Reliable and ultra-low power time synchronization becomes more and more important with the popularity of energy harvesting sensor nodes. This paper proposes an untethered and probabilistic ultra-lower power time synchronization method for energy intermittent sensor network. It avoids the frequent RF communications with the assistance of a solar clock. The SATS system consists of two main parts: the synchronizer, a low power solar clock module for time synchronization, and the S3-Mapping, an offline sequence matching algorithm. Furthermore, we develop an improved version of S3-Mapping, which reduces the computation complexity from exponential to linear using the redundancy models and the onion peeling method. The SATS system is validated by both simulations and a prototype, which shows that the second level synchronization precision can be achieved under reasonable probability. What's more, the energy consumption of time synchronization is reduced by over 1 ~ 2 magnitudes compared with the up-to-date low power time synchronization protocol.
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