Attitude Determination and Control System (ADCS).

Attitude Determination Attitude Determination and Control System and Control System

(ADCS)(ADCS)

What is a satellite anyway?What is a satellite anyway?

Satellite is any object that orbits or revolves around another object.Satellite is any object that orbits or revolves around another object.

Example of Communication SatelliteExample of Communication Satellite

Antennas and transceivers Antennas and transceivers send and receive send and receive radio signals to and from the Earth or another radio signals to and from the Earth or another satellite; satellite;

Rocket motors Rocket motors move the satellite in space; move the satellite in space; Fuel tanks Fuel tanks store the fuel for the rocket; store the fuel for the rocket; Solar panels Solar panels use solar cells to turn the sun's use solar cells to turn the sun's

energy into electricity; energy into electricity; Batteries Batteries store the electricity generated by the store the electricity generated by the

solar panels; and. solar panels; and. On-board processors On-board processors provide the “brain” of the provide the “brain” of the

satellite and tell the satellite to do what humans satellite and tell the satellite to do what humans want it to do. want it to do.

Iridium NetworkIridium Network There will be 66 Iridium satellites in all that will provide mobile telephone There will be 66 Iridium satellites in all that will provide mobile telephone

and paging services worldwideand paging services worldwide

Use of SatelliteUse of Satellite

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

All satellites have two principal All satellites have two principal subsystems: subsystems:

The platform The platform The payload The payload

PlatformPlatform

The platform is the basic frame of the The platform is the basic frame of the satellite and the components which allow satellite and the components which allow it to function in space, regardless of the it to function in space, regardless of the satellite's mission. satellite's mission.

Structure of the satellite Structure of the satellite Power Power Propulsion Propulsion Stabilization and Attitude Control Stabilization and Attitude Control Thermal Control Thermal Control Environmental Control Environmental Control Telemetry, Tracking and Command Telemetry, Tracking and Command

Payload Payload

Communications Communications Position/Navigation Position/Navigation Reconnaissance, Surveillance and Reconnaissance, Surveillance and

Target Acquisition (RSTA) Target Acquisition (RSTA) Weather and Environmental Weather and Environmental

Monitoring Monitoring Scientific/Experimental Scientific/Experimental Manned Manned

Low Earth OrbitLow Earth Orbit Satellites in LEO are just 200 - 500 miles (320 - 800 kilometers) high 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 Satellites in LEO speed along at 17,000 miles per hour (27,359 kilometers

per hour)! They can circle Earth in about 90 minutesper hour)! They can circle Earth in about 90 minutes

Polar OrbitPolar Orbit For this reason, satellites that monitor the global environment, like For this reason, satellites that monitor the global environment, like

remote sensing satellitesremote sensing satellites and certain and certain weather satellitesweather satellites, are almost always , are almost always

in polar orbit.in polar orbit.

Geosynchronous Equatorial OrbitGeosynchronous Equatorial Orbit

A satellite in geosychonous equatorial orbit (GEO) is located directly above 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 equator, exactly 22,300 miles out in space. At that distance, it takes

the satellite a full 24 hours to circle the planet.the satellite a full 24 hours to circle the planet.

Elliptical OrbitElliptical Orbit One part of the orbit is closest to the center of Earth (perigee) and 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 the other part is farthest away (apogee). A satellite in this orbit

takes about 12 hours to circle the planet.takes about 12 hours to circle the planet. Communications satellites in elliptical orbits cover the areas in the high Communications satellites in elliptical orbits cover the areas in the high

northern and southern hemispheres that are not covered by GEO satellitesnorthern and southern hemispheres that are not covered by GEO satellites

Satellite AnatomySatellite Anatomy

Satellite Orbits

Satellite AnatomySatellite Anatomy

ADCS can be divided into 2 subsystems i.e. Attitude ADCS can be divided into 2 subsystems i.e. Attitude Determination (ADS) and Attitude ControlDetermination (ADS) and Attitude Control (ACS) (ACS)

ADS will give information on the orientation of spacecraft through ADS will give information on the orientation of spacecraft through the sensor measurementsthe sensor measurements

ACS is to measure and control the orientation of the spacecraft ACS is to measure and control the orientation of the spacecraft and its instrument throughout the missionand its instrument throughout the mission

Attitude Determination and Control System (ADCS)

Schematic Diagram of a Schematic Diagram of a Satellite ADCSSatellite ADCS

Control Algorithms Actuators

Sensors

Reference Signal

Spacecraft dynamics

Attitude Determination Attitude Determination SystemSystem

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

Where ADS in the satellite Where ADS in the satellite systems?systems?

Block diagram for satellite subsystem

Attitude Determination Attitude Determination ComponentsComponents

The analysis of attitude The analysis of attitude determinationdetermination

The attitude sensorsThe attitude sensors The attitude determination methodThe attitude determination method The mathematical model for attitude The mathematical model for attitude

determinationdetermination

ADS SensorsADS Sensors

To determine the attitude of the satellite with To determine the attitude of the satellite with respect to a defined reference frame.respect to a defined reference frame.The attitude sensor includes :The attitude sensor includes :

(1) Earth sensor(1) Earth sensor(2) Sun sensor(2) Sun sensor(3)Star sensor(3)Star sensor(4)Rate and integrating sensor, based on (4)Rate and integrating sensor, based on

gyroscope, laser or other solid state principlesgyroscope, laser or other solid state principles(5) Horizon scanners(5) Horizon scanners(6) Magnetometer(6) Magnetometer

Sun SensorSun Sensor Sun sensors are used for Sun sensors are used for

providing a vector providing a vector measurement to the Sun measurement to the Sun

Earth SensorEarth SensorThe 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

MagnetometerMagnetometer

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 SensorsSelection of ADS Sensors Magnetometers: low accuracyMagnetometers: low accuracy Earth sensors: low cost, low risk, moderate Earth sensors: low cost, low risk, moderate

accuracyaccuracy Sun sensors: low cost, low risk, moderate Sun sensors: low cost, low risk, moderate

accuracyaccuracy Star sensors: Star sensors: high cost, high risk, high accuracyhigh cost, high risk, high accuracy Gyroscopes: Gyroscopes: high cost, high risk, high accuracyhigh cost, high risk, high accuracy Directional antennasDirectional antennas

In flight dynamics, the orientation is In flight dynamics, the orientation is often described using three angles often described using three angles

called roll, pitch and yaw.called roll, pitch and yaw.

Satellite OrientationSatellite Orientation

Orbit coordinate and body Orbit coordinate and body coordinatecoordinate

Attitude Control System Attitude Control System (ACS)(ACS)

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

Satellite ActuatorSatellite Actuator

To modify the attitude of a spacecraft To modify the attitude of a spacecraft orbiting the Earth, there is only three direct orbiting the Earth, there is only three direct ways:ways:

* Firing opposed thrusters to gain angular * Firing opposed thrusters to gain angular momentum without perturbing the orbit;momentum without perturbing the orbit;* Accelerating or decelerating a momentum * Accelerating or decelerating a momentum wheel inside the spacecraft;wheel inside the spacecraft;* Applying current to magnetic torques. * Applying current to magnetic torques.

Types of Satellite ActuatorTypes of Satellite Actuator

Momentum wheels Momentum wheels Fire thrustersFire thrusters MagnetotorquerMagnetotorquer

Attitude Control SystemAttitude Control System Formultion of Roll-Formultion of Roll-

Pitch-Yaw for Satellite Pitch-Yaw for Satellite OrientationOrientation

Attitude Control LawsAttitude Control Laws

Extended Kalman FilterExtended Kalman Filter Fuzzy ControlFuzzy Control Neural Network ControllerNeural Network Controller Sliding Mode ControlSliding Mode Control Projection based Control Law.Projection based Control Law.

Space DisturbanceSpace Disturbance

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

Stabilization and attitude Stabilization and attitude controlcontrol

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

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

Three axis stabilizationThree axis stabilization

Roll, pitch and yawRoll, pitch and yaw Satellite controllers send signals to the Satellite controllers send signals to the

satellite to fire thrusters in short spurts to satellite to fire thrusters in short spurts to control roll, pitch, yaw and to make control roll, pitch, yaw and to make corrects in orbital altitudecorrects in orbital altitude

To reduce size, mass, complexity and cost To reduce size, mass, complexity and cost some small satellites are designed to some small satellites are designed to tumble freely through space without any tumble freely through space without any stabilization or attitude control. stabilization or attitude control.

Two axis stabilizationTwo axis stabilization

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

3.0 Introduction3.0 Introduction

Figure 1: Concept of operations with ADCS subsystems required to meet the attitude modes of operations

Magnetometer

Magnetorquer

Driver

OBC ACS

Torque command for the MT

Processed attitude data

Attitude Ref