With the rapid development of software-defined radio (SDR) technology, a digital, software-reconfigurable, and flexible solution is provided for microwave radiometers, particularly suitable for atmospheric water vapor and oxygen detection with wideband, multi-channel requirements, significantly improving system efficiency. Meanwhile, digitization helps improve channel consistency and address nonlinearity issues, while the digital zero-balancing mechanism implemented through adaptive integration is more suitable for digital platforms. This paper proposes a digital Dicke-type radiometer system based on an SDR platform, using Xilinx RFSoC XCZU47DR (AMD, San Jose, CA, USA) as the core hardware to achieve single-chip integration of RF signal sampling, digital local oscillator generation, and signal processing. The system implements a 46-channel channelized receiver (23 channels each for K-band and V-band) on an FPGA using a polyphase filter bank. The prototype filters achieve 70 dB stopband attenuation and 0.5 dB passband ripple, with each polyphase branch requiring only 25 coefficients, significantly reducing hardware resource consumption. An adaptive integration method is proposed, where an adaptive switch controller dynamically adjusts the hot source injection time ratio by calculating the power difference between adjacent integration periods, enabling the Dicke zero-balancing mechanism to operate entirely in the digital domain. Furthermore, a complete hardware transfer model is established for three signal branches (antenna, hot source, and matched load), and full-chain calibration of all 46 channels is performed using a liquid nitrogen cold source, with calibration reliability verified through blackbody measurements. Experimental results demonstrate brightness temperature consistency better than 0.7 K, with a sensitivity of less than 0.15 K for the K-band and less than 0.21 K for the V-band at 1 s integration time.

Ground-based scatterometers are widely used for quantitative microwave backscattering measurements in soil moisture retrieval, vegetation monitoring, and satellite scatterometer validation. However, low-cost software-defined radio (SDR) transceivers provide limited instantaneous bandwidth, making it difficult to transmit and process signals with bandwidths on the order of hundreds of MHz for fine range resolution, especially for systems requiring real-time onboard processing. To address this problem, this paper presents a vehicular, fully polarimetric, SDR-based scatterometer that achieves an equivalent wideband response by sequentially transmitting adjacent narrow subbands and coherently synthesizing them onboard. To enable real-time operation on a resource-limited field-programmable gate array/system-on-chip (FPGA/SoC) platform, we adopt a frequency-domain synthesis-pulse-compression pipeline that avoids interpolation and eliminates repeated matched filtering across subbands. A slot-based online phase calibration is performed within the settling window after each fast lock to estimate and compensate random local oscillator (LO) phase offsets, preserving coherent stitching. In addition, pulse repetition within each subband and coherent accumulation are integrated to improve the signal-to-noise ratio (SNR) under real-time throughput constraints. A Zynq-based implementation demonstrates deterministic onboard range-profile output, with a minimum processing latency of about 1.57 ms per frame. Loopback and outdoor experiments validate the equivalent 200 MHz bandwidth (five 40 MHz subbands), achieving approximately 0.75 m resolution and yielding sidelobe metrics consistent with the designed windowing, including a peak sidelobe ratio (PSLR) of −27.43 dB and an integrated sidelobe ratio (ISLR) of −12.38 dB. Field scans over farmland further show consistent σ0 trends across incidence angle and azimuth, indicating reliable onboard quantitative backscattering measurement. These results demonstrate that the proposed method provides a feasible solution for deterministic real-time equivalent wideband scatterometry on a low-cost SDR platform.

The ionosphere is part of the space above 60 to 1000 km from the ground and is an important part of near-earth space. The study of the ionosphere is conducive to providing better understanding the coupling interaction features within the lithosphere, atmosphere and ionosphere, exploring the possible associations between earthquake precursors and ionosphere disturbances, providing services for human activities and more approaches for disaster prevention and mitigation.A chaotic coding ionosonde was developed in Yinchuan, Ningxia Hui Autonomous Region, China, in 2021. The ionosonde scans in 1~30 MHz frequency range, with the distance resolution of 1.5km and detection height from 67.5 km to 560 km. It utilizes technologies of pulse compression and chaotic coding, suppresses clutter interferences successfully and obtains high quality ionograms. This ionosonde operates automatically and produces an ionogram every 15 minutes. A multiscale transformer neural network is utilized for the extraction of echo traces and accurate inversion of the ionospheric parameters, such as the critical frequence of F2 layer, the minimum reflection height, the separation of traces of the F layer's O/X waves as well as the electron density profile based on an improved bottom inversion model of the International Reference Ionosphere.Several strong ionospheric disturbances were observed in 2023 and 2024. In April and November 2023, massive solar flare eruptions caused geomagnetic disturbances, and the F2 layer responded to the disturbances obviously in term of the critical frequence and the traces of the F layer's O/X waves. In December 2023 two earthquakes with ML ≥ 4 happened in Gansu province, and also there were solar flare eruptions during that period. Some ionospheric disturbances were observed by the ionosonde approximately two or three weeks before the earthquakes. Besides, the fluxgate sensors and magnetometers installed on the geomagnetic stations in Gansu province and Ningxia Hui Autonomous Region also recorded the disturbances in the daily curves, synchronous with the ionosonde records. In January and February 2024, some typical U-shaped and sickled-shaped traces are observed in the ionograms, which are considered to be the phenomena caused by traveling ionospheric disturbances (TIDs). Some other disturbance phenomena are also recorded by the ionosonde, including the traces diffusion, partial disappearance, abnormal shapes, etc., worthy of research in multiple fields combining the lithosphere, atmosphere, ionosphere and space physics.

Radio occultation observation has garnered significant attention owing to its low-cost, all-weather, and global coverage feature. However, traditional occultation inversion methods lead to error accumulation due to assumptions that are not entirely suitable in the real ionospheric environment, resulting in poor performance in the low ionosphere (D, E layers). In this article, we propose a new method for inverting the electron density in low ionosphere using high-precision 50 Hz occultation data. This method can eliminate the fixed constant term of 50 Hz data and obtain a sharper weighting function through epoch differencing. The inversion results have a good consistency with the results of the ionosonde, with a correlation coefficient of 0.92 and a determination coefficient of 0.85. In addition, the new method can retrieve local details of electron density profiles and capture sporadic E layer (Es), providing support for the study of Es layer morphology and structure.

The sea surface height (SSH) measured by the satellite radar altimeter is determined by the distance from the satellite to the sea surface and the altitude of the satellite above the reference ellipsoid. The former is measured by altimeter and the latter is determined by precision orbit determination. If the two systems use different clocks, there may be a time tag bias between their time series. This time tag bias reduces the accuracy of SSH measurements by satellite radar altimeter. To solve the problem of time tag bias of the HY-2 altimeter, we conducted nearly 20 in-orbit calibration experiments from November 2021 to September 2022 using a reconstructive transponder, and statistically analysed the effect of time tag bias on the altimeter height measurement. The results of time tag bias of HY-2B is 0.03 ms, that of HY-2C is 0.07 ms, and that of HY-2D is 0.1 ms. As an independent calibration method, the reconstructive transponder can be deployed on land, which makes it unaffected by error sources introduced by sea surface dynamics, resulting in calibration with more accuracy. The altimeter system delay after correcting the time tag bias is also given, which provides a guarantee for the quality of the subsequent data products.

This study is based on the echo data from the Yinchuan vertical ionosonde.The ionosonde supports scanning in the frequency range from 1 to 30 MHz,with a distance resolution of 1.5 km and a reception window ranging from 67.5 km to 560.1 km.It utilizes pulse compression technology and en-codes the transmission signal using Bernoulli mapping sequences,successfully resolving the issue of echo signal mixture with strong clutter interference in practical detection,thus obtaining Ionograms of high quality.In order to extract key information of ionosphere from the ionograms,the signal processing prob-lem is transformed into a semantic segmentation task in computer vision,constructing an original iono-grams dataset,and undergoing preprocessing such as discretization and manual annotation.By training a cGAN neural network to analyze the characteristic parameters of each layer's traces in the ionograms,the goal of segmenting different traces is achieved.The network is suitable for processing various types of ionograms under calm conditions,with an accuracy rate of over 95％,effectively saving time in manual parameter measurement and improving processing efficiency.An improved bottom inversion model of the International Reference Ionosphere and the NeQuick top model is used to invert the electron density pro-file above the ionosonde,while the top calculation results are corrected according to the actual measure-ment data from"CSES-1".By comparing the total electron content calculated with the data results pub-licly available from CDDIS,the accuracy of the ionosonde data is verified.On this basis,combined with the geomagnetic data acquired by the Gaoshaowo magnetometer,the ionosonde successfully observed the entire process of ionospheric anomalies during the geomagnetic storm on 23-24 April,2023,and provid-ed the results of the total electron content changes,offering accurate and reliable observational data for exploring the electromagnetic environment changes in western China.

The echo traces in the ionograms contain key information about the ionosphere. Therefore, the accurate extraction of these traces is crucial for the subsequent work. This paper transforms the original signal processing problem into a semantic segmentation task, combines it with the currently popular deep learning techniques, and proposes a multiscale Transformer network to achieve pixel-level trace extraction. To train the proposed model, we built a dataset by discretizing the original echo data, labeling, and other preprocessing work. A series of advanced semantic segmentation networks are utilized for comparative experiments. The analysis of the results indicates that the proposed network excels in performance, achieving the highest scores on key semantic segmentation evaluation metrics, including mIoU, Kappa, Dice, and AUC-ROC. In addition, this paper also designs a series of ablation experiments to observe the changes in network performance and to evaluate the rationality of the network design. The experimental results demonstrate the effectiveness of the network in the trace extraction task, which plays a positive role in the subsequent electron density reversal work.

China’s marine dynamic satellite constellation was formed in 2021 with the launch of HY-2D, with existing HY-2C and HY-2B. The SSH (Sea Surace Height) calibration by tide-gauges, buoys as well as range calibration utilizing transponders have been performed extensively. In order to correlate the SSH and range calibration in one experiment, search the association between the two biases, furtherly, to compile the range bias obtained by transponders into the altimeters data products, a novel calibration method was developed utilizing a coastal transponder. In the new method, the altimeter works at normal SSH tracking mode, and the transponder captures the pulses instantaneously, transmits them back entering the altimeters receiving windows. From the altimeter spectra, the correlation between the SSH and the transponder echoes are achieved, and the two biases could be validated. In this paper, all the China’s coastal tracks of HY-2B/C/D were examined, and four sites were selected. In March and August in 2023, two calibration missions were carried out. Besides the range bias, the SSH validation was analyzed for the first time. The new method shows potentials in combing the transponder and the sea surface methods. At present, some new calibration techniques based on the signal-rebuilt transponder are in research, involving the fully-focused SAR processing and the absolute measurements of ranges and the instrument resolutions. These new methods are expected to serve for the next advanced altimeters which would be launched in a few years.

HY-2C (Haiyang) and HY-2D were launched on 21 September 2020 and 19 May 2021 separately, which form China’s marine dynamic satellite constellation program with HY-2B, launched on October 25, 2018. The calibration mission for HY-2B altimeter was carried out in 2019, and a precision of less than 1 cm was achieved. For further validation, a comparison between HY-2B and Jasons was performed in SSH (Sea Surface Height). To obtain consistent and comparable calibration results, a mobile transponder was utilized in different sites for HY-2B/C/D altimeters. Two experiment missions were carried out in 2021 and 2022 separately, each for a month. The result shows the range calibration precision for HY-2B/C/D is less than 4 cm, and USO (Ultra Stable Oscillator) drift on board is very small and negligible compared with HY-2A. Further, the range bias correction of the altimeter instrument is defined and will be compiled into the L1 data to improve the quality of L2 data product. In March this year, a novel calibration method is developed on offshore, with the altimeter switched to ocean calibration mode. This paper gives a detailed description of the multi-calibration of HY-2B/C/D altimeter using a mobile transponder. The results are validated with the ocean mode calibration, which shows a high consistency. In the following mission, the mobile transponder will be installed offshore on the altimeter ground track permanently, performing regular range and sigma zero calibration.

In this paper, a matching method for altimeter and transponder signals in Sub-optimal Maximum Likelihood Estimate (SMLE) tracking mode is proposed. In the in-orbit calibration of the altimeter in SMLE tracking mode using the reconstructive transponder, it is necessary to separate the forwarding signal from the ground echo signal. At the same time, the fluctuations in the received signal of the altimeter, which are caused by the forwarding signal of the transponder, can be eliminated. The transponder generates a bias when measuring the arrival time of the transmitting signal from the altimeter and embeds this bias in both the transponder-recorded data and the altimeter-recorded data. Therefore, the two sets of data have one-to-one correspondence, and they are superimposed using the sliding sum method. Moreover, the distance between the altimeter and the transponder is a parabolic geometric relationship, and the outliers are eliminated by the fitting error minimization decision, and the transponder signal is separated from the ground echo. The final altimeter transmitting–receiving signal path is obtained. Furthermore, the principles underlying this method can be used for any transponder that can adjust the response signal delay during calibration.

External interference in ionospheric sounding seriously degrades the quality of echo signals and data; thus, it should be eliminated. This paper presents a method for suppressing interference using chaotic coding with a set of Bernoulli map sequences; compared with other commonly used coding methods such as Barker code, complementary code, and Barker-like codes, through simulation, the ambiguity function (AF) of Bernoulli map codes has better performance in terms of peak sidelobe level (PSL), integral sidelobe ratio (ISL), noise suppression (NS), and signal-to-noise ratio (SNR). Experimental tests were performed using a vertical ionosonde in Yinchuan, Ningxia Hui Autonomous Region, China, and the ionosonde was operated by alternating 40-bit Barker-like coding and 40-bit Bernoulli map coding each day to compare the effectiveness of interference suppression. The results showed that using Bernoulli map coding could remove interference and improve SNR significantly, thereby improving the data quality of the resulting ionograms.

HY-2B (Haiyang) is the follow-on mission to HY-2A, the Chinese first sea satellite for oceanography, launched in October 2018. The main payloads onboard are similar to HY-2A. For the altimeter, an in-orbit calibration mode was designed to achieve higher calibration precision. An improved transponder was built and deployed for the calibration campaign. The purpose of this article is to describe the in-orbit calibration and validation of HY-2B altimeter based on an improved transponder. A signal-rebuilt transponder was developed and employed in HY-2A altimeter calibration. It played an important role in the quantitative analysis of HY-2A altimeter data products. For HY-2B altimeter, a newly designed calibration mode was added, and an improved transponder was built. It has some advantages over the previous one in the rebuilt signal forms and modulation ways. Two calibration campaigns were carried out, in April 2019 and October 2020 at different sites. In the two campaigns, the precision of the altimeter range calibration is obtained less than 1 cm. For further validation and assessment of the result, a comparison between HY-2B and Jason-3 was performed in terms of sea surface height on the tens of cross track points in the open ocean. The comparison result shows high degree of accuracy and stability of HY-2B altimeter instrument. In China's next five-year plan (2021–2025), many more transponders and related calibration facilities would be installed in the ocean calibration fields, serving a variety of different satellite altimetry missions.

雷达高度计的主要功能是测量全球平均海平面高度(SSH:Sea Surface Height),它通过准确测量收发脉冲之间的时间间隔来实现精准的星地距离测量,时间测量的准确度取决于高度计时钟的准确性.卫星在轨运行期间,雷达高度计时钟频率会产生缓慢漂移,时钟频率漂移会影响星地距离的测量准确度,造成对海面高程的测量偏差.在基于重建型有源定标器对HY-2A卫星高度计时钟偏差在轨测试方法的基础上,提出了从星地斜距函数曲线时间偏移量中提取高度计时钟偏差的估计方法,应用于HY-2B卫星雷达高度计在轨定标测试中,测量了雷达高度计时钟频率漂移量,精度优于0.001 Hz.结果 表明:HY-2B高度计原子钟性能稳定,时钟频率偏差造成的测距偏差为毫米量级,平均测距漂移率为2.95×10-7 m/d.

An improved transponder for HY-2B (Haiyang) radar altimeter is presented in this paper. HY-2B is the follow-on mission to HY-2A, the Chinese sea satellites for oceanography. An improved transponder was developed. It has some advantages over the HY-2A one. The paper describes the development and integration test of the transponder. Some new techniques are employed. Its performance is stable and qualified for HY-2B altimeter calibration.

HY-2B (Haiyang) is the follow-on mission to HY-2A, which is the Chinese first sea satellite for oceanography. This paper describes the in-orbit calibration and validation of HY-2B altimeter by utilizing an improved transponder. It has some advantages over the HY-2A one. The calibration campaign has been carrying out from April this year. The altimetry calibration precision of better than 1 cm has been achieved. And a comparison between HY-2B and Jason3 was done in term of SSH (Sea Surface Height). The result shows high degree of consistency through all the cross track points since April to September this year.

Images retrieved from the passive millimeter-wave synthetic aperture interferometric radiometer (SAIR) with small satellites formation suffer from the artifacts due to the very sparse sampling data in the u-v plane. To reduce the artifacts, we firstly propose a UV-interpolation method to obtain more data with accurate amplitude and phase values in u-v plane; and secondly propose the local CLEAN method to keep the object structure in the spatial domain. Experimental results show that the proposed method can reconstruct the ground object with higher accuracy.

A return signal simulator (RSS) for HY-2B altimeter is described in the paper. HY-2 is the second one of China's serial oceanic remote sensing satellites, which purpose is for ocean dynamic parameter measurement. The main payload on board to measure the sea surface height is a dual frequency altimeter. In order to test and calibrate the altimeter before its launch, a RSS is developed and a series of tests are performed. The RSS could generate simulated echoes of sea surface with accurate time delay and flexible power, therefore provides thoroughly test and function validation for the altimeter. The RSS's development and integration, pre-launch test and function verification to the altimeter are presented in this paper.

The aim of the paper is to describe a fully functional return signal simulator (RSS) for thoroughly test and functional verification of HY-2B scatterometer prior to its launch. HY-2 is the second one of China's serial oceanic remote sensing satellites, which purpose is for ocean dynamic parameter measurement. One of the main payloads onboard is a Ku band radar scatterometer. In order to test and calibrate the scaterometer before its launch, a fully functional RSS is developed. The RSS has advantages of complete signal process, flexible time delay and accurate echoes simulation, therefore provides thoroughly test and function validation for the scatterometer. The RSS's development and performance analysis, testing and function verification to the scatterometer are presented in this paper. Also, some of the echo simulation techniques are put forward and discussed.

In this paper, we present the experimental results of a DBF (digital beam forming) scatterometer demonstrator performance on the purpose of development and verification of innovative DBF concepts and advanced operational modes for the future space borne MICAP (microwave imager combined active and passive), China's ocean salinity mission. The hardware configuration of the demonstrator is introduced. The ground-based experiments, verifying the DBF function on receiving and transmitting separately are carried out on the open field using the demonstrator. Analysis of the experimental results verifies the validity of the DBF technique on the receiving and transmitting. At last, the problems caused by the unbalance of the amplitude and phase between channels are discussed. The function verification experiment is the very first step towards the future space borne MICAP, and further researches are in progress.

The aim of the paper is to describe a ground-based microwave scatterometer demonstrator dedicated for the functional verification of MICAP (Microwave Imager Combined Active and Passive). MICAP has been selected to be one of the main payloads for future Chinese ocean salinity mission. The MICAP scatterometer is for estimating ocean surface roughness which measurement will enable the removal of the wind effect from the ocean surface brightness temperature measurement being used to retrieval ocean salinity. The primary technical challenge is its end to end system stability, on the order of 0.1dB. A demonstrator was developed to verify functionality and provide support to trouble shooting in the following project phases. The demonstrator's development and integration, as well as the stability calibration are discussed in the paper.