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Satellites

Satellites. There are over 8,000 artificial objects orbiting the Earth. 2,500 are operative or inoperative satellites. The rest is junk….eg. hatch covers, rocket bodies, payloads that have disintegrated or exploded, and objects that are lost from manned spacecraft during operations. Sputnik I.

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Satellites

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  1. Satellites There are over 8,000 artificial objects orbiting the Earth. 2,500 are operative or inoperative satellites. The rest is junk….eg. hatch covers, rocket bodies, payloads that have disintegrated or exploded, and objects that are lost from manned spacecraft during operations

  2. Sputnik I Sputnik I -- 60 cm (about 2 ft.) diam. sphere with straight-wire antennas

  3. Explorer I Explorer I -- 1 m. long and 20 cm in diam., spin stabilized (like a gyroscope), with flexible antennas

  4. A generic military/meteorological/communications satellite 1-3 m. on each side, stabilized with internal gyroscopes or external thrusters

  5. Dual-spin stabilized satellite 1-3 m. in diameter, up to several meters tall; lower section spins to provide gyroscopic stability, upper section does not spin

  6. 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

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

  8. Interaction Matrix Satellite Subsystems • Scientific instruments (optional) • Power • Thermal • Attitude • Command & Data Handling(Computer(s)) • Communications • Structure • Launch vehicle • Ground control • Propulsion 11.Environmental Control and Life Support (optional) 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)

  9. The Key Point blah ssszzzz blah blah blah . . . EVERY subsystem affects EVERY other subsystem . . . blah blah sszzzzzsstt blah blah ssszzzzz zzzssszzzzzz zzzzzssss

  10. 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

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

  12. Spacecraft Propulsion Subsystem • Uses of onboard propulsion systems • Orbit Transfer • (Low Earth Orbit) LEO to (Geosynchronous Earth Orbit) GEO • LEO to Solar Orbit • Drag Makeup • Attitude Control • Orbit Maintenance

  13. Types of Propulsion • Chemical Propulsion • Performance is energy limited • Propellant Selection • Electric Propulsion • Electrostatic—Ion Engine • Electrothermal—ArcJet • Electomagnetic—Rail gun

  14. Types of Propulsion • Solar Sails • Would use large (1 sq. km.) reflective sail (made of thin plastic) • Light pushes on the sail to provide necessary force to change orbit. • Still on the drawing board, but technologically possible! • Nuclear Thermal

  15. Power • Provides, stores, distributes, and controls electrical power. • Need power for (basically everything) communications, computers, scientific instruments, environ. control and life support, thermal control, and even for propulsion (to start the rocket engine)

  16. 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 less than 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

  17. Power • Dynamic Power Sources • Like power plants on Earth. • Fuel Cells • Think of these as refillable batteries. • The Space Shuttle uses hydrogen-oxygen fuel cells.

  18. Power • The design is highly dependent on: • Space Environment (thermal, radiation) • Shadowing • Mission Life

  19. Thermal • Thermal Control System • Purpose—to maintain all the items of a spacecraft within their allowed temperature limits during all mission phases using minimum spacecraft resources.

  20. Thermal • Passive • Coatings (control amt of heat absorbed & emitted) • can include louvers • Multi-layer insulation (MLI) blankets • Heat pipes (phase transition)

  21. Thermal • Active (use power) • Refrigerant loops • Heater coils

  22. Communications • Transmits data to ground or to relay satellite (e.g. TDRS) • Receives commands from ground or relay satellite

  23. Communications • Radios (several for redundancy) • voice communications if humans onboard • data sent back to Earth from scientific instruments • instructions sent to s/c from Earth • Video (pictures of Earth, stars, other planets, etc.) • various antennas: dish, dipole, helix

  24. Attitude Sensing and Control • Senses and controls the orientation of the spacecraft.

  25. Attitude Sensing • star sensor – • The light from stars and compares it to a star catalog.

  26. Attitude Sensing • sun sensor measures angle between "sun line"

  27. Attitude Sensing • gyroscopes -- spinning disk maintains its orientation with respect to the fixed stars -- onboard computer determines how the s/c is oriented with respect to the spinning disk.

  28. Attitude Control • Thrusters -- fire thrusters (small rockets) in pairs to start rotation, then fire opposite pair to stop the rotation. 

  29. satellite wheel motor Attitude • gyroscopes -- use electric motor in s/c

  30. 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

  31. Command and Data Handling • Principal Function • Processes and distributes commands; processes, stores, and formats data • Other Names • Spacecraft Computer System • Spacecraft Processor

  32. 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

  33. Command and Data Handling • provide automatic capability for s/c, reducing dependence on expensive ground control • must include backups or redundant computers if humans onboard • need to be protected from high-energy radiation • cosmic rays can alter computer program (bit flip) without human ground controllers realizing it.

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

  35. 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

  36. Launch System • System selection process • Analyze capable systems • Maximum accelerations • Vibration frequencies and amplitudes • Acoustic frequencies and amplitudes • Temperature extremes • LV/satellite interface • Kick motor needed?

  37. Delta II Rocket Image:http://www.boeing.com/companyoffices/gallery/images/space/delta_ii/delta2_contour_08.htm

  38. Titan IV Rocket Image: www.spaceline.org/galleries/cpx-40-41/blowup41.jpg.html

  39. 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

  40. Payload Operations Control Center NASA Marshall Space Flight Center, Huntsville Alabama

  41. Mission Control Center NASA Johnson Spaceflight Center, Houston Texas

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