## 3 Results  
Figure 2 shows a screen shot of the graphical user interface (GUI) developed to display the position of the robot in an air slot. Air slots are bifurcated structures shown by the blue lines in the diagram on the left-hand side of Figure 2. The red lines show the AP-type tank-bottom weld pattern in relation to air slots. Figure 1: Conceptual design of a robot to move sensors through air slots. Three-unit design allows the robot to navigate bends. Cameras on crawler units are shown in yellow. The red unit between crawlers carries sensor and actuator. Reprinted from [11] with permission. Figure 2: Graphical User Interface to display the robot position in the air slot (**left**), a video frame from a camera on the robot (**top right**), and data like that expected from non-destructive evaluation (NDE) of double-shell tanks (DSTs) (**bottom right**). The black dot indicates the position of the robot in the air slot. Tank AP-107 has 24-inch annulus-access risers labeled 030 and 031 at the 90- and 270-degree positions, respectively, in the air-slot diagram on the left-hand side of Figure 2. The black dot on the air slot near annulus-access riser 030 shows how tracking of the robot's location in an air slot will be dynamically displayed on the GUI. The top-right panel in Figure 2 is the part of the GUI reserved for real-time display of video footage from a camera on the robot. In Figure 2, this function is illustrated by a video frame from visual inspection of air slots in tank AP-107 [6]. The image shown in the bottom-right panel of Figure 2 illustrates various types of data that can be transmitted from the robot to inspection personnel. This portion of the GUI will most often be used to display real-time data from the GWPA sensor and messages to the operator. Several methods of position tracking are currently under investigation, including cable layout and ultrasonic reflections. We have focused on laser ranging augmented by video analysis. Due to poor illumination in the air slot, video analysis is not practical as a stand-alone system because absolute location of the robot must be the accumulation of many relative measurements of motion toward markers in the region currently illuminated by light sources on the robot. Such a method is vulnerable to large error due to the accumulation of error in relative distances. Laser ranging is not limited by illumination of the air slot but may fail if movement of the robot carrying the laser causes it to bounce off the walls of the air slot rather than the end of the channel that is a known distance from the entrance. Video analysis complements laser ranging in this situation because the spot illuminated by the laser pointer is present in the video, which can be used to monitor misalignment leading to faulty readings. When this happens, video analysis takes over location tracking using the last accurate report from laser ranging as the absolute location of the robot. Restoration of proper alignment, which can be seen in the video, signals return to tracking by laser ranging. Triangle similarity [10] is the most common approach to distance measurement in image analysis. It requires calibration of the camera, which can be performed offline, to determine the apparent focal length F = PD/W, where D is the distance to an object of known width W, and P is the width of the image of the object in pixels. Given F, we can calculate distance D = WF/P to any mark of known width W from its image width in pixels. If P\({}_{1}\) and P\({}_{2}\) are image widths of the mark in earlier and later frames, respectively, then the robot has moved a distance WF/P\({}_{1}\)-WF/P\({}_{2}\) toward the mark in the time between those frames. In our laboratory experiments, we use a rear camera pointed toward the channel entrance. In this arrangement, the distance moved away from a mark is WF/P\({}_{2}\)-WF/P\({}_{1}\).