## 9 Conclusions  
Ultrasound trilateration was identified as a very promising approach for indoor mobile positioning. Signals travelling at the speed of sound offer very good accuracy, well below one meter, even with standard smartphone speaker hardware. In addition, the 20-22 kHz range is normally inaudible to humans but can be reproduced by mobile phone speakers and captured with cheap, standard \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Orientation** & **Check Point 1** & **Check Point 2** & **Check Point 3** & **Check Point 4** \\ \hline back-blocked & 0.293 \(\pm\) 0.109 & 0.353 \(\pm\) 0.115 & 0.424 \(\pm\) 0.071 & 0.460 \(\pm\) 0.075 \\ back-direct & 0.330 \(\pm\) 0.085 & 0.305 \(\pm\) 0.082 & 0.318 \(\pm\) 0.081 & 0.907 \(\pm\) 0.232 \\ forward-blocked & 1.464 \(\pm\) 0.149 & 1.428 \(\pm\) 0.131 & 1.771 \(\pm\) 0.160 & 1.074 \(\pm\) 0.103 \\ forward-direct & 0.266 \(\pm\) 0.080 & 0.270 \(\pm\) 0.108 & 0.295 \(\pm\) 0.044 & 1.539 \(\pm\) 0.222 \\ up-blocked & 0.142 \(\pm\) 0.020 & 0.172 \(\pm\) 0.047 & 0.110 \(\pm\) 0.025 & 0.141 \(\pm\) 0.041 \\ up-direct & 0.114 \(\pm\) 0.015 & 0.135 \(\pm\) 0.030 & 0.139 \(\pm\) 0.033 & 0.141 \(\pm\) 0.016 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Difference between MPV and true position \(\pm\) standard deviation for each check point, user location, and orientation. First column identifies combination of phone orientation and user location. Columns 2–5 contain results for each of the four check points in meters. microphones. Because no specialized hardware is used on the client side, the user only needs to make a software (app) installation to use the Lok8 positioning system. The highly directional nature of ultrasound, susceptibility to certain high frequency background noises, and the need for line-of-sight between speaker and receiver were identified as the biggest obstacles to positioning accuracy. A prototype of the positioning system was developed and tested in an office environment for accuracy and other possible shortcomings. Despite the obstacles listed above, it was possible to get full positioning coverage of a 7 \(\times\) 7 m room with four microphones placed directly below the ceiling corners and achieve a satisfactory degree of reliability. On average the system produced accuracies approaching \(\pm\)10 cm, an order of magnitude better than contemporary indoor locationing approaches. The developed indoor positioning system can be evaluated in terms of the user scenarios given in Section 2. Scenarios 1 and 2 involve pointing the phone at an object and doing directional querying. Scenarios 2-4 involve guiding a user around the building. Centimeter-level accuracy means that objects the size of a vase can be correctly identified and queried, provided that orientation of the phone can also be determined with sufficient accuracy. In addition, a positioning fix can be done almost instantly without the need for the system to run continuously in the background, which is ideal for scenarios like directional querying and for improving battery life. The fact that the phone should be momentarily flipped upside-down (_i.e._, speaker facing up) to get the best accuracy is a disadvantage. Doing a positional fix with the screen up (_i.e._, speaker facing down), results in positional accuracy dropping to 30-40 cm. Although ultrasound positioning, in most cases, will not work well from inside a bag or pocket, this is not a problem in any of the given scenarios because there is no need for tracking the user continuously in the background. As soon as the user takes out the phone and starts to interact with the app, his position will be immediately calculated and used by the program. The ability to work in the background is only important if we want to track and record how people move in the building, which was not among our objectives. The Lok8 positioning system was tested in a typical university office/lab environment where it was possible to maintain a direct line-of-sight to all four microphones.