Attitude Control

Question 1

What is meant by the “attitude” of a satellite and diagram

Answer 1

The attitude of a satellite is the orientation of the satellite in space about its centre of mass

Question 2

Describe main Attitude Control functions

Answer 2

Attitude steering involves a constant bias or law of progression about one or more axes can be needed to fulfil mission or orbit requirements

Attitude stabilization is the process of maintaining a spacecraft’s attitude at a desired orientation. This is typically done with thrusters, reaction wheels, or magnetorquers.

Question 3

When the steering function of the Attitude Control is essential?

Answer 3

performing a manoeuvre that requires precise control of its orientation and attitude, such as a rendezvous or docking manoeuvre.

The Attitude Control system allows the spacecraft to maintain a predetermined attitude and orientation in space, which is important for the successful completion of such manoeuvres.

Question 4

Describe methods of measuring the attitude of a satellite

Answer 4

The current attitude can be measured with respect to external objects and using internal inertial
sensors

Reference sensors:
1. Sun sensors; 2. Earth sensors; 3. Star sensors; 4. Radio-frequency sensors; 5. GPS; 6. Laser
detectors; 7. Magnetometer

Inertial sensors:
1. Orbital gyrocompass; 2. Laser ring gyroscopes (LRG); 3. Fiber Optic Gyroscope (FOG)
The inertial sensors are subject to drifts and bias errors while mechanical devices have a
limited lifetime (hence no good for conventional missions of 10 years)

Question 5

. Describe nature of the disturbing torques acting on a satellite

Answer 5

The external disturbance torques are:

1. Solar radiation pressure;
2. Satellite antenna radiation: very small torque
3. Gravitational forces;
4. Atmosphere drag (for LEO h<500km).
5. Terrestrial magnetic field interacting with satellite’s electric loops;
6. Lack of alignment of the thrust of the orbit control actuator with respect to the position of the centre of mass
7. Impact by a meteoroid or debris: short time high torque + other damage possible

Question 6

Explain principal axes of a body.

Answer 6

moments of inertia depend on the choice of the reference frame. An orientation always can be found in which 𝐼𝑥y,𝐼𝑦z,𝐼𝑧x are all 0.

Axes of the reference frame are called principal axes.

if the body is symmetric, the principal axes coincide with the axes of symmetry

Question 7

Explain the principle of three axis stabilisation of satellite attitude

Answer 7

Three-axis stabilization is a process used to keep a satellite’s attitude, or orientation, stable while in orbit. The three axes of a satellite are the pitch, yaw, and roll axes.

The satellite’s attitude is kept stable by using three separate attitude control systems, which use thrusters or reaction wheels to keep the satellite pointing in the right direction, with each one controlling a different axis.

Three-axis stabilization helps to ensure that a satellite remains pointed in the right direction, which is necessary for its mission.

Question 8

Explain how Gyroscopic stabilisation is used to limit the motion of a satellite about the pitch, roll and yaw axes. What is the direction of the correcting torque for roll and yaw control?

Answer 8

Gyroscopes measure angular velocity and provide a restoring torque when a satellite deviates from its desired attitude. This restoring torque opposes the undesired motion, thereby stabilizing the satellite.

For roll and yaw control, the direction of the correcting torque is always perpendicular to the axis of rotation. For example, in roll control, the correcting torque is applied in the yaw direction, while in yaw control, the correcting torque is applied in the roll direction.

Question 9

Describe method of active attitude control and its advantages and disadvantages

Answer 9

requires:
1 - sensors to measure vehicle attitude
2 - actuators to apply the torques needed to re-orient vehicle
3 - algorithms to command actuators
method:
1 - measurement of attitude
2 - correction of the attitude

advantages:
ability to orient quickly and precisely
allows for greater flexibility in attitude control
disadvantages:
expensive to build and maintain parts
large amount of fuel or power
can be difficult to use for large attitude changes

Question 10

Describe method of passive attitude control and its advantages and disadvantages

Answer 10

Passive attitude control can involve:
(i) gravity (a elongated satellite tends to orient toward the earth by its longest axis);
(ii) satellite spinning;
(iii) gyroscope;
(ii) and (iii): enhanced resistance to the disturbing torque.

Advantage: no propellant consumption, no complicated electronic equipment is needed.

Disadvantage: accuracy obtained is small (at best, a few degrees); the response time is large.

Question 11

Describe Angular Momentum Devices as Attitude Control Actuators.

Answer 11

Angular Momentum Devices:

1. Reaction wheel (not spinning) device which aim a spacecraft in different directions without firing rockets or jets. useful when the spacecraft must be rotated by very small amounts,
2. Momentum wheels are a different type of actuator, mainly used for gyroscopic stabilization of spacecraft: momentum wheels have high rotation speed and mass
3. A control momentum gyroscope (CMG) is used in spacecraft attitude control systems. A CMG consists of a spinning rotor and one or more motorized gimbals that tilt the rotor’s angular momentum. As the rotor tilts, the changing angular momentum causes a gyroscopic torque that rotates the spacecraft.

Question 12

A satellite with a wheel spinning about the pitch direction is supplied with six pairs of thrusters. indicate the spinning wheel uploading thrusters and also correcting thrusters for the roll and the yaw

Answer 12

Spinning uploading thrusters: 1-1’ and 2-2’.
Roll correcting thrusters: 3-3’ and 5-5’.
Yaw correcting thrusters: 4-4’ and 6-6’.