Similar questions

1.Short note on 3-Axis stabilization

2.Explain spin stabilization and 3–Axis stabilisation methods. Mention their merits and demerits.

Marks: 5 M, 10 M

Year: May 2012, Dec 2012, May 2014, Dec 2014

When the satellite is placed into stable orbit then the attitude of the satellite is set by firing the jet thrusters present in the satellite. Therefore, once the initial attitude of the satellite is set it must be maintained is this position. This is achieved by stabilization technique.

There are two different stabilisation techniques:

(1) The spin stabilisation: For cylindrical satellites

• Cylindrical satellites may be made to spin on their axis. Once the satellite is in proper orbit a jet thrusters is fired to begin spinning the satellite.
• A typical spin stabilised satellite rotates between 30 and 120 rpm creating an inertial stiffness, which maintains the satellite spin axis perpendicular to the equatorial plane.
• The spinning causes a gyroscopic or flywheel effect that keeps the satellite spin action is north south direction.
• The spin stabilisation has the disadvantage that it requires the use of a de-spun antenna, so that the antennas mounted are constantly directed towards the earth.

For non-cylindrical satellites:

• In this type of stabilisation a large flywheel is included at some point on the satellite body.
• Once the satellite is in the position and its antenna, solar panels and sensors are oriented, the flywheel is put into motion. Again the gyroscopic effect keeps the satellite oriented with proper attitude.

(2) The three-axis body stabilisation

• The three axis stabilisation as the name suggest use three axis called as pitch, roll and yaw to achieve attitude control.
• Yaw axis: Directed towards the earth’s centre.
• Pitch axis: Normal to the orbital plane.
• Roll axis: Tangent to the orbit.
• In this technique, three heavy flywheels one for each axis is put into motion to provide gyroscopic effect to stabilize satellite.
• Any of the axes is corrected by firing thrusters in proper direction controlling the flywheel motor speed.
• This technique is accurate and therefore used where pinpoint accuracy of antenna pointing is needed.
• This technique uses sensors to observe external reference points like sun remote stars etc. These optical sensors operate on an electronic control system that determines when attitude control is needed, which then fires thrusters for the same purpose when needed.

Another method for achieving three-axis stabilization is to use electrically-powered reaction wheels also called momentum wheels. Massive wheels are mounted in three orthogonal axes aboard the spacecraft.

They provide a means to trade angular momentum back and forth between spacecraft and wheels. To rotate the vehicle in one direction, you spin up the proper wheel in the opposite direction. To rotate the vehicle back, you slow down the wheel.

Excess momentum that builds up in the system due to external torques, caused for example by solar photon pressure or gravity gradient, must be occasionally removed from the system by applying torque to the spacecraft, and allowing the wheels to acquire a desired speed under computer control.

This is done during maneuvers called momentum desaturation, (desat), or momentum unloads maneuvers. Many spacecraft use a system of thrusters to apply the torque for desats.

Advantages / Disadvantages

There are advantages and disadvantages to both spin stabilization and three-axis stabilization.

• Spin-stabilized craft provide a continuous sweeping motion that is desirable for fields and particles instruments, as well as some optical scanning instruments, but they may require complicated systems to de-spin antennas or optical instruments that must be pointed at targets for science observations or communications with Earth.
• Three-axis controlled craft can point optical instruments and antennas without having to de-spin them, but they may have to carry out special rotating maneuvers to best utilize their fields and particle instruments. If thrusters are used for routine stabilization, optical observations such as imaging must be designed knowing that the spacecraft is always slowly rocking back and forth, and not always exactly predictably.
• Reaction wheels provide a much steadier spacecraft from which to make observations, but they add mass to the spacecraft, they have a limited mechanical lifetime, and they require frequent momentum desaturation maneuvers, which can perturb navigation solutions because of accelerations imparted by their use of thrusters.
• No matter what choices have been made—spin or three-axis stabilization, and what sort of control force for 3- axis stabilizaton: thrusters or reaction wheels; or any combinations—the task of attitude and articulation control falls to an AACS computer running highly evolved, sophisticated software.