  Adding location to the available information enables a new category of
applications. With the constrained battery on cell phones, energy-efficient
localization becomes an important challenge. In this paper we introduce a
low-energy calibration-free localization scheme based on the available internal
sensors in many of today's phones. We start by energy profiling the different
sensors that can be used for localization. Based on that, we propose GAC: a
hybrid GPS/accelerometer/compass scheme that depends mainly on using the
low-energy accelerometer and compass sensors and uses the GPS infrequently for
synchronization. We implemented our system on Android-enabled cell phones and
evaluated it in both highways and intra-city driving environments. Our results
show that the proposed hybrid scheme has an exponential saving in energy, with
a linear loss in accuracy compared to the GPS accuracy. We also evaluate the
effect of the different parameters on the energy-accuracy tradeoff.
