Satellites There are over 8,000 artificial objects orbiting the Earth. 2,500 are operative or...
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Transcript of Satellites There are over 8,000 artificial objects orbiting the Earth. 2,500 are operative or...
SatellitesThere 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
Sputnik I -- 60 cm (about 2 ft.) diam. sphere with straight-wire antennas
Explorer I
Explorer I -- 1 m. long and 20 cm in diam., spin stabilized (like a gyroscope), with flexible antennas
A generic military/meteorological/communications satellite
1-3 m. on each side, stabilized with internal gyroscopes or external thrusters
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
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
Hubble Space Telescope
http://www.stsci.edu/hst/proposing/documents/cp_cy12/primer_cyc12.pdf
1. Scientific instruments (optional)
2. Power3. Thermal4. Attitude5. Command & Data Handling
(Computer(s))6. Communications7. Structure8. Launch vehicle9. Ground control10. Propulsion11.Environmental Control and
Life Support (optional)
Satellite SubsystemsInteraction Matrix
1 2 3 4 5 6 7 8 9 10
1
2
3
4
5
6
7
8
9
10
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)
blah blah ssszzzzz zzzssszzzzzz zzzzzssss
blah ssszzzz blah blahblah . . . EVERY subsystem affects EVERY other subsystem . . . blahblah sszzzzzsstt
The Key Point
Designing a Satellite
• Bottom-up method • Top-down method
Product
A BC
components
1. design up from component level2. interactions not handled well3. costs:short-term – low
long-term – high (low reliability)
System
A B
interactions
1. design down from system reqmnts2. consider interactions at each step3. costs:short-term – high
long-term – lower(high reliability)
subsystems
C
Propulsion
• Provides force needed to change satellite’s orbit.• Includes thrusters and propellant.
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
Types of Propulsion
– Chemical Propulsion• Performance is energy limited• Propellant Selection
– Electric Propulsion• Electrostatic—Ion Engine• Electrothermal—ArcJet• Electomagnetic—Rail gun
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
• 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)
Power
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
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.
Power
• The design is highly dependent on:– Space Environment (thermal, radiation)– Shadowing– Mission Life
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.
Thermal
• Passive– Coatings (control amt of heat absorbed & emitted)
• can include louvers
– Multi-layer insulation (MLI) blankets– Heat pipes (phase transition)
Thermal
• Active (use power)– Refrigerant loops– Heater coils
Communications
• Transmits data to ground or to relay satellite (e.g. TDRS)
• Receives commands from ground or relay satellite
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
Attitude Sensing and Control
• Senses and controls the orientation of the spacecraft.
Attitude Sensing
• star sensor – – The light from stars and compares it to a star
catalog.
Attitude Sensing
• sun sensor measures angle between "sun line"
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.
Attitude Control
• Thrusters -- fire thrusters (small rockets) in pairs to start rotation, then fire opposite pair to stop the rotation.
Attitude
• gyroscopes -- use electric motor in s/c
wheel
motor
satellite
Attitude Determination and Control
• Sensors– Earth sensor (0.1o to 1o)
– Sun sensor (0.005o to 3o)
– star sensors (0.0003o to 0.01o)
– 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
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
• Principal Function– Processes and distributes commands;
processes, stores, and formats data
• Other Names– Spacecraft Computer System– Spacecraft Processor
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
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.
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
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?
Delta II Rocket
Image:http://www.boeing.com/companyoffices/gallery/images/space/delta_ii/delta2_contour_08.htm
Titan IV Rocket
Image: www.spaceline.org/galleries/cpx-40-41/blowup41.jpg.html
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
Payload Operations Control Center
NASA Marshall Space Flight Center, Huntsville Alabama
Mission Control Center
NASA Johnson Spaceflight Center, Houston Texas