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.

Multimodal image fusion has recently garnered increasing interest in the field of remote sensing. By leveraging the complementary information in different modalities, the fused results may be more favorable in characterizing objects of interest, thereby increasing the chance of a more comprehensive and accurate perception of the scene. Unfortunately, most existing fusion methods tend to extract modality-specific features independently without considering intermodal alignment and complementarity, leading to a suboptimal fusion process. To address this issue, we propose a novel interactive generative adversarial network (IG-GAN), for the task of multimodal image fusion. IG-GAN comprises guided dual streams tailored for enhanced learning of details and content, as well as cross-modal consistency. Specifically, a details-guided interactive running-in module (GIR1) and a content-guided interactive running-in module (GIR2) are developed, with the stronger modality serving as guidance for detail richness or content integrity, and the weaker one assisting. To fully integrate multigranularity features from dual-modality, a hierarchical fusion and reconstruction branch is established. Specifically, a shallow interactive fusion (SIF) module followed by a multilevel interactive fusion (MIF) module is designed to aggregate multilevel local and long-range features. Concerning feature decoding and fused image generation, a high-level interactive fusion and reconstruction module (HRM) is further developed. In addition, to empower the fusion network to generate fused images with complete content, sharp edges, and high fidelity without supervision, a loss function facilitating the mutual game between the generator and two discriminators is also formulated. Comparative experiments with 14 state-of-the-art methods are conducted on three datasets. Qualitative and quantitative results indicate that IG-GAN exhibits obvious superiority in terms of both visual effect and quantitative metrics. Moreover, experiments on two RGB-IR object detection datasets are also conducted, which demonstrate that IG-GAN can enhance the accuracy of object detection by integrating complementary information from different modalities. The code will be available athttps://github.com/flower6top.

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.

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.

We report a new observation of daytime ionospheric irregularities on December 27, 2009, at low latitudes in China using multistatic HF radar. The transmitter, with a horizontal polarization log-periodic antenna pointing westward, is located at Wanning (18.97° N, 110.50° E, dip angle 19°). Three separate receivers are in Sanya (18.20° N, 109.49° E), Qiongzhong (19.04° N, 109.77° E), and Tunchang (19.42° N, 110.13° E). Doppler spectra, frequency power spectra, and wavenumber power spectra from the three receivers were calculated. The main features of these observations are: 1) the irregularities observed at 12:10 (LT) had typical lengths on the scale of tens of meters and the altitude was in the range of 105-175 km, which is located in the E region and bottom of the F region; 2) the power distributions from the three receivers are consistent with the irregularities being field aligned; and 3) features consistent with a cascade of irregular spatial scales were observed. We suggest that the observed irregularity could be due to the intermediate layer modulated by electrodynamic instability.

The high-frequency hybrid sky-surface wave radar (HF HSSWR) has recently been used to monitor large-area sea states. However, most of the HF HSSWR detection methods are based on the assumption of a no-tilt and constant height ionospheric model, and the influences caused by uneven electron density are ignored. This paper proposes a new surface current inversion scheme for the HF distributed HSSWR system, which considers the unknown ionospheric state as a black box and extracts the key parameters to compute the surface current based on a scattering model. The computational formula of the component of the current vector is explored using spatial scattering theory instead of an approximate bistatic model. In addition, the Fourier series expansion method is applied to the HF data to extract the real first-order Bragg frequency. Subsequently, the grazing angle and the bistatic angle can be found by inversion using the first-order Bragg frequency formula after searching out the common scattering patch of two receiving stations. Simultaneously, the coordinate registration of the currents can also be determined. The feasibility and effectiveness of this new algorithm are verified with field experimental results by comparing the current vectors derived from HSSWR and traditional HF SWR. The RMS differences of the magnitude and direction of the current vectors within the core common area of the two detection systems are about 10.2 cm/s and 9.5°, respectively.

In this letter, a novel radar waveform is proposed for a low-power high-frequency monostatic radar used for ionospheric sensing and aircraft detection. In this proposed waveform, a hybrid modulation scheme is adopted, where the pulse-to-pulse phase is coded by the WG sequence, and the frequency of each subpulse is linearly modulated with a fixed chirp rate. Compared with the conventional binary-phase-coded waveform, larger signal processing gain, higher range resolution, and lower range sidelobes can be simultaneously achieved with the proposed waveform. Furthermore, it is more insensitive to the Doppler shifts and is not subject to significant range-Doppler cross coupling. With this newly proposed waveform, the ionospheric backscatter ionogram with clear leading and trailing edges can be obtained in the range of 500-2300 km, and the aircraft target can be also clearly detected.

In this letter, a novel ionospheric oblique-incidence sounding network consisting of the ionospheric oblique backscatter sounder and the parasitic oblique-incidence sounder is presented. With the new configuration, the oblique ionogram and the backscatter ionogram can be provided at the same time. Due to the usage of a novel waveform combining the pseudorandom phase coding and the chirp modulation, about 130-dB signal processing gain can be usually obtained to improve the signal-to-noise ratio for the low-power transmission. Furthermore, through choosing different types of pseudorandom sequences for each transmitter, many oblique ionograms that depict the information of different propagation paths can be simultaneously achieved at a single receiver site. In addition, the digital beamforming technique is implemented on the receiving antenna array to simultaneously receive the echoes produced by each transmitter, to achieve a good space isolation to enhance the echo discrimination, and to suppress the strong interferers appearing in the high-frequency band. Details of the system configuration, the ambiguity function of the sounding waveform, and the signal processing algorithm are described. The initial experimental results show that multiple oblique ionograms can be simultaneously obtained at a single receiver site with this sounding network.

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.

In this paper, a novel oblique-incidence ionosonde (Wuhan Ionospheric Oblique-Incidence Sounding System) and its new application in the localization of the ionospheric irregularities are presented. Due to the usage of the binary-phase-coded waveform, a large signal processing gain, a high Doppler and range resolution, and a large unambiguous detection range can be achieved in this ionosonde. This ionosonde also adopts the peripheral component interconnect extensions for instruments (PXI) bus technology and is designed as a small-sized PXI-based system. Furthermore, a high-performance oven-controlled crystal oscillator that is disciplined by the Global Positioning System is used to achieve a good time and frequency synchronization. With multichannel digital receiver and multiple receiving sites, this ionosonde can be applied in the localization of the ionospheric irregularities. The details of the system configuration, the ambiguity function of the sounding waveforms, the signal processing algorithm, and the time and frequency synchronization method are described. The experimental results show that the virtual height along with the ground position of the ionospheric field-aligned irregularities can be preliminarily localized with this ionosonde.

The Wuhan Ionospheric Oblique Backscattering Sounding System (WIOBSS) is a monostatic high-frequency sky-wave radar used for ionospheric remote sensing. The sweep frequency backscatter ionogram (SFBI) recorded by the WIOBSS contains both backscatter echo scattered by distant terrestrial surface and vertical incidence (VI) echo reflected by local ionosphere over the sounding station. The approach for SFBI inversion introduced in this letter requires input of leading edge and peak height derived from local VI echo. The final output of this SFBI inversion approach is the 2-D electron density distribution in a vertical plane aligned in the direction of sounding. In addition, the time and geographic variation of foF2 can be obtained from a series of SFBI inversion results. Two experiments have been utilized to validate the SFBI inversion approach, and the SFBI inversion results are found to be very close to the ionosonde data. The fast-converged feature of the approach makes it possible for applications in real time. The success of the SFBI inversion approach enables us to use the WIOBSS as a vehicle-mounted system to obtain ionosphere electron density profile over a large geographic area.

Monostatic radar, whether using interpulse or intrapulse coded pulse trains, will suffer from the blind zones caused by eclipsing. This letter proposes a novel biphase interpulse coded radar waveform using diverse Pulse Repetition Intervals (PRIs) instead of a consistent PRI within each coherent processing interval (CPI). This waveform can achieve long unambiguous range, high-range resolution and high Doppler resolution without blind zones, and is specially suitable for a monostatic ionosonde.

Vertical-incidence (VI) ionosonde is a kind of high-frequency radar. The sounding waveform is an important characteristic for measurement by ionosondes. This letter compares, both theoretically and practically, four different biphase-coded pulse compression radar waveforms which can be employed in the Wuhan Ionosphere Comprehensive Sounding System at VI sounding, including interpulse-coded m sequence, interpulse-coded Wolfman-Goutelard sequence, intrapulse-coded Barker sequence, and intrapulse-coded complementary sequence pairs. The experimental results demonstrate that among the four waveforms, the intraphase-coded complementary-sequence pairs have the best performance.