Can We Estimate Air Density of the Thermosphere with CubeSats?

The measurement of air density in the Earth’s thermosphere has a wide range of scientific applications from space weather to upper atmosphere dynamics, but also technical applications from satellite control to predictions of atmospheric reentry of space debris. This study models the torques applying on a three-unit CubeSat in low Earth orbit to infer the capability of such platforms to measure the air density along their orbit. Realistic noise levels of available CubeSat components are used, and sensitivity to the various noise sources is presented. The precise knowledge of the spacecraft attitude, angular acceleration, residual magnetic dipole, and center of gravity is critical to allow proper air density retrieval. Winds in the thermosphere also have a significant impact on the thermosphere density retrieval, suggesting that this parameter can also be constrained. Attitude control is not necessary if the attitude itself is properly known. The application to the EntrySat CubeSat predicts that such retrieval is possible at altitudes lower than 200 km with errors lower than 30%. The air density retrieval from CubeSat platforms will open new capabilities to infer upper atmosphere dynamics

ALINA Moon Lander GNC - Architecture, Design and Test Results

This paper describes the architecture and design of the AOCS/GNC system developed for the ALINA Moon lander by DLR and PTS. Driving requirements are the need for complete autonomy during the landing phase, the required absolute landing accuracy of 500 m x 750 m and the touch down conditions (i.e. horizontal speed limit, vertical speed limit, yaw/pitch angle limits). The paper gives an overview about the overall mission and describes the actuator and the sensor configurations as well as the computer architecture. The AOCS modes are defined as well as the objectives, the initial, target and exit conditions and the actuator/sensor configuration for each mode. The paper expands on the 3-DoF system used to control the attitude/rate during the Cruise & Orbital Phase and the 6-DoF GNC system controlling the position/velocity (outer-loop) and attitude/rate (inner-loop) during the Descent and Landing Phase, utilizing a Crater Navigation (CNav) based navigation filter and an optional Hazard Detection and Avoidance (HDA) system. An overview about the redundancy concepts and general FDIR approach on AOCS level is given. The paper offers a high-level overview about the approaches and technologies used in all major algorithmic areas. Finally, Model in the Loop test results are presented, showing an expected landing accuracy of 20 meters and an HDA maneuver execution accuracy of about 10 meters