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Satellite Attitude and Orbit Control System or AOCS in a Nutshell

Updated: Jun 9, 2023


Satellite

Do you wonder how satellites stay on their course? Well, the answer lies within something known as the "Attitude and Orbit Control System" (AOCS). Let's dive in and explore this fascinating technology that makes our everyday lives easier, from weather prediction to global communication.


First up, let's decipher these two key terms: "Attitude" and "Orbit". In the context of space, "attitude" doesn't mean a satellite's grumpy or sunny disposition. Instead, it refers to a satellite's orientation or alignment in space with respect to a reference. "Orbit," on the other hand, is the path that a satellite follows around a planet. So, basically, the AOCS is a system that keeps a satellite facing the right way (attitude control) and going the right way (orbit control).


Now, why is this important? Well, imagine you're trying to take a selfie but your camera keeps turning away from you. Frustrating, right? In the same way, most satellites need to keep their instruments or antennas pointed towards a specific direction to capture data or communicate with the Earth. That's where the AOCS comes in.


The AOCS uses various instruments for this purpose. Sensors like sun sensors, star trackers, and inertial measurement units provide information about the satellite's current attitude. Then, actuators such as reaction wheels, control moment gyros, and thrusters are used to adjust this attitude. For example, if a satellite needs to turn towards Earth, the AOCS might spin up a reaction wheel in the opposite direction to turn the satellite. Thanks, laws of physics!

Hubble

Now, let's bring these abstract concepts down to Earth with some real-world examples. Ever heard of the Hubble Space Telescope? Of course, you have! It's one of the most famous space telescopes, capturing breath-taking pictures of distant galaxies and nebulae. But did you know that the Hubble's AOCS plays a critical role in its operation? The system keeps the telescope stable and pointed at its celestial target for hours or even days, enabling it to capture sharp, detailed images.


Another interesting example is the Mars rovers, like Perseverance. While technically not satellites, they use similar technology to maintain the correct orientation during their perilous descent and landing on the Red Planet. Without a reliable attitude control, the rovers could end up in a spin, landing the wrong way, or worse, crash.


The Actuators Actuators are the movers and shakers of the space world. Let's dive into some of the most commonly used ones and see what makes them tick.


The Reaction Wheels

Reaction Wheel

Imagine you're sitting on a swivel chair with a bicycle wheel in your hands. If you spin that wheel, your chair starts to rotate in the opposite direction. That's the principle behind reaction wheels!

In space, electric motors spin these wheels at high speeds. When a satellite needs to turn, the speed of these wheels is adjusted. This action causes a reaction, making the satellite rotate in the opposite direction (thank you, Newton's third law!). This method is fantastic for precision maneuvers and doesn't consume any propellant.

However, there's a catch: these wheels can only spin so fast before they reach their maximum speed. When this happens, we say the wheel is "saturated," and we need to use another method, like thrusters, to 'desaturate' them. Plus, their moving parts can wear out or break, which is a significant downside in the harsh conditions of space.


The Control Moment Gyros (CMGs)

Control Moment Gyros

Control moment gyros are a bit like reaction wheels on steroids. Instead of just spinning in place, they're mounted on gimbals, allowing them to tilt while spinning. This tilt generates a torque that can rotate the satellite.

CMGs can produce more torque than reaction wheels and don't saturate as quickly. They're used on larger spacecraft, like the International Space Station. However, they're more mechanically complex and heavier, which makes them harder to fit on smaller satellites. And, like reaction wheels, they can break down over time.


The Thrusters

Thrusters

When you think of spacecraft moving, you probably think of thrusters - jets of gas that push the spacecraft around. These are a type of propulsion system that expels mass in one direction, causing the spacecraft to move in the opposite direction, again thanks to Newton's third law.

Thrusters are great for large adjustments and can help desaturate reaction wheels. They're also simple and reliable. But they have a big drawback: they need fuel. Once you run out of fuel, the thrusters can't do anything. Plus, the force from thrusters is hard to control precisely, making them less suitable for fine-tuning a satellite's attitude.


The Magnetic Torquers

Mangetic Torquer

These clever devices use Earth's magnetic field to change a satellite's attitude. By running a current through a coil, they generate a magnetic field. Interacting with Earth's magnetic field, this produces a torque, causing the satellite to rotate.

Magnetic torquers don't need any propellant and can be used to desaturate reaction wheels. They're often used on small, low Earth orbit satellites. However, their effectiveness depends on the strength of the Earth's magnetic field, which gets weaker as you move away from Earth. Therefore, they're not useful for spacecraft in higher orbits or around other planets.

So, there you have it! The AOCS uses a mix of these actuators, selecting the best one for the job depending on the situation. It's a vital system that keeps satellites doing their cosmic ballet in the vacuum of space.


The Sensors

While actuators are the muscles of a satellite, sensors are the eyes and ears. They observe the environment and inform the AOCS about the satellite's current status. Here's a look at some of the most commonly used sensors in space.


Star Trackers


Star Tracker

Just like sailors of old, modern satellites often use the stars to find their way. A star tracker is a specialized camera that takes pictures of the stars. By comparing these pictures to a star catalog, it can determine the satellite's orientation with high precision.

Star trackers are great for high-accuracy applications. They're used on a lot of scientific satellites and on the Mars rovers to help with navigation. However, they can be blinded by the Sun, Earth, or Moon, and they're also vulnerable to stray light from the spacecraft itself. They also require a lot of computational power to process the star images.


Sun Sensors


Sun Sensor

As the name suggests, these sensors detect the direction of the Sun. They come in many forms, from simple photodiodes that generate a current when exposed to sunlight, to sophisticated digital devices that can measure the Sun's angle with high precision.

Sun sensors are a reliable and straightforward way to determine a spacecraft's orientation, especially for smaller satellites. They're often used in combination with other sensors for redundancy. However, they're less precise than star trackers and are, of course, useless in the shadow of a planet or during lunar night.


Inertial Measurement Units (IMUs)


Inertial Navigation Unit

IMUs measure changes in velocity (acceleration) and changes in rotation (angular velocity). They contain accelerometers and gyroscopes to accomplish this. By integrating these measurements over time, they can track a spacecraft's motion.

IMUs are essential for high-dynamic maneuvers, like rocket launches and landings, where the spacecraft's attitude can change rapidly. However, the integration process introduces cumulative errors, meaning the IMU's accuracy decreases over time. This drift has to be corrected with other sensors.


Earth Sensors


Earth Sensor

Earth sensors detect the infrared radiation emitted by the Earth. Since the Earth's infrared signature is quite distinct, these sensors can determine where the Earth is in relation to the satellite. They're often used on Earth-observing satellites and geostationary communication satellites.

While Earth sensors are great for keeping an eye on our home planet, they're less useful for interplanetary missions. They're also less precise than star trackers and can be affected by changes in the Earth's infrared radiation due to weather or seasonal variations.

So, those are the main tools in a satellite's sensor kit! Together, they provide a comprehensive view of the spacecraft's attitude. The AOCS takes this information, crunches the numbers, and commands the actuators to keep the satellite on track and oriented correctly. Both of the sensors and actuators are controlled by the onboard computer.


The Onboard Computer


Onboard Computer

The onboard computer is like the brain of the AOCS. It takes inputs from sensors, processes this information, and sends out commands to the actuators. Let's delve into the workings, usage, and limitations of these essential components.

The onboard computer runs what's known as the "control algorithm." This set of mathematical instructions takes the current attitude of the spacecraft, provided by the sensors, and compares it to the desired attitude. If there's a discrepancy (known as an "error"), the control algorithm calculates the required correction. The onboard computer then sends the appropriate commands to the actuators to implement this correction.


There are different types of control algorithms that an onboard computer can use. Proportional-Integral-Derivative (PID) control is one of the most common. This algorithm adjusts the control inputs (commands to the actuators) based on the current error, the cumulative past error, and the rate of change of the error. This ensures a fast response to changes while minimizing overshoot and oscillations.


The onboard computer for AOCS is a crucial component for the successful operation of any satellite or spacecraft. From ensuring a Mars rover aligns properly during descent to maintaining a communication satellite's alignment for optimal signal transmission, the onboard computer is always at work. It ensures the spacecraft's sensors and actuators work in harmony to keep the spacecraft in the right orientation and on the correct path.


Limitations

Despite their essential role, onboard computers aren't without limitations. One of the main challenges is the harsh environment of space. Radiation can cause bit flips in memory or even damage the computer, leading to what's known as a Single Event Upset (SEU). Hence, space computers often need to be "radiation-hardened" to withstand these effects.

Another challenge is power. Running a computer takes energy, which is a precious resource on a spacecraft. Therefore, the onboard computer needs to be efficient and only use as much power as necessary.

Lastly, the onboard computer needs to be highly reliable and autonomous. Unlike your home computer, you can't just reboot a space computer if something goes wrong! It needs to be able to handle faults, operate independently, and keep the spacecraft safe, even in unexpected situations.

In conclusion, the onboard computer is the command and control center of the AOCS. It performs the critical task of integrating the data from various sensors, processing it in real-time, and then driving the actuators to maintain the spacecraft's correct attitude and orbit. Its role in space missions, from Earth observation to interstellar explorations, is absolutely indispensable.

So, that's pretty much the AOCS in a nutshell! Apart from AOCS, there are other important systems like power systems and telemetry systems, but let's reserve them for next article. Keep exploring!


Cite this article as:

Kumar, Yajur. “Satellite Attitude and Orbit Control System or AOCS in a Nutshell.” Space Navigators, 28 May 2023, www.spacenavigators.com/post/satellite-attitude-and-orbit-control-system-or-aocs-in-a-nutshell.



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