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.

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.