EP3452781A1

EP3452781A1 - Self-calibrating and autonomous magnetic observatory

Info

Publication number: EP3452781A1
Application number: EP17717453.9A
Authority: EP
Inventors: Jean RASSON; Alexandre GONSETTE; François HUMBLED
Original assignee: Institut Royal Meteorologique De Belgique
Current assignee: Institut Royal Meteorologique De Belgique
Priority date: 2016-05-04
Filing date: 2017-04-18
Publication date: 2019-03-13
Also published as: US11346667B2; CN109073380A; CN109073380B; BE1023739B1; EA201892176A1; US20190162537A1; CA3022175A1; EA036173B1; WO2017190951A1

Abstract

The invention relates to an autonomous magnetic observatory that comprises: a scalar magnetometer for measuring the modulus of the local magnetic field vector F; an angular magnetometer for measuring the vertical direction, the direction of geographic North, and the direction of the local magnetic field vector F; a variometer for measuring three variations in the local magnetic field vector F; a clock; and a controller. In which observatory the controller is configured to control and manage the orientation of sensors, to acquire the measurements of the variometer, of the scalar magnetometer, of the angular magnetometer and of the variometer, and to process the acquired measurements in order to obtain automatically the local magnetic field vector F and the errors in the measurements associated with each instrument.

Description

Autonomous and self-calibrated magnetic observatory Technical field

According to a first aspect, the invention relates to magnetic observatories (of the local terrestrial magnetic field) and more particularly to autonomous magnetic observatories.

According to a second aspect, the invention relates to a method for automatically obtaining and self-calibrating the local (terrestrial) magnetic field vector.

State of the art

There are a number of magnetic observatories (of the local terrestrial magnetic field) said autonomous. In practice, these observatories are rather partially autonomous. For example, the article "On the Calibration of a Vectorial 4He Pumped Magnetometer" by O. Gravrand, A. Khokhlov, JL Le Mouel, and JM Léger in Earth Planets Space, 53, 949-958, 2001 discloses a vector magnetometer with optical pumping in helium vapor and a calibration method for this magnetometer. This magnetometer makes it possible to provide an absolute measurement of the intensity of the local magnetic field as well as the variations of three orthogonal components of the local magnetic field measured simultaneously and in the same place. This observatory also provides vertical attitude control. The method of data processing of the magnetometer makes it possible, from the three variations of the components and the absolute value of the modulus of the local magnetic field vector, to obtain a calibration of scale factors making it possible to obtain the variations of the three components of the magnetometer. vector local magnetic field in units SL

However, this observatory does not provide automatic measurement of the direction of the local magnetic field. Nor does it provide the vertical of the place, nor the direction of the geographic North. Drifts due to temperature variations are not corrected. Finally, the method does not make it possible to correct orthogonality defects occurring over time. [0005] A second partially autonomous magnetic observatory is disclosed in "GAUSS: Geomagnetic Automated System" by HU AUSTER, M MANDEA, A. HEMSHORN, KORTE and E. PULZ in PUBLS. INST. Geophys. POL. ACAD. SC., C-99 (398), 2007. This publication discloses an apparatus for automatically measuring the orientation of the local magnetic field vector. This autonomous observatory also provides a horizontal and vertical self-monitoring by detecting the local vertical.

However, this observatory does not automatically provide the value of the local magnetic field vector module, nor the variations of the local magnetic field vector in three directions, nor the direction of the geographic North. No self-calibration method for correcting temperature effects, measuring scale factor and performing orthogonal calibration is proposed.

In conclusion, although these magnetic observatories are often called autonomous observatories, they are actually partially autonomous and require external intervention to obtain the local magnetic field vector.

Summary of the invention

According to a first aspect, one of the objects of the present invention is to provide a device for the autonomous and self-calibrated measurement of the local magnetic field vector, the local magnetic field to be included in the local magnetic field direction of the Earth .

For this purpose, the magnetic observatory for the measurement of the local magnetic field vector comprises: a scalar magnetometer for the absolute measurement of the local magnetic field vector module, a magnetic variometer recording variations of three components mathematically independent of the vector field local magnetic, a clock, an angular magnetometer and a controller. The angular magnetometer comprises a first rotatable support according to a first axis of rotation, said main axis, making it possible to obtain a horizontal orientation of the first orientable support, the so-called first orientable support comprising: a main motorisation for modifying the horizontal orientation of the first support orientable about the main axis, an inclination sensor, a second support orientable along a second axis of rotation called secondary axis, orthogonal to the main axis, for obtaining a vertical orientation of the second orientable support said second support steerable device comprising: a secondary motor for changing the vertical orientation of the second steerable support about the secondary axis, a magnetic sensor for measuring the direction of the local magnetic field vector, a North search device. The angular magnetometer further comprises means for controlling the main and secondary motorization and a device for measuring and angularly acquiring the horizontal and vertical orientations of the first and second orientable supports.

The second steerable support may include the tilt sensor. The inclination sensor can be mounted on the first orientable support or on the second orientable support.

The magnetic observatory is made autonomous and capable of self-calibrating the measurements of the local magnetic field by means of the controller which is configured to automatically control the main motor and the secondary motor, to manage the orientation: of the inclination sensor for the measurement of the direction of the vertical, the North search device for measuring the direction of the geographic North and the magnetic sensor for measuring the direction of the local magnetic field vector. It is also configured to acquire: the angles of the direction of the local magnetic field vector with respect to the geographic North and to the Vertical according to the horizontal and vertical orientations of the first and second orientable supports measured with the measuring device and angular acquisition , the three variations of the local magnetic field vector measured by the variometer and the values of the modulus of the local magnetic field vector measured by the scalar magnetometer. Finally, it is configured to process the data previously acquired to automatically obtain the local magnetic field vector and the measurement errors associated with each instrument.

Traditional magnetic observatories are often installed in buildings with rooms whose temperature is controlled to avoid excessive deviations of instruments. In addition, some measurements are done manually by a highly qualified operator. These observatories therefore can not be installed anywhere. An autonomous observatory has the advantage of being able to be installed where you want it and above all, does not require the presence of a qualified operator. The cost of such an observatory is therefore much lower and it can be used more easily. Measurements can also be performed with a higher frequency and more reliably. Compared to the previous "autonomous" observatories, the advantage lies in the presence of a set of instruments which, combined with each other, allow self-calibration and thus obtain a precision much better than their simple juxtaposition.

Preferably, the first and second steerable supports, the main and secondary engines, the North search device, the tilt sensor, the control means of the main and secondary motorization and the measurement and acquisition device. angular consist of non-magnetic components, defined such that the magnetic susceptibility of the materials is between -1 and 1, preferably between -10 ^"1 and 10 ^{" 1} , even more preferably between -10 ^"3 and 10 ^{" 3} .

For example, the non-magnetic components are materials selected from: ceramic, aluminum, arcap, titanium, copper, ertalon, nylon, ertacetal, peek.

The choice of these materials is crucial to ensure a good measure of the local magnetic field, undisturbed by the instruments themselves.

Preferably, the scalar magnetometer is of the type: proton, Overhauser, atomic, optically pumped.

Preferably, the magnetic sensor is a sensor type: fluxgate, fluxset, rotating electrical circuit or a scalar magnetometer polarized by a magnetic device.

Preferably, the North search device is of the type: GNSS, GPS, target of a target, aim of a star, gyroscope, absolute rotation detector, polarization of sunlight.

[0016] Preferably, the observatory comprises a plurality of non-magnetic pillars, preferably of concrete, whose average dimensions [thickness, width, length] are between [1, 10, 10] cm and [6, 10, 10] m, preferably, the average dimensions [width, length, depth] are between [10, 20, 20] cm and [1, 2, 2] m, even more preferably between [15, 25, 25] cm and [0.25, 0.5, 0.5] m, said pillars being separated by an average distance of between 0 and 10 m, preferably between 1 and 6 m, even more preferably between 2 and and 4 m. The non-magnetic pillars support at least one of the following: the scalar magnetometer, the angular magnetometer, the variometer, the clock (201), the controller.

Although the observatory is self-calibrated and can be installed anywhere on the Earth's crust, it is best to install it on a stable structure that is not subject to vibration in order to obtain better quality measurements. .

Preferably, the observatory comprises at least one non-magnetic shelter surrounding the plurality of pillars and comprising an insulated wall whose average thickness is between 1 and 60 cm, preferably between 2 and 30 cm, even more preferably between 5 and 10 cm. At least one magnetic shelter protects at least one of the following: the scalar magnetometer (MS), the angular magnetometer (AM), the variometer (MV), the clock (201), the controller (202).

Just as the stable structure is important, limiting the temperature variations improves the quality of the measurements and decreases the drift of the instruments.

In a second aspect, the invention relates to a method for automatically obtaining the local magnetic field vector. This method comprises the step of providing a magnetic observatory as described above. Then, the controller carries out the steps of acquiring values, controlling the engines, calculating values, calibrating measurements and finally calculating the value of the local magnetic field vector.

First, the controller acquires measurements of the modulus of said local magnetic field vector measured with the scalar magnetometer at different times ti, and acquires measurements of the three mathematically independent components of the local magnetic field vector dU, dV, dW, measured with the variometer at different times ti. Then, the controller controls the main operator to change the horizontal orientation of the first steerable support and, depending on the indications of the inclination sensor, measures the vertical direction, V, it also controls the secondary motorization for modifying the vertical orientation of the second steerable support and, according to the indications of the North search device, measures the direction of the geographic North N, and it controls the main and secondary motorization to modify the horizontal and vertical orientations of the angular magnetometer and, depending on the indications of the magnetic sensor obtained at different times ti, measures two angles, D ^* and, corresponding to the direction of the local magnetic field vector. Thirdly, the controller calculates the absolute values of the two angles from the directions previously obtained such that: D = D ^* -N and I = Γ-V, it also calculates baselines of the variometer at a time t, average of the instants different ti, using the absolute modulus of the local magnetic vector and angles, all three obtained as described above. Fourthly, the controller calibrates the scaling factors of each mathematically independent component of the variometer and calibrates the orthogonality and spatial orientation of the three mathematically independent components of the variometer and calculates Eulerian rotation matrices. Finally, the controller calculates the value of the local magnetic field vector by obtaining oriented measurements by applying Eulerian rotations to the three measures of the variometer: dU, dV, and dW, by obtaining scaled measurements by multiplying the three oriented measurements of the variometer by the respective scale factors and by obtaining 3 components of the local magnetic field vector by summing the baselines to the three oriented measures and to scale.

The method described above makes it possible to obtain a measurement of the local magnetic field automatically and in a self-calibrated manner.

Automation eliminates the need for a skilled operator, which greatly reduces costs and improves measurement accuracy. Auto-calibration improves the quality of measurements. This self-calibration also contributes to the reduction of costs because it allows the installation of the autonomous observatory in an inexpensive structure in comparison with traditional observatories. The combination of automation and auto-calibration thus brings significant added value to the instruments taken separately. Preferably, the measurement of the two angles, D ^* and, characterizing the direction of the field in the horizontal and vertical plane respectively is obtained by the controller by modifying the horizontal orientation of the first orientable support (rotation of the support in the plane horizontal) until the magnetic sensor indicates a zero, acquire the first angle D ^* and changing the vertical orientation of the second orientable support (rotation of the support in the vertical plane) until the magnetic sensor indicates a zero, acquire the second angle I ^* .

Even more preferably, the measurement of the two angles, D ^* and, is obtained by the controller by modifying the horizontal orientation of the first orientable support so that the magnetic sensor indicates a zero measurement value and acquire the orientation. D1 of the first orientable support corresponding to this measurement, then modifying the horizontal orientation of the first orientable support of 180 ° and adjusting the horizontal orientation so that the magnetic sensor indicates a zero measurement value and acquire the orientation D2 of the first orientable support corresponding to this measurement, then by modifying the vertical orientation of the second orientable support by 180 ° and by adjusting the horizontal orientation of the first orientable support so that the magnetic sensor indicates a zero measurement value and acquire the orientation D3 of the first orientable support corresponding to this measurement, by modifying the horizontal orientation of the first orientable support of 180 ° and adjusting the horizontal orientation so that the magnetic sensor indicates a zero measurement value and acquire the orientation D4 of the first orientable support corresponding to this measurement and finally calculating the first angle corresponding to the horizontal direction of the magnetic field local average of the four measures: D ^* = (D1 + D2 + D3 + D4) / 4.

Similarly, the measurement of I ^* is obtained by modifying the horizontal orientation of the first orientable support in the direction D ^* -90, then performing the same first three steps as for D ^* in which the horizontal and vertical orientations are reversed. and wherein the roles of the first and second steerable supports are reversed, and thereby acquire the orientations 11, 12, 13, 14 by changing the horizontal orientation of the first steerable support in the direction D ^* -90 ° and adjusting the orientation vertical of the second orientable support so that the magnetic sensor indicates a value of zero measurement, the orientation 14 of the second orientable support corresponding to this measurement. Finally, the second angle corresponding to the direction of the local magnetic field in the vertical plane is calculated according to the average of the four measurements: I ^* = (11 + 12 + 13 + 14) / 4.

Preferably, the North search device is an absolute rotation detector type searcher and the controller controls the secondary motorization to modify the vertical orientation of the second orientable support and, according to the indications of the North search device. , measures the direction of the geographic north N by changing the horizontal orientation of the first steerable support until the absolute rotation detector indicates a zero measurement value and acquire the orientation N of the first steerable support corresponding to this measurement.

Even more preferably, the measurement of the direction of the geographic north N is obtained in a manner similar to D ^* , the magnetic sensor being replaced by the absolute rotation detector and the orientations D1, D2, D3, D4 and D ^* being replaced by N1, N2, N3, N4 and N in the method described above.

These measurement methods have the advantage of canceling all the internal alignment errors of the instrument and thus improve the quality of the measurements.

Preferably, the controller calculates the base lines of the variometer based on the three measures of the variometer: dU, dV, and dW, the absolute modulus of the local magnetic field vector F, the two angles characterizing the direction of the magnetic field vector. local (F): the inclination I and the declination D, and optionally functions gU, gV, gW, making it possible to change the coordinates D, F, I to U, V, W coordinates. The controller calculates the baselines U0, V0, W0, according to:

Uo = gu (F, D, l) - dU

V ₀ = gv (F, D, 1) - dV

W ₀ = gw (F, D, 1) - dW.

Preferably, the controller calibrates the scaling factors of each component of the variometer by measuring the amplitudes of the variations of the baselines with respect to the amplitudes of the signals of each of the three components of the local magnetic field vector (F) during a pre-set time, then multiply three measures of the variometer (MV): dU, dV, and dW by corrective factors, scaling factors, fu, fv, fw, then subtracting the corrected values and calculating baselines according to:

Uo = gu (F, D, l) - fu ^* dU

V ₀ = gv (F, D, I) - fv ^* dV

Wo = gw (F, D, l) - f _w ^* dW

Then, the controller increases or decreases the respective scale factors to reduce the variations of the respective baselines and corrects the variometer measurements by multiplication by the respective scale factors. These steps may be repeated iteratively until the increase or decrease in scaling factors is less than a predetermined value.

[0024] Preferably, the controller calibrates the orthogonal ity and the orientation in space of the three mathematically independent components of the variometer and calculates Eulerian E rotation matrices by examining the variation of the baseline of a component. depending on the signal amplitude of the other components for a predetermined period of time, calculating Eulerian rotation matrices E and adjusting the orientations until the variation of the baseline is less than a predetermined value.

Preferably, the scalar magnetometer and the variometer perform measurements with a frequency of between 0.01 Hz and 100 Hz, preferably between 0.05 Hz and 10 Hz, even more preferably between 0.1 Hz and 1 Hz.

Preferably, the measurements of the orientations of the local magnetic field vector are carried out with a frequency of between 10 ^-7 Hz and 10 ^-2 Hz, preferably between 10 ^-6 Hz and 10 ^-3 Hz, even more preferably between 10 ^"5 Hz and 10 ^{" 4} Hz.

[0027] Preferably, the autonomous observatory implements a method as described above. Brief description of the drawings

These and other aspects of the invention will be clarified in the detailed description of particular embodiments of the invention, reference being made to the drawings of the figures, in which:

- Figure 1 shows a local magnetic field vector and its decomposition into different mathematically independent components. ;

FIG. 2 shows an example of an autonomous magnetic observatory according to the invention;

FIG. 3 shows an exemplary embodiment of an angular magnetometer according to the invention;

Figure 4 shows an example of a North search device based on a sighting principle;

FIG. 5 shows an example of a North search device based on an absolute rotation detector principle;

FIG. 6 shows an example of orientation of the sensitive axis of a magnetic sensor with respect to the local magnetic field;

FIG. 7 shows a second example of an autonomous magnetic observatory according to the invention;

FIG. 8 shows a diagram of an exemplary method for obtaining the local magnetic field vector according to the invention;

- Figure 9a shows a leveling defect along an east-west axis corresponding to a rotation about an axis X;

FIG. 9b shows an example of baseline measurement of a Z component of the field presenting a leveling defect in the east-west direction.

Figure 10 shows an example of an algorithm for the determination of scale factor and variometer orientation defects.

The drawings of the figures are not to scale. Generally, similar elements are denoted by similar references in the figures. The presence of reference numbers in the drawings can not be considered as limiting, even when these numbers are indicated in the claims. DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS OF THE INVENTION

Figure 1 shows an example of local terrestrial magnetic field vector, hereinafter local magnetic field vector F and its decomposition into different mathematically independent components. The local magnetic field F being a vector field, the knowledge of its intensity is not sufficient to characterize it completely. In fact, it must define an orientation with respect to reference directions. These reference directions are preferably the local vertical (or, equivalently, a horizontal plane) and the direction of the geographic North N. The magnetic variation "D" is then defined as the angle between the geographic North N and the projection of the magnetic vector in a horizontal plane. Magnetic inclination "I" is defined as the angle between the horizontal plane and the magnetic vector in a vertical plane containing this vector.

In Cartesian coordinates, XYZ, X points to the geographical North N, Y to the geographical east E, and Z vertical, down. In cylindrical coordinates, HDZ, we have H, the horizontal component, D the magnetic declination and Z the vertical component (noted positive downwards). In spherical coordinates, FDI, we have F, the modulus of the local magnetic field vector (vector intensity), D, the magnetic declination and the magnetic inclination. Whatever the chosen coordinate system, three mathematically independent components must be determined to fully characterize the local magnetic field vector F. Finally, let us note that we have the following relationships between the different values: H = F cos /, Z = F sin / , X = H cos D, Y = H sin D.

Figure 2 shows an example of autonomous magnetic observatory 200 according to the invention. Preferably, this observatory 200 comprises: a scalar magnetometer MS, an angular magnetometer MA, a variometer MV, a clock 201, and a controller, 202.

Preferably, the scalar magnetometer MS serves to measure the intensity of the magnetic field, or in other words to measure the modulus of the local magnetic field vector F. The scalar magnetometer can be described as an absolute instrument in the sense that it measures a quantity not relative to a other. The scalar term designates a property that remains unchanged during a change of reference system. The scalar magnetometer MS is, for example, a magnetometer of the type: proton, overhauser, atomic, optically pumped. Preferably, the measurements of the local magnetic field vector module F are carried out with a frequency of between 0.01 Hz and 100 Hz, preferably between 0.05 Hz and 10 Hz, even more preferably between 0.1 Hz and 1 Hz. , the accuracy of the measurements is such that the measurement error is less than 1 nT, even more preferably less than 0.5 nT, ideally less than 0.2 nT.

The local magnetic field F varying in time, a clock 201 for defining a time reference is used for the synchronization of the measurements of the different instruments. This clock is, for example, an internet time server, an atomic clock or a GNSS signal. The synchronization of the measurements must be better as the temporal variation of the field is high.

Figure 3 shows an embodiment of an angular magnetometer MA according to the invention. The angular magnetometer makes it possible to determine the direction of the local magnetic field vector F.

In the example of FIG. 3, the angular magnetometer MA comprises:

A first steerable support 320 along a first axis of rotation, called the main axis 321;

A second steerable support 310 along a second axis of rotation called secondary axis 311, substantially orthogonal to the main axis;

· A tilt sensor 313;

• a North 324 research facility;

A magnetic sensor 323 for measuring the direction of the local magnetic field vector F.

Preferably, the first support comprises: a main motor 322 for modifying the orientation of the first orientable support 320 around the main axis, the inclination sensor 313, the second orientable support 310. The two axes of rotation 321 and 311 , allow a displacement in two planes (horizontal and vertical) essentially orthogonal. These displacements allow to obtain a horizontal orientation of the first orientable support and a vertical orientation of the second orientable support. Preferably, the second steerable support 310 comprises a secondary motorization 312 for modifying the orientation of the second orientable support around the secondary axis 311, a magnetic sensor 323 for measuring the direction of the local magnetic field vector F and a device for searching the North 324. Preferably, the angular magnetometer MA also comprises a control means 340 of the main motor 322 and secondary 312 and an angular measurement and acquisition device 350 of the horizontal and vertical orientations of the first and second orientable supports 320, 310. Preferably, the angular magnetometer has two degrees of freedom in rotation, and is capable of orienting the magnetic sensor 323 in an arbitrary direction. In particular, it is able to orient the magnetic sensor according to the magnetic field.

Preferably, the main motor 322 and secondary 312 have a negligible magnetic footprint so that they do not disturb the measurement of the magnetic field. For example, these engines are piezoelectric motors.

The angular measurement and angular acquisition device 350 makes it possible to measure an angle of rotation of the first and second orientable supports 320 and 310 respectively about each of the main axes 321 and secondary 311. This device is, for example, an optical encoder. This device can be used as a feedback signal for slave angular displacement.

Preferably, the position of the primary axis 321 can be adjusted using screws "calantes". Other solutions are possible such as a cardan joint, a ball joint or a suspended pendulum type system.

The tilt sensor 313 measures the orientation of the primary axis 321 relative to the vertical. The inclination sensor makes it possible to obtain a first reference direction. This sensor is, for example, an electrolyte level or an accelerometer measuring gravity. Preferably, the inclination sensor is secured to the rotating reference frame with the secondary axis 311 so that it is integral with the North search device 324 and the magnetic sensor 323. The inclination sensor can also be placed in a frame integral with the main axis 321.

In an improved version of the device, the angular magnetometer MA further comprises an auxiliary tilt sensor 314 that can be installed on the fixed part of the instrument. Preferably, the auxiliary tilt sensor has two degrees of freedom.

The angular magnetometer MA comprises a North 324 search device. This North search device makes it possible to determine the direction of the geographic North N and thus to determine a second reference direction. Preferably the North search device has two degrees of freedom, that is to say it can perform rotations along the main axes 321 and secondary 311.

The North 324 search device is, for example, a target sighting system as shown in FIG. 4. Such a device points a direction, Target, whose angle, Az, with respect to the geographic north N , is previously known. We thus deduce the direction of geographic North N. Such a device may be a laser pointing a retro-reflector. The return beam is then captured by a device such as a photocell or a PSD (Position Sensitive Detector). Other solutions are also possible as a CCD camera. GPS or GNSS beacons can also be used. Figure 4 also illustrates the magnetic north direction Nmag, and the declination angle D.

Alternatively, the North 324 search device may be a sun sighting device (sunshot): it is possible to derive the direction of the geographical north N by pointing the sun at a given moment (and this even if the sun is hidden by clouds, for example thanks to polarizing filters). Known methods require knowledge of sidereal time, declination and right ascension of the sun. These data are available in astronomical tables or derived from models. Other devices can also be used as an astronomical sighting device: the above principle can be applied to any other visible object. A star camera: in a fixed reference point relative to the observatory, the stars do not appear fixed in the sky. An image of them at a given moment then makes it possible to deduce the direction of the geographic North N. Advantageously, the North 324 search device is an absolute rotation detector as shown in FIG. 5. This type of sensor is capable of detecting the rotation of the Earth Ω _β . This detector measures the projection of the horizontal component 510 of the earth rotation vector along its sensitive axis 520. FIG. 5 shows an example of projection for a latitude Θ, and an orientation angle of the sensitive axis ψ measured relative to Geographical North N (not shown). These two directions are in the same horizontal plane. By moving this sensitive axis in a horizontal plane, its output describes a sinusoid whose maximums lie in the north-south direction and the zeros in the east-west direction.

These sensors or gyroscopes may be in particular of the mechanical type, optical fiber (FOG), laser (RLG), hemispherical resonance (HRG), MEMS or atomic.

The magnetic sensor 323 is a directional magnetic field sensor whose sensitive axis can be oriented by the main axis 321 of rotation and the secondary axis 311 of rotation of the angular magnetometer MA. This magnetic sensor may be, for example, fluxgate type, fluxset, rotating electrical circuit or a scalar magnetometer polarized by a magnetic device. The output T of the magnetic sensor 323 is proportional to the scalar product of the local magnetic field F by a unit vector parallel n to its sensitive axis (see Figure 6): T = k (n ^■), where k is a proportionality factor or scale factor of the magnetic sensor.

The magnetic sensor 323 may be capable of measuring the intensity of the magnetic field (scalar measurement). The scalar magnetometer MS can therefore be included in the angular magnetometer MA. Such a sensor may be a full-field fluxgate mono or triaxial probe, a fluxset triaxial probe, or a scalar magnetometer. The scalar measurement can also be performed by an auxiliary magnetic sensor included in the instrument. Preferably, the measurements of the orientation of the local magnetic field vector are carried out with a frequency of between 10 ^-7 Hz and 10 ^-2 Hz, preferably between 10 ^-6 Hz and 10 ^-3 Hz, even more preferably between 10 ^" ⁵ Hz and 10 ^{" 4} Hz. Preferably, the accuracy of the angular measurements in the horizontal plane is such that the measurement error is less than 20 arc seconds, even more preferably less than 10 seconds. 'bow, ideally less than 6 arc seconds. Preferably, the accuracy of the angular measurements in the vertical plane is such that the measurement error is less than 10 arc seconds, even more preferably less than 5 arc seconds, ideally less than 1 second of bow.

The variometer MV, also called vector magnetometer, measures the variations of three components mathematically independent of the local magnetic field F at regular intervals. Preferably, these measurements of three components mathematically independent of the local magnetic field F are carried out with a frequency of between 0.01 Hz and 100 Hz, preferably between 0.05 Hz and 10 Hz, even more preferably between 0.1 Hz and 1 Hz. Preferably, the accuracy of the measurements is such that the measurement error is less than 2 nT, and even more preferably less than 1 nT, ideally less than 0.5 nT. These measurements are more accurate than the absolute measurements made by the scalar and angular magnetometers but they must be calibrated with respect to a reference. The variometer MV and the scalar magnetometer MS can be combined in the same instrument.

The autonomous observatory also comprises a controller 202. This controller is configured to:

a) automatically control the main motor 322 and secondary 312;

(b) manage the direction of

1) inclination sensor 313 for measuring the direction of vertical V;

2) North 324 search device for the measurement of the North Geographic N direction;

3) magnetic sensor 323 for measuring the direction of the local magnetic field vector F;

(c) acquire

1) the angles of the direction of the local magnetic field vector F with respect to the geographic north N and the vertical V according to the horizontal and vertical orientations of the first and second orientable supports; 2) the three variations of the local magnetic field vector F measured by the variometer MV;

3) the values of the local magnetic field vector module F measured by the scalar magnetometer MS;

d) processing the data acquired in step c) to automatically obtain the local magnetic field vector F and the measurement errors associated with each instrument.

Preferably, the first and second steerable supports 320 and 310 the main motor 322 and secondary 312, the North search device 324, the motor control means 340, and the measuring device and angular acquisition 350 consist of non-magnetic components, that is to say that the magnetic susceptibility of the materials constituting them is between -1 and 1, preferably between -10 ^"1 and 10 ^{" 1} , even more preferably between - 10 ^"3 and 10 ^{" 3} . For example, the materials are materials selected from: ceramic, aluminum, arcap, titanium, copper, ertalon, nylon, ertacetal, peek.

Figure 7 shows a second example of autonomous magnetic observatory 700 according to the invention. In this example, the autonomous magnetic observatory comprises an angular magnetometer MA, a scalar magnetometer MS, a variometer MV, a clock 201, a controller 202 as described above. The observatory 700 further comprises a power source 701, and the controller 202 is further capable of acquiring and transferring data from the instruments (MA, MS, MV) to an external network via a communication system 702 Preferably, the magnetometers are remote from the different electronics so as not to be disturbed by them. The angular magnetometer MA and the variometer MV can be inserted in enclosures with high thermal inertia and insulation. A sandwich structure can also be envisaged. The angular magnetometer MA and the variometer MV can also be installed on a stable support such as a pillar anchored on a solid base 730. The base may be rock or a slab made of non-magnetic materials. In the example of FIG. 7, two non-magnetic housings 710 surround a plurality of pillars 711 and comprise an insulated wall 720 whose average thickness is between 1 and 60 cm, preferably between 2 and 30 cm, so that even more preferred between 5 and 10 cm. Preferably, the plurality of non-magnetic pillars, preferably of concrete, whose average dimensions [thickness, width, length] are between [1, 10, 10] cm and [6, 10, 10] m, preferably the average dimensions [thickness, width, length] are between [10, 20, 20] cm and [1, 2, 2] m, even more preferably between [15, 25, 25] cm and [0.25, 0.5 , 0.5] m, said pillars being separated by an average distance of between 0 and 10 m, preferably between 1 and 6 m, even more preferably between 2 and 4 m. In this example, one of the shelters 710 comprises the vector magnetometer MV and the other the angular magnetometer MA. Preferably, the observatory comprises a single shelter 710.

Preferably, the non-magnetic pillars 711 support at least one of the following elements: the scalar magnetometer MS, the angular magnetometer MA, the variometer MV, the clock 201, the controller 202. Preferably, the non-magnetic shelter protects at least one of the following: the scalar magnetometer MS, the angular magnetometer MA, the variometer MV, the clock 201, the controller 202.

The autonomous magnetic observatory according to the invention can be installed on land or under water, for example at the bottom of the sea. When immersed, the instruments (MA, MS, MV), the clock 201 and the controller 202 are protected by one or more sealed protective housings. The instruments (MA, MS, MV), the clock 201 and the controller 202 are installed in a single housing, or, preferably, in separate housings. The housing or cases prevent the external water from damaging or destroying their contents. For example, an underwater magnetic observatory can be supplied with electricity via batteries, a submarine cable, a surface buoy connected to the observatory by a conductive cable, a marine electric generator.

According to a second aspect, the invention relates to a method for obtaining the local magnetic field vector F.

FIG. 8 shows a diagram of an exemplary method 800 for obtaining the local magnetic field vector F according to the invention. This method includes the steps:

a) S810: provision of a magnetic observatory as described above; b) S820: the controller 202 acquires measurements of the local magnetic field vector module, F, measured with the scalar magnetometer MS at different times ti;

c) S830: The controller acquires measurements of the three mathematically independent components of the local magnetic field vector

F, dU, dV, dW, measured with the variometer MV at different times ti;

d) S840: the controller controls the main motor 322 to change the horizontal orientation of the first steerable support 320 and, depending on the indications of the inclination sensor, measures the vertical, V; e) S841: the controller controls the secondary motorization 312 to change the vertical orientation of the second steerable support 310 and, based on the indications of the North search device 324, measures the direction of the geographic North N;

f) S842: the controller controls the main and secondary motorization to modify the horizontal and vertical orientations of the angular magnetometer MA and, according to the indications of the magnetic sensor 323 obtained at different times ti, measures two angles, D * and P, corresponding to the direction of the local magnetic field vector F;

g) S843: the controller calculates the absolute values of the two angles from the directions obtained in steps d) to f), such that: D = D * -N and I = l * -V; h) S850: the controller calculates baselines of the variometer MV at a time t, average of the different instants ti, using the absolute modulus of the local magnetic vector F, obtained as in step b) and the angles obtained as at step g);

i) S860: the controller calibrates the scaling factors of each mathematically independent component of the MV variometer;

j) S861: The controller calibrates the orthogonality and spatial orientation of the three mathematically independent components of the MV variometer and calculates Eulerian E rotation matrices;

k) S870: The controller calculates the value of the local magnetic field vector

F by performing the steps of: 1) S871: obtaining the oriented measurements by applying Eulerian rotations E, obtained in step j), to the three measurements of the variometer MV: dU, dV, and dW;

2) S872: obtaining scaled measurements by multiplying the three oriented measurements of the variometer MV by the respective scale factors obtained in step i);

3) S873: obtaining the 3 components of the local magnetic field vector F by adding the baselines to the three oriented measurements and to the scale.

The local magnetic field vector F varies over time, which is why measurements must be made at different times ti.

The scalar magnetometer MS is an instrument capable of supplying the value of the local magnetic field module F with an accuracy such that the measurement error is generally less than 1 nT, preferably less than 0.5 nT, ideally less than 0.2 nT. . The controller 202 of the autonomous observatory is configured to acquire the values provided by the scalar magnetometer MS.

Preferably, the controller 202 controls the main motor 322 and secondary 312 to measure the vertical direction V with the tilt sensor. The controller is also configured to control the main 322 and secondary 312 motorizations to measure the North Geographic N direction using the North 324 search facility.

Preferably, the North 324 search device is an absolute rotation detector and the search for the geographic North N consists of modifying the horizontal orientation of the first orientable support 320 until the absolute rotation detector indicates a zero measurement value. The controller 202 then acquires the orientation of the first medium indicating the geographic North N.

Alternatively, the North search device is an absolute rotation detector and the search for the geographic North N includes the steps:

a) change the horizontal orientation of the first steerable support 320 so that the absolute rotation detector indicates a zero measurement value and acquire the orientation N1 of the first orientable support corresponding to this measurement;

b) changing the horizontal orientation of the first orientable support 320 by 180 ° and adjusting the horizontal orientation so that the absolute rotation detector indicates a zero measurement value and acquire the orientation N2 of the first orientable support corresponding to this measurement;

c) changing the vertical orientation of the second orientable support 310 by 180 ° and adjusting the horizontal orientation of the first orientable support 320 so that the absolute rotation detector indicates a zero measurement value and acquire the orientation N3 of the first orientable support 320 corresponding to this measure;

d) changing the horizontal orientation of the first orientable support 320 by 180 ° and adjusting the horizontal orientation so that the absolute rotation detector indicates a zero measurement value and acquire the orientation N4 of the first orientable support corresponding to this measurement;

e) calculate the direction of the geographic North N according to the average of the four measurements: N = (N1 + N2 + N3 + N4) / 4.

The measurement of the geographic North N and the vertical V makes it possible to define a reference frame in which the local magnetic field vector F will be expressed.

Once the direction of the geographic North N and the known vertical V, we have a local reference frame in which to express two orientations of the local magnetic field vector F which are determined by the controller 202 using the angular magnetometer MA and of its magnetic sensor 323. Preferably, the magnetic sensor 323 of the angular magnetometer MA is a directional sensor so that only a component of the local magnetic field parallel to the sensitive axis of the sensor is measured. The output of the sensor is then given by the following scalar product: T = k (n ⁺ F), with T, the sensor output usually expressed in nanoTesIa (nT), k, a scale factor of the sensor, a unit vector parallel to the sensitive axis of the sensor and F, the local magnetic field vector. For example, an orientation of the magnetic sensor 323 normal to the magnetic field leads to a zero dot product. This is called the zero method. Controller 202 is therefore configured to measure the two angles, D ^* and, by changing the horizontal orientation of the first steerable support 320 until the magnetic sensor indicates a minimum value which is ideally a zero, which gives a first orientation D ^* . The controller then changes the vertical orientation of the second steerable support 310 until the magnetic sensor indicates a minimum value which is ideally a zero, giving a second orientation I ^* .

Preferably, several zeros are measured in order to reduce measurement errors as well as certain errors due to the instruments. In general, the component of the magnetic field measured by the magnetic sensor 323 can be expressed as a function of the two degrees of freedom as well as different misalignments using the model known to those skilled in the art of Lauridsen (Lauridsen, KE , 1985. Experiments with the Danish Meteorological Institute Geophysical Papers, R-71):

T = H cos (a- D) (cos (/?) - e sin (/?)) - γΗ sin (a - D) + Z (e cos (/?) + Sin (/?))

+ T ₀

with: T: the output of the magnetic sensor 323, T0: a possible offset, H: the horizontal component (in the plane perpendicular to the main axis 321) of the local magnetic field vector F, Z: the vertical component (parallel to the main axis) of the local magnetic field vector F, D: the magnetic declination, a: angle of the angular magnetometer MA in the horizontal plane relative to the geographic north, β: angle in the vertical plane with respect to vertical, e: defect of alignment of the magnetic sensor 323 with respect to its sensitive axis in a vertical plane, γ: misalignment of the magnetic sensor relative to its sensitive axis in the horizontal plane when the sensitive axis is horizontal. The angular directions of the magnetic field are then determined by seeking magnetic sensor orientations substantially perpendicular to the magnetic field. Methods

(Lauridsen, Kerridge: Kerridge, DJ, 1988. Theory of the fluxgate-theodolite, British Geological Survey Technical Report, Geomagnetism Series, WW / 88/14) are used to compensate for misalignment and offset of the sensor. For example, the controller 202 controls the main and secondary motorization to modify the horizontal and vertical orientations of the angular magnetometer MA and searches for zeros corresponding to 4 orientations (+/- 180 ⁰ depending on the horizontal and +/- 180 ° depending on the vertical ). For example, for the search for declination D, the controller performs the steps:

a) modifying the horizontal orientation of the first orientable support 320 so that the magnetic sensor 323 indicates a zero measurement value and acquire the orientation D1 of the first orientable support corresponding to this measurement;

b) changing the horizontal orientation of the first orientable support 320 by 180 ° and adjusting the horizontal orientation so that the magnetic sensor 323 indicates a zero measurement value and acquire the orientation D2 of the first orientable support corresponding to this measurement;

c) modifying the vertical orientation of the second orientable support 310 by 180 ° and adjusting the horizontal orientation of the first orientable support 320 so that the magnetic sensor 323 indicates a zero measurement value and acquire the orientation D3 of the first orientable support corresponding to This measure ; d) changing the horizontal orientation of the first orientable support 320 by 180 ° and adjusting the horizontal orientation so that the magnetic sensor 323 indicates a zero measurement value and acquire the orientation D4 of the first orientable support corresponding to this measurement;

e) calculating the first angle corresponding to the horizontal direction of the local magnetic field according to the average of the four measurements: D ^* = (D1 + D2 + D3 + D4) / 4.

Similarly, the controller 202 obtains the inclination I by performing the steps:

f) changing the horizontal orientation of the first steerable support 320 in the direction D ^* -90 °;

g) performing steps a) to c) in which the horizontal and vertical orientations are reversed and in which the roles of the first and second steerable supports 320, 310 are reversed, and thus acquire the orientations 11, 12, 13. h) changing the horizontal orientation of the first steerable support 320 in the direction D ^* -90 ° and adjust the vertical orientation of the second steerable support 310 so that the magnetic sensor 323 indicates a zero measurement value and acquire the orientation 14 of the second orientable support corresponding to this measurement;

i) calculating the second angle corresponding to the second direction of the local magnetic field according to the average of the four measurements: = (Ι1 + Ι2 + Ι3 + Ι4) / 4.

Alternatively, a second method consists in looking for a direction in which the magnetic sensor 323 indicates a measurement value close to zero but which has a residue dT. The components of the local magnetic field whose orientations are sought can be converted according to the following relationships:

1 18H0U fdCLTT \\

ÔD = asin -

π H)

With H representing the intensity of the horizontal component of the local magnetic field vector F and F the modulus of the local magnetic field vector. Each measurement is then corrected for its residue.

The orientations D and I are then calculated by the controller 202 in the geographic North reference system N and the vertical such that: D = D ^* -N and I = l ^* -V.

The controller 202 acquires values of the three components of the local magnetic field vector F: dU, dV, dW, measured with the variometer MV at different times ti. In a magnetic observatory, the measurement of the magnetic field is preferably also carried out by means of a vector magnetometer, also called MV variometer, which measures the variations of the three components of the magnetic field at regular intervals (eg 10 Hz, 1 Hz). Hz, 0.05 Hz). These are relative measurements against a reference. These measurements must therefore be calibrated using absolute measurements provided by an angular magnetometer MA and a scalar magnetometer MS.

The values of the three components of the local magnetic field vector F: dU, dV, dW, measured with the variometer MV are component variations around a reference value. This reference value, generally called the LDB "baseline" is determined by subtracting the temporal variation of the component from its "absolute" or complete value. The temporal variation is the relative value of the component measured by the MV variometer. For a component C, the relation is: C ₀ = C (t) - 5C (t), with: C ₀ : the LDB of a component C supposed to be independent of time, C (t): the "absolute" value of C at time t (determined by the absolute measurements of the angular magnetometer MA and the scalar magnetometer MS) and 5C (t): the variation of C at the instant t measured by the variometer MV. The calculation of the baselines of the variometer MV at a time t, which is the average of the different instants ti, is thus obtained by using the absolute modulus of the local magnetic vector F and the angles D and I.

The value of LDB depends only on the implementation of MV and not on the magnetic field itself. However, this value may vary over time due, for example, to temperature. Regular absolute measurements then allow its adjustment. The calculation of the LDBs can also involve a change of coordinates, for example coordinates D, F, I to any coordinates U, V, W. The controller 202 then calculates the baselines of the variometer MV on the basis of the three measurements of the variometer MV: dU, dV, and dW, of the absolute modulus of the local magnetic field vector F, of the two angles characterizing the direction of the local magnetic field vector F: the inclination I and the declination D, and, of functions gu, gv, gw, making it possible to change the coordinates D, F, I to U, V, W coordinates. The controller calculates the base lines Uo, Vo, Wo, according to:

Uo = gu (F, D, l) - dU

V ₀ = gv (F, D, 1) - dV

W ₀ = gw (F, D, 1) - dW.

The variometer MV may also have a scale factor defect (one per component) which must be calibrated by the controller 202. Indeed, the measurement of the field is generally performed through a chain of acquisition transforming the basic magnetic signal into a signal usable by the controller, for example, a current, a voltage or digital bits. A scale factor is generally applied to interpret the output signal of the acquisition chain. The determination of the baseline at sufficiently high resolution makes it possible to highlight this effect and the correct. For example, the controller calibrates the scaling factors of each component of the MV variometer by performing the steps of:

a) measuring the amplitudes of the baseline variations with respect to the amplitudes of the signals of each of the three components of the local magnetic field vector F for a predetermined period of time;

b) multiplication of the three measures of the variometer MV: dU, dV, and dW by corrective factors, scale factors, fu, fv, fw;

c) subtraction of the corrected values and calculation of the baselines according to:

Uo = gu (F, D, l) - fu ^* dU

V ₀ = gv (F, D, I) - fv ^* dV

W ₀ = gw (F, D, 1) - fw ^* dW.

d) increasing or decreasing the respective scale factors to reduce the variations of the respective baselines;

e) correction of MV variometer measurements by multiplication by the respective scale factors;

steps a) to d) being repeatable until the increase or decrease in scale factors is less than a predetermined value. For example, in the case of declination D, we have:

Do (t) = D (t) - δd ^* (t)

5D ^* (t) = (1 + a) SD (t)

D ^* (t) = D it) - SD (t) - a * SD (t)

D ^* (t) = D ₀ - a * SD (t)

with: D _Q C: the calculated LDB with a wrong scale factor and a: the scale factor error. The preceding equations can be transposed to the other components of the magnetic field.

Finally, the MV variometer may have an orthogonal or orientation defect or be mis-oriented in order to measure "unconventional" components. These defects must be corrected and calibrated by the controller 202. In this case, it is possible to detect a correlation between a component measured by the variometer MV and the LDB of another component by means of regular absolute measurements. Again, the correction tends to minimize this correlation. The correction consists of applying a base change matrix to find the basic components of the magnetic field. The base change matrix is called matrix Eulerian, E. The controller thus calibrates the orthogonality and spatial orientation of the three mathematically independent components of the MV variometer and calculates Eulerian E rotation matrices by performing the steps of:

a) examining the variation of the baseline of a component as a function of the signal amplitude of the other components for a predetermined period of time;

b) calculating the Eulerian rotation matrices E and adjusting the orientations until the variation of the baseline is less than a predetermined value.

The aforementioned defects may be present during the implementation of the observatory, vary or appear in time. The MV variometer can, for example, be installed on an unstable pillar. The correction made must then be made as and when the operation of the observatory. In particular, the present invention can control and correct these defects in real time.

Once the various calibrations have been made, the controller 202 calculates the value of the local magnetic field vector F by performing the steps of:

a) S871: obtaining oriented measurements by applying Eulerian E rotations to the three MV variometer measurements: dU, dV, and dW;

b) S872: Scaling measurements by multiplying the three oriented measurements of the MV variometer by scale factors;

c) S873: obtaining the 3 components of the local magnetic field vector F by adding the baselines to the three oriented measurements and to the scale.

For example, the variometer MV can measure the variations of the components X, Y, Z, H and F generally expressed in nanoTesla (nT) or angular D and I usually expressed in arcmin, arcsec, degrees-minutes-seconds or degrees decimal. The modulus of the local magnetic field is given by the scalar magnetometer MS, the absolute values of the angular components, preferably the declination D and the inclination I, are provided by the angular magnetometer MA.

Case of an observatory in XYZ configuration The variometer MV measures variations of the Cartesian components of the field, each sensitive axis is oriented in the direction of the component to be measured. The baselines of each component are: X ₀ = Xt - SXt, Y ₀ = Ytt - SYt, Z ₀ = z (t) - SZ (t). These baselines (LDB) are assumed to be constant. However, it is possible that the scaling factors of the measurements are erroneous. There is a correlation between LDB and its component. An iterative process then makes it possible to adjust the scale factor. For example, for X, we have that:

X ^* t = X t - (1 + a) SX (t), X0 t = X ₀ - a * SX (t, corr {X ₀ ^* (t), X (t)) ≠ 0, with corr, the linear correlation coefficient. One can either try to minimize the correlation coefficient (or covariance) or directly minimize the variations of LDB or a combination of both. One possible solution is to perform several absolute measurements per day (eg every 30 min or each hour) and observe a diurnal variation in LDB corresponding to the diurnal variation of its component. We then determine:

ti = 1 1 = maxOo (t)), t ₂ = t \ X (t ₂ ) = min (t))

~ fi * (t) -fiy (t ₂ )

Alternatively, the measurements can be spread over several days and can also be averaged or smoothed.

The measurements may have a defective orientation. Again, one solution is to measure the LDB at a frequency to highlight diurnal variation. Figure 9a shows an example of a leveling defect φ along an east-west axis corresponding to a rotation around the X axis. The sensor output then becomes (in matrix notation):

A rotation matrix is found around an X axis. Calculating the LDB as presented above then requires the inverse transformation which is none other than the transpose of the previous matrix.

The value of φ can be determined empirically or by calculation (possibly iterative) by considering the LDB. For example, we can try to minimize the variance of the baseline or the covariance of the LDB with another component. In the case of a weak orientation fault, we can assume: cos (0) "1 and sin (0)" φ (in radians):

o = Z - δΖ Ζ ₀ ^* = Ζ- {δΖ- φδΥ), Z ₀ ^* = Z ₀ + φδΥ.

The variation of LDB is therefore proportional to the Y component of the MV variometer measurements. Considering Z ₀ constant, we can estimate the value of φ:

Z ₀ - Z _Q

φ ⁼ -δΫ ^~

However, Z ₀ is not directly accessible. One solution is to take the variations in Z and _Q ^* δΥ "peak-to-peak", possibly filtered or smoothed to avoid measuring noise effects:

t = 11 S (ti) = max (Zo (t)), t ₂ = t \ Z £ (t ₂ ) = min (Zo (t)), φ

Figure 9b shows an example of a baseline measurement, Z ₀ of a Z component of the field having a leveling defect in the east-west direction. In particular, Figure 9b illustrates the case of a leveling defect of 5 ° in the east-west direction. The variations of the baseline lines Z ₀ (in nano Tesla, nT) uncorrected baseline and corrected baseline are represented over a period of three days (in abscissa, day). Case of an observatory in any orientation

In the case of any orientation of the variometer, a possible algorithm is presented in FIG. 10. It consists of successively applying matrices of rotations Rx and Ry around the X and Y axes, respectively, so as to mathematically correct the Z-axis. These virtual rotations are determined so as to minimize a particular coefficient such as the variance of Z _Q ^* or the covariance of Z _Q ^* and δΥ or δχ or a combination of both. For example, the two parameters to be minimized can be determined as follows:

ζ _ζγ = jvAR (z ^* ) ² + VOC (Z ;, ÔY) ²

ζ _ζχ = ^ VAR (z ^* ) ² + cov z; ;, δχγ

With VOC (A, B) the covariance of A and B. Then, a rotation matrix around the Z axis is applied in order to minimize the variance of X _Q ^* or Y _Q OR the covariance of these LDBs with δΥ or δχ or a combination of these parameters.

A similar process can be used upstream or downstream of the correction guidance to adjust scale factors. The whole algorithm can have several iterations.

Case of an observatory in DFI configuration

The variometer MV measures variations of the field components corresponding to a spherical coordinate system. The variometer MV measures variations of the local magnetic field in three orthogonal directions, one of which is oriented parallel to the magnetic field and measures its intensity in the same way as in a cartesian configuration. The other two directions are in a plane perpendicular to the magnetic field. In particular, the direction allowing the relative measurement of D, lies in a horizontal plane perpendicular to the magnetic field while that allowing the measurement of I lies in the magnetic meridian. These two axes record the projection of the magnetic field (H for the measurement of D and F for the measurement of I). A transformation similar to that used during the absolute measurements D and I then makes it possible to go from a residual to an angular value.

As with Cartesian configuration, MV measurements may be affected by a scale factor error. A similar procedure is then used to adjust them, such as: performing high frequency LDB measurements (eg every hour) and adjusting the scale factor of a (relative) component to minimize its LDB variation . The linear correlation coefficient can be used.

A misalignment of MV produces effects that depend on the measured component. For F and I, the effect is not critical because it is the total field F that is projected. So we have a second order relation. On the other hand, the relative measure of D uses the projection of H (sensitive axis in the horizontal plane). A horizontal defect induces a projection of the Z component of the field. We then obtain a dependence of LDB as a function of sin (/). The adjustment therefore consists in determining a correlation between the computed component Z = F sin (/) and the LDB of D.

Case of an observatory in HDZ configuration

The variometer MV measures variations of the field components corresponding to a cylindrical coordinate system. She may be considered intermediate to the two previous ones. A generalization can be done in the case of any orientation of MV.

The present invention has been described in relation to specific embodiments, which have a purely illustrative value and should not be considered as limiting. In general, the present invention is not limited to the examples illustrated and / or described above. In particular, the invention also relates to combinations of the technical characteristics of the embodiments described above. The use of the verbs "to understand", "to include", "to include", or any other variant, as well as their conjugations, can in no way exclude the presence of elements other than those mentioned. The use of the indefinite article "a", "an", or the definite article "the", "the" or "Γ", to introduce an element does not exclude the presence of a plurality of these elements. The reference numerals in the claims do not limit their scope.

In summary, the invention can also be described as follows. An autonomous magnetic observatory 200 which comprises a scalar magnetometer MS for measuring the modulus of the local magnetic field vector F, an angular magnetometer MA for measuring the direction of the vertical V, the geographical north direction N, the direction of the vector field local magnetic F, a variometer MV to measure three variations of the vector local magnetic field F, a clock 201, and a controller, 202. Observatory in which the controller is configured to control and manage the orientation of sensors, acquire the measures of the variometer MV, the scalar magnetometer MS, the angular magnetometer MA and the variometer MV, process the acquired measurements to automatically obtain the local magnetic field vector F and the measurement errors associated with each instrument.

Claims

claims

Magnetic observatory (200) for measuring the local magnetic field vector (F) comprising:

a) a scalar magnetometer (MS) for the absolute measurement of the modulus of the local magnetic field vector (F);

b) an angular magnetometer (MA) comprising:

1) a first steerable support (320) along a first axis of rotation, said main axis (321) making it possible to obtain a horizontal orientation of the first steerable support (320), said first steerable support (320) comprising:

i. a main motor (322) for changing the horizontal orientation of the first steerable support (320) about the main axis (321);

ii. a second steerable support (310) along a second axis of rotation called secondary axis (31 1) for obtaining a vertical orientation of the second steerable support (310) orthogonal to the main axis (321), said second steerable support (310) comprising: a secondary motor (312) for changing the vertical orientation of the second steerable support (310) around the secondary axis (31 1), a magnetic sensor (323) for measuring the direction of the vector field local magnetic

(;

2) control means (340) of the main and secondary engines (322 and 312);

3) a device for measuring and angular acquisition (350) of the horizontal and vertical orientations of the first and second orientable supports (320, 310);

c) a magnetic variometer (MV) recording variations of three components mathematically independent of the local magnetic field vector (F);

d) a clock (201);

characterized in that The first orientable support (320) of the angular magnetometer (MA) further comprises: an inclination sensor (313), preferably mounted on the first orientable support (320) or on the second orientable support (310);

The second orientable support (310) of the angular magnetometer (MA) further comprises: a North search device (324);

The observatory comprises a controller (202) configured to:

a) automatically control the main motor 322 and secondary 312;

(b) manage the direction of

1) inclination sensor (313) for measuring the direction of the vertical (V);

2) North search facility (324) for the measurement of the North (N) geographical direction;

3) magnetic sensor (323) for measuring the direction of the local magnetic field vector (F);

(c) acquire

1) the angles of the direction of the local magnetic field vector (F) relative to the geographic north (N) and the vertical (V) according to the horizontal and vertical orientations of the first and second orientable supports measured with the measuring device and angular acquisition (350);

2) the three variations of the local magnetic field vector (F) measured by the variometer (MV);

3) the values of the local magnetic field vector module (F) measured by the scalar magnetometer (MS);

d) processing the data acquired in step c) to automatically obtain the local magnetic field vector (F) and measurement errors associated with each instrument.

Observatory according to claim 1, characterized in that the first and second steerable supports, the main and secondary engines, the North search device (324), the inclination sensor, the means of control (340) of the main and secondary motor and the measurement and angular acquisition device (350) consist of non-magnetic components, defined such that the magnetic susceptibility of the materials is between -1 and 1, preferably between -10 ^"1 and 10 ^{" 1} , even more preferably between -10 ^"3 and 10 ^{" 3} .

3. Observatory according to claim 2, wherein the non-magnetic components are materials selected from: ceramic, aluminum, arcap, titanium, copper, ertalon, nylon, ertacetal, peek.

4. Observatory according to any one of the preceding claims, wherein the scalar magnetometer (MS) is of the type: proton, Overhauser effect, atomic, optically pumped.

5. Observatory according to any one of the preceding claims, wherein the magnetic sensor (323) is a sensor type: fluxgate, fluxset, rotating electrical circuit or a scalar magnetometer polarized by a magnetic device.

6. Observatory according to any one of the preceding claims, wherein the North search device (324) is of type: GNSS, GPS, sighting of a target, aim of a star, gyroscope, absolute rotation detector, polarization of solar light.

7. Observatory according to any one of the preceding claims, comprising a plurality of non-magnetic pillars, preferably made of concrete, supporting at least one of the following elements: the scalar magnetometer (MS), the angular magnetometer (MA), the variometer (MV) , the clock (201), the controller (202); the non-magnetic pillars having average dimensions [thickness, width, length] of [1,

10, 10] cm and [6, 10, 10] m, preferably between [10, 20, 20] cm and [1, 2, 2] m, even more preferably between [15, 25, 25] cm and [0.25, 0.5, 0.5] m, and being separated by a mean distance of between 0 and 10 m, preferably between 1 and 6 m, even more preferably between 2 and 4 m.

8. Observatory according to claim 7, comprising at least one non-magnetic shelter surrounding the plurality of pillars and comprising an insulated wall whose average thickness is between 1 and 60 cm, preferably between 2 and 30 cm, still more preferably between 5 and 10 cm, the at least one magnetic shelter protecting at least one of the following elements: the scalar magnetometer (MS), the angular magnetometer (MA), the variometer (MV), the clock (201), the controller (202).

9. Method of obtaining the local magnetic field vector (F) comprising the steps:

a) providing a magnetic observatory according to any one of the preceding claims;

b) the controller (202) acquires measurements of the modulus of said local magnetic field vector, F, measured with the scalar magnetometer (MS) at different times ti;

c) the controller (202) acquires measurements of the three mathematically independent components of the local magnetic field vector

(F), dU, dV, dW, measured with the variometer (MV) at different times ti;

d) the controller (202) controls the main motor (322) to change the horizontal orientation of the first steerable support (320) and, based on the indications of the inclination sensor, measures the vertical direction, V;

e) the controller (202) controls the main motor (322) and secondary motor (312) to change the horizontal orientation of the first orientable (320) and vertical support of the second orientable support (310) and, depending on the indications of the North search (324), measures the direction of North Geographic N;

f) the controller (202) controls the main and secondary actuators to change the horizontal and vertical orientations of the angular magnetometer (AM) and, depending on the indications of the magnetic sensor (323) obtained at different times ti, measures two angles, D ^* and, corresponding to the direction of the local magnetic field vector (F); g) the controller (202) calculates the absolute values of the two angles from the directions obtained in steps d) to f), such that: D = D ^* -N and I = -V;

h) the controller (202) calculates baselines of the variometer (MV) at a time t, average of the different times ti, using the absolute modulus of the local magnetic vector F, obtained as in step b) and the angles obtained as in step g);

i) the controller (202) calibrates the scaling factors of each mathematically independent component of the variometer (MV);

j) the controller (202) calibrates the orthogonality and spatial orientation of the three mathematically independent components of the variometer (MV) and calculates Eulerian E rotation matrices;

k) the controller (202) calculates the value of the local magnetic field vector

(F) by performing the steps of:

1) obtaining oriented measurements by applying Eulerian rotations E, obtained in step j), to the three measures of the variometer (MV): dU, dV, and dW;

2) obtaining scale measurements by multiplying the three oriented measures of the variometer (MV) by the respective scale factors obtained in step i);

3) obtaining the 3 components of the local magnetic field vector

(F) by adding the baselines to the three measures oriented and to the scale.

The method of claim 9 wherein in step f) the controller (202) measures the two angles, D ^* and P, by performing the steps of:

a) changing the horizontal orientation of the first steerable support (320) until the magnetic sensor (323) indicates a zero, acquiring the first angle D ^* ;

b) changing the vertical orientation of the second steerable support (310) until the magnetic sensor (323) indicates a zero, acquisition of the second angle P.

1 1. A method according to any one of claims 9 to 10 wherein the controller (202) calculates baselines of the va ometer (MV) based on the three measurements of the vahometer (MV): dU, dV, and dW, of the absolute modulus of the vector local magnetic field F, of the two angles characterizing the direction of the local magnetic field vector (F): the inclination I and the declination D, and, optionally, functions gu, gv, gw, making it possible to change the coordinates D, F, I at coordinates U, V, W, the controller (202) calculates the baselines Uo, Vo, Wo, according to:

Uo = gu (F, D, l) - dU

V ₀ = gv (F, D, 1) - dV

W ₀ = gw (F, D, 1) - dW.

The method of any one of claims 9 to 11 wherein the controller (202) calibrates the scaling factors of each component of the vahometer (MV) by performing the steps of:

a) measuring the amplitudes of the baseline variations with respect to the amplitudes of the signals of each of the three components of the local magnetic field vector (F) during a preset time;

b) multiplication of the three measurements of the vahometer (MV): dU, dV, and dW by corrective factors, scale factors, fu, fv, fw;

Uo = gu (F, D, l) - fu ^* dU

V ₀ = gv (F, D, I) - fv ^* dV

Wo = gw (F, D, l) - f _w ^* dW

e) correction of the vahometer (MV) measurements by multiplication by the respective scale factors;

steps a) to d) being repeatable until the increase or decrease in scale factors is less than a predetermined value.

The method of any one of claims 9 to 12 wherein the controller (202) calibrates the orthogonality and orientation in the space of the three mathematically independent components of the va (MV) and calculates Eulerian E rotation matrices by performing the steps of: a) examining the variation of the baseline of a component as a function of the signal amplitude of the other components during a predetermined period of time;

14. The method of claim 9 wherein, step f) consists of: the controller (202) controls the main and secondary motorization to change the horizontal and vertical orientations of the angular magnetometer (AM) and, depending on the sensor indications magnetic circuit (323) obtained at different times ti, measures two angles, D ^* and, corresponding to the direction of the local magnetic field vector (F) by performing the steps of:

a) changing the horizontal orientation of the first orientable support (320) so that the magnetic sensor (323) indicates a zero measurement value and acquire the orientation D1 of the first orientable support (320) corresponding to this measurement;

b) changing the horizontal orientation of the first orientable support (320) by 180 ° and adjusting the horizontal orientation so that the magnetic sensor (323) indicates a zero measurement value and acquire the orientation D2 of the first orientable support (320) corresponding to this measure;

c) changing the vertical orientation of the second steerable support (310) by 180 ° and adjusting the horizontal orientation of the first steerable support (320) so that the magnetic sensor (323) indicates a zero measurement value and acquire the orientation D3 the first orientable support (320) corresponding to this measurement;

d) changing the horizontal orientation of the first orientable support (320) by 180 ° and adjusting the horizontal orientation so that the magnetic sensor (323) indicates a zero measurement value and acquire the orientation D4 of the first orientable support (320) corresponding to this measure; e) calculating the first angle corresponding to the horizontal direction of the local magnetic field according to the average of the four measurements: D ^* = (D1 + D2 + D3 + D4) / 4;

f) changing the horizontal orientation of the first steerable support (320) in the direction D ^* -90 °;

g) performing steps a) to c) in which the horizontal and vertical orientations are reversed and in which the roles of the first and second steerable supports (320 and 310) are reversed, and thus acquire the orientations 11, 12, 13.

h) changing the horizontal orientation of the first steerable support (320) in the D ^* -90 ° direction and adjusting the vertical orientation of the second steerable support (310) so that the magnetic sensor (323) indicates a zero measurement value and acquiring the orientation 14 of the second orientable support (310) corresponding to this measurement;

i) calculation of the second angle corresponding to the second direction of the local magnetic field according to the average of the four measurements: l ^* = (M + l2 + l3 + l4) / 4.

The method of any one of claims 9 to 14 wherein the North search device (324) is an absolute rotation detector searcher and wherein step e) of claim 9 consists of: the controller ( 202) controls the main (322) and secondary (312) actuators to change the horizontal orientation of the first steerable (320) and vertical support of the second steerable support (310) and, depending on the indications of the North search device (324 ), measures the direction of the geographic north N by changing the horizontal orientation of the first steerable support (320) until the absolute rotation detector indicates a zero measurement value and acquire the orientation N of the first steerable support (320). ) corresponding to this measure.

The method of any one of claims 9 to 15 wherein the North search device (324) is an absolute rotation detector searcher and wherein step e) of claim 9 comprises steps a) to e) of claim 14, wherein the sensor magnetic (323) is replaced by the absolute rotation detector and the orientations D1, D2, D3, D4 and D ^* are replaced by N1, N2, N3, N4 and N.

17. Method according to any one of claims 9 to 16 wherein the scalar magnetometer (MS) and the variometer (MV) perform measurements with a frequency between 0.01 Hz and 100 Hz, preferably between 0.05 Hz and 10 Hz, even more preferably between 0.1 Hz and 1 Hz.

18. Method according to any of claims 9 to 17 wherein the measurements of the orientation of the local magnetic field vector (E) are carried out with a frequency between 10 ^"7 Hz and ^{10" 2} Hz, preferably between 10 ^"6 Hz and 10 ^"3 Hz, still more preferably between ^{10" 5} Hz and 10 ^"4 Hz.

19. Observatory according to any one of claims 1 to 8, characterized in that it implements a method according to any one of claims 9 to 18.