Scalar Magnetometer Research Articles

Phase-error-free atomic magnetometer and vector measurement method based on demodulated signal phase in geomagnetic environment

The performance of double-beam atomic magnetometer in the geomagnetic environment is greatly affected by the vectorial direction of magnetic field, which leads to the shift of the phase of the demodulation signal. We propose a phase-error-free scalar double-beam atomic magnetometer. When the RF field is along the direction of probe beam, the fluctuation of the phase of the demodulation signal can be avoided. For the practical measurement environment, we propose a phase extraction method for atomic magnetometer. Additionally, based on the phase of the demodulated signal when the RF field is applied in the directions of pumping, probing, and perpendicular to the pumping-detection plane, we propose a method for vector magnetic field measurement. The measurement results indicate that under a 10 μT bias magnetic field, the deviations θ and φ are within 0.1°. The theory and method proposed are of great significance to the application of magnetometer in geomagnetic magnetic field.

Total Root Electron Content: A New Metric for the Ionosphere Below Low Earth Orbiting Satellites

AbstractPowerful lightning strikes generate broadband electromagnetic signals. At Extremely Low Frequencies (ELF), the signal partly leaks into the ionosphere and produces whistlers that can be detected by satellites. Indeed, the satellites of the European Space Agency (ESA) Swarm Earth Explorer mission can detect those signals during 250 Hz burst‐mode acquisition campaigns of their Absolute Scalar Magnetometers (ASM). The dispersion of these whistlers depends on their propagation path and the distribution of ionization in the ionosphere crossed along that path. In this paper, we introduce a technique to derive a new measure of ionosphere electron content, the Total square‐Root Electron Content (TREC), using the arrival times of two frequencies of the whistler signal. We validate this approach by using data from ionosondes and from in situ measurements of the electron density at Swarm location. This technique brings new opportunities for sounding the ionosphere in regions poorly observed by other techniques.

Accuracy of the scalar magnetometer aboard ESA's JUICE mission

Abstract. This paper discusses the accuracy of the scalar Coupled Dark State Magnetometer on board the Jupiter Icy Moons Explorer (JUICE) mission of the European Space Agency (ESA). The scalar magnetometer, referred to as MAGSCA, is part of the J-MAG instrument. MAGSCA is an optical omnidirectional scalar magnetometer based on coherent population trapping, a quantum interference effect, within the hyperfine manifold of the 87Rb D1 line. The measurement principle is only based on natural constants; therefore, it is in principle drift-free, and no calibration is required. However, the technical realisation can influence the measurement accuracy. The most dominating effects are heading characteristics, which are deviations of the magnetic field strength measurements from the ambient magnetic field strength. These deviations are a function of the angle between the sensor axis and the magnetic field vector and are an intrinsic physical property of the measurement principle of the magnetometer. The verification of the accuracy of the instrument is required to ensure its compliance with the performance requirement of 0.2 nT (1σ) with a data rate of 1 Hz. The verification is carried out with four dedicated sensor orientations in a Merritt coil system, which is located in the geomagnetic Conrad Observatory (COBS). The coil system is used to compensate the Earth's magnetic field and to apply appropriate test fields to the sensor. This paper presents a novel method to separate the heading characteristics of the instrument from residual (offset) fields within the coil system by fitting a mathematical model to the measured data and by the successful verification of the MAGSCA performance requirement.

Research on magnetic localization and tracking algorithm based on total magnetic field

There are few studies on the total magnetic field localization and tracking algorithm based on the domestic magnetic dipole model, and the scalar magnetometer’s measurement of magnetic targets is not affected by its own posture. Therefore, a magnetic localization and tracking algorithm based on the total magnetic field is proposed., this method is based on the magnetic dipole model and uses the particle swarm algorithm and the extended Kalman auxiliary algorithm to achieve magnetic localization and tracking. The effectiveness of this method has been verified through simulation experiments. The relative localization error is small and maintained in the range of about 10%.

Underwater target correlation detection and location method based on normalized magnetic moment

For the dynamic magnetic detection platform represented by airborne magnetic anomaly detection, the magnetic field measured by the vector magnetometer is easily affected by the rapid attitude change. Therefore, traditional anti-submarine aircraft are equipped with scalar magnetometers that are less affected by changes in the carrier attitude. However, the scalar magnetometer can only measure the total magnetic field signal, and its measurement information is single. It contains a time-varying environmental magnetic field signal, including the geomagnetic field, which is difficult to effectively reject. This brings great difficulties to the single real-time location of a magnetic anomaly target. This paper proposes a target correlation detection method based on the normalized magnetic moment that can effectively solve the above problems, considering the rotational invariance of magnetic anomaly measurements for dynamic magnetic exploration platforms and the efficient detection methods required for the single real-time positioning of targets. Correlation detection based on the normalized magnetic moment characteristic function and characteristic time search based on the correlation detection are included. Simulation and experimental results verify the single-point real-time positioning ability of the improved detection method within an effective operating distance. This method has strong usability for actual magnetic anomaly detection of underwater targets.

Attitude-Independent Route Tracking for Subsea Power Cables Using a Scalar Magnetometer under High Sea Conditions

To overcome the shortcoming wherein the accuracy of subsea cable detection can be affected by the determination of the bias vector, scale factors, and non-orthogonality corrections of the vector magnetometer, a real-time attitude-independent route tracking method for subsea power cables is investigated theoretically and experimentally by means of scalar magnetic field checking. The measurement of the magnetic field Bc produced by the current in a cable is made immune to the influence of the platform attitude by extracting the component of Bc along the geomagnetic field using a high-bandwidth self-oscillating optically pumped magnetometer. The self-oscillating frequency is proved to be independent of the attitude of the magnetometer with the theoretical model. Experiments are carried out to test the attitude-independent performance, and the effectiveness of route tracking is verified by the results of the sea experiment. The proposed method will effectively improve the ability to locate subsea cables under high sea conditions.

Light-narrowed parametric resonance magnetometer with the fundamental sensitivity beyond the spin-exchange limit.

In the field of biomagnetic measurements, one of the most important recent challenges is to perform measurements in a magnetically unshielded environment. This first requires that atomic magnetometers can operate in a finite magnetic field, and have enough high sensitivity. To meet these requirements, we develop a light-narrowed parametric resonance (LPR) magnetometer. By adding a modulation magnetic field to the large longitudinal magnetic field, our LPR magnetometer can measure small transverse magnetic fields with an intrinsic sensitivity of 3.5 fT/Hz1/2 in a longitudinal magnetic field of μT range. Moreover, we have also demonstrated that in contrast to the previous light-narrowed scalar magnetometers, our LPR magnetometer has the potential to achieve higher sensitivity. Because in our case spin-exchange relaxation suppression by using light narrowing can lead to an improvement of fundamental sensitivity limit regardless of which quantum noise is dominant, and hence the fundamental sensitivity is no longer limited by spin-exchange, and approaches the fundamental limit set by the spin-exchange and spin-destruction cross sections.

Tracking Magnetic Target Based on Internative Multi-Model Square Root Unscented Kalman Filter

Scalar magnetic sensors are widely used in magnetic detection and tracking because of their high sensitivity, wide detection range, and small attitude influence. Magnetic anomaly tracking usually uses scalar magnetometers arrays to retrieve the magnetic target position because the incomplete measurement of the geomagnetic field can’t be converted into a filtering problem. However, the workload of calculation is time-consuming, the target is easy to lose. Focusing on filtering divergent and targets easy to lose, tracking magnetic target based on Internative Multi-model Square Root Unscented Kalman Filter is proposed. Firstly, the magnetic anomaly signal is modeled based on moving magnetic target. Subsequently, the initial positioning of the magnetic target is based on dual-magnetometer trajectory planning. Finally, the IMM-SRUKF filter method is used to track magnetic targets adaptively. Both positioning and tracking of magnetic target based on IMM-SRUKF have been verified by simulation and experiments, the result shows that the magnetic target based on IMM-SRUKF can track target stably for a long time and adaptive to the target’s changed state, which indicates that the target is hard to lose.

Feedback Methods for Vector Measurements Using an All-Optical Atomic Magnetometer.

In this work, we look to compare three methods of feedback for the ultimate purpose of measuring the transverse vector components of a magnetic field using a synchronous light-pulse atomic scalar magnetometer with a few tens of fT/Hz sensitivity in Earth-field-scale magnetic environments. By applying modulation in the magnetic field to orthogonal axes, the respective vector components may, in principle, be separated from the scalar measurement. Success of this technique depends in significant part on the ability to measure and respond to these perturbations with low measurement uncertainty. Using high-speed least-squares fitting, the phase response of the atomic spins relative to the first harmonic of the optical pump pulse repetition rate is monitored and correspondingly adjusted into resonance with the natural Larmor precession frequency. This paper seeks to motivate and compare three distinct methods of feedback for this purpose. As a first step toward the full development of this technique, the present work uses a simplified version with modulation applied only along the bias field. All three methods investigated herein are shown to provide results that match well with the scalar magnetometer measurements and to depend on both the applied modulation amplitude and optimal feedback response to achieve low relative uncertainty.

Swarm A and C Accelerometers: Data Validation and Scientific Interpretation

AbstractThe fourth European Space Agency Earth Explorer (Swarm) was launched in 2013 and it comprises a constellation of three identical satellites. The primary mission objective is analyzing and modeling the geomagnetic field and its temporal evolution, based on the measurements of a vector and a scalar magnetometer on board. Each spacecraft (A, B, and C) carries an accelerometer, supposed to measure the non‐gravitational forces. However, this instrument did not work as expected; the processing of the related data sets has a significant latency. The focus was primarily on Swarm C, because its signal‐to‐noise ratio is the best amongst the three spacecrafts. For this satellite, the data are available for the whole mission timeline. At the end of 2021, a batch of Swarm A accelerometer data set was disseminated. Swarm A and C have flown side‐by‐side for most of the mission and, therefore, their measurements are supposed to be nearly identical after calibration. The work presented in this paper shows that such a correspondence occurs: at high frequency similar signatures are visible in both accelerometer data sets, near the equator and the poles. These features agree with measurements of other instruments on board and with earlier findings based on CHAllenging Minisatellite Payload and Gravity Recovery and Climate Experiment data. The equatorial structures show a correlation between the equatorial mass anomaly and the equatorial ionization anomaly. At the poles, the data are related to the polar cap index and to field‐aligned‐currents enhancements measured by the magnetometers on board. This research shows, for the first time, scientific results obtained from the Swarm accelerometers.

Experimental and theoretical investigations on the modulation capabilities of a sample of vertical cavity surface emitting laser diodes for atomic vapour applications

This paper discusses various frequency modulation and intensity modulation capabilities within a sample of direct laser current modulated identical vertical cavity surface emitting laser diodes. The presented analysis is based on measurements of the spectral amplitudes as a function of the applied modulation power at a constant modulation frequency of 3.517 GHz. Their evaluation by Bessel function fits produces the three modulation parameters: frequency modulation index, intensity modulation index (via the alpha parameter) and the side band asymmetry of the first order side bands. The variation of the laser diode’s modulation capability is discussed. It is found that the individual laser diodes show a significant variations in their modulation capabilities. Thus an experimental preselection of the laser diode is required to find laser diodes which are suitable as a light source for atomic vapour applications, with special emphasis on a coherent population trapping-based scalar magnetometers.

A novel geomagnetic satellite constellation: Science and applications

The European Space Agency (ESA)’s Swarm constellation of a trio of geomagnetic survey satellites in nearly circular polar orbits at altitude about <inline-formula><tex-math id="M1">\begin{document}$500$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-zhangkeke_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-zhangkeke_M1.png"/></alternatives></inline-formula> km was launched on 22 November 2013 and has been mapping the Earth’s global magnetic field in unprecedented details, helping scientists better understand how the geomagnetic field is generated and maintained inside the Earth’s fluid core and how the Earth’s external magnetic environment is changing. This review discusses a new novel constellation of the geomagnetic survey satellites that consists of at least four satellites: two satellites are in lower-latitude and nearly circular orbits at altitude about <inline-formula><tex-math id="M2">\begin{document}$450$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-zhangkeke_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-zhangkeke_M2.png"/></alternatives></inline-formula> km; two further satellites are marked by nearly polar but strongly eccentric orbits with perigee about <inline-formula><tex-math id="M3">\begin{document}$200$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-zhangkeke_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-zhangkeke_M3.png"/></alternatives></inline-formula> km and apogee about <inline-formula><tex-math id="M4">\begin{document}$5000$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-zhangkeke_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-zhangkeke_M4.png"/></alternatives></inline-formula> km. The new geomagnetic satellites are equipped with highly stable optical benches, high-precision fluxgate magnetometers and scalar magnetometers which are capable of mapping the Earth’s three-dimensional magnetic field in unprecedented accuracies and details. The new constellation will help elucidate different contributions to the measured geomagnetic field: the core dynamo field, the lithospheric magnetic field, the magnetic fields produced by currents in the ionosphere and the magnetosphere as well as by the currents coupling the ionosphere and magnetosphere, and the magnetic fields induced from the electrically conducting mantle, lithosphere and oceans. In comparison to the Swarm mission, it will provide higher-accuracy, higher-resolution and higher-dimension measurements of the geomagnetic field required for shedding new insights into the core dynamo processes and the Earth’s space magnetic systems along with a wide range of important applications.

Numerical simulation of the sensor output on Macau Science Satellite-1

Macau Science Satellite-1 (MSS-1) will be launched at the early of 2023 into a near-circular orbit. The mission is designed to measure the Earth’s geomagnetic field with unpreceded accuracy through a new perspective. The most important component installed on the satellite, to ensure high accuracy, is the deployable boom (Optical Bench). A Vector Field Magnetometer (VFM), an Advanced Stellar Compass (ASC), and a Couple Dark State Magnetometers (CDSM) are deployed on the deployable boom. In order to maximize the mission’s scientific output, a numerical simulator on MSS-1’s deployable boom was required to evaluate the adaptability of all devices on the deployable boom and assist the satellite’s data pre-processing. This paper first briefly describes the synthesis of the Earth’s total magnetic field and then describes the method simulating the output of scalar and vector magnetometers. Finally, the calibration method is applied to the synthetic magnetometer data to analyze the possible noise/error of the relevant instruments. Our results show that the simulator can imitate the disturbance of different noise sources or errors in the measuring system, and is especially useful for the satellite's data processing group.

Validation of equatorial electrojet derived from Swarm observations using ground based magnetometers

This paper presents the analysis of the Equatorial electrojet (EEJS) and Counter equatorial electrojet (CEJS) at quiet times (Kp⩽3) deduced from the absolute scalar magnetometers (ASM) of the Swarm satellites. The EEJS and CEJS were compared with their corresponding values, EEJG and CEJG respectively, from ground magnetometers in the South American, West African, and Asian sectors during the years 2014–2018. The results show that EEJS highly correlated with EEJG, over South American, West African and Asian sectors with correlation coefficients of 0.91, 0.77 and 0.92 respectively. CEJS with CEJG also showed a reasonably good correlation of 0.78, 0.98 and 0.83 over South American, West African and Asian sectors respectively. For the first time, the Swarm observations of CEJ and EEJ are validated against CEJ and EEJ from ground magnetometers, over South America, West Africa and Asia. Both the occurrence rates of CEJS and CEJG show similar diurnal and seasonal trends, with the highest occurrence rates in the morning (7:00–9:00 LT) and evening (15:00–19:00 LT) times and the lowest occurrence rates around noon (11:00 to 12:00 LT) time. The seasonal occurrence rates of CEJS and CEJG were high in the solstice months and the lowest rates were during equinox months for all the sectors. The longitudinal variability of CEJS and CEJG show highest occurrence rates in the West African sector and the lowest occurrence rates in the Asian sector. This longitudinal variability of CEJS and CEJG occurrence rates were attributed to the effect of the non-migrating tides, migrating tides and stationary planetary waves. The annual occurrence rates of CEJS and CEJG seemed anti-correlated with F10.7.

A method for estimating magnetic target location by employing total field and its gradients data

In this paper, we present a magnetic target localization method by measurement of total field and its spatial gradients. We deduce an approximate formula of the target’s bearing vector expressed by the total field and its gradients. The total field and its gradient can be measured by a scalar magnetometer array and the approximate value of the bearing vector can be calculated. An iterative method is introduced to improve the localization accuracy of the magnetic target. Simulations experiments have been done to evaluate the performance of the proposed method. The results show that the relative errors of the bearing vector estimated by the iterative method can be kept below the level of 5%. In addition, when difference root-mean-square (RMS) noise is added to the magnetometers, the relative errors of the bearing vector only vary from 0.8 to 6%, which indicates that the proposed method has a high tolerance to the noise of the magnetometers.

UAV magnetometer survey in low‐level flight for archaeology: Case study of a Second World War airfield at Ganacker (Lower Bavaria, Germany)

AbstractUnmanned aerial vehicle (UAV)‐based magnetometer systems became more and more attractive for large‐scale archaeological prospection in recent years. Although their sensors exhibit the same sensitivity than the ground‐based prospecting systems, UAV prospecting is seriously handicapped by the magnetic and mechanical disturbances of the drone and by limitations of a low‐level flight. To minimize these disturbances, scalar magnetometers are attached only on a tether 2.5–10 m beneath the drone to be flown as close as possible above the ground. First, test measurements with UAV‐fixed fluxgate magnetometers provide more accurate results than the scalar magnetometers in any configuration but have to overcome disturbance by vibrations. Here, we present a case study choosing the compact set‐up of the Sensys MagDrone R4. The high sampling rate of 200 Hz of the three axis fluxgate sensors of the R4 allows sufficient filtering of the interferences generated by the UAV and external disturbances. High‐precision flight control of the drone allows operating the sensors by radar‐controlled flight height ∼1 m above the ground, which is a fundamental and indispensable prerequisite for archaeological prospecting. For our test, we choose the site Ganacker (southern Bavaria), where we expected a large range of archaeological structures and features with high magnetic contrast. We compare and verify the magnetogram with historical and recent geodata. Our results show that the R4 system offers an outstanding step forward regarding a successful application for archaeological prospection. Already now, the system is well suited for the fast mapping of large areas and archaeological sites with intense magnetic anomalies.

An Aeromagnetic Compensation Method for Suppressing the Magnetic Interference Generated by Electric Current with Vector Magnetometer.

In aeromagnetic detection, the magnetic interference conducted by electric currents in onboard electronic (OBE) equipment is gradually being taken seriously with the development of aeromagnetic compensation technology. Here, we propose a compensation method based on the synthetically total magnetic field (STMF) measured by an onboard fluxgate vector magnetometer. In this method, a compensation model is firstly built to suppress the electric current magnetic interference (ECMI) which is jointly measured by a scalar magnetometer and a fluxgate vector magnetometer. The singular spectrum analysis (SSA) method is introduced to accurately extract the characteristic signal of the ECMI from the compensated STMF. In addition, in order to better suppress the geomagnetic gradient interference, the International Geomagnetic Reference Field (IGRF) model is introduced to modify the existing geomagnetic gradient compensation model. Based on these, a novel compensation model including the traditional aeromagnetic compensation model, modified geomagnetic gradient model, and ECMI compensation model is proposed. The results in the field experiment show that this model has better compensation performance than the TLG model, which is extended from the T–L compensation model.

Magnetization Structure and its Temporal Change of Miyakejima Volcano, Japan, Revealed by Uncrewed Aerial Vehicle Aeromagnetic Survey

Miyakejima volcano experienced its latest eruption in 2000 with the summit subsidence, and the next event is expected in the near future. An aeromagnetic survey in Miyakejima was conducted in March 2021 in order to investigate the current state of its magnetization structure to identify the potential for another eruption and, thus, mitigate volcanic disaster. The survey flight was conducted using an uncrewed aerial vehicle (UAV), a multirotor drone, to deploy a scalar magnetometer. After processing geomagnetic field data from this survey, in combination with data from previous surveys conducted by using another UAV, an uncrewed helicopter, the average magnetization intensity was determined to be 12.4 A/m. Further, the surrounding area of the crater was relatively highly magnetized; however, the crater rim had a low magnetization intensity. Temporal variation was detected between 2014 and 2021 and dominated the central part of the observation area. Decreased magnetization intensity was identified beneath the caldera, which may become recently demagnetized due to heat supply traveling through fractures in the impermeable layer from the deep heat reservoir.

A real-time calibration method of magnetometer array’s misalignment errors with ac modulated three-axis coil

The misalignment errors of magnetometer array are vulnerable to vibrations, which deteriorates the magnetic anomaly detection performance severely and brings strong demands for real-time calibrations. We propose a real-time calibration method of magnetometer array’s misalignment errors, which utilizes a standard three-axis coil to generate an alternating current magnetic field with varied magnitudes and directions. Then the misalignment errors are calibrated by minimizing the difference between experimentally calibrated magnetic fields of magnetometers and theoretically calculated magnetic fields of coils with the Levenberg–Marquardt algorithm and Biot-Savart’s law. It does not require the expensive high-precision non-magnetic rotation platform, homogenous magnetic environment and additional scalar magnetometer compared to the conventional methods, which facilitates the real-time calibration with ambient magnetic fluctuations. The investigations demonstrate that the X-axis magnetic field gradient is decreased from 3665 nT to 68 nT and localization error is reduced from 1.6 m to 0.11 m with the proposed calibration method.

High-Speed Magnetic Surveying for Unexploded Ordnance Using UAV Systems

Using Uncrewed Aerial vehicles (UAVs) to rapidly scan areas for potential unexploded ordnance (UXO) can provide an efficiency increase while minimizing detonation risks. We present a complete overview of how such mappings can be performed using scalar magnetometers, including initial sensor testing, time stamping validation, data positioning, noise removal, and source model inversion. A test survey was performed across disarmed UXO targets, during which three scalar magnetometers were towed in an airframe (“bird”) 10 m below a small (&lt;25 kg) high speed (∼10 m/s) UAV to avoid magnetic disturbances from the UAV itself. Data were collected across ∼58 min of flight, with each sensor traversing ∼31.7 km to acquire dense data coverage across a 600 m × 100 m area. By using three individual magnetometers in the bird, UXO detection results across single-sensor data and several different multi-sensor configurations can be compared. The data obtained exhibited low apparent noise floors (on the order of tens of picoTesla) and retained a precision that enabled targeted modelling and removal of high-frequency noise with amplitudes of ±5 picoTesla. All of the different gradiometer configurations tested enabled recovery of most targets (including all major targets), although the horizontal configuration performed significantly worse in comparison.