## 3 Results and Discussion
### Answering RQ3 to RQ5
The markerless tracking method also takes the second place in dominant tracking methods for AR-based solutions in manufacturing, with 33 articles, equivalent to 16.5% of 200 selected articles or 25% of 133 tracking relevant articles. Next comes the hybrid tracking method that is applied in 21 articles, equaling 10.5% of 200 selected articles or 16% of 133 articles containing the tracking method. As in Figure 15, it also shows that there is a tendency towards using the hybrid tracking method in manufacturing in the last three years. Similar to hybrid tracking, model-based tracking has significantly increased in recent years, which is due to the built-in model tracking packet of popular AR software development platforms such as Unity and Vuforia. This method is implemented in 13 articles--6.5% of 200 selected articles or 10% of 133 articles contributing to tracking content. The sensor-based tracking method is the least favorite tracking technique in implementing AR solutions for manufacturing, with only 3 articles. This can be explained by the high cost and complex hardware that is usually required \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Classification Criteria**} & \multicolumn{2}{c}{**References**} \\ \hline \multirow{8}{*}{\begin{tabular}{c} Tracking \\ method \\ \end{tabular} } & Marker-based & [7,55,64,77,79,81,84,88,93,94,97,98,100] \\  & tracking & [103,104,109,112,121,122,125,130,132] \\  & \begin{tabular}{c} Marker-based \\ tracking \\ \end{tabular} & [142,145,147,149,152,155,157,158,160] \\  & \begin{tabular}{c} [164,165,168,169,171,174,177,178] \\ \end{tabular} \\  & & [180,184,186,193,198,199,202,205] \\  & & [206,209,211,212,214,215,218,221,223] \\ \cline{2-3}  & \
\begin{tabular}{c} [30,51,54,57,61,62,65,66,69,72,74] \\ \end{tabular} \\  & Markerless tracking & [119,138,150,156,159,162] \\  & & [127,127,157,189,192,197] \\  & & [201,203,204,210,211] \\  & & [216,217,219,220,225] \\ \cline{2-3}  &
\begin{tabular}{c} Model based tracking \\ \end{tabular} & [8,67,68,73,86,139,141,153] \\  & & [154,191,194,222,235] \\ \hline \multicolumn{2}{c}{Sensor-based tracking} & [11,161,200] \\ \hline \multirow{2}{*}{Hybrid tracking} & [52,53,56,58,59,71,90,12,127] \\  & [140,163,166,167,173,179,188,190] \\  & & [195,196,208,213] \\ \hline \hline \end{tabular}
\end{table} 
Table 15: Articles on Tracking methods from 2010–2021. for indoor tracking, utilizing ultrasound, magnetic sensors, etc. In addition, the indoor environments of factory plants, production lines or laboratories usually consist of several types of equipment, machines, objects and so on, which could block sensor signals, thus reducing this tracking method's effectiveness [245]. This is why it only appeared in 1.5% of 200 selected articles or 2% of 133 articles regarding tracking methods. In order to gain a more comprehensive understanding of how these tracking methods have been applied in AR-based solutions in manufacturing, some representative works relevant to each type of tracking method are depicted ij more detail in the following, while the rest are listed in Table 15.  
The marker-based tracking method is fast, simple and robust, thus it is broadly utilized for various scenarios in the manufacturing context. For instance, it was implemented to facilitate a human-centered intelligent manufacturing workplace with an AR instructional system for assembly processes regarding highly customized products in some cases [64; 93; 142; 165; 164]. On the other hand, markers could be added to machines as a priori for tracking to assist maintenance processes [77; 143; 144] or to provide useful instructions to newcomer shop-floor operators [199]. In an innovative case, the AR marker-based method was applied for measuring and evaluating casting quality through hand-pouring motion tracking [193].