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Launching, Orbital Effects & Satellite Subsystems Joe Montana IT 488 - Fall 2003

Launching, Orbital Effects & Satellite Subsystems Joe Montana IT 488 - Fall 2003. Agenda. Satellite Subsystems. SPACECRAFT SUBSYSTEMS. Attitude and Orbital Control System (AOCS) Telemetry Tracking and Command (TT&C) Power System Communications System Antennas.

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Launching, Orbital Effects & Satellite Subsystems Joe Montana IT 488 - Fall 2003

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  1. Launching, Orbital Effects & Satellite Subsystems Joe Montana IT 488 - Fall 2003

  2. Agenda • Satellite Subsystems

  3. SPACECRAFT SUBSYSTEMS • Attitude and Orbital Control System (AOCS) • Telemetry Tracking and Command (TT&C) • Power System • Communications System • Antennas More usually TTC&M - Telemetry, Tracking, Command, and Monitoring We will look at each in turn

  4. AOCS • AOCS is needed to get the satellite into the correct orbit and keep it there • Orbit insertion • Orbit maintenance • Fine pointing • Major parts • Attitude Control System • Orbit Control System Look at these next

  5. ORBIT INSERTION - GEO TWO BASIC TYPES OF GEO INSERTION: • High-Energy Apogee Kick Motor firing • A few minutes, symmetrical about apogee • Low-Energy AOCS burn • Tens of minutes to > one hour burns, symmetrical about apogee • Uses Dual-Mode thrusters; i.e. thrusters used for both orbit raising and attitude control

  6. ORBIT MAINTENANCE - 1 • MUST CONTROL LOCATION IN GEO & POSITION WITHIN CONSTELLATION • SATELLITES NEED IN-PLANE (E-W) & OUT-OF-PLANE (N-S) MANEUVERS TO MAINTAIN THE CORRECT ORBIT • LEO SYSTEMS LESS AFFECTED BY SUN AND MOON BUT MAY NEED MORE ORBIT-PHASING CONTROL

  7. ORBIT MAINTENANCE - 2 • GEO STATION-KEEPING BURNS ABOUT EVERY 4 WEEKS FOR  0.05o • DO N-S AND E-W ALTERNATELY • N-S REQUIRES  10  E-W ENERGY • RECENT APPROACH USES DIFFERENT THRUSTERS FOR E-W AND N-S

  8. FINE POINTING • SATELLITE MUST BE STABILIZED TO PREVENT NUTATION (WOBBLE) • THERE ARE TWO PRINCIPAL FORMS OF ATTITUDE STABILIZATION • BODY STABILIZED (SPINNERS, SUCH AS INTELSAT VI) • THREE-AXIS STABILIZED (SUCH AS THE ACTS, GPS, ETC.)

  9. DEFINITION OF AXES - 1 • ROLL AXIS • Rotates around the axis tangent to the orbital plane (N-S on the earth) • PITCH AXIS • Moves around the axis perpendicular to the orbital plane (E-W on the earth) • YAW AXIS • Moves around the axis of the subsatellite point

  10. DEFINITION OF AXES - 2 Earth o Equator s Yaw Axis Roll Axis Pitch Axis

  11. TTC&M TTC&M is often a battle between Operations (who want every little thing monitored and Engineering who want to hold data channels to a minimum • MAJOR FUNCTIONS • Reporting spacecraft health • Monitoring command actions • Determining orbital elements • Launch sequence deployment • Control of thrusters • Control of payload (communications, etc.)

  12. TELEMETRY - 1 • MONITOR ALL IMPORTANT • TEMPERATURE • VOLTAGES • CURRENTS • SENSORS • TRANSMIT DATA TO EARTH • RECORD DATA AT TTC&M STATIONS NOTE: Data are usually multiplexed with a priority rating. There are usually two telemetry modes.

  13. TELEMETRY - 2 • TWO TELEMETRY PHASES OR MODES • Non-earth pointing • During the launch phase • During “Safe Mode” operations when the spacecraft loses tracking data • Earth-pointing • During parts of the launch phase • During routine operations NOTE: for critical telemetry channels

  14. TRACKING • MEASURE RANGE REPEATEDLY • CAN MEASURE BEACON DOPPLER OR THE COMMUNICATION CHANNEL • COMPUTE ORBITAL ELEMENTS • PLAN STATION-KEEPING MANEUVERS • COMMUNICATE WITH MAIN CONTROL STATION AND USERS

  15. COMMAND • DURING LAUNCH SEQUENCE • SWITCH ON POWER • DEPLOY ANTENNAS AND SOLAR PANELS • POINT ANTENNAS TO DESIRED LOCATION • IN ORBIT • MAINTAIN SPACECRAFT THERMAL BALANCE • CONTROL PAYLOAD, THRUSTERS, ETC.

  16. POWER SYSTEMS - 1 • SOLAR CELLS • 1.39kW/m2 available from sun • Cells 10 - 15% efficient (BOL=Beginning Of Life) • Cells 7 - 10% efficient (EOL=End of Life) • SOLAR CELL OUTPUT FALLS WHEN TEMPERATURE RISES • 2mV/degree C; Three-Axis hotter (less efficient) than a spinner

  17. POWER SYSTEMS - 2 • BATTERIES NEEDED • DURING LAUNCH • DURING ECLIPSE (<70mins) • BATTERY LIMITS • NiCd 50% (DOD=depth of discharge) • NiH2 70% DOD NOTE: ISS uses 110V bus and will need 110 kW; 30 minute eclipses per day; 55 kW required from batteries Solution: using Fuel Cells

  18. POWER SYSTEMS - 3 • BATTERIES ARE “CONDITIONED” BEFORE EACH ECLIPSE SEASON • BATTERIES DISCHARGED TO LIMIT • BATTERIES THEN RECHARGED • TYPICAL NiH2 BATTERY CAN WITHSTAND 30,000 CYCLES (AMPLE FOR GEO; WOULD BE 5 YEARS IN LEO)

  19. COMMUNICATIONS SUB-SYSTEMS • Primary function of a communications satellite (all other subsystems are to support this one). • Only source of revenue • Design to maximize traffic capacity • Downlink usually most critical (limited output power, limited antenna sizes). • Early satellites were power limited • Most satellites are now bandwidth limited.

  20. SPACECRAFT ANTENNAS • SIMPLE:GLOBAL BEAM, ~17O WID LOW GAIN, LOW CAPACITY • REGIONAL: NARROW BEAM FROM REFLECTOR ANTENNA, TYPICALLY 3o  3o OR 3o  6o • ADVANCED: MULTIPLE NARROW BEAMS STATIONARY, SCANNED, OR “HOPPED”

  21. ANTENNA TYPES • HORN • Efficient, Low Gain, Wide Beam • REFLECTOR • High Gain, Narrow Beam, May have to be deployed in space • PHASED ARRAY • Complex • Electronically steered

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