Archive for the ‘GEO Subsystems’ Category

2-Axis GEO Satellites

This type of satellite is spin stabilized and gyroscopically stable.  Each satellite manufacturer has its own configuration so this is a basic description of the concept of the 2-Axis stabilized GEO Satellite.  In this application the body of the satellite is spun at a given rate providing gyroscopic stiffness in the Roll (X) and Yaw (Z) Axis.  The communications system is mounted on a platform that is counter spun (de-spun) to maintain communications.  Spin rate control is used to maintain the Pitch (Y) Axis by applying positive and negative torques to the de-spun motor.  The Antenna Control System hardware is used to adjust the position of the communications antenna (reflector) to account for variations in the Roll X-Axis due to the orbit perturbations.  These satellites require 3 types of maneuvers to maintain their orbit. Inclination maneuvers are scheduled to maintain the alignment of the orbit plain with the equator minimizing North/South drift. Delta Velocity (Delta-V)  Maneuvers keep the orbit circular to minimize East/West drift.  Finely Attitude Adjustment Maneuvers to align the spin axis to 90 degrees from the orbit plain minimizing Roll and Yaw errors. The frequency of each type of maneuver is determined by the orbital slot and the pointing requirements the satellite is being operated by.  Typically the frequency ranges from 1 to 3 weeks.

Attitude Control System (ACS) 3-Axis GEO Satellites

Each satellite manufacturer has its own configuration so this is a basic description of the concept of the Attitude Control System on GEO Satellites.  The ACS hardware can be broken down to 2 basic categories, sensors and actuators.  Sensors collect measurements and information in reference to the spacecraft’s axises. This information is converted into error signals used to control each axis.  Sensors include but are not limited to, Earth Sensors, Sun Sensors and Gyros. Actuators allow control of movement in the axis. Typical hardware used as actuators includes but is not limited to,  Momentum Wheel Assemblies, Reaction Wheel Assemblies, Magnetic Torque  Rods and Reaction Control Thrusters. Antenna tracking systems and Solar Array systems have also been used as actuators, but are not part of the basic set of actuators normally used.  Although they are predominately used in 2-Axis GEO Satellites.

A third category, being the ACS software is normally referred to when talking about Attitude Determination and Control System (ADCS). The ACS software will collect the error signals from the sensors and convert them into control signals to be applied to the actuators to maintain control of each axis.

Satellite 3-Axis control

With GEO Satellite control it is important to know the definition of the 3-Axis being controlled. Typically the Axis is referenced from the CM (Center of Mass) or CG (Center of Gravity) of the satellite  to the orbit plain.  Normally the X-Axis points East and West. The Y-Axis points North and South. And the  Z-Axis points at the Earth and away from the Earth. Sensors are located on the spacecraft and aligned to produce positive and negative error signals in each axis for use by the Attitude Control System (ACS). This drawing depicts a typical satellite’s X,Y and Z axis.

Movement around the Z-Axis is Yaw

In 3-Axis satellite control movement around the Z-Axis is called Yaw. Errors in this Axis will shift the pointing of the communications transmit pattern of the satellite in what appears to be a clockwise/counterclockwise movement on the earth. Through the normal progression of an orbit Yaw will translate to Roll in a quarter of the orbit.  Both errors can be managed by active control and typically Roll control is chosen to be controlled if only one of the two Axis is to be controlled. The diagram and animation show a visual representation of this movement.

Movement around the Y-Axis is Pitch

In 3-Axis satellite control movement around the Y-Axis is called Pitch.  Errors in this Axis will shift the pointing of the communications transmit pattern of the satellite in what appears to be an east/west movement on the earth. The diagram and animation show a visual representation of this movement.

Movement around the X-Axis is Roll

In 3-Axis satellite control movement around the X-Axis is called Roll.  Errors in this Axis will shift the pointing of the communications transmit pattern of the satellite in what appears to be a north/south movement on the earth.  Through the normal progression of an orbit Roll will translate to Yaw in a quarter of the orbit.  Both errors can be managed by active control and typically Roll control is chosen to be controlled if only one of the two Axis is to be controlled.  The diagram and animation show a visual representation of this movement.

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INFORMATION

Shining light on satellites and how they operate. Drawing from over 30 years of knowledge and experience in all phases of the life of a satellite from concept, to operations, and through end of life. You will find short topics intended to give you an understanding of how they work, the general concepts, and principals used along with information on ground systems. There is also a section dedicated to topics that can be used as basic concept training along with links to animations and 3D models I have created.