GPS usage is notoriously energy-intensive; its energy efficient use as a sensor has been addressed by various studies,
such as the aforementioned Jigsaw system [83] and the Rate-Adaptive Positioning System (RAPS) [93]. RAPS uses various
techniques to duty cycle the GPS; for instance, RAPS turns off the GPS when the accelerometer indicates that the user is
stationary and uses the space–time history of user movements to estimate when to turn it on. RAPS also predicts whether a
GPS fix is likely to be successful given the GSM signature and its related Received Signal Strength reading. In [94], GPS data
from taxicabs is used to detect gas station visits, measure the time spent, and estimate overall demand.
Combining GPS information, Wi-Fi scanning, and accelerometer readings, the SensLoc system [95] offers a context-aware
location service that abstracts location to provide placed detection and recognition as well as on-demand path tracking. GPS