Autonomous optical navigation for DESTINY+: Enhancing misalignment robustness in flyby observations with a rotating telescope

${\text{DESTINY}}^{\text{+}}$ is an upcoming JAXA Epsilon medium-class mission to flyby multiple asteroids including Phaethon. As an asteroid flyby observation instrument, a telescope mechanically capable of single-axis rotation, named TCAP, is mounted on the spacecraft to track and observe the target asteroids during flyby. As in past flyby missions utilizing rotating telescopes, TCAP is also used as a navigation camera for autonomous optical navigation during the closest-approach phase. To mitigate the degradation of the navigation accuracy, past missions performed calibration of the navigation camera’s alignment before starting optical navigation. However, such calibration requires capturing large number of images by the navigation camera and downlinking them to the ground station. In the case of small spacecraft with limited link capacity, such as ${\text{DESTINY}}^{\text{+}}$, such ground-in-the-loop calibration requires significant operational time to complete and imposes constraints on the operation sequence. From the above background, the ${\text{DESTINY}}^{\text{+}}$ team has studied the possibility of reducing operational costs by allowing TCAP alignment errors to remain. This paper describes an autonomous optical navigation algorithm robust to the misalignment of rotating telescopes, proposed in this context. In the proposed method, the misalignment of the telescope is estimated simultaneously with the spacecraft’s orbit relative to the flyby target. To deal with the nonlinearity between the misalignment and the observation value, the proposed method utilizes the unscented Kalman filter, instead of the extended Kalman filter widely used in past studies. The proposed method was evaluated with numerical simulations on a PC and with hardware-in-the-loop simulation, taking the Phaethon flyby in the ${\text{DESTINY}}^{\text{+}}$ mission as an example. In the example case, the misalignment-induced navigation accuracy degradation of 4.6 km-$1\sigma$ can be reduced to 0.1 km-$1\sigma$ by the proposed method. The required time to run one-cycle of the navigation process on the onboard computer for the ${\text{DESTINY}}^{\text{+}}$ mission is less than 0.18 s. These results validate that the proposed method can mitigate the misalignment-induced degradation of the optical navigation accuracy with reasonable computational costs suited for onboard computers.