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Space Systems > ACDS Technology > Flashcards

ACDS Technology Flashcards

1
Q

What are the three Euler angles used to express orientation wrt some external coordinate system?

A

Roll, pitch and yaw

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2
Q

What 5 sensors are used to determine a spacecraft’s attitude?

A

Sun Sensors
Earth Sensors
Star Sensors
Magnetometers
Inertial Sensors

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3
Q

What are the common reference objects used for attitude determination?

A

The Sun, Earth’s horizon, stars, and the geomagnetic field

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4
Q

How many independent fixes are typically required to determine a spacecraft’s orientation?

A

Two independent fixes.

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5
Q

What are the main coordinate frames used in spacecraft attitude determination?

A

The geocentric inertial reference frame and the spacecraft body frame.

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6
Q

What is the geocentric inertial reference frame?

A

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.

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7
Q

What is the spacecraft body frame?

A

A coordinate system fixed to the spacecraft’s center of mass, used to measure orientation relative to other reference frames.

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8
Q

What is a Kalman filter used for?

A

Identifying the correct overlap point between two independent cones generated by two independent sensors, such as a sun and earth sensor

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9
Q

What are Sun sensors used for in ADCS?

A

To measure the Sun’s direction, providing attitude information with high accuracy in non-eclipse conditions.

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10
Q

What is the accuracy of Sun sensors?

A

Approximately 1 minute of arc (1/60th of a degree).

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11
Q

What is a digital Sun sensor and how does it differ from an analogue one?

A

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.

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12
Q

What is the principle of operation of Earth horizon sensors?

A

Detecting Earth’s IR emissions, particularly from atmospheric CO2, to determine the horizon even in eclipse conditions.

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13
Q

What are the types of Earth horizon sensors?

A

Static and scanning Earth horizon sensors.

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14
Q

How do static Earth horizon sensors work?

A

They use fixed optics, such as germanium lenses, to measure the IR emissions from Earth’s horizon.

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15
Q

What role do star sensors play in attitude determination?

A

They provide the most accurate orientation data by observing and identifying star fields or individual stars.

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16
Q

What are the three types of star sensors?

A

Star scanners, star trackers, and star mappers.

17
Q

What is the main limitation of star sensors?

A

They must be shielded from sunlight using optical baffles.

18
Q

What is a magnetometer used for in ADCS?

A

To measure the Earth’s magnetic field vector for attitude determination, particularly in Low Earth Orbit (LEO).

19
Q

What is the principle of a flux-gate magnetometer

A

Measuring asymmetries in induced currents caused by an external magnetic field.

20
Q

What is a limitation of a sun sensor?

A

It cannot be used in eclipse

21
Q

With reference to earth, what are the roll, pitch and yaw coordinates of a satellite?

A
  • 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

22
Q

What are inertial sensors and what are they used for?

A

Devices like gyroscopes that provide attitude references by maintaining orientation relative to inertial space.

23
Q

What are the challenges associated with gyroscopes in space?

A

Issues like drift, friction, and noise, especially in mechanical and MEMS gyros.

24
Q

How do laser ring gyroscopes differ from mechanical ones?

A

They use optical paths to measure rotation, eliminating mechanical wear and increasing precision.

25
Q

Why is the Gray-code used in binary masks for digital Sun sensors

A

To minimize errors when interpreting angular data.

26
Q

What is the dependency of magnetometers on orbit altitude?

A

They are most effective in LEO due to stronger magnetic field strength, which diminishes with altitude.

27
Q

What is the dependency of Earth horizon sensors on spacecraft altitude?

A

Their field of view changes; for LEO, it’s 120-160°, while for GEO, it’s around 17.5°.

28
Q

What is the function of a state estimation filter like a Kalman filter in ADCS

A

To rule out false attitude solutions by combining sensor data with prior knowledge.

29
Q

What accuracy can star cameras typically achieve?

A

Arc-second precision (1/3600th of a degree).

30
Q

How do scanning Earth horizon sensors determine pitch and roll

A

By analyzing the scan cone angle, Earth’s subtended angle, and the timing of IR signals.

31
Q

Why are multiple attitude sensors often combined in ADCS?

A

To provide redundancy and improve accuracy by leveraging complementary data from different sensors.

32
Q

What is the primary use of MEMS gyroscopes in ADCS?

A

Providing lightweight, compact solutions for short-term attitude reference.

33
Q

How does a 4-quadrant sun sensor determine the solar elevation and solar azimuth?

A

theta = func(A/(A+B)) or func(C/(C+D))

Phi = func(C/(A+C)) or func(D/(B+D))

  • At boresight the four photodiodes are equally illuminated
  • as the sun moves in azimuth or elevation, some diodes are more illuminated than others and a corresponding photocurrent is generated by these diodes
  • the output voltage of the photodiodes is then connected to the angles and can be solved for
34
Q

What are the most appropriate technology types for earth horizon sensors?

A

Since they must be sensitive to 15 um IR radiation, the best choices are pyroelectric or thermocouple/thermistor types.

35
Q

What is a Gray-coded binary mask, and why is it important in Sun sensors?

A

It is a coding system where only one bit changes at a time for consecutive angular positions, reducing the risk of misinterpretation due to noise or transition errors. This ensures reliable angular measurements in digital Sun sensors.

36
Q

What is are the two main differences between the analogue and digital sun sensors?

A
  1. Digital sun sensor has an entry slit meaning there is a line that is detected
  2. the photocurrent data can then be read off using a gray-coded binary mask
  • This gives one angle, if we have two sensors orthogonal we can get both angles
37
Q

What is the FoV of a digital sun sensor?

What about when it is 2 back to back 2d ccd bases sun sensors? What must be added for the ccds to be able to detect the sun?

A

+/- 60 degrees (120 total)

180 degrees

Space Systems (22 decks)

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