The outliers of magnetic sensor that are inevitable in special aeromagnetic surveys reduce the robustness of ordinary least-squares (OLS), which is widely used for aeromagnetic compensation. To address this problem, we propose an aeromagnetic compensation algorithm based on Huber loss method that is robust to outliers. In the proposed method, different weights are assigned to the inliers and outliers using an iteratively reweighted least-squares technique. Although OLS performs similarly to the proposed method when only 1% of the data are outliers, it is theoretically verified that the proposed method can increase the goodness-of-fit to 0.9963, from 0.6618 in the case of OLS, in the presence of 10% outliers. An experimental platform was constructed to record real magnetic data, with special measures taken to ensure the presence of outliers in the collected data. The results of a flight test using this experimental platform demonstrate that the proposed method increases the improvement ratio to 4.14 from 2.46 when using OLS.

Aiming to overcome the problem of the frequency error between the reference signal and the measured signal in an analog lock-in amplifier, this paper presented a digital lock-in amplifier (DLIA) with accurate frequency automatic tracking to improve its performance. In this paper, a modular design approach based on DSP Builder was employed to develop a quadrature vector type lock-in amplifier with automatic frequency tracking. A reference signal generator with high-frequency resolution and a digital filter with good performance were implemented. The performance of the design was tested by determining the linearity, the Q value, and the noise immunity of the DLIA. The experimental results showed that the proposed DLIA has good linearity and the Q value can reach up to 82. The relative errors of the signals with amplitudes greater than 10 mV were less than 1% when the equivalent input was a sine wave signal with a frequency of 1 kHz and an amplitude of 500 mV. When the superimposed noise was less than or equal to 400 mV, the relative error was less than 2% in the same condition. The proposed DLIA has higher precision and efficient noise immunity.

Based on the dynamic nuclear polarization (DNP) effect, an alternative design of an Overhauser geomagnetic sensor is presented that enhances the proton polarization and increases the amplitude of the free induction decay (FID) signal. The short-pulse method is adopted to rotate the enhanced proton magnetization into the plane of precession to create an FID signal. To reduce the negative effect of the powerful electromagnetic interference, the design of the anti-interference of the pick-up coil is studied. Furthermore, the radio frequency polarization method based on the capacitive-loaded coaxial cavity is proposed to improve the quality factor of the resonant circuit. In addition, a special test instrument is designed that enables the simultaneous testing of the classical proton precession and the Overhauser sensor. Overall, comparison experiments with and without the free radical of the Overhauser sensors show that the DNP effect does effectively improve the amplitude and quality of the FID signal, and the magnetic sensitivity, resolution and range reach to 10 pT/Hz ${}^{1/2}$ @1 Hz, 0.0023 nT and 20–100 $\mathsf{\mu }$ T, respectively.

Using a prior data to solve for the characteristic parameters of a moving magnetic target is crucial in its detection. For this purpose, we developed a real-time detection method using distributed scalar sensor networks based on a hybrid algorithm combining particle swarm optimization and the Gauss- Newton method. The magnetic anomaly fitting model of the target is established using the magnetic-dipole-moment principle. The PSO’s insensitivity to the initial solution is used to obtain a rough solution of the coeffcients of the fitting, and then a more accurate solution is developed using the Gauss-Newton method, exploiting its advantage in high precision. Furthermore, to evaluate the proposed method, we built a test platform. The detection results show that the goodness-of-fit reaches 0.9565 and only requires 3.4 s; in the 16 m horizontal distance, the calculated trajectory overlaps significantly the actual trajectory, and errors in position and speed are around 10% and 4.25%, respectively.

A miniature magnetic gradient tensor system is presented in this study, which could serve as a ready-to-use device for small spacecraft using in mineral prospection, unexploded explosive ordnance exploration, etc. The system consists of a magneto-inductive (MI) sensors array, a signal extraction circuit, and a micro-controller. A magnetic gradient full-tensor configuration utilizes four MI sensors arranged on a planar cross structure, was employed to further suppress the influence of the orientation and Orthogonal errors. The performance of the system is validated by checking data validation and magnetic anomaly detection. Numerous experimental results demonstrate that the system is able to detect a low-amplitude magnetic field as low as 8.28 nT at 10 Hz, with a typical noise floor of ${0.235 nT/Hz^{1/2}}$ at 1 Hz. Further, this system can configure the nearby ferromagnetic object clearly and shows the potential for identifying multiple objects, which indicates its feasibility for weak magnetic anomaly detection.

The mission of detecting and locating unexploded ordnance (UXO) such as the landmine, the improvised explosive devised, etc., is still a challenge to be overcome. In this paper, a new active approach for UXO detection through measuring electromagnetic (EM) response with a magnetic sensor is proposed. To further enhance the magnetic field strength of the UXO, a current-carrying coil is employed to produce a time-varying or steady magnetic field, and a magnetic field modeling of the solenoid structure is built to select optimal coil parameters. To realize three-dimensional imaging, a magnetic field measurement array of $3\times3$ tri-axial magneto-inductive (MI) sensors is constructed, and its major electrical characteristics are evaluated. Finally, a test platform is developed, and the experimental results indicate that the proposed method can detect a plastic anti-personnel UXO down up to a depth of 20.1 cm, improving 50% approximately when compared with a commercial metal detector.