Transcript ADCS - UniMAP Portal

Attitude Determination
and Control System
(ADCS)
What is a satellite anyway?
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Satellite is any object that orbits or revolves around another object.
Example of Communication Satellite
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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
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There will be 66 Iridium satellites in all that will provide mobile telephone
and paging services worldwide
Use of Satellite
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Our society depends on satellites for
weather information,
communications, navigation,
exploration, search and rescue,
research, and national defense.
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All satellites have two principal
subsystems:
The platform
 The payload
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Platform
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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.
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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
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Low Earth Orbit
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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
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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
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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
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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.
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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
Reference
Signal
Control
Algorithms
Sensors
Actuators
Spacecraft
dynamics
Attitude Determination
System
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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
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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
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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
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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)
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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
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Attitude Control System
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Formultion of RollPitch-Yaw for Satellite
Orientation
Attitude Control Laws
Extended Kalman Filter
 Fuzzy Control
 Neural Network Controller
 Sliding Mode Control
 Projection based Control Law.
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Space Disturbance
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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
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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
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http://www.technion.ac.il/ASRI/techsat/inorbit.html#Figure%201
Three axis stabilization
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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
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Many communications satellites are
designed to rotate about their
longitudinal axis (roll)
3.0 Introduction
Processed
attitude data
Magnetometer
OBC
ACS
Driver
Torque command
for the MT
Magnetorquer
Attitude Ref
Figure 1: Concept of operations with ADCS subsystems required to
meet the attitude modes of operations