## 4 Discussion  
Currently, inspection of DSTs at the Hanford site by a high-resolution ultrasonic sensor is labor intensive. Operators control robot movement while watching multiple computer screens displaying data in different formats. We have developed a GUI that brings some of these data into one display with multiple windows. A large portion of the display contains a map of air slots in relation to tank-bottom welds on which the location of the robot can be dynamically displayed. This alerts the operator to turns in the channel that should be confirmed by the video feed. It also shows the approach of the robot to tank-bottom welds that will have a major effect on the data from the ultrasonic sensor when welds are close to a data collection site. The location of data collection sites needs to be known with a precision of approximately one inch to enable the development of a composite image from data collected at multiple sites.  
Our research has focused on optical methods to track the robot's location in an air slot. A laser range finder that meets the requirements of size, weight, power consumption, etc., to be part of the robot assembly is the centerpiece of our design. Since the tractors that move the ultrasonic sensor in and out of the air slot have no guidance mechanism, off-center and misalignment can move the range finder away from its target. When this happens, a backup location-tracking system is needed until centering and alignment are restored, possibly by operator intervention. We have demonstrated that the analysis of video footage acquired at the same time as laser-range-finder output can serve this purpose. Since the U81 laser range finder has a laser pointer, video analysis can automate the switch from laser to video and back. This is the immediate goal of our ongoing research in the lab. A somewhat more distant goal is to automate the selection and tracking of a feature in the video to be the basis for the triangle-similarity approach to location tracking. Edge detection is expected to be key to this development.  
Conceptualization, C.M., J.M., C.C. and E.C.; methodology, C.C. and E.C.; software, H.W. and C.C.; validation, C.M., E.C. and C.C.; formal analysis, J.M. and C.C.; investigation, C.C. and E.C.; resources and funding, C.M.; writing--original draft preparation, J.M.; writing--review and editing, J.M., C.C., E.C. and C.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Pacific Northwest National Laboratory, grant number 516084.  
The authors gratefully acknowledge assistance from Washington River Protection Solutions staff members who provided data on the structure of Hanford DSTs that enabled us to map the air-slot network in relation to tank-bottom welds and the Pacific Northwest National Laboratory for the realistic air-slot mockup provided to WSU-TC. This mockup enabled demonstration of the validity and effectiveness of our proposed position-tracking techniques.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.