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Thermal Attitude Control System for CubeSat
Transcript of Thermal Attitude Control System for CubeSat
Thermal Attitude Control
Complementary Attitude Control
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
• Neutral species
• Charged particles
• 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
• Spacecraft orientation
• Photonics noise
• Materials degradation
• Meteorite impact
Average Density Model
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.
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.
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
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
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
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
Drag has 2 main effects:
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?
Control Moment Gyroscope
The Future of Antennas
The Ground Station
Telemetry and command subsystems
Transmitting and receiving signals
No radio frequency interference
Network of ground stations is ideal
There are many different mechanisms for finding a satellite's attitude.
The main types are:
Committed sun sensors - photodiodes, etc
Coarse sun sensors - solar panels
University Star Trackers
- microASC - DTU - 2 arcsec
- CubeStar - SU - 26 arcsec
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
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
- Accuracies = arcseconds