Satellite Communication (EC-603 (C)) - Unit 4 Short Notes

UNIT 4: Satellite Communication

I. Orbital Mechanics and Parameters

A. Kepler's Laws of Planetary Motion

1. First Law (Law of Ellipses): The orbit of a satellite around Earth is an ellipse with Earth's center at one focus.

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

   Consequence: 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 ($a$).

$$T^2 = \frac{4\pi^2}{GM} a^3$$

Where $G$ is gravitational constant, $M$ is Earth's mass.

B. Types of Orbits

C. Orbit Perturbations

Causes:

• Gravitational: Earth's non-uniform gravity (equatorial bulge, J₂ effect).

• Lunar/Solar: Gravitational pulls from Moon and Sun.

• Solar Radiation Pressure: Photon pressure from Sun.

• Atmospheric Drag: Significant for LEO satellites.

Effects: Change in orbital elements (eccentricity, inclination, RAAN). Station Keeping: Regular thruster firings to maintain desired orbit (e.g., GEO box: ±0.1° lat/long, ±0.05° inclination).

D. Orbital Calculations

1. Semi-major axis ($a$) & Eccentricity ($e$):

   Given apogee ($$\displaystyle r_a $$) and perigee ($$\displaystyle r_p $$) heights above Earth's surface ($$\displaystyle R_E $$):

$$a = \frac{(r_a + R_E) + (r_p + R_E)}{2}$$

$$e = \frac{(r_a + R_E) - (r_p + R_E)}{(r_a + R_E) + (r_p + R_E)}$$

2. GEO Visibility Limit: Maximum Earth central angle ($$\displaystyle \theta_{max} $$) from satellite to horizon:

$$\theta_{max} = \arccos\left(\frac{R_E}{R_E + h}\right) - \text{elevation angle constraint}$$

For $$\displaystyle h_{GEO} \approx 35,786 $$ km, $$\displaystyle \theta_{max} \approx 81.3^\circ $$ (from satellite nadir). Ground station latitude ($\phi$) limits visibility: $$\displaystyle |\phi| \leq 81.3^\circ $$.

E. Launching Orbits to GEO

II. Satellite Subsystems and Space Segment

A. Overview of Space Segment

• Role: The "space-based repeater." Receives uplink, processes/transposes, retransmits downlink. Enables global, instantaneous communication over vast areas (one GEO covers ~1/3 Earth).

• Revolution: Enabled live global TV, international telephony, internet backbones, real-time weather monitoring.

B. Attitude Control Subsystem (ACS)

• Importance: Maintains correct orientation (pointing) of antennas (toward Earth) and solar panels (toward Sun).

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

• Actuators: Reaction wheels, thrusters, magnetic torquers.

C. Station Keeping Subsystem

• Function: Uses ACS and thrusters to maintain orbital position (longitude/latitude for GEO) and orientation against perturbations. Consumes propellant (fuel life limits satellite life).

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

• Telemetry: Downlink of satellite health/status data (temperature, voltage, pressure).

• Tracking: Uplink signals to measure satellite position/velocity precisely.

• Command: Uplink commands from ground control to adjust orbit, switch payloads, manage anomalies.

E. Transponders

• Function: Core "bent-pipe" or processing unit. Receives uplink signal (in one band), frequency-transposes it (shifts to downlink band), amplifies, and retransmits.

• Types:

  • Bent-pipe (Transparent): Simple frequency translation & amplification. Most common for TV/telephony.

  • Regenerative (On-board Processing): Demodulates, decodes, re-encodes, re-modulates. Enables TDMA and better link performance.

F. Antenna Subsystems

• Types: Parabolic reflectors (high gain, narrow beam for spot beams), Horn antennas (feed elements), Phased arrays (electronic beam steering).

• Antenna Polarization:

  • Linear: Horizontal (H) or Vertical (V). Sensitive to orientation.

  • Circular: Right-Hand (RHCP) or Left-Hand (LHCP). Insensitive to rotation. Most common for satellites (mitigates Faraday rotation).

• Cross-Polarization Discrimination (XPD):

  Measure of antenna's ability to reject the opposite polarization. High XPD (>30 dB) is critical to prevent interference between co-frequency channels using opposite polarizations (frequency reuse).

III. Link Budget and Signal Propagation

A. Effective Isotropic Radiated Power (EIRP)

• Definition: The power that would need to be radiated by an ideal isotropic antenna to produce the same power density as the actual antenna in its direction of maximum gain.

• Calculation:

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

Where $$\displaystyle P_t $$ = transmitter power, $$\displaystyle G_t $$ = transmit antenna gain.

• Significance: Directly determines signal strength at the satellite (uplink) or at the Earth station (downlink). Key for coverage area prediction.

B. Transmission Losses

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

$$FSPL (dB) = 20\log_{10}(d) + 20\log_{10}(f) + 20\log_{10}\left(\frac{4\pi}{c}\right)$$

Simplified: $$\displaystyle FSPL (dB) = 92.4 + 20\log_{10}(d_{km}) + 20\log_{10}(f_{GHz}) $$.

2. Other Losses: Atmospheric absorption (gases, clouds), rain attenuation (Ku/Ka band), polarization mismatch loss, antenna pointing loss, implementation margin.

C. System Noise and Carrier-to-Noise Ratio (C/N)

• Noise Sources:

  • Thermal Noise: $$\displaystyle N = kTB $$ (Boltzmann constant $k$, system noise temperature $T$, bandwidth $B$).

  • Intermodulation Noise: From non-linear transponder amplifiers.

  • Atmospheric Noise: Especially from rain/water vapor.

• C/N Calculation (Overall Link):

  Noise powers add reciprocally (like resistances in parallel).

$$\frac{1}{C/N_{total}} = \frac{1}{C/N_{uplink}} + \frac{1}{C/N_{downlink}} + \frac{1}{C/N_{IM}}$$

Where $$\displaystyle C/N_{IM} $$ is carrier-to-intermodulation noise ratio.

• Effects: Low C/N causes bit errors, reduced voice quality, video artifacts. Required C/N depends on modulation and coding (e.g., QPSK ~ 10 dB for BER $$\displaystyle 10^{-6} $$ with FEC).

D. Atmospheric Effects on Links

• Rain Attenuation:

  • Quantification: Specific attenuation $\gamma$ (dB/km) increases with frequency and rain rate $R$ (mm/hr). Models: ITU-R P.838. Total attenuation $$\displaystyle A = \gamma \cdot L $$, where $L$ is path length through rain.

  • Impact: Severe for Ku-band (12-18 GHz) and Ka-band (20-30 GHz). Causes deep fades (10-30 dB).

  • Uplink Design Challenge: Uplink (ground→sat) power can be increased (up-link power control) to compensate, but downlink is fixed. Site diversity (multiple ground stations) mitigates rain.

• Mitigation: Adaptive Coding and Modulation (ACM), power control, site diversity, larger ground station antennas (higher gain).

E. Intermodulation Noise

• Cause: Non-linear amplification in a TWTA (Traveling Wave Tube Amplifier) when multiple carriers are amplified together. Produces sum/difference frequencies (e.g., $$\displaystyle 2f_1 - f_2 $$) that fall in the desired band.

• Reduction Methods:

  • Input Backoff (IBO): Reduce individual carrier power relative to amplifier saturation.

  • Output Backoff (OBO): Operate amplifier below saturation.

  • Use linear amplifiers (SSPA, but less efficient/powerful than TWTA).

  • Pre-distortion techniques.

F. Link Margin Calculation (VSAT Star Network)

• Definition: Difference between available C/N and required C/N (for target BER).

$$\text{Link Margin (dB)} = (C/N)_{available} - (C/N)_{required}$$

• Purpose: Budget for fades (rain, scintillation), equipment degradation, pointing errors.

• VSAT Star: Hub (large antenna) → Satellite → Many VSATs (small antennas). Uplink (VSAT→Hub) is often the critical link due to low VSAT EIRP. Margin typically 3-6 dB.

IV. Earth Stations

A. Types of Earth Stations

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

   • Setup: Small parabolic dish (60-90 cm), LNB (Low-Noise Block downconverter), IRD (Integrated Receiver/Decoder).

   • Function: Receives digital broadcast signals (DVB-S/S2). LNB converts Ku-band to L-band (950-2150 MHz). IRD demodulates, decodes, outputs to TV.

2. Transmit-Receive Earth Station (Hub/Gateway):

   • Large antenna (10-30 m), high-power amplifiers (TWTA/SSPA), low-noise receivers, modems, multiplexers. Used for network control, uplink of TV channels, internet gateway.

3. VSAT (Very Small Aperture Terminal):

   • Components: Small antenna (0.75-2.4 m), BUC (Block Upconverter), LNB, IDU (Indoor Unit with modem).

   • Transmission: TDMA (time-division multiple access) for star networks; SCPC (Single Channel Per Carrier) for point-to-point.

B. Earth Station Setup and Installation

• Transmit-Receive: Antenna foundation, pedestal, reflector, feed system, RF chain (BUC/LNB), IF cables, IDU, networking equipment. Requires precise pointing (azimuth/elevation) and polarization adjustment.

• VSAT: Similar but smaller. Critical: Clear line-of-sight to satellite (obstruction check).

C. VSAT Network Design and Implementation

1. Network Planning: Determine traffic requirements (number of VSATs, data rates), service type (star/mesh), satellite capacity, frequency band.

2. Equipment Selection: Antenna size (based on EIRP/G/T requirements), BUC/LNB power/noise figure, modem capabilities (modulation, FEC).

3. Installation: Site survey (obstruction, ground quality), antenna mounting, cable routing, grounding, polarization alignment (critical for XPD), commissioning tests.

D. Antenna Installation for DBS-TV

Steps:

1. Site Survey: Use satellite finder tool/app to check azimuth (compass direction) and elevation (angle above horizon). Ensure clear view (no trees/buildings).

2. Mounting: Secure mast/pole, level base.

3. Assembly: Attach dish, LNB arm, LNB (set polarization angle).

4. Pointing: Roughly set azimuth/elevation. Connect receiver, use signal strength meter to fine-tune for maximum signal/quality.

5. Weatherproofing: Seal all connections (F-connectors) with silicone/tape. Obstacle Impact: Buildings, trees cause signal blockage/reflection, leading to multipath (ghosting) or complete loss. Requires relocation or higher mounting.

V. Frequency Allocation and Polarization

A. Frequency Allocation

B. Antenna Polarization (See II.F)

• Linear (H/V): Used in some C-band. Requires precise alignment.

• Circular (RHCP/LHCP): Standard for satellite links (Ku/Ka). XPD > 30 dB required for effective frequency reuse.

VI. Applications and Services

A. Direct Broadcast Satellite Television (DBS-TV)

• System: Digital (DVB-S/S2) broadcast from GEO to small home dishes. Uses MPEG-2/4 compression.

• Error Control:

  • Forward Error Correction (FEC): Convolutional/Turbo/LDPC codes (e.g., rate 3/4, 7/8).

  • Interleaving: Spreads burst errors (from fading) to appear as random errors correctable by FEC.

  • Importance: Maintains broadcast quality (no freezing, artifacts) despite noise/fades.

B. Satellite Radio Broadcasting

• Example: SiriusXM (US), WorldSpace (former). Uses geostationary or highly elliptical orbits.

• Technology: Digital audio broadcasting (DAB). SCPC or MCPC (Multiple Carrier Per Channel). Encryption for subscription services. Large footprint for continental coverage.

C. VSAT Systems for Business Networking

• Connectivity: Provides always-on broadband to remote offices, ships, oil rigs.

• Topology: Star (hub-and-spoke) or mesh (VSAT-to-VSAT).

• Services: Internet, VoIP, VPN, SCADA, credit card transactions. Enables corporate WANs without terrestrial infrastructure.

VII. Case Studies and Specific Systems

A. Morelos and Satmex 5 Satellites

[!TIP] Exam Focus: Be prepared to calculate semi-major axis/eccentricity, explain Kepler's 2nd law with diagram, list GEO advantages, define EIRP/C/N, describe VSAT components, compare frequency bands, and outline DBS-TV error control. Past papers frequently test numerical link budget and orbital parameter calculations.