1 / 20

Bits and Pieces

Bits and Pieces. Spacecraft Systems. Propulsion Already discussed Communications Science instruments Power. Communications. A number of issues: Limited power Large distances Reception Multiplexing. Communications. Typical spacecraft transmission power ~20 W

heidi-hawkins
Download Presentation

Bits and Pieces

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Bits and Pieces

  2. Spacecraft Systems • Propulsion • Already discussed • Communications • Science instruments • Power

  3. Communications • A number of issues: • Limited power • Large distances • Reception • Multiplexing

  4. Communications • Typical spacecraft transmission power ~20 W • Limited power solved in two ways • Large receiving stations • Directional microwaves • Round-the-clock communication possible by the DSN

  5. Communications • How about at the spacecraft? • Cannot have huge antennae - typically 5m diameter • High transmission power from Earth • Highly sensitive amplifiers, narrow band-pass, phase locking, low data rates all used • See “Basics of Space Flight Chs. 10, 11

  6. Communications • Two principal types of spacecraft antennae: • High gain antennae provide primary communications • Highly directional, high data rates possible • Low gain antennae provide wide angle coverage at the expense of gain • Low pointing accuracy needed, hence can be used for initial contact or in the event of problems. Low data rates only.

  7. Communications • Spacecraft receivers/transmitters • Many spacecraft use the “S” or “X” bands (~ 2 and 5 GHz respectively) • See “Basics of Spaceflight” p. 101 • DS1 testing a “Ka” band transponder (~20 GHz) • Advantages; smaller, more directional (hence less power), less suceptible to poor ground station conditions - e.g., bad weather

  8. Instrumentation • Detectors • Remote sensing • Other systems • Basics of Space Flight Chapters 11, 12, 13

  9. Detectors • Charged particle detectors • Measure composition and distribution of interplanetary medium • Plasma detectors • Measure interactions of solar wind with planetary magnetic fields • Dust detectors • Magnetometers

  10. Remote Sensing • Imagers • Spectrometers • Remotely measure compositions • Polarimeters • Determine the size, composition and structures of particles in, for example, planetary rings

  11. Other Systems • Data recording • Record data for later playback • Tape recorders used, now being replaced by high capacity solid-state memories • Fault protection • “Default” procedures to re-establish contact with Earth, etc. if something goes wrong • Redundancy - Duplication of important systems

  12. Power • Typical spacecraft (e.g., Voyager, Galileo etc.) require 0.3-2.5 kW, over possibly decades! • Two currently available methods for long-term power • Photovoltaic cells (solar panels) • Radioisotope Thermal Generators (RTGs)

  13. n-type p-type Power • Solar Panels • Utilise photovoltaic effect across a semiconductor junction • Usually gallium arsenide or silicon http://www.iclei.org/efacts/photovol.htm

  14. Power • At 1 AU, silicon solar panels can provide 0.04 A/cm2 at 0.25 V per cell. GaAs is more efficient. • Solar power can, in practice, be used out to the orbit of Mars. • Output degrades by about 2% per year due to radiation damage - faster if there is high solar activity!

  15. n Radiator Pu-238 p Power • Radioisotope Thermal Generators (RTGs) • Use thermoelectric effect • Heat provided by decay of radioactive isotopes, usually Pu-238

  16. Power • Special issues relating to RTGs • Safety • They cannot “explode” • Design ensures RTG units remain intact, even after re-entry and impact in the event of an accident • PuO2 in insoluable, ceramic form • Get the launch right! • http://www.jpl.nasa.gov/cassini/rtg/

  17. Power • Typical RTG contains 11 kg PuO2 fuel, producing about 300 W of electricity from about 400 W of heat. • Total mass about 60 kg. • Decay rate of about 1-2% per year • e.g., Voyager RTGs provided 470 W at launch (1977), now provide 330 W • Probably still good for at least another 20 years

  18. What Next? • “DS 1” • Launched October 1998 • Test of new technologies, for example... • ion engine • autonomous navigation and operations • Ka band transponder • Asteroid and comet encounters • Solar wind studies • http://nmp.jpl.nasa.gov/ds1/

  19. What Next? • Continued Mars campaign • Launches at each opportunity • 2001 (orbiter) • 2003 (orbiter and rover) • 2005… (orbiters, landers, rovers, sample return…) • Failure of the 1998/99 missions raised a few questions • http://mars.jpl.nasa.gov/

  20. What Next? • “Stardust” comet and interplanetary material sample return • Launched Feb. 1999 • Encounter with comet Wild 2 in Jan. 2004 • Sample return 2006 • http://stardust.jpl.nasa.gov/

More Related