Empirical Determination of Efficient Sensing Frequencies for Magnetometer-Based Continuous Human Contact Monitoring

Abstract

The high linear correlation between the smartphone magnetometer readings in close proximity can be exploited for physical human contact detection, which could be useful for such applications as infectious disease contact tracing or social behavior monitoring. Alternative approaches using other capabilities in smartphones have aspects that do not fit well with the human contact detection. Using Wi-Fi or cellular fingerprints have larger localization errors than close human contact distances. Bluetooth beacons could reveal the identity of the transmitter, threatening the privacy of the user. Also, using sensors such as GPS does not work for indoor contacts. However, the magnetometer correlation check works best in human contact distances that matter in infectious disease transmissions or social interactions. The omni-present geomagnetism makes it work both indoors and outdoors, and the measured magnetometer values do not easily reveal the identity and the location of the smartphone. One issue with the magnetometer-based contact detection, however, is the energy consumption. Since the contacts can take place anytime, the magnetometer sensing and recording should be running continuously. Therefore, how we address the energy requirement for the extended and continuous operation can decide the viability of the whole idea. However, then, we note that almost all existing magnetometer-based applications such as indoor location and navigation have used high sensing frequencies, ranging from 10 Hz to 200 Hz. At these frequencies, we measure that the time to complete battery drain in a typical smartphone is shortened by three to twelve hours. The heavy toll raises the question as to whether the magnetometer-based contact detection can avoid such high sensing rates while not losing the contact detection accuracy. In order to answer the question, we conduct a measurement-based study using independently produced magnetometer traces from three different countries. Specifically, we gradually remove high frequency components in the traces, while observing the correlation changes. As a result, we find that the human coexistence detection indeed tends to be no less, if not more, effective at the sampling frequency of 1 Hz or even less. This is because unlike the other applications that require centimeter-level precision, the human contacts detected anywhere within a couple of meters are valid for our purpose. With the typical smartphone battery capacity and at the 1 Hz sensing, the battery consumption is well below an hour, which is smaller by more than two hours compared with 10 Hz sampling and by almost eleven hours compared with 200 Hz sampling. With other tasks running simultaneously on smartphones, the energy saving aspect will only become more critical. Therefore, we conclude that sensing the ambient magnetic field at 1 Hz is sufficient for the human contact monitoring purpose. We expect that this finding will have a significant practicability implication in the smartphone magnetometer-based contact monitoring applications in ...

Non-Pharmaceutical Interventions against COVID-19 Pandemic: Review of Contact Tracing and Social Distancing Technologies, Protocols, Apps, Security and Open Research Directions

“…First, GPS systems reveal device identity and locations, therefore, are prone to security and privacy-related vulnerabilities. Furthermore, it has been observed that GPS-based systems suffer from poor co-location accuracy, high battery consumption and are not suitable for indoor applications [ 60 , 61 , 62 ]. Despite these weaknesses of GPS-based systems, there are some Apps developed leveraging GPS technology in the fight against the coronavirus.…”

“…On the contrary, the notification is rejected and normal life continues [ 14 ]. However, this app most likely suffers poor accuracy and will not be suitable for indoor applications as applicable to GPS-based systems [ 60 , 61 ].…”

A Smartphone Magnetometer-Based Diagnostic Test for Automatic Contact Tracing in Infectious Disease Epidemics

“…We measured the magnetic field strengths at the default sampling frequency of 10 Hz, a popular magnetometer sensing rate in the literature [17]. The magnetometer readings are obtained in three phone-specific axes: X, Y, and Z.…”

“…The lowpower smartphone magnetometers can only be affected by ferromagnetic structures within a few meters [15], [16]. Only the co-existing smartphones within this distance can bear sufficient similarity in their magnetometer readings [11], [17]. This last characteristics is especially important as many infectious disease transmissions occur in close distances.…”

“…First, many of the operating parameters are still to be determined. In particular, the exiting works [11], [13], [17] consider only two extreme and impractical cases: either continuous contact or no contact during the whole duration of comparison. However, when two people, possibly strangers, make a contact, its duration can be very small compared to the entire span of comparison (which can match the most active transmission period of the disease).…”