Aerobraking Navigation, Guidance and Control

A Comparison of State Variables Performance in Propagation

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Abstract

The amount of propellant required to perform deep-space maneuvers usually consists of a large percentage of the total spacecraft mass. In the early '90s, a new technique was experimented at the end of the Magellan mission, which employed the atmosphere of Mars itself, to reduce the eccentricity of the orbit over a long period of time. Said action was named aerobraking and the design of the required onboard software to perform this maneuver, is the main topic of this thesis.

The navigation part of the software has the purpose of determining the current spacecraft state (position, velocity and attitude). Two navigation systems have been implemented in this research and they differ in the fact that one is purely based on data available onboard (such as accelerometer measurements and a simplified environment model), whereas the other uses observations from an unspecified positioning system, to complement the onboard model.
The tasks of the guidance system can be summarized as: estimating the minimum and maximum values of the pericenter altitude and targeting the midpoint of said range. The first function is achieved by considering the thermodynamic limitations of the spacecraft and the restrictions on aerobraking duration, whereas to attain the second purpose, use is made of a maneuver estimator.
Both an attitude and an orbit controller are designed for the mission. A proportional, integral and derivative controller is used to regulate the rotational motion of the spacecraft, whereas, orbit control is realized by assuming impulsive shots at apoapsis.

Based on the analyses performed, it was concluded that this GNC system partly fulfills its duties. In fact, it is able to 1) reduce the mass of an interplanetary spacecraft, by requiring the employment of 82 instead of more than 1000 m/s of velocity increments, and 2) to maintain a low navigation error (for most of the orbit the position error is within 50 m), but, on the other hand, it is not capable of providing a robust performance. In particular, the excessive heating experienced during the atmospheric phases of the mission, poses a large threat to the thermal and overall integrity of the spacecraft, possibly jeopardizing the whole mission.

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