Satellite Communications

1Satellite CommunicationsIntroduction General conceptsNeeds, advantages, and disadvantagesSatellite characteristicsOrbitsEarth coverageCommunication characteristicsSpectrum and BandwidthChannel capacityFrequency and Wavelength

GPS Satellite - NASA 2012 Raymond P. Jefferis III Lect01Satellite Communications1##2TextTextSatellite Communications, Second Edition, T. Pratt, C. Bostian, and J. Allnut, John Wilen & Sons, 2003. 2012 Raymond P. Jefferis III Lect01Satellite Communications2##3General Concepts of Satellites:They orbit around the earthHave various orbital paths (to be discussed)They carry their own source of powerThey can communicate with:Ground stations fixed on earth surfaceMoving platforms (Non-orbital)Other orbiting satellites 2012 Raymond P. Jefferis III Lect01Satellite Communications3##4Needs, Advantages & Disadvantages Communications needs Advantages of using satellites Disadvantages of using satellites 2012 Raymond P. Jefferis III Lect01Satellite Communications4##Satellite Communications NeedsSpace vehicle to be used as communications platform(Earth-Space-Earth, Space-Earth, Space-Space)Space vehicle to be used as sensor platform with communicationsGround station(s) (Tx/Rx)Ground receivers (Rx only)

5Advantages of Using SatellitesHigh channel capacity (>100 Mb/s)Low error rates (Pe ~ 10-6)Stable cost environment (no long-distance cables or national boundaries)Wide area coverage Coverage can be shaped by antenna patterns6Disadvantages of Using SatellitesExpensive to launchExpensive ground stations requiredMaintenance DifficultLimited frequency spectrumLimited orbital space (GeoStationary)Constant ground monitoring required for positioning and operational control

7Satellite CharacteristicsOrbiting platforms for data gathering and communications position holding/trackingVHF, UHF, and microwave radiation used for communications with Ground Station(s)Signal path losses - power limitationsSystems difficult to repair and maintainSensitive political environment, with competing interests and relatively limited preferred space 2012 Raymond P. Jefferis III Lect01Satellite Communications8##Mission Dependent CharacteristicsOrbital parametersHeight (velocity & period related to this)Orientation (determined by application)Location (especially for geostationary orbits)Power sourcesSolar (principal), nuclear, chemical powerStored gas/ion sources for position adjustment

910Satellite Application ExamplesTelecommunicationsMilitary communicationsNavigation systems Remote sensing and surveillanceRadio / Television BroadcastingAstronomical researchWeather observation 2012 Raymond P. Jefferis III Lect01Satellite Communications10##Orbital PropertiesAltitude (radius to center of the earth)Inclination with respect to the earth axisPeriod of rotation about the earthGround coverage by the satelliteCommunications path length(s)Lect 01 2012 Raymond P. Jefferis III 11Types of Orbit12

Dr. Leila Z. Ribeiro, George Mason UniversityMissions Associated with Orbit TypesGEOPrimarily commercial communicationsMEOMilitary and research usesGlobal Positioning SystemsLEORemote sensingLect 01 - 1314LEO and MEO FeaturesEarth coverage requires multiple passesTypical pass requires about 90 minutesSignal paths relatively short (lower losses)Small area, high resolution ground imageEarth station tracking requiredMultiple satellites for continuous coverage15Geo-Synchronous Satellite (GEO) FeaturesAppears fixed over point on earth equator Each satellite can cover 120 degrees latitudeOrbital Radius = 42,164.17 kmEarth Radius = 6,378.137 km (avg)Period (Sidereal Day) = 23.9344696 hr(86164.090530833 seconds)Long signal path - large path losses16Orbital Altitudes and ProblemsLow Earth Orbit (LEO)80 - 500 km altitudeAtmospheric drag below 300 kmMedium Earth Orbit (MEO)2000 - 35000 km altitudeVan Allen radiation between 200 - 1000 kmGeostationary Orbit (GEO)35,786 km altitude (42,164.57 km radius)Difficult orbital insertion and maintenance 2012 Raymond P. Jefferis III Lect01Satellite Communications16##17Orbital InclinationsEquatorialPrograde inclined toward the east Retrograde inclined toward the westInclinedVarious inclination angles with respect to the spin axis of the earth, including polarGeostationary (on equator; no inclination)Sun synchronousLect 01 2012 Raymond P. Jefferis III 18Earth Coverage CalculationBy the Law of Sines:and,

The elevation angle is approximately,

The total coverage area on the surface of the earth, using the previously calculated value of ) is given by the equation,

Earth Coverage Calculation (continued)Lect 01 2012 Raymond P. Jefferis III 19

Earth Coverage vs Satellite AltitudeLect 01 2012 Raymond P. Jefferis III 20

Satellite Communication CharacteristicsVia electromagnetic waves (radio)Typically at microwave frequenciesHigh losses due to path lengthMany interference sourcesAttenuation due to atmosphere and weatherHigh-gain antennas needed (dish) to make up for path loss and noise21Frequencies of Interest for SatellitesLect 01 2012 Raymond P. Jefferis III 22 Generally between 1 GHz and 30 GHz. The microwave spectrumAllows efficient generation of signal power Energy radiated into spaceEnergy may be focused (beam shaping) Efficient reception over a specified area. Properties vary according to the frequency used: Propagation effects (diffraction, noise, fading) Antenna SizesDesignated Microwave BandsLect 01 2012 Raymond P. Jefferis III 23

WikipediaStandard designationsFor microwave bands

Common bands for satellite communication are the L, C and Ku bands.24Common Microwave Frequency AllocationsL band 0.950 - 1.450 GHzNote: GPS at 1.57542 GHzC band3.7 - 4.2 GHz (Downlink)5.925 - 6.425 GHz (Uplink)Ku band 11.7 - 12.2 GHz (Downlink) 14 - 14.5 GHz (Uplink)25Common Microwave Frequency AllocationsKa band 18.3 - 18.8, 19.7 - 20.2 GHz (Downlink)30 GHz (Uplink)V band40 - 75 GHzLect 01 2012 Raymond P. Jefferis III 26L-BandFrequencies: 0.950 1.450 GHz ( ~30cm)Uses:Radio communicationsGPS devicesFeatures:Patch antenna used for GPS receiversLow rain fade - Low atmospheric atten. (long paths)Low powerSmall receiver configurations27C-Band

Lect 01 2012 Raymond P. Jefferis III 28C-BandFrequencies: 3.7 - 6.425 GHz ( ~5cm)Uses:TV reception VSATFeatures:Large dish antenna needed (3m diameter)Low rain fade - Low atmospheric atten. (long paths)Low power - terrestrial microwave interferences29Ku-Band

2012 Raymond P. Jefferis III Lect01Satellite Communications29##Lect 01 2012 Raymond P. Jefferis III 30Ku-BandFrequencies: 12 - 18 GHz ( ~ 2cm)Uses:Remote TV broadcastingSatellite communicationsVSATFeatures:Rain, snow, ice (on dish) susceptibilitySmall antenna size - high antenna gainHigh power allowedLect 01 2012 Raymond P. Jefferis III 31Ka-BandFrequencies: 18 - 40 GHz Uses:High-resolution radarCommunications systemsDeep space communicationsFeatures:Obstacles interfere (buildings, vegetation, etc.)Atmospheric absorptionLect 01 2012 Raymond P. Jefferis III 32V-BandFrequencies: 40 to 75 GHz. Uses:Millimeter wave radar research (very expensive!)High capacity millimeter wave communicationsFeatures:Rain fadeObstacles block pathAtmospheric absorptionExpensive equipment