Satellite Communication (EC-603 (C)) - Unit 2 Short Notes

UNIT 2: Satellite Communication

I. Orbital Mechanics and Launching

Kepler's Laws of Planetary Motion

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

   • For circular orbits (special case of ellipse), Earth is at the center.

2. Second Law (Law of Equal Areas): A line joining a satellite and Earth sweeps out equal areas in 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) of its orbit.

   • Mathematical form:

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

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

*   For Earth, a simplified constant is often used: 

$$T^2 = \frac{4\pi^2}{GM} a^3 \approx 1.01 \times 10^{-9} a^3$$

(with $a$ in km, $T$ in seconds).

*   **Key Insight:** Higher orbits (larger $a$) have longer periods.

[!TIP] Exam Focus: You may be asked to derive/state the laws or calculate orbital period from semi-major axis. Remember to use consistent units (km, seconds).

Orbit Types and Characteristics

Calculating Elliptical Orbit Parameters:

Given apogee height $$\displaystyle h_a $$ and perigee height $$\displaystyle h_p $$ (above Earth's surface, $$\displaystyle R_e \approx 6371 $$ km):

1. Semi-major axis: $$\displaystyle a = \frac{(R_e + h_a) + (R_e + h_p)}{2} = R_e + \frac{h_a + h_p}{2} $$

2. Eccentricity: $$\displaystyle e = \frac{(R_e + h_a) - (R_e + h_p)}{2a} = \frac{h_a - h_p}{2a} $$

[!TIP] Common Pitfall: Always add Earth's radius ($$\displaystyle R_e $$) to heights to get distances from Earth's center ($$\displaystyle r_a $$, $$\displaystyle r_p $$) before calculating $a$ and $e$.

Orbit Perturbations

Causes: Non-uniform Earth gravity (geopotential harmonics), lunar/solar gravity, solar radiation pressure, atmospheric drag (for LEO). Effects: Gradual change in orbital elements (inclination $i$, eccentricity $e$, right ascension of ascending node $\Omega$, argument of perigee $\omega$). Station Keeping: Regular thruster firings to counteract perturbations and maintain desired orbit (e.g., GEO box: ±0.05° inclination, ±0.05° eccentricity). Fuel consumption limits satellite lifetime.

Launching Orbits for GEO

1. Geostationary Transfer Orbit (GTO):

   • Process: Launch into highly elliptical orbit (perigee ~200-300 km, apogee ~36,000 km). Satellite uses onboard apogee motor at apogee to circularize into GEO.

   • Advantage: Lower initial launch energy; uses smaller, cheaper launch vehicle.

   • Disadvantage: Long transfer time (hours); satellite must survive high-radiation Van Allen belts; requires reliable apogee motor.

2. Direct Injection (Super-synchronous Transfer):

   • Process: Launch vehicle injects satellite directly into a near-GEO circular orbit (or very high apogee ellipse). Final circularization may require minor adjustments.

   • Advantage: Shorter time to service; less radiation exposure.

   • Disadvantage: Requires much more powerful/expensive launch vehicle.

II. Satellite Subsystems

Space Segment Overview

The space segment (the satellite itself) is the core of the system, enabling global, reliable, and instantaneous communication by acting as a high-altitude repeater. It revolutionized communication by providing wide-area coverage from a single asset, enabling live global broadcasting and connectivity to remote areas.

Payload Subsystems

• Transponders:

  • Function: Receive uplink signal, frequency-translate (to avoid interference), amplify, and regenerate (if bent-pipe vs. regenerative) for downlink.

  • Types:

    • Bent-pipe (Transparent): Simple frequency conversion & amplification. Most common.

    • Regenerative (On-board processing): Demodulates, processes, re-modulates. Enables better performance, mesh networks.

  • Key specs: Bandwidth (MHz), EIRP (dBW), G/T (dB/K).

• Antenna Subsystems:

  • Types: Parabolic reflectors (most common for focused beams), phased arrays (electronic beam steering).

  • Coverage Patterns: Global (low-gain, for broadcast), zone (regional), spot (high-gain, narrow).

  • Polarization:

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

    • Circular: Right-Hand (RHCP) or Left-Hand (LHCP). Immune to orientation changes; used for mobile/LEO.

  • Cross-Polarization Discrimination (XPD): Measure of antenna's ability to isolate orthogonal polarizations (e.g., H vs. V). High XPD is critical for frequency reuse (using same frequency for two orthogonal polarizations to double capacity).

Bus Subsystems

• Attitude Control:

  • Importance: Precise antenna pointing (≤0.1° for Ku/Ka) and solar panel orientation.

  • Methods: Spin stabilization (satellite spins for gyroscopic stability), three-axis stabilization (uses reaction wheels, thrusters for precise control), Control Moment Gyros (CMGs).

• Station Keeping:

  • North-South: Controls inclination ($i$). Counteracts gravitational perturbations from Moon/Sun. Major fuel consumer.

  • East-West: Controls eccentricity ($e$). Maintains satellite at correct longitude slot. Counteracts solar radiation pressure.

  • Fuel (propellant) capacity directly determines mission lifetime.

• Telemetry, Tracking, and Command (TT&C):

  • Functions:

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

    • Tracking: Uplink signals to measure satellite orbit/position.

    • Command: Uplink commands for orbit control, switching, etc.

• Power Systems: Solar panels (primary source, degrade over time) + Batteries (eclipse power).

• Thermal Control: Passive (coatings, radiators) and active (heaters) to maintain component temperatures.

III. Link Budget and Signal Propagation

Key Parameters

• Effective Isotropic Radiated Power (EIRP):

  • Definition: Power that would need to be radiated by an ideal isotropic antenna to produce the same signal strength in the direction of maximum radiation as the actual transmitter.

  • Formula:

$$\boxed{EIRP = P_t \cdot G_t}$$

(in linear units: Watts) or

$$EIRP_{(dBW)} = P_t_{(dBW)} + G_t_{(dBi)}$$

*   **Significance:** Determines **signal strength at satellite input** (uplink) or at Earth station (downlink). Higher EIRP = larger coverage/stronger signal.

• System Noise Temperature ($$\displaystyle T_s $$):

  • Sources: Sky noise (cosmic), atmospheric (rain, gases), antenna (spillover), feeder losses, receiver noise figure (NF).

  • Total: $$\displaystyle T_s = T_{sky} + T_{atm} + T_{ant} + T_{feeder} + T_{rec} $$ (in Kelvin).

  • Effect: Raises noise floor, degrades Carrier-to-Noise ratio (C/N), increases Bit Error Rate (BER).

• Carrier-to-Noise Ratio (C/N):

  • Uplink C/N:

$$\left(\frac{C}{N}\right)_{UL} = EIRP_{UL} - L_{UL} + \left(\frac{G}{T}\right)_{sat} - k - \text{Other losses}$$

*   **Downlink C/N:** 

$$\left(\frac{C}{N}\right)_{DL} = EIRP_{DL} - L_{DL} + \left(\frac{G}{T}\right)_{ES} - k - \text{Other losses}$$

*   **Overall C/N (for bent-pipe):** 

$$\frac{1}{(C/N)_{total}} = \frac{1}{(C/N)_{UL}} + \frac{1}{(C/N)_{DL}} + \frac{1}{(C/N)_{IM}}$$

*   Where $k$ is Boltzmann's constant ($-228.6$ dBW/K/Hz), $L$ is total path loss (dB), $G/T$ is figure of merit (dB/K).

*   **Intermodulation Noise (IM):** Caused by transponder amplifier non-linearities when multiple carriers are present. Reduced by **output back-off (OBO)** and careful power allocation.

Propagation Impairments

• Free Space Path Loss (FSPL): Fundamental loss due to beam spreading.

$$\boxed{L_{FSL (dB)} = 92.4 + 20\log_{10}(f_{GHz}) + 20\log_{10}(d_{km})}$$

• Rain Attenuation:

  • Quantification: Specific attenuation $\gamma$ (dB/km) is a function of rain rate R (mm/hr) and frequency. Models: ITU-R P.838-3.

  • Impact: Severe at Ku-band (12-18 GHz) and Ka-band (26-40 GHz). Causes fading (signal drop) with duration and depth statistics (e.g., fade margin needed for 99.9% availability).

  • Mitigation: Link margin, site diversity (multiple Earth stations), adaptive coding and modulation (ACM).

• Atmospheric Effects (Uplink):

  • Gaseous Absorption: Oxygen (60 GHz peak), water vapor (22 GHz peak). Significant at high frequencies.

  • Cloud/Fog: Scattering by water droplets. More severe at >10 GHz.

Transmission Losses (Total Link Loss, $L$)

$$L = L_{FSL} + L_{atm} + L_{rain} + L_{pol} + L_{feeder}$$

• $$\displaystyle L_{FSL} $$: Free space loss.

• $$\displaystyle L_{atm} $$: Atmospheric gaseous loss.

• $$\displaystyle L_{rain} $$: Rain attenuation (fade margin is additional to average).

• $$\displaystyle L_{pol} $$: Polarization mismatch loss.

• $$\displaystyle L_{feeder} $$: Waveguide/cable losses in Earth station/satellite.

Link Margin Calculation (VSAT Star Network Example)

1. Compute Uplink C/N using $$\displaystyle EIRP_{VSAT} $$, $$\displaystyle L_{UL} $$, $$\displaystyle (G/T)_{sat} $$.

2. Compute Downlink C/N using $$\displaystyle EIRP_{sat} $$, $$\displaystyle L_{DL} $$, $$\displaystyle (G/T)_{VSAT} $$.

3. Combine for Total C/N (include IM noise if multi-carrier).

4. Required $C/N$ is determined by modulation & coding scheme (e.g., QPSK 3/4 might require ~9 dB).

5. Link Margin (dB) = Actual $$\displaystyle (C/N)_{total} $$ - Required $$\displaystyle (C/N)_{req} $$.

6. Fade Margin is additional margin allocated for rain (e.g., 5-10 dB for Ku-band).

[!TIP] Exam Problem: Be prepared to calculate overall C/N from given uplink/downlink/IM values (in dB). Use: $$\displaystyle (C/N)_{total(dB)} = -10\log_{10}(10^{-(C/N)_{UL}/10} + 10^{-(C/N)_{DL}/10} + 10^{-(C/N)_{IM}/10}) $$.

IV. Earth Stations

Types and Configurations

• Receive-Only Home TV System (DBS-TV):

  • Components: Small parabolic antenna (0.6-1.2 m), LNB (Low-Noise Block downconverter - converts Ku-band to L-band), set-top box (demodulates, descrambles).

  • Signals: Digital TV (DVB-S/S2), radio, data.

• Transmit-Receive Earth Station (Hub/Teleport):

  • Uplink Path: Baseband → Modulation/Encoding → Upconversion (to RF) → High Power Amplifier (HPA - TWT/Klystron) → Antenna.

  • Downlink Path: Antenna → Low Noise Amplifier (LNA) → Downconversion → Demodulation/Decoding → Baseband.

• VSAT (Very Small Aperture Terminal):

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

  • Topology: Star network (central hub, many remote VSATs). Hub controls network.

  • Transmission Techniques: TDMA (Time Division Multiple Access - most common), FDMA (Frequency Division), Spread Spectrum (CDMA).

Antenna Installation (DBS-TV)

1. Site Survey: Clear line-of-sight to satellite (azimuth & elevation). Use satellite finder tools.

2. Calculate Azimuth (compass direction) & Elevation (angle above horizon) based on latitude/longitude and satellite orbital position.

3. Mount & Secure: Install mast/roof mount, ensure structural integrity.

4. Point & Fine-Tune: Coarse pointing via calculated angles, fine-tuning using signal strength meter on receiver.

5. Weatherproofing: Seal all connections (F-connectors).

• Obstacles: Buildings, trees cause blockage/multipath. Mitigation: Relocate antenna, use taller mount, or choose different satellite orbital slot.

VSAT Network Design & Implementation

1. Network Planning:

   • Traffic Analysis: Number of remotes, data rates per remote, traffic pattern (always-on, bursty).

   • Coverage: Ensure satellite beam covers all remote locations.

   • Link Budget: For both hub-to-satellite and satellite-to-remote (and return). Determines antenna size, HPA power.

2. Equipment Selection:

   • Antenna Size: Based on required $(G/T)$ and rain margin.

   • HPA Power: Based on required EIRP and back-off.

   • Modulation/Coding: QPSK, 8PSK, 16APSK with FEC (LDPC, Turbo codes) for spectral efficiency vs. robustness trade-off.

3. Installation & Configuration:

   • Site prep (foundation, power, grounding).

   • Antenna installation and precise pointing (critical for Ku/Ka).

   • ODU/IDU setup, parameter entry (satellite frequency, polarization, symbol rate).

   • Network Activation: Hub configures remote IDs, bandwidth allocation (TDMA slots), protocols (e.g., DVB-RCS).

V. Applications and Specific Systems

Satellite Television (DBS-TV)

• Error Control Methods:

  • Forward Error Correction (FEC): Adds redundant bits (e.g., Reed-Solomon (RS) outer code, Convolutional or LDPC inner code). Corrects errors without retransmission.

  • Interleaving: Spreads consecutive bits from a codeword over time to combat burst errors (from fading, impulse noise).

  • Scrambling: Prevents unauthorized reception (conditional access).

  • Importance: Maintains broadcast quality (clear picture/audio) by combating noise, fading, and interference in the satellite channel.

Satellite Radio Broadcasting

• Principles: Digital audio signals (e.g., SDARS - Satellite Digital Audio Radio Service) uplinked to satellite (often GEO) and broadcast over large coverage areas.

• Advantages: Wide geographic coverage (continental), mobile reception (cars), program diversity (hundreds of channels), digital quality (CD-like), metadata (song/artist info).

VSAT Networks

• Applications: Remote office connectivity, Internet access (rural/enterprise), SCADA (utility monitoring), POS (retail), maritime/land mobile.

• Design Considerations:

  • Star vs. Mesh: Star (hub-spoke, simpler, hub handles routing) vs. Mesh (remotes talk directly, lower latency, complex).

  • Bandwidth Allocation: Fixed (FDMA), demand-assigned (DAMA), or statistical multiplexing (TDMA).

  • Protocol Efficiency: Need robust MAC (Medium Access Control) and routing protocols for shared medium.

Case Studies: Morelos & Satmex 5

VI. Frequency Allocation and Standards

Frequency Bands in Satellite Communication

Frequency Allocation

• International: Coordinated by ITU-R (International Telecommunication Union - Radiocommunication sector). Radio Regulations treaty divides globe into regions and allocates bands to services (FSS, MSS, BSS).

• Domestic: National administrations (e.g., FCC in USA) assign specific frequencies and orbital slots within ITU framework. Licensing required.

• Uplink/Downlink Separation: To avoid interference, uplink and downlink use different frequency bands (e.g., 6 GHz uplink / 4 GHz downlink for C-band; 14 GHz uplink / 12 GHz downlink for Ku-band). This is called paired bands.

VII. Additional Cross-Cutting Topics

Cross-Polarization Discrimination (XPD)

• Definition: Ratio (in dB) of the power received in the co-polarization (intended) direction to the power received in the cross-polarization (orthogonal) direction.

$$XPD = 10\log_{10}\left(\frac{P_{co}}{P_{cross}}\right)$$

• Importance: Enables frequency reuse. By transmitting two independent signals on the same frequency but with orthogonal polarizations (e.g., H and V), channel capacity is effectively doubled. High XPD (>30 dB) is needed to keep cross-polarization interference low.

Local Mean Solar Time (LMST)

• Concept: Solar time at a given longitude, adjusted for the Equation of Time (difference between apparent solar time and mean solar time due to Earth's elliptical orbit and axial tilt).

• Relevance to SSO: A sun-synchronous orbit is designed so that the satellite's orbital plane precesses (nodal regression) at the same rate as Earth orbits the Sun (~1°/day). This ensures the satellite passes over any given point on Earth at nearly the same local mean solar time each day, providing consistent illumination conditions for Earth observation sensors.

System Noise vs. Link Performance

• Relationship: Higher system noise temperature ($$\displaystyle T_s $$) directly lowers $(C/N)$ (since $$\displaystyle N = k T_s B $$). This forces the use of more robust (lower data rate) modulation/coding to maintain target BER.

• Impact on Eb/N0: The fundamental parameter for digital communication is bit energy-to-noise density ratio ($$\displaystyle E_b/N_0 $$). For a given modulation/coding, there is a minimum required $$\displaystyle E_b/N_0 $$ for a target BER. Since $$\displaystyle C/N = (E_b/N_0) \times (R/B) $$, where $R$ is data rate and $B$ is bandwidth, noise limits the achievable data rate for a given bandwidth and power.

[!TIP] Key Formula Chain: $$\displaystyle EIRP \rightarrow $$ Received $C$; $$\displaystyle T_s, B \rightarrow $$ Noise $$\displaystyle N = k T_s B $$; $$\displaystyle C/N \rightarrow $$ Determines feasible Modulation & Coding (ModCod) $$\displaystyle \rightarrow $$ Achievable Data Rate.