Analysis of radio surveillance in search of missing persons - An empirical study of Bluetooth Low Energy and Wi-Fi characteristics

Abstract: Development of technical and methodological support has been identified as a key area for future research and development within the search and rescue (SAR) of missing persons in Sweden. Previous studies have demonstrated the feasibility of using Wi-Fi signals to locate a missing person, and also developed hardware and software for SAR systems based around Unmanned Aerial Vehicles (UAV). This thesis empirically studies the actual transmission behaviour of three non-connected smartphones over Wi-Fi and Bluetooth Low Energy (BLE), under the assumption that a missing person could be carrying a smartphone with them. Furthermore, the thesis studies the range of relatively low-cost off-the-shelf Wi-Fi and BLE devices in a “best-case” scenario with free line-of-sight and a slightly elevated transmitter. Finally, the thesis studies how antennas and other factors impact range. The results show that all of the tested smartphones transmit so called Wi-Fi probe packets when Wi-Fi is enabled. Though, these probes are, in most cases, transmitted during short events separated by intervals of 5 to 9 minutes, depending on the smartphone. Furthermore, in most cases only a few packets are transmitted in each event. The interval lengths and scarce packets imply significant limitations to the usability of Wi-Fi in SAR. BLE broadcast transmissions, on the other hand, require that some kind of advertising service is enabled, but are transmitted at least every 10.5 seconds, due to the strict regulation in the Bluetooth standard. This was also demonstrated with the studied iPhone that continuously transmitted BLE packets every two seconds. Roughly 60 % of the smartphones in use in Sweden today are Apple devices, thus a significant portion of all the smartphones in use are likely to be detectable via BLE within brief time intervals. In the tested “best-case” scenario with free line-of-sight and a slightly elevated transmitter, at least 80 % of the transmitted Wi-Fi probes are detected up to 500-600 meters with a maximum range, with sparsely detected packets, of up to about 700 meters. Correspondingly, at least 80 % of the BLE advertisements are detected up to 60-110 meters with occasional packets detected up to a maximum range of approximately 210 meters. However, the ranges for both Wi-Fi and BLE are highly dependent on the receiving device. In addition to the receiver device and standard, it is found that the transmission power and antenna directivity of the transmitter have a potentially substantial impact on the range with a possible combined difference in directed transmitted power of up to 49 dB. This corresponds to the difference between a strong signal and no detected signal at all. In summary, there are clear differences between the properties of BLE and Wi-Fi, both in terms of range and how different devices transmit broadcast packets (transmission behaviour), resulting in both having advantages and disadvantages in SAR applications. There are also a number of factors on the transmitter and receiver devices that could have considerable impact on the possibility of detecting a device. Concludingly, both BLE and Wi-Fi could fill a role in SAR of missing persons, but both also have major drawbacks that are severely limiting their possible use.