Augmented-Reality Survey: from Concept to Application <s> Sensor-based approaches <s> The authors describe the design and prototyping steps they have taken toward the implementation of a heads-up, see-through, head-mounted display (HUDset). Combined with head position sensing and a real world registration system, this technology allows a computer-produced diagram to be superimposed and stabilized on a specific position on a real-world object. Successful development of the HUDset technology will enable cost reductions and efficiency improvements in many of the human-involved operations in aircraft manufacturing, by eliminating templates, formboard diagrams, and other masking devices. > <s> BIB001 </s> Augmented-Reality Survey: from Concept to Application <s> Sensor-based approaches <s> This survey paper presents a classification of different challenges and tracking techniquesin the field of augmented reality. The challenges in augmented reality arecategorized into performance challenges, alignment challenges, interaction challenges,mobility/portability challenges and visualization challenges. Augmentedreality tracking techniques are mainly divided into sensor-based tracking, visionbasedtracking and hybrid tracking. The sensor-based tracking is further dividedinto optical tracking, magnetic tracking, acoustic tracking, inertial tracking or anycombination of these to form hybrid sensors tracking. Similarly, the vision-basedtracking is divided into marker-based tracking and markerless tracking. Eachtracking technique has its advantages and limitations. Hybrid tracking provides arobust and accurate tracking but it involves financial and tehnical difficulties. <s> BIB002
The sensor-based approaches are divided into acoustic tracking, magnetic tracking, optical tracking, and inertial tracking. Similarly, the vision-based approaches are divided into marker-based tracking and markerless tracking BIB002 . Both the limitations and the advantages are applicable for each approach. Rabbi et al. BIB002 summarized the characteristics of the different sensors, as shown in Table 4 .  Optical tracking: Optical tracking is a 3D-localization technology that is based on the monitoring of a defined measurement space that is achieved with the use of two or more cameras. One of the advantages of optical tracking is that it allows for a simultaneous tracking of many objects BIB001 .  Magnetic tracking: When it is used for AR, the magnetic-tracker transmitter acts as the origin of the virtual coordinate system, and by mounting a receiver to the viewer, the position and orientation can be determined [22] .  Acoustic tracking: Acoustic tracking measures the length of time that is taken for an acoustic signal to reach a variety of receivers  .  Inertial tracking: Inertial tracking is performed using an accelerometer and a gyroscope. The accelerometer measures the linear acceleration while the gyroscope measures the angular velocity  . 