Attitude Determination and Control System (ADCS)

What is a satellite anyway? Satellite is any object that orbits or revolves around another object.

Example of Communication Satellite Antennas and transceivers send and receive radio signals to and from the Earth or another satellite; Rocket motors move the satellite in space; Fuel tanks store the fuel for the rocket; Solar panels use solar cells to turn the sun's energy into electricity; Batteries store the electricity generated by the solar panels; and. On-board processors provide the “brain” of the satellite and tell the satellite to do what humans want it to do.

Iridium Network There will be 66 Iridium satellites in all that will provide mobile telephone and paging services worldwide

Use of Satellite Our society depends on satellites for weather information, communications, navigation, exploration, search and rescue, research, and national defense.

All satellites have two principal subsystems: The platform The payload

Platform The platform is the basic frame of the satellite and the components which allow it to function in space, regardless of the satellite's mission. Structure of the satellite Power Propulsion Stabilization and Attitude Control Thermal Control Environmental Control Telemetry, Tracking and Command

Payload Communications Position/Navigation Reconnaissance, Surveillance and Target Acquisition (RSTA) Weather and Environmental Monitoring Scientific/Experimental Manned

Low Earth Orbit Satellites in LEO are just 200 - 500 miles (320 - 800 kilometers) high Satellites in LEO speed along at 17,000 miles per hour (27,359 kilometers per hour)! They can circle Earth in about 90 minutes

Polar Orbit For this reason, satellites that monitor the global environment, like remote sensing satellites and certain weather satellites, are almost always in polar orbit.

Geosynchronous Equatorial Orbit A satellite in geosychonous equatorial orbit (GEO) is located directly above the equator, exactly 22,300 miles out in space. At that distance, it takes the satellite a full 24 hours to circle the planet.

Elliptical Orbit One part of the orbit is closest to the center of Earth (perigee) and the other part is farthest away (apogee). A satellite in this orbit takes about 12 hours to circle the planet. Communications satellites in elliptical orbits cover the areas in the high northern and southern hemispheres that are not covered by GEO satellites

Satellite Anatomy

Satellite Orbits

Satellite Anatomy

Attitude Determination and Control System (ADCS) ADCS can be divided into 2 subsystems i.e. Attitude Determination (ADS) and Attitude Control (ACS) ADS will give information on the orientation of spacecraft through the sensor measurements ACS is to measure and control the orientation of the spacecraft and its instrument throughout the mission

Schematic Diagram of a Satellite ADCS Control Algorithms Actuators Sensors Reference Signal Spacecraft dynamics

Attitude Determination System Attitude Determination is one of the most important subsystems on-board a satellite. This component determinates the satellite’s orientation relatively to the Earth, Sun or other object.

Where ADS in the satellite systems? Block diagram for satellite subsystem

Attitude Determination Components The analysis of attitude determination The attitude sensors The attitude determination method The mathematical model for attitude determination

ADS Sensors To determine the attitude of the satellite with respect to a defined reference frame. The attitude sensor includes : (1) Earth sensor (2) Sun sensor (3)Star sensor (4)Rate and integrating sensor, based on gyroscope, laser or other solid state principles (5) Horizon scanners (6) Magnetometer

Sun Sensor Sun sensors are used for providing a vector measurement to the Sun

Earth Sensor The earth sensor is composed of four thermopile-optical assemblies viewing four fields of view through a single 13.5mm cut-on optical filter/window Each earth sensor measures the attitude of the spacecraft relative to its axis

Magnetometer Used mainly for Low Earth Orbit, where the magnetic field of the Earth is well defined and strong a 3-axis magnetometer will provide valuable attitude information.

On Board Magnetometer Sensor

Selection of ADS Sensors Magnetometers: low accuracy Earth sensors: low cost, low risk, moderate accuracy Sun sensors: low cost, low risk, moderate accuracy Star sensors: high cost, high risk, high accuracy Gyroscopes: high cost, high risk, high accuracy Directional antennas

Satellite Orientation In flight dynamics, the orientation is often described using three angles called roll, pitch and yaw.

Orbit coordinate and body coordinate

Attitude Control System (ACS) Keeps the satellite pointed towards the desired location on the Earth. it is very susceptible to external forces of the Sun and Earth that will cause the satellite to move. There are two ways to control a satellite's attitude. First,

Satellite Actuator To modify the attitude of a spacecraft orbiting the Earth, there is only three direct ways: * Firing opposed thrusters to gain angular momentum without perturbing the orbit; * Accelerating or decelerating a momentum wheel inside the spacecraft; * Applying current to magnetic torques.

Types of Satellite Actuator Momentum wheels Fire thrusters Magnetotorquer

Attitude Control System Formultion of Roll-Pitch-Yaw for Satellite Orientation

Attitude Control Laws Extended Kalman Filter Fuzzy Control Neural Network Controller Sliding Mode Control Projection based Control Law.

Space Disturbance Satellites are subjected to a number of forces in space such as particles streaming from the Sun, meteorites, atmospheric drag, gravity from the Moon, gravity gradients and other perturbations.

Stabilization and attitude control The satellite can be spun up or down (usually between 30 and 300 rotations per minute, or “rpm”) around its axis which provides stability and keeps the satellite pointing in the right direction. These satellites are cylindrical in shape and often referred to as “spinners.”

Attitude history of July 2004 http://www.technion.ac.il/ASRI/techsat/inorbit.html#Figure%201

Three axis stabilization Roll, pitch and yaw Satellite controllers send signals to the satellite to fire thrusters in short spurts to control roll, pitch, yaw and to make corrects in orbital altitude To reduce size, mass, complexity and cost some small satellites are designed to tumble freely through space without any stabilization or attitude control.

Two axis stabilization Many communications satellites are designed to rotate about their longitudinal axis (roll)

3.0 Introduction Magnetometer Magnetorquer Driver OBC ACS Torque command for the MT Processed attitude data Attitude Ref Figure 1: Concept of operations with ADCS subsystems required to meet the attitude modes of operations