In an online GPS tracking system, a fundamental trade-off exists between timeliness, the average time interval between a recorded change in location and a change in the reported location; accuracy, the average error between the actual location and the reported location; and uplink usage, the average amount of data used per second of tracking. While tracking efficiency has been addressed in the literature, our thrifty tracking system presents the first unified view of timeliness, accuracy, and uplink usage, allowing the user to specify desired targets for any two of these objectives, while optimizing the third. We also provide a closed-form characterization of this three-way trade-off, and demonstrate that our system converges to the predicted performance.

A typical online GPS tracking system uses a cellular uplink to report the location of a device to a central server, and in a study based on 1.6 billion location updates we find at least 90% are sent with a fixed 1--300 second period. Through experiments with the cost of cellular data transmission we also find that every packet sent incurs significant overhead.

Smartphones with Wi-Fi enabled periodically transmit Wi-Fi messages, even when not associated to a network. In one 12-hour trial on a busy road (average daily traffic count 37,000 according to the state DOT), 7,000 unique devices were detected by a single road-side monitoring station, or about 1 device for every 5 vehicles.

A real-time travel time estimation system is demonstrated that uses Wi-Fi monitors deployed along surface streets to capture Wi-Fi packets from smartphones in passing vehicles. Travel time is estimated based on observations of the same vehicle by two or more monitors.