CONTENTS GROUND SEGMENT Otto Koudelka Institute of Communication Networks and Satellite Communications Graz University of Technology, Austria koudelka@tugraz.at Types of Ground Stations Architecture Subsystems Antenna, Tracking Station Parameters: G/T, EIRP Polarisation Components of Ground Stations Very Small Aperture Terminals (VSATs) TV Receive-only Terminals Topologies 1 2 FREQUENCIES C-Band: 4 / 6 GHz X-Band: 8 / 10 GHz Ku-Band: 11 / 14, 12 / 14 GHz Ka-Band: 20 / 30 GHz Always higher uplink frequency (efficiency reasons) TYPES OF GROUND STATIONS Large stations (operators head-ends, TV feeder links, hub stations) VSATs (very small aperture terminals) one-way interactive TV Receive-Only (TVRO) 3 4 INTELSAT-A TV, telephony, data up to 32 m LARGE STATIONS MEDIUM-SIZED STATIONS VSAT hubs TV uplinks up to 9.5 m antenna diameter 5 6 1
FIXED MEDIUM-SIZE STATIONS VSATs Very Small Aperture Terminals up to 3.7 m antennas typically 1.2...1.8 m 7 8 LOW-COST TERMINAL TRANSPORTABLE STATIONS Trailer-mounted Terminal Satellite News Gathering Terminal 9 10 NOMADIC TERMINAL MOBILE TERMINAL 11 12 2
MOBILE ANTENNA TV RECEIVE ONLY (TVRO) 35...120 cm (Ku) 3 m (C) 13 14 12.5 GHz ARCHITECTURE Low Noise Amplifier (LNA) Receiver 70 / 140 MHz or 950 2050 MHz Frequency Downconverter Demodulator FEC Decoder Data out RX: 12.5 GHz LNA DOWNCONVERTER 1 st IF= 750 MHz BW=40 MHz LO1R: 11.75 GHz LO2R: 680 MHz 2 nd IF= 70 MHz DE Antenna 14 GHz Polarisation separation High Power Amplifier Frequency Upconverter Transmitter 70 / 140 MHz or 950 2050 MHz FEC Encoder Modulator Data in HPA TX: 14.0 GHz Driver BW=40 MHz LO2T: 13.25 GHz LO1T: 680 MHz 15 16 12.5 GHz LNA TEST TRANSLATOR 12.5 GHz LNA TEST TRANSLATOR DOWNCONVERTER DOWNCONVERTER 1.5 GHz 1.5 GHz 14.0 GHz HPA 14.0 GHz HPA 17 18 3
DOWNCONVERTER DE 1 ANTENNAS 12 GHz x y diplexing 14 GHz 1 SIMULTANEOUS OPERATIONS 2 Horn antenna 12 GHz DOWNCONVERTER DE 2 12 GHz 20 GHz Waveguide ( transmission line for microwaves) transport of electromagnetic energy 19 20 PARABOLIC ANTENNAS OFFSET ANTENNA Primary focus feed With subreflector Cassegrain Gregory Advantage: feed close to RF equipment Primary focus offset Offset-Cassegrain Offsetantenne hier Primärfokus und asymmetrischer Reflektor Subreflektor Offset - Cassegrain- Antenne Offset-Gregory Offset-Gregory - Gregory- Antenne Subreflektor 21 22 OFFSET ANTENNAS higher efficiency, less shielding antenna angle steeper, less risk of snow remaining in dish electrical elevation angle is not mechanical elevation angle (subtract antenna offset ~20!) ANTENNA PATTERN Gain plotted versus azimuth (or elevation) angle Antenna sidelobes below certain mask G= 29-25logΘ Gain Reduction [db] Main lobe Azimuth angle 23 24 4
ANTENNA GAIN Depends on diameter, wavelength, efficiency G antenna gain [db] λ...wave lenghth [m], λ = c/f η...efficiency (depends on surface accuracy, shielding) Example: D = 2.4 m, f = 14 GHz, η = 60 % 2 2 2 2 π G D π 2.4 = 10log η = 10log 0.6 = 48. 7dB 2 2 λ 3E8 14E9 25 DUAL POLARISATION Orthogonal polarizations used, same frequency can be used twice Horizontal / vertical (linear) Left-hand / right-hand (circular) Ortho-mode Transducer () separates polarised waves must have a good cross-polar discrimination (XPD) 25 db minimum 30...35 db (@1 deg. off boresight) Otherwise cross-talk, interference to other users Proper alignment of vital 26 Low-cost XPD: 25 db EIRP Effective isotropic radiated power: power emitted by an isotropic antenna to produce same peak power density (in far field) as a (directional) antenna in the direction of maximum gain antenna port receive port EIRP [dbw] = P[dBW] + G[dB] High-performance XPD: 48 db antenna port transmit port 27 28 FIGURE OF MERIT Relationship between antenna gain and receiver noise temperature G/T [in db/k] G/T [db/k] = G [db] 10 log T [K] Good value for a 2.4 m antenna with 80 K LNA : 28 db/k Receive signal/noise ratio derived from link budget: E N b o = EIRP L ( G T ) k[ dbj / K ] B[ + [ dbw ] all[ db] / [ db / K ] L sum of all losses, k Boltzman constant, B bandwidth dbhz] ANTENNA POINTING manual motor-driven (can be used for tracking) azimuth elevation polarization beacon signal from satellite used to optimize pointing 29 30 5
1) determine local position (GPS) 2) calculate azimuth, elevation, polarisation angles 3) adjust elevation angle (inclinometer) 4) coarse alignment of azimuth (compass, GPS) 5) search for beacon (spectrum analyzer) 6) fine adjustment of elevation, azimuth for maximum signal level 7) adjust polarisation TRACKING for larger earth stations compensation of small half-power beamwidth satellite movement compensation of wind force inclined-orbit satellites 31 32 program track step-track monopulse system METHODS beacon signal to find maximum RX signal strength PROGRAM TRACK employ orbit calculations enter spacecraft orbital elements updated regularly after orbital maneuver calculate azimuth, elevation use data to control azimuth/ elevation actuators 33 34 STEP TRACK simple algorithm antenna moved a discrete step if signal increases, carry on in this direction if signal decreases, go back satisfactory for most applications ANTENNA MOUNTS stable withstand wind load non-penetrating mount with frame for semi-permanent installations should have azimuth/ elevation reading grounding! 35 36 6
ANTENNA HEATING COMPONENTS surface heated by foils remove ice, snow high attenuation distortion of antenna diagram control system to keep temperature constant Mixer: analogue multiplier Phase-locked oscillator XTAL reference oscillator Amplifier Waveguide switch 37 38 VERY SMALL APERTURE TERMINALS -VSATs RF Front-end cost-optimised Small outdoor unit, directly mounted on or near feed of antenna Low noise downconverter block (LNB) Upconverter and high-power amplifier LNA Downconverter OUTDOOR UNIT HPA Upconverter 1 st IF= 950... 2050 MHz VSAT 2nd Down converter INDOOR UNIT DE 1st Upconverter 39 40 LOW NOISE AMPLIFIER Solid-state GaAs-FET, HEMT High gain Low noise power T= 65, 80, 120, 160 K (Ku) T = 30 K (C) LNA HIGH POWER AMPLIFIER (HPA) SSPA (solid state power amplifiers) 1,2,4,8,16,40, 80, 100 W for Ku-band up to 300 W for C-band Higher power: Travelling Wave Tube TWTA (100 W...kW) bandwidth: 0.7 GHz (Ku), 3 GHz (Ka) Klystrons (very high power: up to 3 kw) bandwidth: 150 MHz 41 42 7
HIGH POWER AMPLIFIER TV RECEIVE-ONLY (TVRO) simple low-cost outdoor mount water-tight receiver unit single cable for intermediate frequency, power, control Upconverter / Driver High-Power Amplifier 16 W 43 44 LOW-NOISE BLOCK TVRO 950...2050 MHz Antenna SET-TOP BOX Feed POLAR SELECT LNA Low Noise Block Mixer Filter Dielectric Resonator Oscillator Amplifier 9.75, 10, 10.75, 11.3 GHz 950..2050 MHz LNB Power for LNB via RF cable Control signals: - Polarisation - Selection of Band TUNER DE DECOD TV 45 46 LNB / SET-TOP BOX TOPOLOGIES 47 48 8
POINT- TO- POINT STAR Central Hub Station: responsible for capacity assignment network management Internet 49 50 STAR / MESH MESHED E Star topology suitable if data flow is always to/from central site (e.g. Internet via satellite) If satellite terminals need to communicate with each other, traffic has to be routed via central hub station Double satellite hop (ca. 500 ms) prohibits interactive applications (voice, video conference, ) In mesh mode only single hop (250 ms) Any terminal can communicate with any other terminal without a hub 51 52 9