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Earth Stations

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Published in: Electronics
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A short note on Earth Stations

Ankan D / Guwahati

1 year of teaching experience

Qualification: B.Tech/B.E. (NIT Silchar - 2017)

Teaches: Basic Computer, Computer for official job, MS Office, School Level Computer, Algebra, Computer Science, IT & Computer Subjects, Mathematics, Physical Education, Physics, Chemistry, EVS, Science, SBI Exam, IIT JEE Advanced, IIT JEE Mains, WBJEE, C / C++

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  1. Earth stations
  2. Contents O Introduction Types of earth station O Earth station architecture O Earth station design consideration O Earth station Hardware O Antenna High Power amplifier Up/down converter Low noise amplifier O Satellite tracking
  3. Introduction An earth station is a terrestrial terminal station mainly located on O Earth's surface. Airborne or maritime O It may be fixed or movable. It consists of O Transmitter: Complexity depends on the no. of carriers and satellite handled by the earth station Receiver: Complexity depends on the no. of carriers and satellite handled by the earth station Antenna system : Transmission and reception with a multiplex arrangement. Tracking: to ensure that the antenna points to the satellite. Terrestrial interface equipment: Primary power: to run the earth station Test equipment: Routine maintenance of earth station and terrestrial interface.
  4. Types of Earth station Depending on the type of services: O Fixed satellite service (FSS). Broadcasting Satellite Service (BSS); O Earth Exploration Satellite Service (EES); O Space Research Service (SRS); O O Space Operation Service (SOS); Radiodetermination Satellite Service (RSS); O O Inter-Satellite Service (ISS); Amateur Satellite Service (ASS). O
  5. Fixed satellite service (F SS) Under this category, Large earth station (G/T= 40 dB/K). O Medium earth station (G/T = 30 dB/K) O O Small earth station (G/T= 25 dB/K) Very small terminals with transmit/receive functions (G/ T O - 20 dB/K)
  6. Fig. Large earth station Fig. Very small terminal
  7. Fixed satellite service (F SS) (contd••) Very small terminals with receive only functions (G/T=12 O dB/K)
  8. Fixed satellite service (F SS) (contd••) FSS is a term used mainly in North America. O O It involves use of Geostationary satellites for telephony, data communication and radio and television broadcast feeds. O It operates in C band (3.7-4.2 GHz), or the Ku band (11.45 -11.7 GHz and 12.5 - 12.75 GHz in Europe and 11.7-12.2 GHz in United states. Use linear polarization compared to circular polarization O by BSS satellite transponders.
  9. Broadcast Satellite Services (BSS) There are large Earth station (G/T = 15 O dB/K) used for community reception and O Small earth station (G/ T = 8 dB/K) used for individual reception. The frequency bands referred by ITU are: O 10.7-12.75 GHz in ITU region-I (Europe, Russia, Africa). 12.2-12.7 GHz in region-2 (North and South America). 11.7-12.2 GHZ in ITU region-3 (Asia, Australia). Direct broadcast service (DBS) or O Direct-to-home (DTH) service for domestic services. Fig: Very small terminal (Receive only)
  10. Mobile Satellite Service (VIS S) Large Earth station ( G/T = - 4dB/K) O Medium Earth Station (G/T 12 dB/K) O O Small Earth Station (G/T = -24 dB/K) Both large and medium earth station require tracking. O O Satellite phone is the most commonly used MSS. O It is a type of mobile that connects directly to satellite instead of terrestrial cellular sites. MSS is provided by both GEO and LEO. O
  11. Single Function Stations Single function stations are O characterized by a single type of link to a satellite or a satellite constellation. These stations may be O transmit-only, receive-only or both. For e.g. television receive-only O (TVRO) terminals used for TV reception by an individual. Fig. TVRO terminal
  12. Earth station Architecture [1] Major components are . RF section, baseband section and O terrestrial interface. Baseband Section Baseband Processing Circuits Modulator Demodulator Terrestrial Interface User Up-Converter rter Environmental Conditioning RF Section High Power Amplifier Low Noise Amplifier Support Facilities Monitoring Control Antenna : Power Supply Fig. Block schematic arrangement of a generalized Earth station
  13. RF section The job of the up converter is to up-convert the O base-band signal to the desired frequency band . The antenna feed system provides O the necessary aperture illumination, introduces the desired polarization, and isolation between the transmitted and received signals by connecting HPA output and LNA input to the cross-polarized ports of the feed. The terrestrial network could be a fibre optic or O microwave link or combination of two.
  14. Baseband section Performs the modulation/demodulation function. O For e.g. , in the case of a two-way digital communication O link, the baseband section would comprise of a digital modem and a time division multiplexer. O It is connected to the terrestrial network through a suitable interface known as terrestrial interface. The terrestrial network could be O a fibre optic cable link or a microwave link or even a combination of the two.
  15. Key Perfornnance O Transmitting parameter: EIRP (Effective or Equivalent Isotropic Radiated Power): It gives the combined performance of the high power amplifier (HPA) and the transmitting antenna. O Receiving parameter: Figure-of-merit (G/ T): Determines sensitivity and the quality of the received signal.
  16. Earth station Hardware [1] Antenna O High Power Amplifier O Up/down converters O Low noise amplifier O
  17. Antenna Types of antenna: O O O O O The horn antenna; The phased array antenna; The parabolic antenna. The horn antenna allows a high figure of merit to be achieved but it is expensive and cumbersome. This type of antenna was used at the start of space communication for experimental links with the Telstar satellite. This technology is no longer in use. Phased array antennas have an advantage when the beam is in constant movement as is the case for stations mounted on mobiles; however, the technology remains relatively difficult and costly which limits the use of this type of antenna.
  18. The most common antennas are those with parabolic reflectors. O The three principal mountings are: O symmetrical or axisymmetric mounting; offset mounting; Cassegrain mounting
  19. enna with symmetrical parabolic reflector [2] It has symmetry of rotation with respect to the principal axis on which the primary feed is at the focus. The feed supports and the feed itself have a masking effect on the radiating aperture (aperture blocking). This blocking leads to a reduction of antenna efficiency and an increase in the level of the side lobes due to diffraction by the obstacles. The primary feed faces the earth and that part of the radiation pattern of the primary feed which does not intercept the reflector (spillover) easily captures the radiation emitted by the ground. This makes a relatively large contribution to the antenna noise temperature (several tens to around 100 K). tan (CDP) = D/ 4f Parabolic reflector Focal length. t/ Primary feed Spillover Primary feed pattern Ground Fig. Axisymmetric parabolic reflector antenna.
  20. Offset mounting [2] 'It involves the use of that part of the parabola situated on one side of the vertex for the reflector profile 'Enables microwave circuits to be located immediately behind the primary feed without masking effects. With offset mounting, the spillover is again towards the ground and the antenna temperature remains high. Spillover Parabolic reflector I vo Paraboloid vertexs Primary feed Ground antenna.
  21. Cassegrain mounting [2] The phase centre of the primary feed is situated at the first focus S of an auxiliary hyperbolic reflector. 'The other focus R of the auxiliary reflector coincides with the focus of the main parabolic reflector. The Cassegrain antenna is thus less cumbersome although it retains the advantage of antennas with a long focal length. •The antenna noise temperature is low firstly since the greatest part of the' spillover is no longer directed towards the ground but towards the sky. Microwave circuits can easily be located immediately behind the primary feed which is located behind the main reflector Parabolic reflector Z Z u Spillover Primary feed Hyperbolic subreflector Scattered radiation from subreflector Ground
  22. If D is the aperture diameter of the parabolic reflector and its focal length, the solid apex angle under which the reflector is viewed, is given by Fig. Single reflector with equivalent focal length of a dual-reflector Cassegrain antenna
  23. g power Amplifier (HPA) [1] EIRP which is the product of the power of the HPA and the gain of the transmitter is an important parameter. deciding the uplink performance of the earth station. Different types of power amplifier used are: Carrier Combiner Signals Switch Matched Load Switch To Feed System 1. 2. 3. Travelling wave tube (TWT) amplifier Klystron amplifiers Solid state power amplifiers.
  24. Comparison among the power amplifiers: Technologies •rwrA Advantages Medium to large output powers (35—3000 W) • Proven fielded robust performance Good RF/DC efficiency: 30—50% does not decrease quickly with back-off • Stable performances over tempera ture • Instantaneous, broadband capability • Long life No memorv effect (non-linearitv) Disadvantages Limited TVVT production No soft-fail capability in case of failure High voltages required Non-linear, but linearisers exist or operation with back-off
  25. Comparison among the power amplifiers: SSPA Klystron High volume production capability • Built-in soft-fail capabilities in case of single device or module failure • Inherently good linear performance for multicarrier transmission High power (several kW), headroom for back-off for optimal linear performance Good linear performance for multicarrier Cost effective, reliable Good efficien (50%) • Limited output powers (100 s W at C and Ku bands, IOS W at Ka band) Highly inefficient (10—30%) High currents Need to be temperature compensated Large amounts of heat at concen trated locations, heat dissipation problems • Increased size and weight at high power levels due added cooling requirements (air flow, heat sinks, etc) Narrower instantaneous bandwidth (40—90 MHz), but tunable over 500 MHz more
  26. Up/ Down Converters [l] • Converts the IF used in the modem to the operating RF frequency bands (C, Ku and Ka) and vice-versa. Amp. Arnp. BPF BPF Mixer Amp. Local Oscillator (a) Mixer Amp. Local Oscillator (b) BPF BPF Fig. (a) Up converter (b) Down converter.
  27. Low noise e minimum level of signals that can be amplified without noise. It should have higher gain with low noise temperature. Nowadays LNA are used, incorporating gallium arsenide (GaAs) FET and high electron mobility transistors (HEMTs). Uplink frequency band (GHz) Noise figure (dB) Two variations: 14 30 2.2 2.4 (i) LNB: Frequency converter is incorporated, used in TVRO. (ii) LNC: Can be tuned to amplify over the entire bandwidth of a single transponder. RF out
  28. Satellite tracking: [l] • The earth station antenna needs to track the satellite when the beam width of the antenna is only marginally wider than the satellite drift seen by it. Satellite Beacon Earth Station Antenna Autoatrack Feed Receiver System Measured Position Antenna Amplifier Antenna Control System Manual Track Data Program Track Data Fig. Satellite tracking block diagram:
  29. Tracking techniques: [l] Commonly used tracking techniques are: 1. 2. 3. 4. 5. 6. Lobe switching Sequential lobing Conical scan Monopulse tracking Step track Intelligent tracking
  30. Lobe switching: The antenna beam is rapidly switched between two positions around the antenna axis in a single plane . Drawback: Prone to inaccuracies if the object cross section as seen by the antenna changes between different returns in one scan
  31. Sequential lobing: In sequential lobing, the beam axis is slightly shifted off the antenna axis. The squinted beam is sequentially placed in discrete angular positions, usually four round the antenna axis. -€0 beam A retum beam A beam B return beam B rerum Beam Axis Antenna Axis Fig. Principle of Sequential lobing
  32. Conical scan: • Similar to sequential lobing except for the difference that the squinted beam is scanned rapidly and continuously in a circular path. • If the object to be tracked is off the antenna axis, the amplitude of the echo signal varies with antenna's scan position. • The amplitude variation provides information on the amplitude of the angular error And phase delay indicates direction. Squint Ang I e Beam Axis Antenna Axis
  33. Monopulse tracking The drawbacks of the above techniques is that tracking accuracy is severely affected if the cross-section of the object to be tracked changes during the time the beam was scanned . Two types: l. Amplitude Comparison monopulse ll. Phase comparison monopulse.
  34. Amplitude monopulse tracking (a) (b) (a) (d) (b) (c) Fig. Amplitude comparison monopulse tracking — Spot for different angular positions Amplitude comparison monopulse tracking Received wavefront
  35. Phase monopulse tracking Monopulse tracking offers high accuracy and fast response time. Disadvantage includes high cost , large and complex feed system and good RF phase stability. Commonly used in large earth stations.
  36. Step track: Antenna axes are moved in small incremental steps in an effort to maximize the received signal strength. It is simple and low cost and RF phase stability is not important. Best suited for small and medium earth stations.
  37. Intelligent tracking The satellite position is obtained by optimally combinig antenna position estimate data with the prediction on satellite position obtained from a satellite position model. In case of signal amplitude fluctuations, the antenna position may be updated by using prediction data from the satellite position model. Useful for small, medium and large station , particularly those susceptible to scintillation and signal fades.
  38. Refe re n [l]. A. K. Maini and Varsha Agrawal, "satellite Communications", Wiley Publications. [2]. Gerard Maral and Michel Bousquet, "Satellite Communications Systems", Wiley publication, 5th edition.

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