Synchronization is a challenging problem for wireless nodes, especially for applications demanding good synchronization accuracy over wide areas. In that case, the GPS is a valuable solution as the nodes can independently synchronize to UTC. However, the energy consumption of a GPS receiver (over 100 mW when switched on) is not sustainable on a wireless node. Therefore, in this work, we developed a synchronization scheme based on periodic extinctions of the GPS receiver. The goal is to study the GPS power switching effect on the synchronization accuracy. To do so, a node with dedicated timestamping hardware was designed. Two clock models were compared to predict the node time when the GPS is off and the impact of a Kalman filter, to remove the GPS noise, was evaluated. From experimental data, we show that the choice of the clock model depends on the accuracy needed and that the Kalman filter improves the estimation of the clock frequency for both models. In our design, the GPS can be off from 60% up to 95% of the time for mean synchronization errors of 20 ns to 420 ns, respectively. This work demonstrates that GPS power switching is an efficient solution to reduce energy costs while maintaining a high synchronization accuracy.

Last decade, the need for wireless sensors solutions as core-solutions of Structural Monitoring gained in interest. The cost of wireless devices compared to the cost of wiring important structures (bridges, energy-plants,...) is attractive. Most of recent development in WSN domain focused on energy (saving or harvesting), on wireless protocols, on embedded algorithms. But it is a fact that, most of monitoring applications need samples to be time-stamped. According to the application, the wished time resolution could be up to one second for automation monitoring, one millisecond for vibration, one microsecond for acoustic monitoring, one nanosecond for electricity or light propagation... The consequence for a Wireless network of electronic nodes is that, by nature, no common signal could physically provide a synchronization top. But, as each electronic device, a wireless sensor time-base uses a timer incremented by a quartz whose initial value is theoretical up to some p.p.m. and whose period drift on time because of age, temperature,... Two kind of solutions could be regarded : a synchronization signal provided by the wireless protocol itself; an absolute synchronization from a referential source such as: GPS, Frankfurt clock, Galileo,... In the first way, it will be demonstrated the poor accuracy and the need of energy such a mechanism offers. In the second way, the article will details how a deterministic (Universal Time), accurate and resilient algorithm has been implemented. The article also provides specific results of application on acoustic monitoring system and electricity propagation where the accuracy of a WSN has reached up to 10 nanosecond UT. Consequence on energy consumption of this algorithm are given with a description of future works to improve the energy balance while keeping the device sober and synchronized.

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