Time-stamp correction of magnetic observatory data acquired during unavailability of time-synchronization services
Pierdavide Coïsson1, Kader Telali1, Benoit Heumez1, Vincent Lesur1, Xavier Lalanne1, and Chang Jiang Xin21Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS, F-75005 Paris, France 2Lanzhou Geomagnetic Observatory, Lanzhou Institute of Seismology, China Earthquake Administration
Received: 04 Mar 2017 – Accepted for review: 14 Mar 2017 – Discussion started: 17 Mar 2017
Abstract. During magnetic observatory data acquisition, the data time-stamp is kept synchronized with a precise source of time. This is usually done using a GPS controlled Pulse Per Second (PPS) signal. For some observatories located in remote areas or where internet restrictions are enforced, only the magnetometer data are transmitted, limiting the capabilities of monitoring the acquisition operations. The magnetic observatory in Lanzhou (LZH), China, experienced an interruption of the GPS PPS in 2013. The data-logger clock drifted slowly in time: in 6 months a lag of 28 s was accumulated. After a reboot on 2 April 2014 the drift became faster, 2 s per day, before the GPS PPS could be restored on 8 July 2014. To estimate the time lags that LZH time-series had accumulated, we compared it with data from other observatories located in East Asia. A synchronization algorithm was developed. Natural sources providing synchronous events could be used as markers to obtain the time lag between the observatories. The analysis of slices of 15 minutes of 1-s data at arbitrary UTC allowed estimating time lags with an uncertainty smaller than ~ 10 s, revealing the correct trends of LZH time drift. A precise estimation of the time lag was obtained by comparing data from co-located instruments controlled by an independent PPS. In this case, it was possible to take advantage of spikes and local noise that constituted precise time-markers. It was therefore possible to determine a correction to apply to LZH time-stamps to correct the data files and produce reliable 1 minute averaged definitive magnetic data.
Coïsson, P., Telali, K., Heumez, B., Lesur, V., Lalanne, X., and Xin, C. J.: Time-stamp correction of magnetic observatory data acquired during unavailability of time-synchronization services, Geosci. Instrum. Method. Data Syst. Discuss., doi:10.5194/gi-2017-13, in review, 2017.