A resource--efficient time estimation for wireless sensor networks

We study the problem of providing a sensor node with an accurate estimate of the current time, from a novel prospective which is complementary to the well--studied clock synchronization problem. More precisely, we analyze the case in which a sensor node is temporarily unable to run a clock synchronization protocol due to low--energy, or intermittent connectivity, or process failures, but still requires an accurate estimate of the time. We propose and analyze two efficient clock reading methods, one deterministic and the other probabilistic, which are designed to work in synergy with clock synchronization protocols to provide a better time estimate. Our deterministic method achieves a better accuracy by exploiting the sign of the global deviation of the hardware clock from the reference time, and can be applied to reduce the frequency of the periodic clock adjustments by a factor 2, while maintaining the same time uncertainty. The second method based on time series forecasting, is more flexible than the previous one since it is independent of the frequency at which clock synchronization occur.