Hybrid GPS and GSM localization — energy-efficient detection of spatial triggers

For realizing proactive Location-based Services it is necessary to automatically detect when the mobile device enters or leaves certain geographical zones. Assuming terminal-based positioning like GPS, in a rudimentary approach this can be realized by continuous operation of the GPS receiver, where the generated position fixes are always checked against the zone definition. However, as GPS is very energy-consuming, this will lead to a quick battery drain and thus to a bad user experience. As a possible solution, this paper presents a novel technique which dynamically switches on and off the GPS receiver. The goal is to keep the GPS receiver off as long as possible in order to save energy, while still being able to detect the entering or leaving of a zone. The basic idea is to compare the device's current GSM measurements with the ones taken the last time the GPS was switched on. If, based on that comparison, it can be ruled out that the zone has been entered or left, the GPS receiver may remain switched off. For comparing GSM readings, the classical Common Base Station indicator is extended by considering multiple samples and incorporating values of Received Signal Strength. The paper presents results based on simulated as well as real test data, which underline the validity of the method.