Gyroless attitude estimation algorithm for nanosatellites

Abstract

A nanosatellite, or nanosat for short, is a type of small satellite with a mass between 1 and 10 kilograms. They are often built using off-the-shelf components and can be launched relatively inexpensively, making them a popular choice for academic and commercial space missions. Nanosatellites can be used for a variety of applications, including Earth observation, communications, scientific research, and technology demonstration. Nanosatellites face several technical and operational challenges, including limited power, communication capabilities, and computing resources. Due to their small size, they have limited space for equipment and instrumentation, which can make it challenging to implement complex systems. Moreover, they may be more susceptible to radiation and thermal effects in space, which can impact their performance and longevity. Additionally, the short lifespan of some nanosatellites (often just a few years) may limit their usefulness for long-term missions or data collection. Attitude Determination and Control (ADC) subsystem is crucial for nanosatellites to fulfill their mission objectives. Almost 40% of nanosatellites use an active ADC system to control their attitude accurately and reliably. The accuracy of the sensors and the actuator's torque limit determines the ADC subsystem's capabilities. MEMS gyros are preferred for nanosatellites due to their low cost and weight, but they have lower accuracy and stability and may degrade or fail during the mission. Magnetometers and sun sensors are common attitude sensors for nanosatellites, but they face challenges when only one vector measurement is available. The development of higher accuracy and minimal sensor ADC subsystems is a research topic for nanosatellites. Gyroless attitude estimation is a technique used to determine the orientation of a nanosatellite in space without relying on traditional gyroscopes, which are often bulky, expensive, and consume a significant amount of power. Instead, this technique uses a combination of different sensors such as magnetometers, sun sensors, and star trackers, along with advanced algorithms and mathematical models, to estimate the attitude of the nanosatellite. The approach aims to overcome some of the limitations associated with traditional ADC systems and enable more cost-effective and reliable solutions for small satellite missions. There are several advantages of gyroless attitude estimation for nanosatellites, including: • Cost-effectiveness: Gyroless attitude estimation techniques can be less expensive compared to systems that require gyroscopes, which can be relatively expensive and consume significant power. • Reduced size and weight: Gyroscopes can be relatively large and heavy, making them impractical for use on small nanosatellites. Gyroless systems can be smaller and lighter, which is important for satellites with size and weight constraints. • Improved reliability: Since gyroscopes have moving parts, they can be prone to mechanical failure or degradation over time. Gyroless systems are less complex and can be more reliable. Overall, gyroless attitude estimation techniques can provide a practical and cost-effective solution for nanosatellites with limited resources and stringent mission requirements. This thesis presents and evaluates attitude estimation filters that are specifically developed for nanosatellites that do not have gyroscopes. Disturbances can have a significant impact on the accuracy of gyroless attitude estimation for nanosatellites. In particular, non-gravitational disturbances such as residual magnetic dipole moment (RMM), aerodynamic drag, and solar radiation pressure can induce torques on the satellite, causing it to deviate from its desired attitude. These disturbances can affect the accuracy of the attitude estimation algorithm, leading to errors in the estimated attitude. The RMM is the most important disturbance torque for nanosatellites because it interacts with the Earth's magnetic field and causes an external disturbance torque that affects the satellite's attitude. This is particularly important for small satellites like nanosatellites, which have a low moment of inertia and are more susceptible to external disturbances. The RMM can arise from several sources, such as the satellite's magnetic materials, electronic components, or even the solar cells. Modelling and compensating of RMM is essential in the attitude control system to maintain the satellite's stability and accuracy. Accurate modeling of RMM and designing filters capable of precise estimation are crucial in dynamic-based (gyroless) filters, where RMM plays a dominant role. When working with a dynamic-based (gyroless) filter, accurate estimation of the RMM becomes crucial due to its dominant role. As a result, the thesis has focused on developing models that accurately represent the RMM, and designing filters that can precisely estimate it in real-time. The integration of a filtering algorithm with static attitude estimation methods, such as the QUEST algorithm, can improve the accuracy and convergence speed of gyroless attitude estimation for nanosatellites. However, the approach has a major drawback when only one vector measurement is available, such as during eclipse phases when the sun sensor cannot provide a measurement. In these cases, using three-axis magnetometers (TAM) is necessary to ensure accurate attitude estimation. Therefore, TAM only attitude estimation when sun is not available is also proposed.