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