Abstract
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.
Highlights
The Lanzhou Geomagnetic Observatory provides continuous observation of the Earth magnetic field
The acquisition system used for recording LZH data from the VM391 and GSM90 magnetometers includes a GPS receiver that provides a pulse per second (PPS) signal for precise time stamping of the acquired data
When we noticed that time synchronization using GPS PPS was unavailable for LZH data, we first decided to use data readily available at Institut de Physique du Globe de Paris (IPGP) or on INTERMAGNET to see if we could get a reasonable estimate of the time-stamp error of the recorded data
Summary
The Lanzhou Geomagnetic Observatory provides continuous observation of the Earth magnetic field. It is one of the oldest magnetic observatories of China (Yang, 2007) established during the International Geophysical Year initiatives in 1959. The magnetic instruments include a VM391 three-axis and homocentric fluxgate magnetometer providing 1 s vector data (Chulliat et al, 2009) and a GSM90 scalar magnetometer providing 5 s data. Both are controlled by a data logger running on an Acrosser AR-ES0631 fanless embedded system, using designed software
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