## 5 Experiments
### Properties of VL53L1X Proximity Sensor  
An example of a proximity sensor, which could be easily adapted for use in a the presented hybrid localization system concept is the VL53L1X chip produced by ST Microelectronics [36]. It is a small device, which consists of a 940 nm invisible Class 1 laser emitter, SPAD (Single-photon avalanche diode) receiving array and additional optical filters. The main advantage of the sensor is the fact that unlike typical infrared sensors, it employs Time-of-Flight (ToF) method for ranging, which according to the producer is supposed to ensure accurate distance measurement regardless of target's color and surface reflectance [40]. The sensor's properties, which are most important from the localization point of view are listed in Table 1 (the values come from VL53L1X datasheet [36]). The sensor has a range of 4 m, which would be sufficient for indoor localization in typical apartments. Its field of view (FoV) is a conical-shaped beam, the width of which can be programmed in the range of 15-27\({}^{\circ}\). Such property allows to tune the sensor and increase its selectivity. The measurement rate can also be programmed up to 50 Hz, which makes it possible to precisely match the localization rate of a radio subsystem. Additionally, the sensor is small and can be easily incorporated into the radio system's anchor nodes. It is worth noting that the values stored in Table 1 were obtained in particular conditions and for a specific target. As stated in the application note, the measurements were taken for charts of different reflectance, which covered the full sensor FoV. In case of measuring distance from a person, those values might be different. The proposed hybrid algorithms utilize some of the above values (maximum range, field of view). Therefore, to avoid errors resulting from assuming sensor parameters from data sheets, it is necessary to estimate those parameters for reflection from a human body. The performed sensor tests consisted of evaluating its basic properties such as ranging bias, ranging standard deviation and sensor FoV. In the experiment P-NUCLEO-53L1A1 [41] modules consisting of two evaluation boards: STM32F401 RE (nucleo microcontroller board) [42] and X-NUCLEO-53L1A1 (nucleo board expansion with up to three VL53L1X sensors) [43] were used. The module was programmed with the exemplary SimpleRanging application [44]. The application measured distance and sent it to the attached PC over USB. The sensor working mode was set to 'long', ranging rate to 10 Hz and the FoV to the default 27\({}^{\circ}\) width. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline maximum range & 400 cm \\ ranging resolution & 1 mm \\ field of view & 15–27\({}^{\circ}\) \\ maximum measurement rate & 50 Hz \\ package size & 4.9 \(\times\) 2.5 \(\times\) 1.56 mm \\ \hline \end{tabular}
\end{table} 
Table 1: Selected VL53L1X parameters. The first part of the experiment consisted of ranging from a person located at different distances and dressed in different clothes. The sensor was placed at waist level. The measurements were conducted on a sunny day in an office space under two different lighting conditions (blinds opened and closed). The obtained results were compared to the distance measured with an accurate laser distance meter. The distance measurements were conducted for the following persons:  
* person in a black shirt (blinds closed)
* bare-chested person (blinds closed)
* person in a white shirt (blinds closed)
* person in a black shirt (blinds opened)  
For each of the measurements ranging bias and standard deviation were evaluated. The gathered results were used to model bias and standard deviation with polynomial functions, which would be used in the hybrid localization algorithm.