Satellite Communication (EC-603 (C)) - Unit 1 Short Notes

UNIT 1: FUNDAMENTALS OF SATELLITE COMMUNICATION

I. Introduction to Satellite Communication

• Definition: A communication system that uses an artificial satellite in orbit around Earth as a relay station to transmit signals between distant ground stations.

• Significance & Revolution:

  • Enables global coverage, including remote and inaccessible areas.

  • Provides broadcast capability (one-to-many) for TV, radio, and data.

  • Offers high reliability and rapid deployment for emergency and military communications.

  • Revolutionized communication by creating a space-based infrastructure, enabling real-time global connectivity, international telephony, and direct-to-home broadcasting.

• Basic Architecture:

  • Space Segment: The satellite itself (payload + bus subsystems).

  • Ground Segment: Earth stations, control centers, and network operations centers.

  • User Segment: End-user terminals (VSATs, DBS dishes, mobile phones).

II. Orbital Mechanics and Satellite Orbits

A. Kepler's Laws of Planetary Motion

1. First Law (Law of Ellipses): A satellite orbits Earth in an elliptical path, with Earth at one of the two foci.

$$ r = \frac{a(1-e^2)}{1 + e \cos \theta} $$

where $a$ = semi-major axis, $e$ = eccentricity, $\theta$ = true anomaly.

2. Second Law (Law of Equal Areas): A line joining a satellite and Earth sweeps out equal areas during equal intervals of time.

   • Implication: Satellite moves fastest at perigee (closest point) and slowest at apogee (farthest point).

3. Third Law (Harmonic Law): The square of the orbital period ($T$) is proportional to the cube of the semi-major axis ($a$).

$$ T^2 = \frac{4\pi^2}{GM} a^3 \quad \text{or} \quad T^2 \propto a^3 $$

*   For Earth, $T$ in hours, $a$ in Earth radii ($$\displaystyle R_E $$): $$\displaystyle T^2 = \frac{a^3}{4.24} $$ (approx).

B. Orbit Types and Parameters

[!TIP] GEO Visibility Limit: Determined by satellite altitude ($h$) and minimum elevation angle ($$\displaystyle \theta_{min} $$) acceptable at Earth station. Maximum Earth central angle: $$\displaystyle \psi_{max} = \arccos\left(\frac{R_E}{R_E+h} \cos \theta_{min}\right) $$. Earth curvature limits coverage to ~81° from sub-satellite point.

C. Orbit Perturbations

• Causes:

  • Geopotential: Earth's non-uniform gravity (equatorial bulge, $$\displaystyle J_2 $$ term) → drift in inclination & right ascension of ascending node (RAAN).

  • Lunar/Solar Gravity: Causes long-period variations in eccentricity & inclination.

  • Solar Radiation Pressure: Affects eccentricity (especially for large, lightweight satellites).

  • Atmospheric Drag: Significant for LEO orbits → reduces semi-major axis & period.

• Effect: Gradual change in orbital elements (Keplerian parameters).

• Station-Keeping: Periodic thruster maneuvers to maintain orbital slot (GEO: ±0.1° longitude, ±0.05° inclination) and attitude.

D. Launching Orbits for GEO

1. Direct Injection: Launch vehicle places satellite directly into GEO. (Expensive, requires high launch energy).

2. Geostationary Transfer Orbit (GTO): Standard method.

   • Launch into elliptical GTO with apogee at GEO altitude (~35,786 km) and perigee at low Earth orbit (~200-300 km).

   • Satellite uses apogee kick motor at apogee to circularize orbit into GEO.

   • Advantage: More fuel-efficient for launch vehicle.

   • Disadvantage: Satellite carries large kick motor; longer time to operational orbit.

III. Frequency Allocation and Spectrum Utilization

• Governing Body: International Telecommunication Union (ITU) allocates frequency bands globally.

• Common Satellite Bands & Characteristics:

• Frequency Coordination: Process to avoid interference between satellite networks and with terrestrial services. Involves ITU notification, coordination, and registration.

IV. Satellite Subsystems

A. Attitude Control Subsystem (ACS)

• Purpose: Maintain desired orientation (three-axis stabilization) of satellite (antenna pointing, solar array sun-pointing).

• Sensors: Sun sensors, Earth horizon sensors, star trackers, magnetometers.

• Actuators: Reaction wheels (primary), thrusters (momentum dumping), magnetic torquers.

B. Station Keeping

• North-South: Corrects inclination drift caused by lunar/solar gravity (primary GEO requirement).

• East-West: Corrects longitude drift caused by Earth's equatorial ellipticity and solar radiation pressure.

• Fuel: Typically hydrazine monopropellant; lifetime often limited by station-keeping fuel.

C. Telemetry, Tracking, and Command (TT&C) Subsystem

• Telemetry (TM): Downlink of satellite health data (voltages, temperatures, pressures).

• Tracking (T): Ground station measures satellite range, range-rate, and angle to determine precise orbit.

• Command (C): Uplink of control commands (switch on/off, maneuver, mode change).

D. Transponders

• Role: Receive, amplify, frequency-translate, and retransmit signals.

• Types:

  • Bent-Pipe (Transparent): Simple frequency translation (e.g., $$\displaystyle f_{up} \rightarrow f_{down} $$). Most common.

  • Regenerative: Demodulates, decodes, re-encodes, and remodulates. Offers better $C/N$ but introduces delay.

• Typical Chain: LNA → Frequency Converter (Mixer + Local Oscillator) → Power Amplifier (TWTA/SSPA) → Output Filter.

E. Satellite Antenna Subsystems

• Parabolic Reflectors:

  • Global Horn: Simple, illuminates entire Earth disc from GEO.

  • Hemispheric/Zone Coverage: Covers a continent or large region.

  • Spot Beam: High-gain, narrow beam for frequency reuse (HTS).

• Phased Array: Electronically steerable beams (used in some LEO constellations).

V. Earth Stations and User Terminals

A. Types of Earth Stations

1. Receive-Only (RO) Home TV System:

   • Components: Parabolic dish → LNB (Low-Noise Block downconverter) → Set-top box/receiver.

   • Signals: DBS-TV (Direct Broadcast Satellite, Ku-band) and FSS (Fixed Satellite Service, C/Ku-band).

2. Transmit-Receive (Full) Earth Station:

   • Components: High-power amplifier (HPA), low-noise amplifier (LNA), up/down converters, large tracking antenna.

   • Used for network hub or gateway.

3. VSAT (Very Small Aperture Terminal) Networks:

   • Components: Small antenna (0.75m-2.4m), ODU (Outdoor Unit: BUC/LNB), IDU (Indoor Unit: modem).

   • Transmission Techniques: TDMA (Time Division), FDMA (Frequency Division), SCPC (Single Channel Per Carrier).

   • Topologies: Star (hub-and-spoke, most common), Mesh (VSAT-to-VSAT).

   • Design Steps: Network planning → Equipment selection → Site survey (obstruction analysis) → Installation → Commissioning.

B. Antenna Installation for DBS-TV

• Steps: Site selection (clear southern sky) → Mounting → Pointing (azimuth & elevation using GPS/signal meter) → Polarization alignment (skew angle).

• Obstacle Impact: Buildings, trees cause signal blockage or multipath (ghosting). Requires precise line-of-sight to satellite.

VI. Link Budget and Signal Propagation

A. Equivalent Isotropically Radiated Power (EIRP)

• Definition: Power radiated by an ideal isotropic antenna to produce the same power density as the actual transmitter antenna.

• Formula:

$$ \boxed{\text{EIRP} \, (\text{dBW}) = P_t \, (\text{dBW}) + G_t \, (\text{dBi})} $$

• Significance: Key parameter for coverage area and signal strength at satellite receiver.

B. Transmission Losses

1. Free Space Path Loss (FSPL): Inherent loss due to beam spreading.

$$ \boxed{\text{FSPL} \, (\text{dB}) = \left( \frac{4\pi d}{\lambda} \right)^2 = 20\log_{10}(d) + 20\log_{10}(f) + 92.45} $$

($d$ in km, $f$ in GHz).

2. Atmospheric Attenuation:

   • Rain Attenuation: Specific attenuation $$\displaystyle \gamma_R $$ (dB/km) depends on rain rate $R$ (mm/h) and frequency. Severe at Ku/Ka-band.

   • Gaseous Absorption: Oxygen (60 GHz), water vapor (22 GHz).

   • Cloud/Fog: Minor except at very high frequencies.

3. Other Losses: Polarization mismatch, pointing error, antenna misalignment.

C. System Noise

• Sources: Uplink interference, downlink thermal noise (sky + receiver), receiver noise (LNA noise temperature $$\displaystyle T_{sys} $$).

• Effect: Degrades Signal-to-Noise Ratio (SNR), increases bit error rate (BER), reduces system capacity.

• Noise Power: $$\displaystyle N = k T_{sys} B $$ (Boltzmann constant $k$, bandwidth $B$).

D. Carrier-to-Noise Ratio (C/N)

• Uplink C/N: $$\displaystyle C/N_{up} = \text{EIRP}_{up} - \text{FSPL}_{up} - \text{Atten}_{up} + G_{sat,up} - (k T_{sys,up})_{dB} $$

• Downlink C/N: $$\displaystyle C/N_{down} = \text{EIRP}_{sat} - \text{FSPL}_{down} - \text{Atten}_{down} + G_{ES} - (k T_{sys,down})_{dB} $$

• Intermodulation Noise ($$\displaystyle C/N_{im} $$): From transponder nonlinearities (TWTAs).

• Overall C/N (Cascaded):

$$ \boxed{\frac{1}{C/N_{\text{total}}} = \frac{1}{C/N_{\text{up}}} + \frac{1}{C/N_{\text{down}}} + \frac{1}{C/N_{\text{im}}}} $$

• Mitigation: Input Back-Off (IBO) & Output Back-Off (OBO) to operate TWTAs in linear region.

E. Rain Attenuation

• Quantification: $$\displaystyle \gamma_R = k R^\alpha $$ (dB/km), where $R$ = rain rate (mm/h), $k,\alpha$ = frequency-dependent coefficients (ITU-R P.838-3).

• Impact: Causes deep, rapid fades. Requires link margin (excess $C/N$ over threshold).

• Mitigation: Adaptive Coding & Modulation (ACM), power control, site diversity (multiple ground stations).

F. Link Margin

• Definition: Excess $C/N$ (dB) above the minimum required $C/N$ ($$\displaystyle (C/N)_{req} $$) for target BER and modulation.

$$ \boxed{\text{Link Margin (dB)} = (C/N)_{\text{actual}} - (C/N)_{\text{req}}} $$

• Importance: Provides fade margin to maintain link during adverse conditions (rain, mispointing).

VII. Antenna Polarization

• Definition: Orientation of the electric field vector of the radiated wave.

• Types:

  • Linear: Horizontal, Vertical. Sensitive to antenna orientation.

  • Circular: Right-Hand Circular (RHCP), Left-Hand Circular (LHCP). Immune to rotation, used for mobile/LEO.

• Cross-Polarization Discrimination (XPD):

  • Definition: Ratio (dB) of co-polarized gain to cross-polarized gain in a given direction.

  • Importance: High XPD (>30 dB) minimizes interference between orthogonal channels, enabling frequency reuse on same polarization.

VIII. Applications and Systems

A. Direct Broadcast Satellite Television (DBS-TV)

• Architecture: Broadcast from high-power GEO satellite with spot beams to small home dishes (0.5-1m).

• Error Control: Concatenated coding (inner convolutional code, outer Reed-Solomon) + interleaving to combat burst errors from fading. (e.g., DVB-S, DVB-S2).

B. Satellite Radio Broadcasting

• Overview: Digital audio broadcasting (DAB) via satellite (e.g., SiriusXM).

• Technology: Uses S-band (2.3 GHz) or Ku/Ka-band. Satellite diversity (multiple satellites) and terrestrial repeaters for urban coverage.

C. VSAT Networks

• Star Network Design:

  • Hub-to-VSAT Link: High $$\displaystyle EIRP_{hub} $$, large antenna. Downlink is broadcast.

  • VSAT-to-Hub Link: Low $$\displaystyle EIRP_{VSAT} $$, small antenna. Uplink is TDMA/FDMA.

• Link Margin Calculation: Must account for rain fade on uplink (higher frequency) and downlink separately.

• Applications: Enterprise WAN, Internet access (remote/rural), SCADA, POS networks.

D. Case Studies: Morelos & Satmex 5

IX. Error Control in Satellite Links

• Techniques:

  • Forward Error Correction (FEC): Add redundancy at transmitter for error correction at receiver (convolutional, Reed-Solomon, Turbo, LDPC).

  • Automatic Repeat reQuest (ARQ): Error detection + retransmission request (less common in broadcast).

• DBS-TV Specific: Concatenated FEC (e.g., convolutional inner code + Reed-Solomon outer code) + interleaving (to disperse burst errors). Standards: DVB-S (QPSK), DVB-S2 (QPSK/8PSK/16APSK) with LDPC codes.

[!CAUTION] Common Pitfalls in Exams:

1. Confusing EIRP with ERP: EIRP uses isotropic reference (dBi), ERP uses dipole reference (dBd). $$\displaystyle G_{dBi} = G_{dBd} + 2.15 $$.

2. Misapplying C/N combination: Remember it's a power sum ($$\displaystyle 1/C/N_{total} = \sum 1/C/N_i $$), not linear dB addition.

3. GEO altitude: Exact value is 35,786 km above equator, not 36,000 km (approximation may be accepted but exact is better).

4. Kepler's 3rd Law: Ensure consistent units. $T$ in seconds, $a$ in meters for $$\displaystyle GM = 3.986 \times 10^{14} m^3/s^2 $$.

5. Rain attenuation: Only a major issue for Ku/Ka-band; C-band is relatively immune.