Integrated Inertial and Radio Based State and Parameter Estimation for Mars Entry Phase Spacecraft Navigation

Abstract

We address the entry phase navigation of Entry, Descent, and Landing (EDL) operation for future Mars spacecraft missions. The entry phase is characterized by a highly non-linear motion model and is influenced by various aerodynamic forces. Current entry phase navigation systems exclusively use Inertial Measurement Units (IMUs) from atmospheric entry to parachute deployment phase. This results in large error accumulation. Such systems cannot meet the precision landing requirements of future missions. In addition, entry phase filters designed using inertial measurement systems alone cannot observe some states which leads to diverging state estimates. We present an integrated navigation scheme whereby a ground based radio ranging device telemeters spacecraft range and range rate information to a computer on board the spacecraft. This information is fused with inertial measurements to yield a more accurate navigation system than a standalone inertial system. The paper implements the navigation scheme using the continuous-discrete Extended Kalman Filter (EKF) and the Unscented Kalman Filter (UKF). Performance comparisons are made between the two filters in terms of RMS error, runtime, and ease of implementation. Our results show comparable results between the two filters. The UKF provides better estimates of lift to drag ratio and downrange distance while the EKF outperforms the UKF with the rest of the state and parameter estimates. The fact that no Jacobian calculation is required in the case of the UKF renders it a more attractive option than the EKF in terms of ease of implementation for highly non linear systems.