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Thermal Attitude Control System for CubeSat

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Thermal Control

on 28 March 2014

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Transcript of Thermal Attitude Control System for CubeSat

What is a CubeSat?
Considered Effects
CubeSat Design
CubeSats:
Thermal Attitude Control

Our Project
Environmental Effects
Spacecraft Anomalies
Attitude Determination
Complementary Attitude Control
Conclusion
There are timescales on which thermal control will be observable
Fin based design can function in a drag-free environment
Greater potential in GEO and interplanetary space for bigger Satellites
www.uclcubesat.com
• Neutral species
• Charged particles
• Plasmas
• Electric and magnetic fields
• Solar and galactic radiations
• Meteoroids and space debris
• Surface and internal charging
• Single Event Upset
• Total dose effects
• Solar radio frequency interference and telemetry scintillation
• Drag effect
• Debris
• Spacecraft orientation
• Photonics noise
• Materials degradation
• Meteorite impact

Atmospheric Density
Diurnal Variations
Monthly Variations
Latitudinal Variations
Longitudinal Variations
Yearly Variations
Average Density Model
Atmospheric Drag
An important part of testing a satellite, in space, is selecting an appropriate orbit, as different orbits exhibit different properties.

Though several orbits were considered, the two that stood out were the Low Earth and Geostationary Orbit.
Orbits
Geostationary Orbit
GEO
No atmospheric drag
Used for satellite TV and weather forecasting
Low Earth Orbit LEO
Relatively low payload to deploy in this orbit
Communications at this altitude perfectly feasible
Thermal Attitude Control
The main aim is to put to the test the possibility of using a attitude control system which functions by using thermal emission from the satellite surface.
Magnitudes
Torque values of 1 - 10 nNm (nano Newton Meters)
Angular accelerations of 1 - 10 µ.rad/s^2
Approx' 30s for a roataion of 2 Arc seconds (the smallest change detectable by the ADS)
Showing how the forces from thermal emission can produce torques
Time Scales
Angular acceleration must be integrated over a time to produce a rotation
Must be at least 5s - 45s
This is so change in attitude is detectable by the ADS
The longer the better, but conditions of LEO place upper
Sunlight (photon pressure)
Earth's magnetic field
Communication Systems
Costing
Wherever possible, use proven systems
SFL's CanX-2 is a good starting point
But certain aspects need improving...
3U CubeSat Budget Comparison
1U- 10cm x 10cm x 11cm
Up to 3U
Fits inside a P-Pod
Usually used for educational purposes
Design ultra-fine steering system (attitude control)
Using thermal emission control
Potential for full sized Satellites
Fin-Based Design
In LEOs a satellite is constantly colliding with gas molecules at speeds of between 7,000 - 7,800 m/s.

This causes the satellite's orbit to decrease over time

This force is proportional to the atmospheric density
What is drag?
A resistive force which is proportional to the magnitude of velocity

Analysing Drag
Drag has 2 main effects:

Orbital Decay

Creates Torques
Torques & Forces
Calculating the Force
Aerodynamic Simulations very difficult!

Instead an approximation takes a constant called the Ballistic Coefficient and uses this to calculate F
Why is it important?
Thrusters
Control Moment Gyroscope
Piezoelectric Sphere
Reaction Wheel
Transceivers
Antenna Subsystem
Current Subsystems
The Future of Antennas
The Ground Station
Telemetry and command subsystems
Transmitting and receiving signals
Options
COTS
Modified COTS
Custom-built
No radio frequency interference
Network of ground stations is ideal
Torque
There are many different mechanisms for finding a satellite's attitude.

The main types are:

Gyroscopes
Magnetometers
Sun Sensors
Star Sensors
Horizon Sensors
Sun Sensing
Horizon Sensing
Star Sensing
Gyroscopes
Magnetometers
Rate
Rate integration
Laser
Fibre optic
Piezoelectric
MEMS
Fluxgate
Magnetoresistive Sensors
SQUID
Committed sun sensors - photodiodes, etc
Coarse sun sensors - solar panels
Static Earth
Scanning Earth
University Star Trackers
- microASC - DTU - 2 arcsec
- CubeStar - SU - 26 arcsec
Star Mappers
Star Trackers
Orbit Comparison
Gravity gradients
(Compass-like behaviour)
(Pointing downwards)

Can achieve rotation of ~1 milliradians
over an interval ~10 minutes
Component Cost Breakdown
- Measures angular momentum
- Accuracies - around 0.001deg/hr but drift
1 - Measure Earth's magnetic field
2 - Compare to a magnetic model - simple dipole, IGRF, etc.
3 - Determine orientation to 1 degree
- Photodetectors register change in Sun's position
- Accuracies = 0.1 degrees
P-Pod for 3U
Conclusion
The Higher the altitude the lower the drag and smaller the torques

in LEOs in order to minimise drag, the longest axis must stay within a maximum solid angle about the velocity vector

If we want to add fins or have complete rotation freedom we must go to higher orbits to be effective
- Scan the Earth to detect horizon line
- Accuracies = around 0.1 degrees
- Image star positions and compare to star catalogues
- Expensive
- Accuracies = arcseconds
Group 4
Full transcript