aocs

1. The entire satellite including its antenna(s) spins • This means that the satellite's attitude is very stable • Spinning can be achieved using rods with coils of wire around them

2. An advantage of spin stabilized is it requires very little power. • A disadvantage is that the solar panels cannot all collect power all the time because they will spin, so they don't face the sun once every rotation. • Also, the instruments can only take measurements in one direction once every rotation

3. Done using an small thruster jets or rockets • Because its spin is so stable, a sensor to tell the satellite when its ATTITUDE (direction) is changing • The satellite can then correct the problem using thrusters • Thrusters usually contain compressed gas that when sent out of the end of the thruster will move the satellite in space

4. Thrusters, however, are heavy and use up lots of power • The advantage is that the satellite as a whole can point stably in one direction; • for example, the solar panels can always point at the sun, and the instruments can always point at their target

5. Attitude and Orbit Control Subsystem • The attitude and orbit control subsystem (AOCS) provides attitude information and maintains the required spacecraft attitude during all phases of the mission. • starting at spacecraft separation from the launch vehicle and throughout its operational lifetime. • The subsystem consists of • redundant microprocessor-based control electronics, • sun and earth sensors, • gyros, • momentum wheels (MWs), a reaction wheel (RW), magnetic torquers, thrusters, and solar array.

6. Attitude and Orbit Control Electronics The attitude and orbit control electronics (AOCE) contain electronic circuitry and software to collect attitude data from a variety of sensors and to control the attitude of the spacecraft. The AOCE includes a microprocessor that performs attitude data processing and control algorithm calculations to close the loop between the sensors and the actuators. With the exception of the solar array drive electronics, the AOCE receives all commands to the subsystem as derived from the command unit, processes them, and coordinates related hardware functions. The various AOCE control modes are selected by ground command. Most telemetry signals from the AOCS are formatted in the AOCE.

7. The interface electronics portion of the AOCE provides appropriate gates and clocks analog-to-digital and digital-to-analog conversion, magnetic torquer control thruster control Sensors interfaced to the AOCS include coarse analog sun sensors (CASSs), digital sun sensors (DSSs), earth sensors (ESs). The sensors provide attitude data in the form of absolute attitude, attitude error, and rates for processing by the AOCE. These data are also formatted and telemetered to the ground to be used for mission operation checks of attitude determination.

11. The Attitude and Orbit Control System (AOCS) supports the functions of Earth acquisition after launch, three axis body stabilization as well as orbit maintenance throughout the mission life. The AOCS is configured with a Microprocessor based control electronics with hot redundancy. The control electronics receives the attitude error measurements from sun sensors, earth sensors , star trackers, magnetometers, gyroscopes and drives the actuators – reaction wheels, magnetic torques and RCS thrusters to minimize the attitude errors. There are several special logics like auto-acquisition sequence, safe mode, auto-reconfiguration of reaction wheels in case of a single wheel failure etc.,.

13. The Attitude and Orbit Control Electronics uses inputs from the spacecraft receivers, decoders, Sun Sensors, and ground commands to control the spacecraft spin rate, attitude, and manoeuvres. It has four main modes of operation: spin control, Solar Aspect angle control, Earth pointing attitude control using 'conscan', and thruster control by telecommand. TTC Subsystem and uses them to determine the pointing error between the Earth direction and the High Gain Antenna (HGA) boresight, and to determine the direction of this offpointing. This information is then used to generate thruster pulses to move the spacecraft spin axis in the correct direction to keep the spacecraft Earth-pointing.

14. AOCS has the ability to control the spin period under circumstances when it could vary The Attitude and Orbit Control Electronics uses inputs from the spacecraft receivers, decoders, Sun Sensors, and ground commands to control the spacecraft spin rate, attitude, and manoeuvres. It has four main modes of operation: spin control, Solar Aspect angle control, Earth pointing attitude control using 'conscan', and thruster control by telecommand. The Attitude Measurement Electronics processes raw Sun pulses from the Sun Sensors, and provides most of the power switching for the AOCS. This unit also controls power to the valves in the Reaction Control Subsystem and to the heaters used to heat the thrusters before firing.

15. Telemetry, Tracking, and Command Subsystem The transponders provide the sole source of communication between the spacecraft and the Earth. Each transponder consists of a Receiver which receives commands and ranging signals, a modulator, and both an X-band and S-band transmitter which produce the radio frequencies needed for transmission or telemetry and ranging signals to the Earth. Receivers The spacecraft receivers provide acquisition and tracking of the uplink signal, demodulation of telecommands and ranging signals, provide a reference frequency to the transmitters to allow coherent operation, and provide a Conscan signal derived from the coherent Automatic Gain Control (AGC), to the spacecraft Attitude and Orbit Control Subsystem (AOCS).