Using the echo energy difference of symmetric beams and the background three-dimensional (3-D) wind field observed by Wuhan mesosphere–stratosphere–troposphere (MST) radar, this letter analyzes the tilted condition of the atmosphere, studies the disturbance factors of tilted atmosphere based on the Lomb–Scargle method, and analyzes the relation between the atmospheric disturbances and 3-D wind field. The results indicate that: 1) the MST radar is capable of observing the tilting of the scattering layers by using the symmetric radar beams; 2) the main source induced the tilting of the scattering layer is the atmospheric waves with various scales in the troposphere, tidal waves in the lower stratosphere and mesosphere, but the main source in the lower stratosphere is semidiurnal tide, the main source in the mesosphere is diurnal tide; and 3) the wave periods detected by the echo power difference really exist in the atmospheric wind data, so we can use the echo power difference to analyze atmospheric waves based on the MST radar for all the ranges.

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.

The observations of the Wuhan mesosphere, stratosphere, and troposphere radar with different tilted angles and observational modes are used to study the aspect sensitivity of the radar echoes and discuss the echo mechanism in the lower thermosphere, mesosphere, lower stratosphere, and troposphere. This letter indicates that: 1) the variation of radar echoes with different tilted angles is more obvious in the troposphere than that in the lower thermosphere, mesosphere, and lower stratosphere; 2) the values of the aspect sensitivity of radar echoes in the tropopause region and the mesopause region are smaller than that in the other region, which shows the difference of the echo mechanism; 3) the radar echo mechanism has the obvious spatial direction difference (S-N and E-W) in the mesosphere and lower stratosphere. Meanwhile, in the lower thermosphere, the sudden probability of radar enhanced echoes is larger, which may be related to the electronic thermal radiation, the scattering of the electron group, meteor trails, or the breaking of the gravity waves; and 4) in the troposphere and the lower stratosphere, the values of the aspect sensitivity increase with the increase of the tilted angles at all most height regions. However, the variations of the values of the aspect sensitivity are not monotonic in the lower thermosphere and mesosphere, which is much different from that in the troposphere and the lower stratosphere.

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.

We present the observational evidence of Eand Fregion field-aligned irregularities (FAIs), nighttime medium-scale traveling ionospheric disturbances (MSTIDs), and a sporadic E (ES)-layer using the Wuhan very-high-frequency (VHF) coherent scatter radar, Wuhan Global Navigation Satellite System (GNSS) network, and Wuhan ionosonde. We observed simultaneously E and F FAIs by VHF radar and the ES-layer and spread-F by Wuhan ionosonde. We also observed MSTIDs using the Wuhan GNSS network in a southwestward direction of propagation and horizontal propagation velocity of less than 180 m/s, for a period of ~33 min. Simultaneous observations of an ES-layer and FAIs in the E-region and FAIs in the F-region suggested the existence of an electrodynamic coupling between the E and F regions in the midand low-latitude nighttime ionosphere. A polarized electric field associated with nighttime MSTIDs can generate the uplift movements of the F-region's electron density. This uplift effect consequently can excite the gradient drift instability (GDI) with an accompanying enhanced vertical gradient of the F-layer's electron density. Our results indicated that the F-region MSTIDs excited by Perkins instability might be further modulated by the E × B effect and subsequently can evolve to spread-F like FAIs. Our observational investigation provided the first evidence of the full dynamics and links among different ionospheric disturbances in a midand low-latitude nighttime region of China.

Convection is an important source to excite gravity waves. But the activity characteristics of the convection are hard to research, because it is transient and localized. This paper utilizes the five beams operated by mesosphere-stratosphere- troposphere radar to study the radial movements before, during, and after the small-scale convection. The Doppler spectra of radar echoes during the convection are evidently different from that in the clear weather. The strong updrafts (~5.1 m · s-1) and downdrafts (~2.4 m · s-1) make the turbulent motion compositions more complex, so the radial Doppler velocity and Doppler widths of five beams are almost significantly different at the same height. The characteristics of gravity waves are detected by using the quasi-monochromatic inertia gravity wave model. The results indicate that: 1) the kinetic energies of the IGWs during the convection are more powerful than that before and after the convection, and the propagation directions of the IGWs during the convection are also different to that before and after the convection and 2) the propagation parameters of the IGWs in different regions are also different during the convection, which means that the cold zone is at south and the warm zone is at north for this convection case.

By using the 2015-2016 data set of newly built Wuhan very high-frequency (VHF) radar, we investigate the occurrence of E-region field-aligned irregularities (FAIs) in mid-latitude China region for the first time. Wuhan E-region FAIs present the strongest occurrence of quasi-periodic (QP) echoes at postsunset period in the height range of 100-120 km and moderate occurrence of continuous echoes at postsunrise period in the height range of 90-100 km. Characteristics including temporal and altitudinal distribution, Doppler spectra and period of QP echoes are presented from a statistical point of view. We find that E-region FAIs observed by Wuhan VHF radar show strong linkage with the concurrent sporadic E layer observed by ionosonde. In general, Doppler spectra present type 2 feature with velocity in the range of ±50 m/s and width less than 50 m/s. Our results also indicate that mid-latitude E-region FAIs are closely associated with Es layer. Several mechanisms were proposed to interpret the mid-latitude E-region FAIs; however, further investigation is required to understand the generation with different ionospheric background and irregularity types.

Based on the construction of the Chinese Meridian Project, two mesosphere, stratosphere, and troposphere (MST) radars were designed and located in Beijing and Chongyang, Hubei province, China, to investigate the atmospheric dynamics in the troposphere, stratosphere, and mesosphere. The official names of the two radio systems are Beijing MST radar and Wuhan MST radar. They are the monostatic radars with an active phased antenna array consisting of 576 Yagi antennas and operated by a 53.8-MHz frequency. The three-element Yagi antennas are arranged in a square grid of 96-m side length to form the aperture of 9216 m2. This antenna array arrangement forms the five symmetric radar beams of vertex, east, west, south, and north. The beamwidth is 3.2°, the maximum directive gain is 34.8 dB, and the total transmitting peak power is ~172 kW. The zenith angle of the oblique beams is adjustable between 0 and 20° with a 1° step. The average power aperture product of the radars is 3.2×108Wm2. There are three operating modes of the MST radars, including the low, middle, and high modes, applied to observe the troposphere, stratosphere, and mesosphere, respectively. Thus, these two coherent pulse Doppler MST radars have the ability to study the features of the midlatitude atmospheric turbulence and wind field vector from the troposphere to the lower thermosphere with high spatiotemporal resolution. In this paper, the antenna array, system hardware, and signal processing methods, as well as the typical observation results of the troposphere, stratosphere, and mesosphere, are introduced.

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 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.

In this letter, a new high-frequency (HF) skywave multistatic radar that is used for ionospheric sensing and aircraft detection is presented. With multiple receiving sites, this radar can detect the aircraft in much larger surveillance with high probability and can obtain the characteristics of many ionospheric paths simultaneously. In addition, this radar adopts a binary-phase-coded continuous waveform to achieve a high Doppler resolution, a large signal processing gain, and a low probability of interception. Details of the system configuration, the ambiguity function of the sounding waveform, and the signal processing algorithm are described. The experimental results show that backscatter ionograms and oblique ionograms can be simultaneously obtained and that the aircraft with different flying conditions can be clearly detected with an HF skywave multistatic experimental radar.

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.

Vertical ionograms suffer from artifacts and distortion due to other users of the high-frequency channel and the ionosphere. The traces of each layer have variable shapes, so the automatic scaling of the vertical ionograms is rather complicated. New algorithms for signal identification and trace extraction are developed to support automatic scaling. Different features of the signals, such as signal-to-noise ratio, Doppler shift, and virtual height, are converted to certainty factors and combined to identify signals. Kalman filtering, which is widely used in trace extracting, is introduced to form traces and separate the ordinary trace and the extraordinary trace. These algorithms are effective without using information on the wave polarization and can be applied to the ionograms recorded by single-antenna systems.

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.

The dense radio-frequency interference (RFI) distributed on the spectrum in the high-frequency band greatly degrades the performance of radio systems and makes it urgent to find an effective and speedy method to eliminate the interferences. The interference mitigation method introduced in this letter is implemented before any further signal processing. It reconstructs the interference by the exactly estimated frequency, amplitude, and phase parameters in the signal spectrum and then subtracts the artificial interference from the input time-domain complex signal. The procedure steps of the method are illustrated in detail. The data stream of continuous oblique-incident ionospheric detection has been processed effectively and rapidly in real time. Several delay-Doppler ionograms severely contaminated by RFI are selected and presented. Comparisons of the original and mitigated ionograms and the signal-to-noise ratio (SNR) data show that the RFIs are eliminated perfectly and that the SNR is greatly improved.

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.

The ionosphere is not homogeneous, hosting many irregularities with different scales from dozens of meters to dozens of kilometers. The observation and formation mechanism of the irregularities is among the most active domains for understanding the influence of irregularities on propagation of radio waves. The characteristics of ionospheric field-aligned irregularity (FAI) at low-latitude regions were investigated in this letter in terms of analyzing the data obtained from ionospheric sounding experiments using the Wuhan University ionospheric oblique backscattering system, in which the scales and drift speeds of FAIs were specified.

The ionospheric laboratory of Wuhan University, Wuhan, China, has developed a new ionospheric sounding system for about seven years. The first system is used for ionospheric oblique backscattering detection and is called the Wuhan ionospheric oblique backscattering sounding system (WIOBSS). The WIOBSS is a portable monostatic backscatter ionosonde for ionospheric research and high-frequency (HF) channel management, adopting alternate transmitting and receiving patterns to transmit long coded pulse trains for high pulse-compression gain. However, it is difficult to recognize the echo path and mode only with the backscatter ionogram. Therefore, a separate remote digital receiver has been developed for auxiliary detection. The receiver is a compact radio system used to receive the transmitting signal of the WIOBSS, and it includes a Global Positioning System receiver for clock and frequency standard synchronization. Combined with the WIOBSS, the receiver has been used for ionospheric bistatic backscatter sounding and oblique incident sounding. The WIOBSS and several remote digital receivers can compose a detection network for ionospheric research and HF channel management.

A new high-frequency backscatter radar (Wuhan Ionospheric Oblique Backscattering Sounding System) for ionospheric sensing and study has been developed. With the new system design, oblique backscatter sounding and oblique sounding can be achieved in this radar just through simply loading appropriate software. Furthermore, due to alternate transmission and reception in equal interval, this radar can detect the ionospheric state in the range of a few thousand kilometers and achieve the largest gain of applied pseudorandom sequences. In addition, this radar usually obtains 27-dB gain from phase coded pulse compression and 21-dB gain from coherent spectral integration using pseudorandom code, pulse compression, and coherent spectral integration techniques. This radar also exploits merits of Peripheral component interconnect eXtensions for Instruments (PXI) bus technology and is designed as a modular and compact PXI-based system. Details of system design, sounding timing, and signal-processing algorithm are described. Except for the antenna systems and power amplifier, this radar's dimension is 177.8 times 431.8 times 457.2 mm. With this radar, the oblique backscatter ionogram and Dopplerionogram can be clearly obtained in the range of a few thousand kilometers with 100-W power. The oblique ionogram for propagation over a distance of several kilometers can also be clearly acquired at the time of oblique backscatter sounding. The initial observations obtained with this radar are included to demonstrate its performance.