send to earth station Tracking observe and determine position of satellite Command message from earth station to control attitude, position (using gas jet to correct them) and control all communication systems Antennas for Earth Station 1. Offset Paraboloidal Refector Antenna feeder is on the focal point of the dish suitable for small diameter antenna often used in direct satellite TV such as UBC -7-

8 Antennas for Earth Station 2. Cassegrain Antenna very large dia. up to 30 m. high gain suitable for control station Signal Impairments Propagation Impairment Rain Attenuation Atmospheric Losses Refraction Signal Scintillations Reflection Multipath Propagation Delays Intersymbol Interference Signal Depolarization Interference Physical Cause rain and cloud O 2, H 2 O Atmospheric gases Refractivity Fluctuations Objects on the earth surfaces Distance Ducting, Scatter, Diffraction Ice Crystals Signals from other systems Types A B C E F Types of Interference -8-

9 Satellite Access 1. Frequency Division Multiple Access (FDMA) assign specific frequency for each earth station Satellite Access 2. Time Division Multiple Access (TDMA) assign time slot for each earth station Satellite Access 3. Code Division Multiple Access (CDMA) each earth station use its own code -9-

10 VSAT Very Small Aperture Antenna Diameter of antenna 2 meters Such as UBC Mobile Satellite Transportation Logistic TV station MSAT GPS Global Positioning Satellite System Navigation Mapping Surveying -10-

11 Satellite for Earth Observation Natural Resource Surveying 2 Technologies Remote Sensing Using electromagnetic spectrum Need data interpretation Camera Real picture Such as THEOS Thailand Earth Observation System Meteorological Satellite Weather Forecasting Cloud, Rain, Wind, Temperature and Turbidity Various effects in atmosphere Ozone For Disaster prevention Aviation Education Video Conference via Satellite Distance meeting Distance learning -11-

12 More about Orbits GEO Geostationary orbit Satellite appears stationary related to the earth 35,800 km., equatorial plane MEO Medium earth orbit 10,000 km., Inclined orbit or polar orbit LEO Low earth orbit 1,000 km., Inclined orbit or polar orbit Sun synchronous orbit Remains fixed relative to the sun Orbit plane incline with constant angle to the sun Satellite Systems in Thailand THAICOM THAICOM 1A : Spinner, 120 E, for TV, Telephone,Cable TV THAICOM 2: Spinner, 78.5 E, for TV, Telephone, Cable TV THAICOM 3: 3 axis stabilization, damage => released THAICOM 4: IPSTAR, 3 axis, E, more efficiency due to frequency reused, for high speed Internet THAICOM 5: 3 axis, 78.5 E, instead of THAICOM 3, most users are cable TV Satellite Systems in Thailand THEOS Thailand Earth Observation Satellite Owner = GISTDA: Geo Informatics and Space Technology Development Agency LEO Low Earth Orbit and Sun synchronous Orbit 5 years lifetime and 822 km. from surface Time period in 1 round = minutes Use for natural resource management agriculture surveying 2 cameras Panchromatic Telescope(B&W) Multispectral Camera(color) resolution 2 meters resolution 15 meters -12-

13 Satellite Systems in Thailand TMSAT(thai pat) Thai Micro SATellite (50 kg.) Owner = Mahanakorn University of Technology + United Communication Co.Ltd. (UCOM) LEO and Sun synchronous orbit 815 km. Digital Signal with Frequency MHz/ MHz Use for : Engineering Education and Amateur Radio Satellite Systems in Thailand INTELSAT (I VIII) International Telecommunication Satellite Organization GEO : both spinners and 3 axis stabilizations 35,800 km. from earth Early Bird = The first commercial satellite(1965) There are 52 satellites in orbits still be used Use for : International Telephone and Television Satellite Systems in Thailand INMARSAT International Maritime Satellite Organization 7 satellites in GEO Communication services for mobile terminals Provided by CAT telecom Aeroplane, cargo ship, patrol tanker, train Thai Navy, Army, Air Force Video conference, national resource surveying Etc. -13-

14 Satellite Systems in Thailand Others PALAPA Indonesia Commercial services, TV stations etc. ASIASAT Hongkong Commercial services, TV stations etc. THE END -14-