Ambient Noise Cross-Correlation for GPS-Free Clock Offset Recovery in Offline Distributed Sensor Networks

We propose using ambient noise cross-correlation (ANCX) to recover pairwise clock offsets between sensor nodes that share a common propagating background signal, requiring neither GPS, NTP, nor any inter-node message exchange during recording. In controlled simulation (5 nodes, ±2 s random clock offsets, 8 seeds), cross-correlation correction raises Pairwise Ordering Accuracy (POA) from 0.798 to 0.960. In a real-world experiment using IRIS FDSN seismic waveforms from two physically separated broadband stations (IU.ANMO × XR.SC04, 75 km apart), seven injected clock offsets (0–10 s) are all recovered with 0 ms error. The method is directly applicable to GPS-denied sensor networks: underground microseismic monitoring, submarine acoustic arrays, and distributed structural health monitoring. Simulation code and figures are included.