μs-Scale time synchronization for energy-constrained mission-critical DTSNs

It is generally considered a trivial task to synchronize time in a distributed sensor network having a GPS module on board for each sensor node. This is not quite true depending on the application at hand. Commissioned to implement YuShanNet, a delay-tolerant sensor network (DTSN) enabling encounter information collection for hiker search and rescue, we find that the naive use of standard GPSs time acquisition functionalities gives a synchronization error at the scale of 10-100s milliseconds, which is not sufficient given two application-specific considerations: (1) data goodput and (2) energy efficiency. The two requirements are crucial in that the matter is life or death and the hiking trips usually take days. Towards efficient use of the intermittent connectivity in YuShanNet, we design, implement, and evaluate a time synchronization mechanism that improves the synchronization accuracy by approximately two orders of magnitude. This enables TDMA-fashioned MAC for high-goodput data transmission while the GPS is duty-cycled for energy efficiency.