ACDS Technology Flashcards
What are the three Euler angles used to express orientation wrt some external coordinate system?
Roll, pitch and yaw
What 5 sensors are used to determine a spacecraft’s attitude?
Sun Sensors
Earth Sensors
Star Sensors
Magnetometers
Inertial Sensors
What are the common reference objects used for attitude determination?
The Sun, Earth’s horizon, stars, and the geomagnetic field
How many independent fixes are typically required to determine a spacecraft’s orientation?
Two independent fixes.
What are the main coordinate frames used in spacecraft attitude determination?
The geocentric inertial reference frame and the spacecraft body frame.
What is the geocentric inertial reference frame?
A frame with its origin at Earth’s center, ZI aligned with the geometric North Pole, XI towards the Vernal Equinox, and YI completing a right-hand system.
What is the spacecraft body frame?
A coordinate system fixed to the spacecraft’s center of mass, used to measure orientation relative to other reference frames.
What is a Kalman filter used for?
Identifying the correct overlap point between two independent cones generated by two independent sensors, such as a sun and earth sensor
What are Sun sensors used for in ADCS?
To measure the Sun’s direction, providing attitude information with high accuracy in non-eclipse conditions.
What is the accuracy of Sun sensors?
Approximately 1 minute of arc (1/60th of a degree).
What is a digital Sun sensor and how does it differ from an analogue one?
A digital Sun sensor uses Gray-coded binary masks or CCDs to determine Sun angles with a larger field of view and often greater flexibility.
What is the principle of operation of Earth horizon sensors?
Detecting Earth’s IR emissions, particularly from atmospheric CO2, to determine the horizon even in eclipse conditions.
What are the types of Earth horizon sensors?
Static and scanning Earth horizon sensors.
How do static Earth horizon sensors work?
They use fixed optics, such as germanium lenses, to measure the IR emissions from Earth’s horizon.
What role do star sensors play in attitude determination?
They provide the most accurate orientation data by observing and identifying star fields or individual stars.
What are the three types of star sensors?
Star scanners, star trackers, and star mappers.
What is the main limitation of star sensors?
They must be shielded from sunlight using optical baffles.
What is a magnetometer used for in ADCS?
To measure the Earth’s magnetic field vector for attitude determination, particularly in Low Earth Orbit (LEO).
What is the principle of a flux-gate magnetometer
Measuring asymmetries in induced currents caused by an external magnetic field.
What is a limitation of a sun sensor?
It cannot be used in eclipse
With reference to earth, what are the roll, pitch and yaw coordinates of a satellite?
- Roll is tangential to the orbit
- Yaw is perpendicular to roll and points towards earth
- Pitch is then orthogonal to the other two.
For each vector pointing away from the satellite rotation is positive going clockwise around the vector
What are inertial sensors and what are they used for?
Devices like gyroscopes that provide attitude references by maintaining orientation relative to inertial space.
What are the challenges associated with gyroscopes in space?
Issues like drift, friction, and noise, especially in mechanical and MEMS gyros.
How do laser ring gyroscopes differ from mechanical ones?
They use optical paths to measure rotation, eliminating mechanical wear and increasing precision.
