To realize magnetic induction (MI) communication through seawater, a rotating permanent magnet array (RPMA) operating in the ultralow frequency (ULF) band is proposed. The array consists of 5×5 cylindrical Nd-Fe-B magnets driven by servo motors. To maximize the transmitting magnetic moment, the motors are controlled to rotate synchronously through network communication. The design of the magnet and array configuration is implemented by analyzing the torque and magnetic moment. Magnetic forces between the array magnets are analyzed by using the equivalent static magnetic dipole model. Permalloy sleeves are used to reduce the influence of magnetic force on the synchronization of magnets. The total transmitting magnetic moment of the RPMA is 360A∙m ² , and its power efficiency is 14.4 times higher than the conventional transmitting system using coils. Binary frequency shift keying (BFSK) modulation is realized by switching the rotating speed of the magnets and verified by an indoor test. The equivalent orthogonal horizontal magnetic dipole (HMD) model of the array is used, and the magnetic field distribution in layered media is simulated using FEKO. The calculated magnetic induction is 100fT in the position with a distance of 1km and a depth of 5m in the sea. The results of the through-the-sea experiment show that the magnetic field of the RPMA can be detected in a range of 500m by magnetic inductive sensors immersed at a depth of 5m in seawater, and agree well with the simulated results. Transmission bit rates of 10bps and 5bps can be achieved in the positions of 60m and 110m.

An ultra-wideband and high-gain Vivaldi antenna with artificial electromagnetic material, suitable for ground-penetrating radar (GPR) systems, is proposed. Directors loaded inside the antenna gradient slot direct electromagnetic waves by inducing current to improve gain. The artificial electromagnetic material, also called metamaterial, is composed of multiple “H”-shaped units arranged in a certain regular pattern, loaded at the antenna aperture. The artificial electromagnetic units affect the antenna radiation waves by changing the refractive index to improve radiation directivity. The four Vivaldi units are arranged into a horn-shaped array, and each two units are orthogonally fed to realize dual polarization. Experimental results demonstrate that the antenna has good impedance matching of $S_{11}\le -10$ dB in 0.9–4 GHz, and the maximum realized gain can reach 15.2 dBi.

This paper proposes a rotating shutter antenna that can directly generate 2FSK signals in the ULF band and it is expected to be used as the transmitter for magnetic induction (MI) underground communication systems. The antenna was modeled using ANSYS Maxwell and the magnetic field distribution was simulated. The results show that the interaction between the high-permeability shutter and the mutual cancellation of magnets decreased the transmitting magnetic moment of the antenna. A prototype antenna was manufactured and the time and frequency properties of the measured B${}_z$ field were the same as the simulated results, while the magnitude of the measured signal was larger. The propagation characteristics of the antenna in air–soil–rock were simulated using FEKO and the results show that the signal strength was greater than 1 fT at a depth of 450 m from the antenna whose magnetic moment as 1 Am${}^2$. The relationship between different magnetic components and azimuth could be used to enhance the signal strength. The formula of the B${}_z$ field was derived using the measured magnitude versus distance and the path loss was also analyzed. Finally, the 2FSK modulation property of the antenna was verified by indoor communication experiments with a code rate of 12.5 bps in the ULF band.

The accumulation rate is a key parameter in computing the mass balance of glaciers and ice sheets to estimate sea level rise. A shallow-layers-detection ice sounding radar (SLDISR) is developed to measure the accumulation rate and shape of near-surface internal layers with high resolution. With a transmitting frequency from 500 to 2000 MHz, this frequency-modulated continuous wave (FMCW) radar provides a range resolution of about 16 cm in free space by using a Hanning window and a penetrating depth about 150 m under polar ice. The spectral analysis and coherent integration techniques are used to obtain a high processing gain and to improve the signal-to-noise ratio of the system. A phase-locked loop with wideband yttrium iron garnet (YIG) oscillator is applied to generate a sweeping chirp signal as an input source for the transmitter. A stable, low-frequency reference chirp signal is generated with a direct digital synthesizer (DDS) integrated in field-programmable gate array (FPGA). To reduce the high-speed requirement to the analog-to-digital converter (ADC), dechirp technology is adopted at the RF section of the receiver. The implementation of the digital unit is based on an FPGA chip. The designed radar has been successfully deployed in Antarctica during the 31st Chinese Antarctic Research Expedition (CHINARE 31) and CHINARE 33, mainly over the East Antarctic Ice Sheet (EAIS). The echograms indicate the effectiveness of the radar system on detecting clear internal reflecting horizons (IRHs) over ice sheets.

Drift detection is one of the important detection modes in a digital ionosonde system. In this paper, a new data processing method is presented for boosting the automatic and high-quality drift measurement, which is helpful for long-term ionospheric observation, and has been successfully applied to the Chinese Academy of Sciences, Digital Ionosonde (CAS-DIS). Based on Doppler interferometry principle, this method can be successively divided into four constraint steps: extracting the stable echo data; restricting the ionospheric detection region; extracting the reliable reflection cluster, including Doppler filtering and coarse clustering analysis; and calculating the drift velocity. Ordinary wave (O-wave) data extraction, complementary code pulse compression and other data preprocessing techniques are used to improve the signal-to-noise ratio (SNR) of echo data. For the purpose of eliminating multiple echoes, the ionospheric region is determined by combining the optimal height range and detection frequencies obtained from the ionogram. Successively, Doppler filtering and coarse clustering analysis extract reliable reflection clusters. Finally, the weighting factor is brought in, and then weighted least-squares (WLS) is used to fit the drift velocity. The entire data processing process can be implemented automatically without constantly changing parameter settings due to changes in external conditions. This is the first time coarse clustering analysis has been used to extract the paracentral reflection cluster to eliminate scattered reflection points and outer reflection clusters, which further reduces the impacts of external conditions on parameter settings and improves the ability of automatic drift measurement. Compared with the previous method possessed by Digisonde Protable Sounder 4D (DPS4D), the new method can achieve comparable drift detection precision and results even with fewer reflection points. In 2021–2022, several experiments on F region drift detection were carried out in Hainan, China. Results indicate that drift velocities fitted by the new method have diurnal variation and change more gently; the trends of drift velocities fitted by the new method and the previous method are semblable; and this new method can be widely applied to digital ionosondes.

The miniaturized high-gain antenna is required in portable, ultra-wideband radar systems. However, the miniaturization, ultra-wideband and high gain often restrict each other in the antenna design. In this paper, a dual-polarized, double-slot, antipodal tapered slot antenna with a double-layer, dual-loop structure and novel slot edges is presented. The proposed magnetic dual-loop structure has the capacity to reduce the low cut-off frequency of the double-slot tapered slot antenna by weakening the resonance and coupling. In addition, the high gain, low sidelobe level (SLL), and low cross-polarization level are achieved in the boresight direction. A novel gradient slot profile is designed to improve the low-frequency directivity of the tapered slot antenna without affecting the matching. To feed the antenna elements, a kind of wideband, balun-divider structure is designed. The dual-polarized antenna is combined by two orthogonal elements in a cross configuration without galvanic contact or influence to performance. The measured results show that the impedance bandwidth of the proposed antenna is 0.6~4 GHz, and the maximum gain is 11 dBi. The isolation between the two antenna ports is better than 32 dB, and the cross-polarization discrimination (XPD) is better than 20 dB.