Spacecraft attitude and orbit control are essential for the successful performance of any space mission, especially during critical phases such as launch, insertion, maneuvering, docking, and reentry. In this article, you will learn how these functions are achieved using various sensors, actuators, algorithms, and modes of operation.
To control the attitude and orbit of a spacecraft, you need to measure its orientation, position, and velocity relative to a reference frame, such as the Earth or the Sun. For this purpose, you can use different types of sensors, such as gyroscopes, magnetometers, sun sensors, star trackers, GPS receivers, and radar altimeters. Depending on the accuracy, reliability, and power requirements, you can choose the best combination of sensors for your mission. To change the attitude and orbit of a spacecraft, you need to apply forces and torques to it. For this purpose, you can use different types of actuators, such as thrusters, reaction wheels, control moment gyros, magnetic torquers, and solar sails. Depending on the magnitude, direction, and duration of the desired changes, you can choose the best combination of actuators for your mission.
From propulsion perspective, at times when such actuators like gyro or rotation wheels malfunctions then spacecraft is put into thruster control mode. In this thruster control mode small pulses with shorter thruster valve opening time (also known as on time) are fired as per the rotation requirement of satellite. The pulses can be characterised as cold pulses and hot pulses depending upon off time. When off time is more than few seconds, the pulse is called as cold pulse since thruster gets enough time for cooling. When pulse off time is of order of milliseconds, the pulse is called as hot pulse as thruster doesn’t get sufficient time to cool off. These thrusters in pulse mode operation can help orient the spacecraft in desired direction.
To control the attitude and orbit of a spacecraft, you need to implement algorithms that can process the sensor data, calculate the errors, and generate the commands for the actuators. For this purpose, you can use different types of algorithms, such as proportional-integral-derivative (PID) controllers, linear quadratic regulators (LQR), Kalman filters, and artificial neural networks. Depending on the complexity, robustness, and adaptability of the control problem, you can choose the best type of algorithm for your mission. To control the attitude and orbit of a spacecraft, you need to define modes of operation that can handle different situations and objectives. For example, you can have modes for detumbling, pointing, slewing, station keeping, rendezvous, and landing. Depending on the phase, condition, and goal of the mission, you can switch between different modes of operation.
While there are many different systems currently in place to manage critical phases, the advancement of autonomous control systems is interesting to watch. Autonomous systems are able to respond to unforeseen circumstances or carry out predetermined processes autonomously, without the need for human intervention.
The launch phase is one of the most critical and challenging phases of a space mission, as it involves high accelerations, vibrations, and uncertainties. To control the attitude and orbit of a spacecraft during launch, you need to coordinate with the launch vehicle and its guidance system. You also need to ensure that the spacecraft is properly configured and protected from the aerodynamic and thermal loads. You need to monitor the separation events and activate the sensors and actuators as soon as possible. You need to perform a health check and a detumbling maneuver to stabilize the spacecraft and orient it towards the Sun or the Earth.
It’s properly better to use GN&C systems to replace guidance systems only. Normally during the launch phase, sometimes we choice simply programmer as a guidance algorithm.
The insertion phase is another critical and complex phase of a space mission, as it involves precise maneuvers and timing to achieve the desired orbit. To control the attitude and orbit of a spacecraft during insertion, you need to plan and execute a series of burns using the thrusters or the main engine. You also need to adjust the attitude and orbit based on the orbital elements and the perturbations. You need to avoid collisions and interference with other spacecraft and debris. You need to communicate with the ground station and confirm the orbit parameters and the mission status.
The maneuvering phase is a frequent and important phase of a space mission, as it involves changing the attitude and orbit of a spacecraft to meet different objectives and requirements. To control the attitude and orbit of a spacecraft during maneuvering, you need to design and perform optimal and efficient maneuvers using the actuators. You also need to account for the fuel consumption and the mass distribution. You need to cope with the uncertainties and disturbances in the environment and the system. You need to coordinate with other spacecraft and satellites for cooperative or collaborative missions.
The docking phase is a rare and delicate phase of a space mission, as it involves approaching and attaching to another spacecraft or a space station. To control the attitude and orbit of a spacecraft during docking, you need to establish and maintain a relative position and velocity with the target. You also need to align and match the attitude and the docking ports. You need to ensure the safety and compatibility of the spacecraft and the target. You need to follow the procedures and protocols for the docking operation and the transfer of personnel or cargo.
The reentry phase is the final and hazardous phase of a space mission, as it involves returning to the Earth's atmosphere and landing safely. To control the attitude and orbit of a spacecraft during reentry, you need to reduce the orbital speed and altitude using the thrusters or the aerodynamic drag. You also need to control the attitude and the lift-to-drag ratio to steer the spacecraft towards the landing site. You need to protect the spacecraft and its payload from the high temperatures and pressures. You need to deploy the parachutes or the landing gear and touch down smoothly.