Approaches to Autonomous Aerobraking at Mars

Planetary atmospheric aerobraking will most likely be incorporated in every future Mars orbiting mission. Aerobraking requires an intensive workload during operations. To provide safe and efficient aerobraking, both navigation and spacecraft system teams must be extremely diligent in updating spacecraft sequences and performing periapsis raise or lower maneuvers to maintain the required orbital energy reduction without exceeding the design limits of the spacecraft. Automating the process with onboard measurements could significantly reduce the operational burden and, in addition, could reduce the potential for human error. Two levels of automation are presented and validated using part of the Mars Global Surveyor aerobraking sequence and a simulated Mars Odyssey sequence. The simplest method only provides the capability to update the onboard sequence. This method uses onboard accelerometer measurements to estimate the change in orbital period during an aerobraking pass and thereby estimates the beginning of the next aerobraking sequence. Evaluation of the method utilizing MGS accelerometer data showed that the time of the next periapsis can be estimated to within 25% 3σ of the change in the orbital period due to drag. The second approach provides complete onboard orbit propagation. A low-order gravity model is proposed that is sufficient to provide periapsis altitude predictions to within 100–200 meters over three orbits. Accelerometer measurements are used as part of the trajectory force model while the spacecraft is in the atmosphere.