Satellite Subsystems and Link Design: Comprehensive Study Guide

Satellite Subsystems: Attitude and Orbit Control System (AOCS)\n\n* The Altitude and Orbit Control System (AOCS) is a critical satellite subsystem composed of rocket motors and gas jets used for positioning and attitude maintenance.\n* Rocket motors are employed to move the satellite back to its assigned orbit when external forces cause it to drift off-station.\n* Gas jets and inertial devices are used to control the attitude (orientation) of the satellite.\n* Attitude control is essential to ensure that antennas, which often have narrow beams, are pointed precisely at the earth.\n* Internal and External Forces Acting on Satellites:\n * Gravitational forces from the sun, moon, and planets set up rotational moments if the satellite is unbalanced.\n * Solar pressure acting on the antennas, satellite body, and solar sails can create rotational forces.\n * The Earth's magnetic field can produce forces if the satellite has a net magnetic moment.\n* Dynamics and Motion:\n * Cyclic forces vary through a 24-hour period, tending to cause nutation (a wobble), which must be mechanically damped.\n * Gravitational field variations cause not only attitude changes but also acceleration that alters the orbit.\n* Primary Functions of AOCS:\n * Orbit insertion: Placing the satellite into the correct orbit initially.\n * Orbit maintenance: Keeping the satellite in the correct position over time.\n * Fine pointing: Ensuring the accuracy of antenna orientation towards the service area.\n\n# Dynamics and Stability Methods\n\n* There are two primary methods to stabilize a weightless satellite in orbit:\n * Spin Stabilization: The entire body of the satellite rotates at $30$ to $100\,rpm$. This creates a gyroscopic force that stabilizes the spin axis and keeps it pointing in a fixed direction. These satellites are called \"spinners.\" Example: The Hughes 376 (now Boeing 376).\n * Three-Axis Body Stabilization: The satellite is stabilized by one or more momentum wheels, which are solid metal disks driven by electrical motors. Example: Hughes (Boeing) 701 series.\n* Principles of the Momentum Wheel:\n * Based on the principle of conservation of angular momentum, increasing the speed of the momentum wheel causes the satellite to precess in the opposite direction.\n * In three-axis stabilized satellites, a pair of gas jets is needed for each axis to provide rotation in both directions for pitch, roll, and yaw.\n * Rotation about each axis is commanded from the earth station by modulating the speeds of the three momentum wheels.\n\n# Propulsion and Axes of Rotation\n\n* Satellite Reference Axes (Cartesian Coordinate System $X_R, Y_R, Z_R$):\n * Yaw Axis: Directed toward the center of the Earth.\n * Pitch Axis: Normal to the orbital plane.\n * Roll Axis: Tangent to the orbit path.\n* Spinner Satellite Design:\n * Consists of a cylindrical drum covered in solar cells containing power systems and rocket motors.\n * The communication system is mounted at the top of the drum and is despun: driven by an electric motor in the opposite direction of the satellite's rotation to keep antennas pointed at Earth.\n * Satellites are spun up during the launch phase using small radial gas jets on the drum's periphery.\n* Rocket Motor Types:\n * Bipropellant thrusters.\n * Arc jets or ion thrusters.\n* Fuel Usage on GEO Satellites:\n * Fires the Apogee Kick Motor (AKM) for final orbit injection.\n * Maintains the satellite in orbit throughout its lifetime.\n* Pointing Accuracy:\n * Stabilization must often be within $\pm 0.1^{\circ}$ for large narrow-beam antennas.\n * References for attitude control include the outer edge of the Earth's disk (observed via infrared sensors), the sun, or specific stars.\n\n# Telemetry, Tracking, Command, and Monitoring (TTC&M) Subsystem\n\n* The TTC&M system is split between equipment on the satellite and the controlling earth station. It is essential for managing the satellite throughout its lifecycle.\n* Telemetry and Monitoring System:\n * Uses hundreds of sensors to monitor satellite health: fuel tank pressure, PCU voltage/current, subsystem current draw, and communication electronic status.\n * Reports the status of sightings devices used for attitude; allows the earth station to detect failures and switch to spare units via the command system.\n * Telemetry data is digitized and transmitted via Phase Shift Keying (PSK) on a low-power carrier using Time Division Multiplexing (TDM) techniques.\n * Low data rates are used to maintain a narrow bandwidth and high carrier-to-noise ratio ($C/N$).\n* Tracking System:\n * Located at the earth station to provide range, elevation, and azimuth angle data.\n * Computes orbital elements to detect changes in the satellite's path.\n * Techniques include measuring Doppler shift of the telemetry carrier and establishing position via triangulation using multiple earth stations (accuracy within $10\,m$).\n * Active ranging is done by transmitting pulses or sequence of pulses (ranging tones) and measuring the time delay.\n* Command System:\n * Provides a secure structure to change position, attitude, antenna pointing, and configuration switches.\n * Safeguards: A command word is sent in a TDM frame, checked for validity in the satellite, sent back to Earth for a second check, and only then is an \"execute\" instruction sent.\n\n# Satellite Power Systems\n\n* Primary Source: Communications satellites derive all electrical power from solar cells converting sunlight into energy.\n* Power Categories:\n * Solar: Standard for commercial satellites.\n * Chemical: Backup power used specifically during solar eclipses.\n * Nuclear: Used for deep space exploration satellites leaving Earth orbit; not used for communication satellites due to the risk of nuclear spread in case of launch failure.\n* Solar Radiation and Efficiency:\n * Solar intensity in GEO is $1.39\,kW/m^2$.\n * Efficiency of cells is $20$ to $25\%$ at Beginning of Life (BOL).\n * Efficiency falls over time due to aging and micrometeorite impacts; satellites are designed with $15\%$ extra solar cell area to meet requirements at End of Life (EOL).\n* Array Configurations:\n * Cylindrical Arrays (Spinners): Only half of the cells are illuminated at any time. Non-illuminated cells stay cool ($20$ to $30^{\circ}C$), maintaining higher efficiency than hot cells.\n * Solar Sails (Three-Axis Stabilized): Maintain normal incidence to sunlight by rotating sails with motors every 24 hours. They produce more power (up to $10\,kW$ vs $6\,kW$ for large spinners) but run hotter ($50$ to $80^{\circ}C$), causing a voltage drop.\n\n# Satellite Batteries and Eclipses\n\n* Eclipses occur in GEO twice a year during spring and fall equinoxes, lasting up to $70\,min/day$.\n* Battery Specifications:\n * Type: Sealed Nickel-Hydrogen ($NiH_2$) batteries are preferred for reliability, long life, and lack of gassing during charge.\n * Safe Discharge: Can discharge to $70\%$ of capacity.\n * Parameters: Voltages of $20$ to $50\,V$; capacities of $20$ to $100\,Ah$.\n* Operational Adjustments:\n * Shutting down non-critical communication loads during eclipses saves weight by reducing battery size.\n * TV broadcasting satellites often shut down during eclipses. Placing a satellite $20^{\circ}$ west of the service area's longitude shifts the eclipse to occur after $1\,a.m.$ local time, when shutdown is more acceptable.\n\n# Communications Subsystem and Transponders\n\n* Function: To provide a GEO platform for relaying voice, video, and data via receiving, amplifying, and retransmitting signals.\n* Link Constraints:\n * Downlink is the most critical design part.\n * Link distance is approximately $36,000\,km$.\n * Received power is low, rarely exceeding $10^{-10}\,W$.\n * Signal-to-Noise Ratio ($S/N$) must be $5$ to $25\,dB$ above noise depending on modulation and data rate.\n* Transponder Elements:\n * Input Pass Band Filter: Rejects adjacent and same-satellite interference; typical bandwidth is $500\,MHz$.\n * Low Noise Amplifier (LNA): Provides initial gain ($\approx 20\,dB$) with minimal noise addition.\n * Down Converter: Non-linear mixer that translates uplink frequency to a lower downlink frequency. Stability of the Local Oscillator (LO) is typically $1\,PPM$.\n * Input Multiplexer: Sets of band-pass filters (BPFs) that split the wideband signal into sub-bands.\n * Power Amplifier: Traveling Wave Tube Amplifiers (TWTA) used for power $> 20\,W$; Solid State Power Amplifiers (SSPA) used for lower power (less space/weight, lower voltage).\n * Output Multiplexer (OMUX): Combines HPA outputs for the transmit antenna.\n* Transponder Types:\n * Bent Pipe (Linear): Amplifies and retransmits at a lower frequency.\n * Baseband Processing: Converts digital signals to baseband, processes them, and retransmits digital data.\n * On-Board Processing (OBP): Combined with switched-beam technology to increase capacity and allow direct communication between small earth stations.\n\n# Satellite Antennas: Types and Parameters\n\n* Wire Antennas: Monopoles and dipoles; used at VHF/UHF for TTC&M systems. Aimed at providing omnidirectional coverage though patterns often have nulls.\n* Horn Antennas: Used for wide microwave beams (global coverage); gain is limited to $\approx 23\,dB$ and beamwidth to $\approx 10^{\circ}$.\n* Reflector Antennas: Usually paraboloids; use one or more feeding horns to create a uniform phase front for high gain and narrow beams.\n* Array Antennas: Groups of small antennas properly spaced; Phased Arrays allow electronic steering of the beam by changing element phases.\n* Antenna Math:\n * Gain of aperture antenna: $G = \frac{4π A_e}{\lambda^2} = \frac{η 4π A}{\lambda^2}$\n * Circular aperture area: $A = \frac{π D^2}{4}$\n * Aperture Efficiency ($η$): Typical values are $65-80\%$ for horns; $50-65\%$ for single-feed reflectors.\n\n# Antenna Selection Criteria\n\n* Selection depends on:\n * Structure: Fixed (rigid) for launch or space-erectable for deployment.\n * Beam Type: Spot beams, global, elliptical, or steerable.\n * Feed Type: Dipole, horn, flared horn, or helical.\n * Polarization: Linear or circular (used for frequency reuse).\n * Directive Gain: High gain is required for satellite operations.\n * Efficiency: Parabolic reflectors are typically $55\%$ efficient.\n * Environmental Factors: use of composite materials for zero thermal expansion.\n\n# Satellite Link Design: Propagation and Power\n\n* Design Factors: Driven by satellite weight (half of which may be fuel for a 15-year life), DC power availability (solar cells), allocated frequency bands, and antenna size limits.\n* Basic Transmission Theory:\n * Flux Density (Isotropic): $\psi = \frac{P_t}{4π R^2}\,W/m^2$\n * Flux Density (with Gain): $\psi = \frac{P_t G_t}{4π R^2}\,W/m^2$\n * Effective Isotropic Radiated Power: $EIRP = P_t G_t$\n * Effective Aperture ($A_e$): $A_e = η A$\n * Power Received ($P_r$): $P_r = \psi A_e = \frac{P_t G_t G_r λ^2}{(4π R)^2}$\n * Free Space Path Loss ($L_p$): $L_p = 20\log(\frac{4π R}{\lambda})$\n * Link Equation: $P_r = EIRP + G_r - L_p - L_a - L_{ta} - L_{ra}\,dBW$ (where $L_a$ is atmospheric loss).\n\n# Noise and Performance Metrics\n\n* Noise Power ($P_n$): $P = k T_p B$, where $k$ is Boltzmann’s constant, $T_p$ is physical temperature, and $B$ is bandwidth.\n* System Noise Temperature ($T_s$): Representing all noise sources at the input of a noiseless receiver.\n* Noise Figure ($F$) and Temperature Relation: $T_e = T_0(F - 1)$, where $T_0 = 290\,K$.\n* Figures of Merit:\n * $G/T$ ratio: The ratio of receiver gain to system noise temperature; a higher value indicates better receiver performance.\n\n# Earth Station Receiver Configuration\n\n* Superheterodyne Receiver: Consists of an RF amplifier (LNA), mixer/LO for frequency conversion, and an IF amplifier stages.\n* Double Conversion Receiver: Used in many earth stations to translate signals to a first IF (e.g., $900-1400\,MHz$) and then a second tunable IF (e.g., $70\,MHz$ or $20\,MHz$ for TV) for channel selection.\n* Low Noise Block Converter (LNB): A component mounted behind the antenna feed combining the RF amplifier and downconverter.\n\n# Link Budgets and Performance Continuity\n\n* Link budget Calculation: Uses decibel units for simplified addition/subtraction. Calculated individually for transponders and for the four separate links in a two-way system.\n* Design Objectives:\n * Continuity: Guarantee link percentage (e.g., $99.5\%$ to $99.99\%$) with a required $S/N$.\n * Cost Optimization: Balancing antenna/receiver costs with performance.\n* Worst-Case Scenario Factors:\n * Station at the edge of coverage (loss of $\approx 3\,dB$).\n * Maximum path length and low elevation angles with high atmospheric attenuation.\n * Maximum rain attenuation causing signal loss and increased noise temperature.