# Evaluation of the SteamVR Motion Tracking System with a Custom-Made Tracker
## 2 Tracker and Measurement Setup
As a result, 65 different orientations were obtained, and tested at each of the nine points in the room, which give a total of 585 tested launcher positions. A greater concentration of measurements would be too time-consuming, due to the need to manually set the tripod for each measurement. A model of the launcher was developed based on a PVC pipe with a diameter of 75 mm, on which the other elements, made by 3D printing, were mounted. To mount the tracker on the launcher model, a Picatinny rail (STANAG 2324) was used, which in the real launcher is used for mounting the scope. The model was equipped with a handle that allows it to be stably mounted on the tripod head (Figure 3). The model of the user's head, along with the HMD goggles from the HTC VIVE set, were mounted on the launcher. These are the goggles that will be used in the simulator application, so that real conditions can be reproduced more faithfully. Before the measurements were taken, the SteamVR system's working area was calibrated, which determined the location of its coordinate system. The controller from the HTC VIVE set was used for the calibration. The origin of the \(Y_{v}\) axis, denoting height, was determined at floor level by measuring the height of the controller placed on it. The \(X_{v}\) and \(Z_{v}\) axes were set parallel to the boundaries of the rectangular working area. This area was  
Figure 3: Photo of the launcher model with the mounted tracker and the user’s head model. Figure 2: Photo of the laboratory room with a tracker and a model of the launcher on a tripod (**a**); and diagram of the distribution of the measuring points (“x” symbols) (**b**). The blue line marks the walls of the room, and the red and green circles symbolise the base stations. determined by measuring the position of the controller at the four corner points so that the origin of the SteamVR coordinate system (\(O_{v}\)) coincides with the centre of the working area (\(O_{r}\)). In turn, the \(X_{v}\) and \(Z_{v}\) axes should be parallel to their limits. During preliminary measurements, however, to appeared that the SteamVR coordinate system did not coincide with the assumed coordinate system. The differences concerned both the shift between the origins (\(X_{c}\), \(Y_{c}\), \(Z_{c}\)) as well as the rotations (\(A_{c}\), \(B_{c}\), \(\Gamma_{c}\)); as a result, the directions of the individual axes of both systems did not coincide (Figure 4). Such a result was expected, as similar problems were noted by other authors of the SteamVR system research [26; 27]. The values of translation and rotation shown in the above diagram, once the calibration was corrected, did not exceed a few centimetres and a few degrees for each respective axis. The translation and rotation values for the individual axes changed when the SteamVR was restarted, or when synchronisation between the base stations was lost. The resulting errors could be compensated by additional calibration before using the SteamVR system. This issue is discussed in greater detail throughout the paper. Data acquisition consisted of manually setting the launcher position on a tripod in accordance with the guidelines calculated by the program. After setting, the user started the acquisition of results containing information about the position and orientation of the tracked objects, such as the tested tracker, the HMD goggles, and the base stations. The measurement method described above is an extension of the method used in [25], where the tripod was also set at subsequent points in the room. In the method described here, however, a tripod head rotation element has been added to assess potential problems with tracker visibility more accurately. In work on motion tracking systems, measurements with an industrial robot arm are often used [21; 28], but this method makes it difficult to perform tests in a larger working area.