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

Designing a Satellite Bottom-up method Top-down method Product A B C components 1.design up from component level 2.interactions not handled well 3.costs:short-term – low long-term – high (low reliability) System A B interactions 1.design down from system reqmnts 2.consider interactions at each step 3.costs:short-term – high long-term – lower (high reliability) subsystems C

1.Scientific instrumentsScientific instruments 2.PowerPower 3.ThermalThermal 4.AttitudeAttitude 5.Command & Data HandlingCommand & Data Handling 6.CommunicationsCommunications 7.StructureStructure 8.Launch vehicleLaunch vehicle 9.Ground controlGround control 10.PropulsionPropulsion Satellite Subsystems Interaction Matrix 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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LIONSAT Local 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)

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 20 o behind Earth 3 spacecraft separated by l = 5 x 10 6 km Will detect spatial strain of  l/ l =   l = 5 x 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) mirror thrusters

Hubble Space Telescope

Power Solar array: sunlight  electrical power –max. efficiency = 17% (231 W/m 2 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 238 PuO 2 ) 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

Attitude Determination and Control Sensors –Earth sensor (0.1 o to 1 o ) –Sun sensor (0.005 o to 3 o ) –star sensors ( o to 0.01 o ) –magnetometers (0.5 o to 3 o ) –Inertial measurement unit (gyros) Active control (< o ) –thrusters (pairs) –gyroscopic devices reaction & momentum wheels –magnetic torquers (interact with Earth’s magnetic field) Passive control (1 o to 5 o ) –Spin stabilization (spin entire sat.) –Gravity gradient effect x y Earth sensor photocells wheel motor satellite Motor applies torque to wheel (red) Reaction torque on motor (green) causes satellite to rotate rotation field of view

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)