How Does the ACS Work

Another aspect that influences the ACS design is how the ACS operates to achieve these functions. Figure 8.1 shows a typical ACS operation, and introduces the main hardware components that comprise the ACS. Starting at the top of the figure, torques act on the spacecraft and cause it to rotate. There are two types of torque. First, there are those we apply deliberately to control the spacecraft's rotation using the on-board ACS hardware. These are produced by the control torquers that we mentioned above. Second, there are disturbance torques, which are the rotational equivalent of orbit perturbations. As we saw earlier, the orbit motion of the spacecraft was altered by perturbing forces (see Chapter 3). The spacecraft is also disturbed in terms of its rotation by naturally occurring torques, produced by the spacecraft's interaction with its environment. For example, Figure 8.2 shows how a disturbance torque is generated by a spacecraft's interaction with the atmosphere. As we saw in Chapter 3, atmospheric drag causes a decrease in orbit height, but it can also produce a disturbance torque that causes an unwanted rotation in the spacecraft, which needs to be corrected by the ACS.

The torques act on the spacecraft and cause its attitude to change, that is, cause it to rotate. Moving around Figure 8.1, we see that this rotation is detected and measured by attitude sensors, which are hardware components of the ACS, usually optical sensors looking out at reference objects, such as the Sun or stars. To picture how these work, imagine being inside a box with windows, which is good description of an airplane. Imagine sitting in a

Disturbance torques

Control torquers

Ground control

Figure 8.1: A block diagram showing how the ACS operates.

Disturbance torques

Control torquers

Torque demands

Torque demands

On-board computer

Ground control

Figure 8.1: A block diagram showing how the ACS operates.

window seat on a night flight. If the cabin lights are down, the stars can be seen easily, and while the airplane is not turning (rotating), the stars look as if they are stationary in the window. However, as soon as the aircraft starts to turn, they appear to move across the window. In the same way, the sensors look out of the spacecraft and interpret movement of reference objects, such as the stars, as a rotation of the vehicle—so allowing measurement of the rotation.

The sensor measurements of the spacecraft's rotation are passed to the on-board computer, which itself can be considered to be another piece of ACS hardware. The measurements are processed by control software, which is essentially fancy mathematics coded into the computer to calculate the spacecraft's attitude. This estimate of the attitude is then compared with the attitude required to achieve the pointing mission, and if they differ, the computer's control software calculates what torques are required to correct the spacecraft's attitude. These torque demands are then passed to the control torquers (continuing our walk around Fig. 8.1), which then apply the required torques to bring the spacecraft's attitude back to where it should be.

For example, the pointing mission for the spacecraft may be to point the antenna of a communications satellite to a ground station to facilitate intercontinental telephone calls. If the satellite is disturbed, the pointing of

Rotation resulting from disturbance torque

Spacecraft

Spacecraft

Spacecraft motion

'Moment arm'

Figure 8.2: An example of a disturbance torque—in this case that produced by aerodynamic drag.

"71 Spacecraft

Spacecraft motion

'Moment arm'

Drag force on solar panel

Figure 8.2: An example of a disturbance torque—in this case that produced by aerodynamic drag.

the antenna to the ground station will be affected, but this disturbance will be detected by the ACS sensors. The computer will then process the sensor measurements to command the on-board torquers to correct the pointing error, thus maintaining the communications link. One thing to note about the ACS operation shown in Figure 8.1 is that it operates in a loop, and on most spacecraft it does this automatically many times a second, so that the pointing mission is continuously monitored and maintained. This loopingtype operation is referred to as a feedback loop by the ACS engineer.

At the bottom of Figure 8.1 there is a ground intervention in this automated process. For example, to operate a space telescope, the astronomers on the ground need to command the telescope to point at a particular galaxy, for example. They can do this by typing the position of the object of interest into a computer on the ground, and this information is then up-linked to the spacecraft and processed by the on-board computer to produce torque demands, which are then executed by the control torquers to rotate the spacecraft to direct the telescope to the required segment of sky.

The main functions of the ACS help other subsystems in their operation (e.g., pointing a solar panel to the Sun to help the power subsystem to do its job). Similarly, if we look at the typical ACS operation outlined in Figure 8.1, we can see that other subsystems help the ACS do its job (e.g., sensors, computers, and control torques need electricity from the power subsystem to work). So, in the design process, the ACS engineer has to work together with many other subsystem engineers, resulting in a very interactive design process.

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