# Retro-Reflective-Marker-Aided Target Pose Estimation in a Safety-Critical Environment
###### Abstract  
Visual technologies have an indispensable role in safety-critical applications, where tasks must often be performed through teleoperation. Due to the lack of stereoscopic and motion parallax depth cues in conventional images, alignment tasks pose a significant challenge to remote operation. In this context, machine vision can provide mission-critical information to augment the operator's perception. In this paper, we propose a retro-reflector marker-based teleoperation aid to be used in hostile remote handling environments. The system computes the remote manipulator's position with respect to the target using a set of one or two low-resolution cameras attached to its wrist. We develop an end-to-end pipeline of calibration, marker detection, and pose estimation, and extensively study the performance of the overall system. The results demonstrate that we have successfully engineered a retro-reflective marker from materials that can withstand the extreme temperature and radiation levels of the environment. Furthermore, we demonstrate that the proposed maker-based approach provides robust and reliable estimates and significantly outperforms a previous stereo-matching-based approach, even with a single camera. safety critical; teleoperation; eye-in-hand; optical tracking; retro-reflective markers; marker detection; stereoscopic; pose estimation [PERSON], [PERSON], A.  
been successfully used for maintenance and assembly tasks in extra-vehicular space applications [4]. In the aforementioned work, the computed six degrees of freedom (DOF) transformation between the current and desired end-effector positions is overlaid on the camera feed showed to the operator as either graphics or text. The application that motivated our work is remote handling of ITER's ([[https://www.iter.org/](https://www.iter.org/)]([https://www.iter.org/](https://www.iter.org/))) divertor cassette locking system. ITER constitutes one of the world's most hostile remote handling environments and is characterized by the unpredictable nature of its maintenance situations and tasks [6]. In ITER, all tasks are to be performed using a human-in-the-loop telemanipulation approach where the operator is always in control, although remotely located to minimize the risk of ionizing radiation exposure [6]. Operation in this environment differs from a common industrial environment, since high radiation levels and temperatures and strong magnetic fields strictly limit the materials that can be used in the activated areas, including the hardware that can be used for sensing the environment. This deems necessary the use of radiation-tolerant cameras, which are often characterized by a low resolution, gray-scale output, and high level of noise, which degrades with exposure to radiation over the lifetime of the camera. The operation of the divertor cassette locking system requires the alignment of several tools to their respective slots. Both the tools and the manipulator are carried in by a transporter that moves on rails throughout the facility. The tolerances for error in tool-slot alignment are small, with \(\pm 3\) mm of maximum allowed error in translation. Taking into account the difficulty in making a visual alignment in this range, there is a need to develop an accurate machine-vision-based aid that can comply with the strict requirements of the application. Our aim here is to develop a system that calculates, using a set of one or two cameras attached to the wrist of the robotic manipulator, the transformation between the robot end-effector and the target (cassette). This information is intended to be used to update the models behind the virtual-reality-based remote handling platform [7; 8]. Most man-made structures in the ITER environment, like the divertor cassettes, have large, flat, textureless surfaces, which are often specularly reflective and look different from different perspectives.