Accurate outdoor ground truth based on total stations

In robotics, accurate ground-truth position fostered the development of mapping and localization algorithms through the creation of cornerstone datasets. In outdoor environments and over long distances, total stations are the most accurate and precise measurement instruments for this purpose. Most total station-based systems in the literature are limited to three Degrees Of Freedoms (DOFs), due to the use of a single-prism tracking approach. In this paper, we present preliminary work on measuring a full pose of a vehicle, bringing the referencing system to six DOFs. Three total stations are used to track in real time three prisms attached to a target platform. We describe the structure of the referencing system and the protocol for acquiring the ground truth with this system. We evaluated its precision in a variety of different outdoor environments, ranging from open-sky to forest trails, and compare this system with another popular source of reference position, the Real Time Kinematics (RTK) positioning solution. Results show that our approach is the most precise, reaching an average positional error of 10 mm and 0.6 deg. This difference in performance was particularly stark in environments where Global Navigation Satellite System (GNSS) signals can be weaker due to overreaching vegetation.

I. Introduction

Access to precise and accurate ground-truth pose reference during experiments is vital in mobile robotics. For instance, knowing the pose of a robot, that is its position and orientation, facilitates development and testing of localization, mapping and control algorithms. For indoor experiments, motion capture such as the Vicon [1], [2] or Optitrack [3] systems have become the de facto standard. They are based on tracking reflective or active markers by infrared cameras and offer an accuracy of sub-millimetre precision and high sampling rates. However, the area that they can cover is limited by the number of cameras available. Moreover, they struggle in direct sunlight. Another high-fidelity source of reference pose is laser trackers based on laser interferometry, such as the one used by Sang et al. [4]. In exchange for the limitation of tens of meters in range, they offer position measurements accurate down to microns. The possibilities of pose reference are more limited outdoors. The approaches most prominent in the literature use Global Navigation Satellite System (GNSS) [5], total stations [6], and qualitative comparison of trajectories to maps [7]. Besides, these outdoor referencing systems do not provide direct orientation measurements.