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
The Lanzhou Geomagnetic Observatory provides continuous observation of the
Earth magnetic field. It is one of the oldest magnetic observatories of China
The magnetic instruments include a VM391 three-axis and homocentric fluxgate
magnetometer providing 1 s vector data
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. Like all recent
computers, the data logger is equipped with a material clock: it includes a
64-bit counter that starts when the system is switched on and computes
incremental values
A GPS antenna is installed on the roof of the observatory, connected to a GPS receiver that provides a PPS signal to the data logger via a RS232 link. The width of the PPS signal can be configured between a few microseconds and a few milliseconds. After every PPS emission, the GPS receiver provides also the complete date in UTC hours through the same link. This time stamp pertains to its previous PPS and thus it corresponds to an integer number of seconds. This is also the desired time for obtaining magnetometer readings.
Since the frequency of the quartz oscillator depends on its temperature, it
is necessary to keep track of the drift of the computed time
In the case of a failure of the PPS signal, the data logger uses the last
values of
When we noticed that time synchronization using GPS PPS was unavailable
for LZH data, we first decided to use data readily available at IPGP or on
INTERMAGNET to see if we could get a reasonable estimate of the
time-stamp error of the recorded data. We first selected observatories on the
same longitudinal sector as Lanzhou: the nearest observatory available is the
one at Phu-Thuy (PHU) in Vietnam at nearly 1700
Locations of geomagnetic observatories. Geomagnetic coordinates from
IGRF model for year 2014
Whenever magnetic pulsations were recorded simultaneously at the various
observatories, these signals were used to evaluate the time lag between the
various time series. The lag is defined as
lag
Time series of the standardized magnetic
The bottom panel of Fig.
Cross-correlation between all analysed observatories over 1 h of data (11:30–12:30 UTC) on 6 July 2014, one of the last days without GPS synchronization for LZH data logger. The second acquisition system of Lanzhou observatory is labelled LZ2.
To obtain the estimation of the time lag, the filtered time series of each
measured component at all observatories have been cross-correlated in pairs.
An example for that same day, during local night, is shown in
Fig.
Time lags calculated every day during 1 h around 17:30 UTC
comparing Lanzhou and Kakioka data for each component of the magnetic field
From this first analysis, it appeared clearly possible to use distant
observatories for verifying the data synchronization, but the precision is
not sufficient for the purpose of correcting the data time stamps. We
computed estimates of the time lag for each day between January 2013 and
July 2014, obtaining coherent trends for all pairs of observatories.
Figure
Time lags calculated for every hour using the data of the two
acquisition systems available at Lanzhou observatory for each component of
the magnetic field
It was therefore decided to use the data of the second acquisition system
available in Lanzhou inside the same building for computing the time lags
suitable for correcting the time stamps. Two different sensors were deployed
on the second acquisition system, one in 2013 and another in 2014, making it possible
to generate a complete dataset for comparison. The resulting curves, shown
in Fig.
After computing all the time lags, it was decided that only the period up to
2 April 2014 was suitable for time-stamp correction, since the clock drift
was very slow during that time. Data for the period between 2 April and
8 July 2014, when the time drift was of 2
The time synchronization of LZH VM391 and GSM90 instruments was lost on 7
March 2013, but it was noticed only 1 year later since the drift of the
data logger clock was very slow. At that time, the acquired data had
accumulated a lag of 28
Seasonal variation of the data logger energy card temperature during 2013 and 2014. This temperature is measured every minute and we assumed that it is similar to the oscillator temperature.
The most significant part of the lag was accumulated during the summer
months, when the temperature of the data logger was the highest
(Fig.
To avoid having similar issues in the future, some options are possible:
Use a data logger with a temperature-compensated crystal oscillator
(TCXO). Improve the software for clock correction to save the corrections to the quartz
frequency so that they are available even after a reboot of the data logger and possibly also include temperature
correction. Improve the monitoring tools of the observatory so that similar failures could be detected more easily.
It should be pointed out that this particular situation occurred because, at that time, the log files were not routinely transmitted along with the data files.
LZH monitoring data are now regularly transmitted to IPGP and checked to prevent future occurrences of unnoticed time synchronization unavailability.
The GPS time synchronization of LZH magnetic observatory was lost on
7 March 2013. Over 1 year, the time-stamp attribute to the acquired data
had accumulated a lag of 28
Magnetic observatory data used for this paper are available
at the Bureau Central de Magnetisme Terrestre (BCMT) website
(
PC analysed the data, produced the corrected files and wrote the article; BH identified the problem and produced definitive data; KT and XL developed the LZH observatory acquisition system and interpreted the instrument response; VL validated the methodology; and CJX processed the data. PC prepared the paper with contributions from all co-authors.
The authors declare that they have no conflict of interest.
This article is part of the special issue “The Earth's magnetic field: measurements, data, and applications from ground observations (ANGEO/GI inter-journal SI)”. It is a result of the XVIIth IAGA Workshop on Geomagnetic Observatory Instruments, Data Acquisition and Processing, Dourbes, Belgium, 4–10 September 2016.
The results presented in this paper rely on the data collected at Lanzhou,
Phu Thuy, Da Lat, Kakioka and Cheongyang observatories. We thank IPGP,
Chinese Earthquake Administration, Vietnam Academy of Science and
Technology, Japan Meteorological Agency, Korea Meteorological Administration
for supporting their operations and INTERMAGNET for promoting high standards
of magnetic observatory practice (