The observation of the ionosphere is of great significance for geophysics, radio propagation mechanisms, and related engineering applications. In response to the low range and Doppler resolution of the scattering function map obtained by traditional ionosonde with fewer coherent soundings, this letter proposes a super-resolution analysis method. By utilizing the coherence between the frequency components of the echo signal and using phase weighted summation, the super-resolution synthesis in the range dimension was achieved. In the frequency dimension, the Capon spectral super-resolution estimation method was employed. In this way, super-resolution is achieved both along the range and Doppler axis of the scattering function map simultaneously. It breaks the limitations of spatial and temporal sampling rates on observation resolution. Simulation and experiment results have verified the performance of this method. With a tenfold increase in resolution, it achieves the distinction of multilayer echo traces and the identifies their subtle Doppler difference. This method provides a favorable means for efficient and accurate detection of the ionosphere.

The ionosonde is one of the most widely used ionospheric detection devices. It has significant implications for the study of space physics and wireless communication technology. To solve the problem of low resolution with traditional iososonde imaging method, this letter proposes a high-resolution 2-D spatial-frequency spectral estimation imaging method. By reconstructing the echo signal matrix and constructing the guidance vector that simultaneously reflects the range and motion characteristics of the target, high-precision estimation of the range-Doppler spectrum is achieved in 2-D Capon estimation. The analysis of experimental sporadic-E (Es) sounding data of August 13, 2021 in Wuhan ($114^{\circ } 22^{\prime } $E,$30^{\circ } 30^{\prime } $N), China, demonstrates its ability of the precise measurement for single-layer echoes and the identification ability for multilayer fine structures. This method further enhances the potential of traditional ionosonde for precise observation and analysis of the ionosphere.

WIOBSS is a low-power instrument for monitoring and researching the ionosphere. Unfortunately, as operate in the High-Frequency (HF) band, its performance suffers from a lot of unwanted interferences, which can result in low signal to interference and noise ratios (SINR) and low ionogram quality. In this letter, an analysis of the range-domain correlation of radio frequency interference (RFI) in WIOBSS is presented. Based on this, a novel eigen-subspace based filtering approach for ionospheric oblique backscatter sounding is proposed. It slides the range domain sequence into the submatrix to construct the subspace and uses an improved source number estimation approach to divide the subspace according to the mode criterion. Then, the range bins with signals are projected into the eigen-subspace to suppress RFI. Experiment results show that this method is still effective under limited snapshot conditions. It can improve the SINR by about 25 dB at some typical frequencies. Furthermore, the proposed method is more robust than typical methods while the SINR fluctuates violently.

Improving the quality of the oblique sounding ionogram under the low transmit power and miniaturized antenna array structure has always been a meaningful research content. Here we try to use the adaptive beamforming technology combined with Wuhan Multi-channel Ionospheric Sounding System (WMISS) to improve the situation effectively. First, an L-shaped antenna array is employed to receive the oblique sounding signal; subsequently, the precise angle of the arrival signal is estimated by the multiple signal classification (MUSIC) algorithm. Then three typical adaptive beamforming algorithms are employed to synthesize the multi-channel signals. The experimental results show that for the ionospheric signals of oblique incidence sounding (OIS), SNR can be improved to tens of dBs, and the interference can be significantly suppressed. Especially for the swept-frequency ionogram of OIS, the trace gets clearer and more continuous, which will provide great convenience for further interpretation and ionospheric parameter inversion. In addition, by comparing the application effects of the three typical beamforming algorithms, this letter provides a reference for selecting the synthesized methods of the ionospheric signals of OIS received by the miniaturized array.

Total precipitable water (TPW) values derived from Constellation Observing System for Meteorology, Ionosphere and Climate-2 (COSMIC-2) are compared with those derived from Special Sensor Microwave Imager Sounder (SSMIS), Global Precipitation Measurement (GPM) Microwave Imager (GMI), and Advanced Microwave Scanning Radiometer-2 (AMSR-2) over the ocean from October 1, 2019 to February 16, 2020. The overall comparison results indicate that TPW values derived from SSMIS, AMSR-2, and GMI have a good correlation and agreement with COSMIC-2 TPW values with the correlation coefficients greater than 0.99 and root mean square (rms) no greater than 2.7 mm. We compare TPW derived from three different microwave radiometers with COSMIC-2 TPW over the subtropical and tropical oceans. The differences illustrate that TPW values derived from three different microwave radiometers are more consistent with COSMIC-2 TPW values over the subtropical ocean than those over the tropical ocean. In addition, we also analyze the relationship between the TPW retrieval accuracy derived from three different microwave radiometers and environmental factors, including cloud, rain rate, wind speed, and surface temperature. The results indicate that four environmental factors have an important influence on the TPW retrieval from three different microwave radiometers.

Despite a great progress has been made in 3D point cloud recognition, recent studies find that deep models are vulnerable to adversarial attacks generated through various point cloud transformations. However, existing spoofing attack methods neglect the influence of point cloud topology on the model recognition accuracy. In this paper, we propose a novel adversarial point cloud generation network, named Topology Destruction Network (TD-Net), which destroys topological structure of a point cloud by selectively dropping points leading to the formulation of holes on the surface. The network consists of an encoder and a decoder. The encoder first leverages a PointNet architecture to extract geometric information of the original point cloud, from which a topological adjacency matrix is encoded describing adjacency relationships between points. The decoder selects a specific point to drop associated with its neighboring points derived from the learned adjacency matrix, resulting in an adversarial point cloud with a destructed topology. Experiments on the ModelNet40 dataset demonstrate that the proposed method surpasses existing adversarial attacks in terms of reducing model recognition accuracy.

Large-scale ionospheric irregularities, which occurred on September 8, 2017, were detected by the ionosonde and the Global Navigation Satellite System (GNSS) receiver network in this letter. The Wuhan ionospheric sounding system (WISS), deployed at Puer (PUR, 22.7°N, 101.05°E), Leshan (LSH, 29.6°N, 103.7°E), and Zhangye (ZHY, 39.4°N, 100.13°E), detected these large-scale ionospheric irregularities. The 2-D maps of the rate of the GNSS total electron content (TEC) index (ROTI) further confirmed that the large-scale ionospheric irregularities can cover a broad area (from the equator to middle latitude, ~80°E to ~120°E, ~20°N to ~45°N). The ionospheric variation recorded by the WISS shows that the large-scale ionospheric irregularities were driven by the super eastward electric field, which was formed by the eastward prompt penetration electric field during a storm superimposed on the normal prereversal enhancement during the postsunset hours. Moreover, observations by the WISS show the higher the latitudes at which the ionospheric irregularities/spread F occurred, the shorter the duration time.

A novel land-based high-frequency (HF) geolocation system is proposed. The geolocating signals are transmitted from the stations with known geographic coordinates. Because of the ionospheric refraction, the signals propagate as skywave in the over-the-horizon (OTH) manner. Therefore, a wide spatial coverage can be achieved. The receiving station uses a small 2-D antenna array to estimate the angle of arrival (AOA) of each transmitter's signal. An intersecting geolocation algorithm based on the geometric relations between the transmitters, and the receiver is described, which only relies on the coordinates of the transmission stations and the azimuth angles of the signals. A novel specific multichannel system structure is also described. It has a simple structure and low hardware cost, as well as good flexibility. The preliminary experimental results prove the feasibility of this geolocation method and its engineering application significance.

In this letter, an ultrawideband ionospheric sounding system was designed for ionospheric research and communication channel management. The frequency band of this ionosonde ranges from high frequency to very high frequency. It can be flexibly used at both monostatic and bistatic stations as a GPS receiver module was integrated. The system design includes three major components: control system, transmitting system, and receiver system. The operational parameters, such as frequency, waveform, and modulation type, can be manually reset. Correlation and Fourier algorithms are used to obtain a high signal-to-noise ratio and enlarge the detection range. The typical experimental results demonstrate that the proposed system can be used to study the ionosphere effectively.