S/C System Design Overview

S/C System Design Overview Robert G. Melton Department of Aerospace Engineering

Bottom-up method Top-down method System Product C A B subsystems C A B components interactions • design up from component level • interactions not handled well • costs:short-term – low • long-term – high • (low reliability) • design down from system reqmnts • consider interactions at each step • costs:short-term – high • long-term – lower • (high reliability) Designing a Satellite

Interaction Matrix Satellite Subsystems • Scientific instruments • Power • Thermal • Attitude • Command & Data Handling • Communications • Structure • Launch vehicle • Ground control • Propulsion Designers must fill in all the squares! Modes of Interaction • spatial (shadowing, motion restraints) • mechanical (vibrations) • thermal • electrical • magnetic • electromagnetic • radiative (ionizing radiation) • informational (data flow) • biological (contamination)

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LIONSATLocal IONospheric Measurements SATellite • will measure ion distrib. in ram and wake of satellite in low orbit • student-run project • (funded by Air Force, NASA and AIAA) • www.psu.edu/dept/aerospace/lionsat

LionSat (exploded view) Created by Christopher Borella and Rachel Larson for LionSat

LISA (Laser Interferometer Space Antenna) Space-based detector of gravity waves from black hole binaries Formation will orbit Sun, but 20o behind Earth 3 spacecraft separated by l = 5 x 106 km Will detect spatial strain of l/ l = 10-23  l = 5 x 10-14 m. (both images from lisa.jpl.nasa.gov)

The LISA orbits simulation by W. Folkner, JPL

- - - - Challenges for LISA • Electrical charging • Radiation pressure from sunlight • Self-gravity • New technology thrusters (micro-Newton) + thrusters mirror

Hubble Space Telescope http://www.stsci.edu/hst/proposing/documents/cp_cy12/primer_cyc12.pdf

Power • Solar array: sunlight  electrical power • max. efficiency = 17% (231 W/m2 of array) • degrade due to radiation damage 0.5%/year • best for missions  1.53 AU (Mars’ dist. from Sun) • Radioisotope Thermoelectric Generator (RTG): nuclear decay  heat  electrical power • max. efficiency = 8% (lots of waste heat!) • best for missions to outer planets • political problems (protests about launching 238PuO2) • Batteries – good for a few hours, then recharge

Thermal • Passive • Coatings (control amt of heat absorbed & emitted) • can include louvers • Multi-layer insulation (MLI) blankets • Heat pipes (phase transition) • Active (use power) • Refrigerant loops • Heater coils

satellite wheel motor Attitude Determination and Control y Earth sensor • Sensors • Earth sensor (0.1o to 1o) • Sun sensor (0.005o to 3o) • star sensors (0.0003o to0.01o) • magnetometers (0.5o to 3o) • Inertial measurement unit (gyros) • Active control (< 0.001o) • thrusters (pairs) • gyroscopic devices • reaction & momentum wheels • magnetic torquers (interact with Earth’s magnetic field) • Passive control (1o to 5o) • Spin stabilization (spin entire sat.) • Gravity gradient effect x field of view photocells rotation • Motor applies torque to wheel (red) • Reaction torque on motor (green) causes satellite to rotate

Command and Data Handling • Commands • Validates • Routes uplinked commands to subsystems • Data • Stores temporarily (as needed) • Formats for transmission to ground • Routes to other subsystems (as needed) • Example: thermal data routed to thermal controller, copy downlinked to ground for monitoring

Communications • Transmits data to ground or to relay satellite (e.g. TDRS) • Receives commands from ground or relay satellite Interconnections! • Data rate  power available  attitude ctrl. • Data rate  antenna size  structural support • Data rate  pointing accuracy  attitude ctrl.

Structure • Not just a coat-rack! • Unifies subsystems • Supports them during launch • (accel. and vibrational loads) • Protects them from space debris, dust, etc.

Launch Vehicle • Boosts satellite from Earth’s surface to space • May have upper stage to transfer satellite to higher orbit • Provides power and active thermal control before launch and until satellite deployment Creates high levels of accel. and vibrational loading

Ground Control • MOCC (Mission Operations Control Center) • Oversees all stages of the mission (changes in orbits, deployment of subsatellites, etc.) • SOCC (Spacecraft Operations Control Center) • Monitors housekeeping (engineering) data from sat. • Uplinks commands for vehicle operations • POCC (Payload Operations Control Center) • Processes (and stores) data from payload (telescope instruments, Earth resource sensors, etc.) • Routes data to users • Prepares commands for uplink to payload • Ground station – receives downlink and transmits uplink

Propulsion • Provides force needed to change satellite’s orbit • Includes thrusters and propellant

Effects of Power on Attitude Control • Provide properly regulated, adequate levels of electrical power for sensors and actuators • Failure to meet these requirements could result in incorrect satellite orientation (which affects astron. observations!)

Effects of Attitude Control on Power • Proper attitude (orientation) needed for solar arrays • some arrays track sun independently but still depend upon overall satellite orientation control • Spin-stabilized satellites require electrically switched arrays • high spin rates  faster switching (cheaper attitude ctrl) (more complex electronics)

S/C System Design Overview