Articles | Volume 6, issue 2
Geosci. Instrum. Method. Data Syst., 6, 311–317, 2017

Special issue: The Earth’s magnetic field: measurements, data, and applications...

Geosci. Instrum. Method. Data Syst., 6, 311–317, 2017

Research article 01 Sep 2017

Research article | 01 Sep 2017

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 Xin2 Pierdavide Coïsson et al.
  • 1Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS, 75005 Paris, France
  • 2Lanzhou Geomagnetic Observatory, Lanzhou Institute of Seismology, China Earthquake Administration, Lanzhou, China

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 unnoticed interruption of the GPS PPS starting 7 March 2013. The data logger clock drifted slowly in time: in 6 months a lag of 27 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 1 h of 1 s data at arbitrary UTC allowed estimating time lags with an uncertainty of ∼ 11 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 min averaged definitive magnetic data.

Short summary
Data loggers of magnetic observatories use GPS receivers to provide accurate time stamping of recorded data. Typical sampling rate is 1 s. A failure of the GPS receiver can result in erroneous time stamps. The observatory of Lanzhou, China, accumulated a lag of 28 s over 1 year. Using magnetic data recorded at other locations in a radius of 3000 km it was possible to estimate the diurnal lag and correct the time tamps to produce reliable 1 min averages of magnetic data.