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As the observation accuracy of parameters in the ionosphere cannot be directly checked, the comparison with other observations is the main way to evaluate the data quality of satellite measurements. Through the comparative analysis between the in situ electron density (Ne) observed by the China Seismo-Electromagnetic Satellite (CSES) and Ne at about 500 km altitude detected by Qujing Incoherent Scatter Radar (ISR), it was found that the pattern of CSES Ne is consistent with that of ISR Ne, and the correlation coefficient between the two sets of data is above 0.88 for different groups according to the magnitude. The value of CSES Ne is lower than that of ISR Ne, and the median value of the ratio for the difference between the conjugate data is 84.04%. Based on the comparison in the daytime between CSES Ne and ionosonde observations in China, it was found that the trend of the two datasets is mostly similar, and the correlation coefficient in some locations can reach up to 0.7. The distribution of CSES Ne and correlation coefficients at different latitudes show that the relationship is relatively better around the peak of the equatorial ionization anomaly (EIA). The differences in the value between CSES Ne and ionosonde data also exist, the relative change of which is about 80–95% in the daytime. [ABSTRACT FROM AUTHOR]

A new incoherent scatter radar (ISR) has recently been dedicated at Sanya (SYISR), China (geographic 18.34°N and 109.62°E) to detect the low‐latitude ionospheric plasma by conducting continuous operation and electronic beam steering on a pulse‐to‐pulse basis. This paper provides an overview of the processing procedure of SYISR ion line data and presents some preliminary observational results under various transmitting signal schemes and different working modes, such as the zenith‐directed mode and meridional and zonal scanning modes, with comparison to the international reference ionosphere model (IRI‐2016), ionosonde and Ionospheric Connections Explorer (ICON) satellite measurements. The diurnal and altitude versus latitude variation characteristics of electron density (Ne), electron temperature (Te), and ion temperature (Ti) are in accordance with those of other ISR measurements in mid‐low latitudes, and IRI shows some discrepancies with SYISR observations, including the sunrise and sunset enhancement in SYISR_Ti and the overestimated IRI_Te. The comparison between the daytime SYISR_Ti and ICON_Ti shows good consistency based on 31 observational samples. The measurement of line‐of‐sight velocity (Vi) for long pulse can reveal the properties of the medium scale traveling ionospheric disturbances manifesting the radar's potential to study the mesoscale ionospheric variation in the Sanya area. The derived vector velocities with a better measurement accuracy during the day are generally in line with ionospheric plasma physics. The results show that SYISR can obtain continuous ionospheric parameters through multi‐beam scanning measurements at ∼700 km horizontal scale, which provides important information for studying the atmosphere‐ionosphere‐magnetosphere coupling and ionospheric scintillation at low latitudes in East Asia. Plain Language Summary: China's first phased‐array incoherent scatter radar was completed in 2021 in Sanya, which is called SYISR. SYISR aims to investigate fundamental scientific problems in the ionosphere, including atmosphere‐ionosphere‐magnetosphere coupling, regional ionospheric properties and ionospheric irregularities in southern China. SYISR can directly measure electron density, electron and ion temperature, and drift velocity of the ionospheric plasma and can be used to indirectly obtain the temperature of the neutral atmosphere, wind field and irregularity information. This paper describes in detail the data processing from the IQ signal to the derived ionospheric parameters for SYISR. Ionospheric experiments in various scanning modes have been performed, mainly including a single beam in zenith‐directed mode and multiple beams in meridian and zonal scanning modes, and the obtained results are evaluated. These experimental results are evaluated through comparison with the IRI‐2016 model, digital ionosonde and Ionospheric Connections Explorer satellite. The comparison results indicate that SYISR is able to provide reliable, unique, and important ionospheric information for the study of ionospheric science at low latitudes in East Asia. Key Points: Sanya incoherent scatter radar (SYISR) is a new phased array system dedicated to the ionospheric observations at low latitudes in Asia sectorSYISR is operational, providing high‐quality ionospheric measurements in multiple radar scanning modes and radio pulse schemes modesData processing procedures and initial ion‐line results using typical experiment modes are reported [ABSTRACT FROM AUTHOR]

Led by the Institute of Geology and Geophysics, Chinese Academy of Sciences, we have built a brand‐new modular active digital phased array, with all solid‐state transmission and digital receiving incoherent scatter radar (ISR) in Sanya (18.3°N, 109.6°E), a station in low latitude China called Sanya ISR (SYISR) since 2015. The development of SYISR involved the indoor design and development of key components, an outdoor prototype test, and the production and debugging of the entire array. The unique features of SYISR include a single‐channel directly connected T/R unit and antenna, a radar array monitoring and calibration network, environmental adaptability design and open architecture. The entire radar has 4,096 channels and 5,930 modules in total. All the technical indices, mainly including a >2 MW peak power, a 43 dBi antenna gain and a <120 K noise temperature, either meet or are superior to the designed value through an independent evaluation. Waveforms of single pulse, linear frequency modulation, Barker code, long pulse and alternating code have been implemented to meet multiple purposes. Four observational modes for ionospheric experiments, including zenith stare, perpendicular to geomagnetic field, meridian scan, and all sky scan, have been developed. We have implemented time domain decoding, frequency domain decoding, and statistical inversion methods in calculating the autocorrelation function and power spectra in signal processing. The preliminary experimental results on ionospheric parameters, plasma lines, irregularities and hard targets are reasonable and encouraging, which greatly enhances our confidence in achieving our scientific goals in the future. Plain Language Summary: The Earth's ionosphere (∼60–1,000 km above the sea level) is composed of dense plasma, which shows complicated variations versus altitude, geographic location, and solar activity level. When a ground‐based radio wave is transmitted onto these plasmas, it will generate stimulated radiation, with a small portion of them radiating back to the ground. If this backscattered radiation is collected by a ground‐based radar, a variety of ionospheric parameters, including plasma density, temperature, and movement speed, can be derived through a complicated signal processing algorithm. An ionospheric monitoring method called incoherent scatter radar (ISR) was therefore developed. However, since this backscattered radiation is very weak, the ISR should be built with sufficiently large power (usually megawatts) and apertures (usually hundreds of square meters). This leads to the high cost of building an ISR, with only a few having been built in the world thus far. In this paper, we describe a newly built ISR, called Sanya ISR (SYISR), in low latitude China by applying modern radar technologies, such as modular active digital phased arrays, all solid‐state transmitting and digital receiving. The development of SYISR followed the principle of a gradual and orderly progress, including indoor design, outdoor prototype testing and the entire radar construction and debugging. The radar technical indices have been evaluated, and all meet the designed value. We also developed a signal processing algorithm and a variety of observational modes. Finally, preliminary experimental results are shown, which are reasonable and encouraging. In the future, we will gradually achieve our scientific goals via effective data accumulation and analysis. Key Points: Sanya incoherent scatter radar (SYISR) is a newly built incoherent scatter radar in low latitude China with modular active electronic scanning, digital phased array, solid‐state transmitting, and digital receivingThis paper details the hardware, key parameters, data processing and preliminary experimental results of SYISRThe key parameters meet or are superior to the original design, and the preliminary observations are encouraging [ABSTRACT FROM AUTHOR]

Incoherent scatter radar (ISR), as one of the most powerful ionospheric detection methods on the ground, can obtain a variety of important ionospheric parameters with high spatiotemporal resolution over the entire ionosphere. A high-power large-aperture phased array (PA) ISR, named Sanya ISR (SYISR), is under construction at Sanya (18.3°N, 109.6°E), China. Two remote receive-only arrays of SYISR will be built in the future to form a tristatic ISR system, which will be the only multistatic ISR in a low-latitude region. Estimating the signal-to-noise ratio (SNR) is the key task for analyzing the detection accuracy and evaluating the radar performance. In this article, the SNR of the SYISR tristatic system is simulated for the first time using a suitable PA bistatic radar equation. The parameters of the designed SYISR tristatic system are used in the simulation. Subsequently, the SNR spatial variability of the bistatic PA radar and bistatic equivalent parabolic dish (EPD) radar is analyzed and compared. The results show that the radar system constants, effective scattering volume, and spatial variable terms are the three different sources of factors affecting the SNR. They jointly determine the spatial distribution characteristics of the SNR. Moreover, the PA SNR is significantly different from the EPD SNR numerically and morphologically. The PA SNR is weaker than the EPD SNR in the whole observational space. In the horizontal direction, the PA SNR has a single-peak structure, while the EPD SNR has a double-peak structure. The root cause of these differences is that the beamwidth of a PA changes with the beam directions, while that of the EPD is fixed. This research provides a theoretical basis for the design of tristatic PA ISR in hardware optimization and error analysis, which will be shown in another article. [ABSTRACT FROM AUTHOR]

The bistatic radar system has been one of the effective methods to detect the space debris in low earth orbit (LEO). Tianlai radio array with cylindrical-parabolic antennas is designed for dark energy detection, which has large field of view and high sensitivity, offering a fan-beam during the observation. We propose a bistatic radar system, which consists of Tianlai radio array and an incoherent scattering radar (ISR) assumed as a transmitter in the Qujing city of China, to detect space debris. In this paper, we calculate and analyze the detection capabilities of this system. The results show the bistatic radar system has the potential to detect small space debris of less than 10 cm in LEO. We provide a space debris detection method to obtain the position of the cross-beam satisfying the observation requirement with the TLE data of the space debris. The method can solve the problem of space synchronization between the radio array and ISR. We used the long-short baseline method of the radio array to locate the space target. The relationship among positioning error, the azimuth and the elevation angle are also discussed. [ABSTRACT FROM AUTHOR]

• A novel model, CIG, is proposed for soil EC retrieval using CYGNSS data. • CIG outperforms other models for soil EC retrieval. • The L-band signal of CYGNSS is pretty sensitive to soil EC. • CYGNSS can be an effective way for soil salinization monitoring on a large scale. Soil salinization, which occurs mainly in arid and coastal regions, hampers agricultural development, and threatens food security in these regions. Therefore, acquiring the spatial distribution of soil salinization with high accuracy is paramount. However, obstacles continue to hinder attempts to use satellite remote sensing. Cyclone Global Navigation Satellite System (CYGNSS) provides a new alternative opportunity for soil salinization retrieval. In this study, soil electrical conductivity (EC) was chosen as the proxy of soil salinity. A CYGNSS-based model for soil EC retrieval (CIG) was proposed, and has been tested in the Yellow River Delta, China. The CIG model consisted of two modules. First, we modified the geometrical optics model to derive the surface's Fresnel reflectivity from the incoherent scattering signal of the land surface with vegetation attenuation, and horizontal and vertical surface roughness calibrations. Second, soil EC was retrieved based on the gradient boost regression tree (GBRT) algorithm with the inputs of the surface's Fresnel reflectivity and other ancillary variables. The in-situ measurements were used to compare the CIG retrieved soil EC against the retrievals from other classical machine learning approaches and the coherent model using CYGNSS data, and the optical SVIs model. CYGNSS signal was found to be highly sensitive to in-situ soil EC, and the proposed CIG model outperformed other models, with R = 0.730, RMSE = 1.318 mS/cm, and MAE = 0.570 mS/cm. In addition, several issues related to the model were also discussed, including the interactions between the incidence angle of the CYGNSS signal and soil EC, and the advantages and limitations of CYGNSS. The result suggested that the proposed CIG model, along with new CYGNSS data, can provide a promising method for monitoring land salinization on a large scale. [ABSTRACT FROM AUTHOR]

In the development of Sanya Incoherent Scatter Radar (SYISR), which is a major scientific instrument project founded by the National Natural Science Foundation of China (NSFC), we established a prototype system of 8 subarrays (named SYISR-8 hereafter), including 256 transceiver modules totally, beginning in July 2018 to verify the functions of the radar. Based on SYISR-8, we configured the system to detect the meteor echo with high range resolution. During a continuous observation interval of 16 h, 55 meteor echo events were found. Among these events, 34 cases were head echoes, 18 cases were specular trail echoes, and 3 cases were head echo and nonspecular trail echo concurrent events. We have observed 2 potential direct meteoroid fragmentation cases of all 55 meteor events. It demonstrates that potential direct observation of meteoroid fragmentation could be observed by a relatively small aperture and power using pulse compression processing. However, the occurrence ratio of meteoroid fragmentation in our experiment is approximately 6%, which is considerably lower than that observed in previous studies, which needs further investigation. • Meteor echoes observation results by SYISR-8 are demonstrated in a high range resolution mode. • The SYISR-8 was configured to detect meteoroid with 37.5m resolution through pulse compression technique. • Two potential direct meteoroid fragmentation cases were observed for the first time to our knowledge using the SYISR-8. • Typical meteor echoes could be observed by a high range resolution radar with relatively small aperture and power. [ABSTRACT FROM AUTHOR]

The response of the ionosphere to geomagnetic activities is still a challenging topic. Geomagnetic disturbances drive the ionosphere deviating severely away from its normal state. A positive phase storm is called the situation when the deviation of the ionosphere from the normal state is positive, and a negative phase storm is designated to the opposite case. In this work, we report an investigation on the responses of the F2 peak and the topside ionosphere to the October 2002 geomagnetic storms. The data are the first 31 days long-duration observations of the Millstone Hill incoherent scatter radar in October 2002. The observations indicate the altitudinal behavior of the ionosphere over Millstone Hill during the storms. It reveals the relative contributions to ionospheric variations from changes in atmospheric compositions and dynamic processes.AcknowledgmentsThe data of Millstone Hill incoherent scatter radar observations are provided by the Haystack Observatory through the Madrigal data system. This research was supported by National Natural Science Foundation of China (41774161, 41621063). [ABSTRACT FROM AUTHOR]