Absolute magnetic measurements are of great importance in magnetic observatories. They allow not only instrument calibration but also data quality checking. They require the vertical and the geographic or true north as reference directions, usually determined by means of a level and by pointing an azimuth mark, respectively. We present here a novel system able to measure the direction of the magnetic field and of the vertical and true north. A design of a north seeker is proposed taking into account sensor bias as well as misalignment errors. Different methods are derived from this model and measurement results are presented. A measurement test at high latitude is also shown.
Measuring the magnetic declination is realized by determining, in a horizontal plane, both magnetic field and geographic or true north direction (in the rest of this paper, the term true north will be employed). Then the angle between them is computed. In magnetic observatory as well as in the field, this value is measured by an observer during the so-called “absolute” measurement step (Rasson, 2005). This procedure consists of two main steps in manipulating a DI-flux instrument. First, the instrument is oriented relative to the magnetic field in order to establish its direction in space. Practically, the magnetic field sensor mounted on the telescope is placed in the horizontal plane. The sensor output is then the projection of the field horizontal component along the sensor sensitive axis or, in other words, the scalar product of both. The most sensitive direction is therefore perpendicular to the magnetic field. Then, the true north is determined by pointing at a target whose azimuth is already known. Finally the observer extracts the magnetic declination from both readings. The target azimuth can be established by different methods: by a gyrotheodolite, by pointing at a celestial body such as the Sun in combination with a clock or by using a GNSS system (Newitt et al., 1996). In any case, this target azimuth value is measured prior to the declination measurement and is assumed constant until it is checked again.
In the last few years, efforts have been made in order to automatize absolute magnetic measurements. Niemegk observatory developed the Geomagnetic AUtomated SyStem (GAUSS) based on a three-axis fluxgate sensor rotating sequentially around two known directions (Auster et al., 2007). At the same time, Dourbes observatory successfully attempted to robotize the DI-flux absolute measurement procedure, leading to the AutoDIF instrument (Rasson and Gonsette, 2011). Today several are operational in different observatories. However, both GAUSS and AutoDIF use the target pointing principle for the true north measurement. The development of an automatic observatory will allow its deployment in remote areas but consequently raises new challenges that were not considered up to now. What would happen if no target were available or if it were not stable? Arctic regions are good candidates to host autonomous systems (Marsal et al., 2017) but drifting ice and permafrost require a constant azimuth update (Eckstaller et al., 2007). Furthermore the idea of automatic observatories also creates a need for automatic true north direction determination. The system described in this paper is an automated DI-flux instrument based on AutoDIF architecture in which the target pointing system has been replaced by an embedded true north seeker.
A fiber-optic gyroscope (FOG) is an absolute rotation sensor and may be able to detect the Earth's rotation. Its principle is based on the Sagnac effect (Arditty and Lefèvre, 1981). Briefly, let us imagine two balls rolling at the same speed but in opposite directions at the circumference of a disc. If the disc is static, an external observer would see both balls crossing each other after half a turn and again at the start point. If the disc is put into rotation, the balls will not reach the start point relative to an inertial frame at the same time. The delay is therefore proportional to the disc rotation speed. FOG-based sensors use a similar principle: two light beams traveling at the same speed along an optical fiber are injected from each end. The phase shift between the two optical waves gives the sensor rotation speed.
North seeker methods are usually sorted in two categories: static
(Liu et al., 2014) and dynamic (Xu and Guo, 2010). In both cases,
the sensitive axis of the FOG is directed horizontally and the
projection of the Earth's rotation vector on it is given by
In the static method, two opposite directions are pointed in order to
compensate for the bias. Due to the
In the dynamic method, the FOG's sensitive axis is also kept
horizontal but continuously turns around a vertical axis. The
phase shift of the FOG output gives the true north direction (
It is also possible to combine both methods by performing static
measurements at regularly spaced angular positions (Abbas, 2013). In
this hybrid case, the sampling time can be optimum. The output is
therefore a discrete sinus curve whose amplitude is given by
The above true north methods do not consider a possible FOG
misalignment. However, it is evident that a horizontal
misalignment has a direct impact on the measurement. Again,
since the sensor is supposed to measure the horizontal
component of the Earth's rotation vector (see Eq. 1), a vertical
misalignment also leads to an error due to the orthogonal
projection of vertical component of the Earth's rotation vector
onto the plane of measurement of the FOG sensor. Many FOG-based
north seekers only have the possibility to rotate around the
vertical axis so that they do not have the opportunity to take
misalignment into account. When looking to the accuracy of
magnetic declination required by international standards like
those established by Intermagnet (Intermagnet, 2012), it
appears evident that such error must be compensated. Indeed,
the 5 nT maximum allowed error on the
Because a DI-flux instrument has 2 principal degrees of freedom, a FOG sensor mounted in the same reference frame as magnetic sensor (i.e., on the telescope in the case of conventional DI-flux instruments such as Zeiss 020) can be oriented in any direction in space. Moreover, the FOG magnetic signature contributes to the magnetic sensor offset and is compensated by the declination/inclination measurement protocol (Gilbert and Rasson, 1998).
The GyroDIF instrument is a non-magnetic robotized platform able
to orient sensors in any direction. It is based on the AutoDIF
system. A fluxgate sensor and a FOG are mounted on the horizontal
axis. Neglecting misalignment errors, both have their sensitive
direction parallel. Piezoelectric motors can rotate the horizontal
and vertical axes with a resolution up to 0.001
GyroDIF instrument.
In the middle of the 1980s, Kring Lauridsen (Lauridsen, 1985)
and David Kerridge (Kerridge, 1988) established a model
mathematically describing the magnetic field vector in the DI-flux
sensor reference frame. The theodolite was supposed to have 2
degrees of freedom, perfectly leveled and free of mechanical
errors such as orthogonality errors or play in axes. They
included a sensor offset and two angles describing the
misalignment of the fluxgate sensitive axis relative to the
telescope optical axis. Kerridge model leads to the following
equation:
However, considering a platform like the GyroDIF with two orthogonal
rotation axes, a similar model can be implemented. Furthermore, this
system also records its tilt angle, which could be modeled by 2
angular degrees of freedom. The Earth's rotation vector can be
expressed in the FOG sensor reference frame with
Considering the GyroDIF as shown in the Fig. 1, the FOG output is given by
computing the third component of previous equation
The static method can be adapted in order to compensate for the
FOG misalignment. For an arbitrary direction
Combining Eqs. (
The four- (or eight-) position method requires to roughly know
a priori the true north direction. Moreover, instrument
uncertainties (angular sensors and FOG) will cause an error even
with an interpolation procedure. Comparatively a hybrid method
combining static and dynamic methods ranges the whole circle and
performs a measurement at regular intervals (e.g., each
10
There are different ways to implement the hybrid method in the
case of GyroDIF. For instance, we can choose to perform all
measurements with
Allan variance plot giving the FOG output SD according to the acquisition time. The minimum value gives the bias stability and the acquisition optimum time.
Long-term series of interpolated four-position gyro-north-seeker measurement (trace on horizontal circle).
The interpolated four-position method has been tested first. A cost-effective FOG has been used for validating the theory. The optimum
acquisition time and bias stability have been defined from Allan
variance (Fig. 2). They are, respectively, 500 s and
0.05
Standard deviation (SD) is about
Baseline D0 comparison. Blue dots are computed from GyroDIF measurements. Red dots are computed from conventional DI-flux instrument (Zeiss 010-B).
Series of true north measurements (trace on horizontal circle) at Sodankylä magnetic observatory. The angle readings correspond to horizontal circle value when instrument is pointing true north.
Result of the intercomparison session organized during the
XVIIth IAGA Workshop on Geomagnetic Observatory instruments, data
Acquisition and Processing. Each value corresponds to the mean result of an
observer/instrument series performed on pillar D05. Eastern component
The presence of
Fiber-optic gyro output signal due to Earth's rotation when its
sensitive axis scans the horizontal plane in Dourbes. The maximum of the
sine function corresponds to true north. Blue: hybrid method
Series of true north measurements (trace on horizontal circle) obtained by means of hybrid method (Dots). The solid line corresponds to the true north determination after passing through a Kalman filter.
Blue: Dourbes LEMI 025 baselines computed from GyroDIF
measurements (red). The true north direction used in the
Different sources may contribute to the uncertainty measured in
Sect. 4.1. The angular accuracy of AutoDIF and thus GyroDIF is
around 6 arcsec (Poncelet et al., 2017). Both vertical and
horizontal angles uncertainties contribute to the global
error. Moreover, this estimated uncertainty is a statistical value
computed over a whole turn while the four-position method always
uses the same positions, leading to a systematic error that could
be slightly different from the statistical one. In the case of
conventional measurements, the observer's eyesight and ability to
point the target in the same way as a colleague is seldom better
than 5 arcsec and also depends on the telescope optics. Other
sources of uncertainty are the pillar difference;
time synchronization between variometer and absolute instrument,
including scalar instrument; and magnetic cleanliness of the
absolute room or the observer. It should be noted that
intercomparing absolute instruments by performing parallel
measurements using a variometer baseline as a yardstick rarely
secures accuracies better than
The hybrid method has also been implemented. A four-position
protocol is executed every 10
The series of measurements presented in Fig. 8 has an SD
The hybrid method has been compared to the conventional
measurements (Fig. 9). The magnetic (declination and inclination)
phase has been executed every night between 00:00 and
03:00 UTC while the rest of the time
was used for the true north measurement. As for the interpolated
four-position method, comparison is performed on different pillars
and the same remarks apply here. Results seem better than in
Sect. 4.1 since the difference in
In this paper, we presented a new improvement in automation of
magnetic observatories. Different methods for automatically finding
true north have been established and demonstrated. It appears that
the hybrid method is more in accordance with the concept of an
automatic setup. Moreover, a series of instrument uncertainties are
smoothed during the sinus fitting step. Results presented here have
been obtained with a low-cost FOG sensor. A more sensitive device
may lead to better and faster result. In particular, high-latitude
observatories need accurate FOG as
Data are available upon request from the corresponding author at agonsett@meteo.be.
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.
We would like to acknowledge the Royal Meteorological Institute of Belgium, which allowed this research. We also acknowledge the editor and the reviewers who contributed to the improvement of this article. Edited by: Kusumita Arora Reviewed by: Heinz-Peter Brunke and Christopher Turbitt