Smartphones represent an ideal platform for any human-centric sensing application thanks to their programmability,
their flexibility, and their extensive application distribution opportunities. As illustrated in [70], smartphones have been
equipped with sensors to improve various aspects of the user’s phone experience: accelerometers and gyroscopes [71] can,
for example, detect the orientation of the phone (to adjust the display or to enhance the gaming experience), light sensors
can serve to adjust the brightness of the display, and proximity sensors can detect whether the phone is close enough to
the user’s face so that the touchscreen may be turned off or unwanted touch input may be prevented. Other sensors such
as microphones, cameras, electronic compasses [72], and GPS receivers have been included to provide complementary services.
Launched in April 2013, Samsung’s Galaxy S4 was the first smartphone to feature a barometer, a thermometer, and a
hygrometer.