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Nanosatellite and microsatellite refer to miniaturized satellites in terms of size and weight, in the range of 1-10 Kg and 10-100 kg, respectively. These are the fastest growing segments in the satellite industry. ‘CubeSat’ is one of the most popular types of miniaturized satellites. CubeSats were made possible by the ongoing miniaturization of electronics, which allows instruments such as cameras to ride into orbit at a fraction of the size of what was required at the beginning of the space age in the 1960s. The missions of micro-satellites range from early military usages to weather forecast, resources exploration, communication, and scientific experiment.
The trend toward small-sized spacecraft continues in government applications and is even increasing in commercial space endeavors that are funded by venture capital. Small spacecraft, including nanosatellites, microsatellites, and small satellites (smallsats), are an attractive alternative to traditional, larger spacecraft due to reduced development costs, decreased launch costs, and increased launch opportunities. CubeSats reduce launch costs in two fundamental ways. They don’t weigh that much, which means a rocket doesn’t need a lot of fuel to heft them. In most cases, they also share a rocket with a larger satellite, making it possible to get to space on the coattails of the heavier payload.
Microsatellites and nanosatellites have revolutionized the way we explore space. With their smaller size and lower cost, they have made it possible for a wider range of organizations and individuals to participate in space missions. However, their compact size also means that they are more susceptible to disturbances in their orientation, which can cause them to drift off course or even lose control.
This is where Attitude Determination and Control Systems (ADCS) come in. These systems play a crucial role in ensuring that microsatellites and nanosatellites remain stable and maintain their intended orbit. In this blog post, we will explore the importance of ADCS in microsatellite and nanosatellite missions.
What is ADCS?
Before we dive into the importance of ADCS, let’s first understand what it is. ADCS is a system that is responsible for controlling the orientation and stabilization of a satellite. It does this by using sensors and actuators to monitor and adjust the satellite’s attitude, which refers to its orientation with respect to a reference frame.
The attitude Determination and Control System deals with the position and orientation of the satellite in space, which is required for maintaining stability and maneuvering for imaging and communications. The attitude control system architecture is a crucial subsystem for any satellite mission since precise pointing is often required to meet mission objectives.
The Importance of ADCS in Microsatellite and Nanosatellite Missions
The accuracy and precision requirements are even more challenging for small satellites where limited volume, mass, and power are available for the attitude control system hardware.
ADCS plays a critical role in the success of microsatellite and nanosatellite missions. Here are a few reasons why:
- Maintaining the desired orbit: One of the primary objectives of microsatellite and nanosatellite missions is to maintain a stable orbit. Any deviation from the intended orbit can significantly impact the success of the mission. ADCS helps to ensure that the satellite maintains its intended orbit by adjusting its attitude as needed.
- Minimizing disturbances: Microsatellites and nanosatellites are more susceptible to disturbances in their orientation due to their smaller size and mass. These disturbances can come from a variety of sources, such as solar radiation pressure, atmospheric drag, and magnetic fields. ADCS helps to minimize these disturbances by continuously monitoring the satellite’s orientation and making adjustments as necessary.
- Pointing and tracking: Microsatellites and nanosatellites are often used for imaging and remote sensing applications. ADCS plays a critical role in these applications by enabling the satellite to point and track its target accurately.
- Data transmission: Microsatellites and nanosatellites often transmit data to Earth using antennas. ADCS helps to ensure that the antenna is pointed in the right direction, which is crucial for maintaining communication with the ground station.
For in-depth understanding on ADCS technology and applications please visit: Mastering Spacecraft Attitude Control Systems: From Fundamentals to Advanced Techniques
Elements of ADCS
The Control System has several objectives, including microsatellite angular motion control, onboard equipment control, computation of current orientation parameters, and computation of current navigation parameters. These objectives are achieved through the orientation and stabilization control subsystem, attitude determination subsystem, autonomous magnetometric navigation subsystem, and onboard equipment control subsystem.
To achieve these objectives, microsatellites and nanosatellites require Attitude Determination and Control Systems (ADCS) that consist of attitude sensors, attitude control actuators, and attitude control algorithms. The selection of a specific ADCS depends on the mission requirements, but a well-designed ADCS is crucial for the success of any microsatellite or nanosatellite mission.
Different types of sensors, including solar sensors, horizon sensors, and star trackers, are used to control attitude in space. The limitations of microsatellites require minimizing unnecessary sensors and controllers to ensure precision in attitude control.
The sensors are the main resource when it comes to determining the attitude of a vehicle or object with precision. The so-called actuators are also necessary to provide the torque that is needed to position the satellite in the desired attitude, with the algorithms that can interpret the data and drive actions in the Spacecraft.
Reaction wheels are the standard AOCS actuators for most earth observation and science missions with high-resolution optical payload. Reaction Wheel assembly (RWL) consists of 5 wheels in a skewed configuration, 4 mainly used and 1 for cold redundancy. The main set of 4 wheels provides actuation during any operating mode with the exclusion of ASM. For the specific application, key parameters are the allowed control torque and the momentum storage capacity.
MANY Earth-orbiting spacecraft have a nadir-pointing requirement that requires full three-axis attitude stabilization. Typical nadir-pointing stabilization systems use passive gravity gradient momentum wheel and magnetic torquers, or fully active systems that include a suite of reaction wheels, control moment gyros, or thrusters to implement full three-axis control. Traditional attitude stabilization systems are impractical for micro and nanosatellites . Reaction wheels and momentum wheels do not fit within its limited weight and power budget. Magnetic torque coils can be used, but it is difficult to guarantee global stability with only magnetic torque because of the inability to torque about the local magnetic field direction.
If the design does not require orbital maneuvering capability and, therefore, they do not include thrusters. At altitudes below 400 km, aerodynamic drag torque tends to overwhelm the gravity gradient torque for practical lightweight deployable boom designs.
The limitations of micro-satellite due to the limits of weight and power and requirements of low design cost and high precision requires reducing unnecessary attitude sensors and controllers to ensure the precision of attitude control.
Microsatellites and nanosatellites typically carry an Inertial Measurement Unit (IMU) that uses accelerometers and gyroscopes to measure the inertial components of the system. The data collected by the IMU can be used to calculate the position and orientation of the vehicle and study the vibration characteristics of the system. The onboard computer acts as the heart and brain of the satellite, controlling the payload operations and scheduling its activities. The ground station can send commands to the satellite based on the requirements for the operation of the payloads, but current microsatellites and nanosatellites are capable of autonomous operation, meaning they do not require intervention from the ground station for basic survival.
Here are some of the benefits of using an ADCS for microsatellites and nanosatellites:
- Improved mission success: An ADCS can help to ensure that a microsatellite or nanosatellite is properly aligned with its target, which can improve the chances of a successful mission.
- Reduced risk of damage: An ADCS can help to protect a microsatellite or nanosatellite from harmful radiation, which can reduce the risk of damage to the spacecraft.
- Increased operational flexibility: An ADCS can allow a microsatellite or nanosatellite to perform its mission objectives more effectively, which can increase the operational flexibility of the spacecraft.
Conclusion
In conclusion, Attitude Determination and Control Systems (ADCS) are crucial for the success of microsatellite and nanosatellite missions. These systems help to ensure that the satellite maintains its intended orbit, minimizes disturbances, points and tracks accurately, and transmits data efficiently. As the use of microsatellites and nanosatellites continues to grow, the importance of ADCS will only continue to increase.
