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With the continuous expansion of the high-speed railway network in China, long-span railway bridges carrying multiple tracks demand reliable and fast testing procedures and techniques. Bridge dynamic behavior analysis is a critical process in ensuring safe operation of structures. In this study, we present some experimental results of the vibration monitoring of a four-track high-speed railway bridge with a metro–track on each side: the Nanjing–Dashengguan high-speed railway bridge (NDHRB). The results were obtained using a terrestrial microwave radar interferometer named IBIS-S. The radar measurements were interpreted with the support of lidar point clouds. The results of the bridge dynamic response under different loading conditions, including high-speed trains, metro and wind were compared with the existing bridge structure health monitoring (SHM) system, underlining the high spatial (0.5 m) and temporal resolutions (50 Hz–200 Hz) of this technique for railway bridge dynamic monitoring. The detailed results can help engineers capturing the maximum train-induced bridge displacement. The bridge was also monitored by the radar from a lateral position with respect to the bridge longitudinal direction. This allowed us to have a more exhaustive description of the bridge dynamic behavior. The different effects induced by the passage of trains through different tracks and directions were distinguished. In addition, the space deformation map of the wide bridge deck under the eccentric load of trains, especially along the lateral direction (30 m), can help evaluating the running stability of high-speed trains.

Surface Water and Ocean Topography (SWOT) is a next-generation radar altimeter that offers high resolution, wide swath, imaging capabilities. It has provided free public data worldwide since December 2023. This paper aims to preliminarily analyze the detection capabilities of the Ka-band radar interferometer (KaRIn) and Nadir altimeter (NALT), which are carried out by SWOT for internal solitary waves (ISWs), and to gather other remote sensing images to validate SWOT observations. KaRIn effectively detects ISW surface features and generates surface height variation maps reflecting the modulations induced by ISWs. However, its swath width does not completely cover the entire wave packet, and the resolution of L2/L3 level products (about 2 km) cannot be used to identify ISWs with smaller wavelengths. Additionally, significant wave height (SWH) images exhibit blocky structures that are not suitable for ISW studies; sea surface height anomaly (SSHA) images display systematic left-right banding. We optimize this imbalance using detrending methods; however, more precise treatment should commence with L1-level data. Quantitative analysis based on L3-level SSHA data indicates that the average SSHA variation induced by ISWs ranges from 10 cm to 20 cm. NALTs disturbed by ISWs record unusually elevated SWH and SSHA values, rendering the data unsuitable for analysis and necessitating targeted corrections in future retracking algorithms. For the normalized radar cross section, Ku-band and four-parameter maximum likelihood estimation retracking demonstrated greater sensitivity to minor changes in the sea surface, making them more suitable for ISW detection. In conclusion, SWOT demonstrates outstanding capabilities in ISW detection, significantly advancing research on the modulation of the sea surface by ISWs and remote sensing imaging mechanisms.

Jakobshavn Isbræ, a major outlet glacier in Greenland, lost its protective ice shelf in 2002 and has been speeding up and retreating since. We image its grounding line for the first time with a terrestrial radar interferometer deployed in 2016 and detect its migration at tidal frequencies. The southern half of the glacier develops a floating section (3 km × 3 km) that migrates in phase with the tidal difference, up to a distance of 2.8 km, far more than previously expected. We attribute the migration to kilometer‐scale seawater intrusions, 10–20 cm in height, with the tide. The intrusions reveal that the glacier bed may be up to 800 m deeper than expected on the south side, which illustrates that our knowledge of bed topography remains limited for this glacier. We expect seawater intrusions to cause rapid melt of basal ice and play a major role in the glacier evolution. Plain Language Summary The transition boundary between grounded glacier ice and floating glacier ice, or grounding line, has never been mapped in much detail on the largest, fastest outlet glaciers of Greenland because available satellite radar imagery does not provide short enough repeat pass data. Here, we use a terrestrial radar interferometer which images the glacier every 2 min to map the grounding line repeatedly with differential interferometry. Surprisingly the glacier develops a small floating section on the south side where the grounding line migrates over considerable distances—0.5 to 2.8 km—during the tidal cycle, which is 10 times farther than previously expected from flotation. We attribute the migration to seawater intrusions over a bed 100–800 m deeper than previously known. Seawater intrusions will carry sufficient ocean heat to melt basal ice vigorously, a factor that has not been incorporated in modeling studies of this glacier. Key Points We present the first mapping of the grounding line of Jakobshavn Isbr $\\ae$ , Greenland at ocean tidal frequencies The grounding line migrates over kilometers, far more than expected from flotation, which we attribute to kilometer‐size seawater intrusions Seawater intrusions along a bed several hundred meters deeper than expected must be included in future glacier modeling studies

Ground-based/terrestrial radar interferometry (GBRI) is a scientific topic of increasing interest in recent years. This article is a bibliographic review, as much complete as possible, of the scientific papers/articles published in the last 20 years, since the pioneering works in the nineties. Some statistics are reported here about the number of publications in the years, popularity of applications, operative modalities, operative bands. The aim of this review is also to identify directions and perspectives. In the opinion of authors, this type of radar systems will move forward faster modulations, wider view angle, MIMO (Multiple Input Multiple Output) systems and radar with capability to detect the vector of displacement and not only a single component.

Traditional nadir altimeters struggle with coastal water surface elevation (WSE) measurement and fine‐scale river‐estuary interactions, due to land‐water signal interference and their wide inter‐track spacing. The wide‐swath Surface Water and Ocean Topography (SWOT) mission, using a new Ka‐band radar interferometer, aims to address these issues by delivering 2D WSE measurements with unprecedented spatial resolution, accuracy, and precision. However, the mission's effectiveness in coastal WSE retrieval and its error characteristics remain unverified. This study leverages gauge and airborne LiDAR data to validate SWOT's WSE in the Bristol Channel and Severn Estuary. Assuming error‐free in situ data, SWOT ocean products exhibit a standard deviation of difference (STD) of 13 cm within a 3 km radius of tide gauges. Compared to LiDAR, SWOT's PIXC measurements have STD of 37 cm, improving to 14 cm over 100 m grids and 9 cm over 1 km2 areas. This meets the SWOT science requirement of 10 cm STD at 1 km2 scale and extends satellite‐based WSE monitoring into complex coastal environments.

The SWOT (Surface Water Ocean Topography) mission will provide high-resolution and two-dimensional measurements of sea surface height (SSH). However, despite its unprecedented precision, SWOT’s Ka-band Radar Interferometer (KaRIn) still exhibits a substantial amount of random noise. In turn, the random noise limits the ability of SWOT to capture the smallest scales of the ocean’s topography and its derivatives. In that context, this paper explores the feasibility, strengths and limits of a noise-reduction algorithm based on a convolutional neural network. The model is based on a U-Net architecture and is trained and tested with simulated data from the North Atlantic. Our results are compared to classical smoothing methods: a median filter, a Lanczos kernel smoother and the SWOT de-noising algorithm developed by Gomez-Navarro et al. Our U-Net model yields better results for all the evaluation metrics: 2 mm root mean square error, sub-millimetric bias, variance reduction by factor of 44 (16 dB) and an accurate power spectral density down to 10–20 km wavelengths. We also tested various scenarios to infer the robustness and the stability of the U-Net. The U-Net always exhibits good performance and can be further improved with retraining if necessary. This robustness in simulation is very encouraging: our findings show that the U-Net architecture is likely one of the best candidates to reduce the noise of flight data from KaRIn.

The Surface Water and Ocean Topography (SWOT) mission aims to measure the sea surface height (SSH) at a high spatial resolution using a Ka-band radar interferometer (KaRIn). The primary oceanographic objective is to characterize the ocean eddies at a spatial resolution of 15 km for 68% of the ocean surface. This resolution is derived from the ratio between the wavenumber spectrum of the conventional altimeter (projected to submesoscale) and the SWOT SSH errors. While the 15-km threshold is useful as a global approximation of the spatial scales resolved by SWOT (SWOT scale), it can be misleading for regional studies. Here we revisit the problem using a high-resolution (~2-km horizontal grid spacing) tide-resolving global ocean simulation and map the SWOT scale as a function of location and season. The results show that the SWOT scale increases, in general, from about 15 km at low latitudes to ~30–45 km at mid- and high latitudes but with a large geographical dependence. A SWOT scale smaller than 30 km is expected in the high-latitude energetic regions. The SWOT scale varies seasonally as a result of the seasonality in both the noise and ocean signals. The seasonality also has a geographical dependence. Both eddies and internal gravity waves/tides contribute significantly to the SWOT scale variation. Our analysis provides model predictions for interpreting the anticipated observations from SWOT and guidance for the development of analysis methodologies.

The Surface Water and Ocean Topography (SWOT) satellite mission enables, for the first time, two‐dimensional (2D) mapping of significant wave height Hs $\\left({H}_{s}\\right)$ at kilometer‐scale resolution. Using data from SWOT's Ka‐band Radar Interferometer (KaRIn), this study investigates interactions between surface waves, winds, and currents across diverse dynamic regimes, including western boundary currents, mesoscale turbulence, tropical cyclones, and wave group modulation. SWOT reveals unprecedented 2D spatial gradients in Hs ${H}_{s}$, capturing fine‐scale variability previously identified only in numerical models. These observations highlight the critical role of currents in shaping the wave field and show strong agreement with theoretical predictions. SWOT's high‐resolution wave data represent a transformative advance in understanding Hs ${H}_{s}$ gradients, paving the way for refining operational models and addressing challenges in characterizing the influence of sea state gradients on coupled air‐sea processes.

This paper presents three refinements in ground-based radar interferometer (GB-radar) measurement for bridge load testing: (1) GB-radar phase jumps were detected for the first time on bridge tower displacement monitoring, and a recovery method is presented to obtain the correct unwrapped value; (2) a precise displacement projection method considering target deformation was exploited, and a case study of the Fifth Nanjing Yangtze River Bridge (FNYRB) GB-radar campaign shows that a centimeter-level compensation can be achieved; (3) a post-construction settlement phenomenon was found during the FNYRB static load tests, characterized by 0.31 mm/min, which accumulated up to 25 mm. In addition, the dynamic monitoring capabilities of GB-radar for the bridge tower and girder were verified, highlighting its potential for bridge structural health monitoring (SHM). The insights gained from this study offer valuable recommendations for future GB-radar bridge displacement monitoring.

The Ka-band Radar Interferometer (KaRIn) on the Surface Water and Ocean Topography (SWOT) satellite will revolutionize satellite altimetry by measuring sea surface height (SSH) with unprecedented accuracy and resolution across two 50-km swaths separated by a 20-km gap. The original plan to provide an SSH product with a footprint diameter of 1 km has changed to providing two SSH data products with footprint diameters of 0.5 and 2 km. The swath-averaged standard deviations and wavenumber spectra of the uncorrelated measurement errors for these footprints are derived from the SWOT science requirements that are expressed in terms of the wavenumber spectrum of SSH after smoothing with a filter cutoff wavelength of 15 km. The availability of two-dimensional fields of SSH within the measurement swaths will provide the first spaceborne estimates of instantaneous surface velocity and vorticity through the geostrophic equations. The swath-averaged standard deviations of the noise in estimates of velocity and vorticity derived by propagation of the uncorrelated SSH measurement noise through the finite difference approximations of the derivatives are shown to be too large for the SWOT data products to be used directly in most applications, even for the coarsest footprint diameter of 2 km. It is shown from wavenumber spectra and maps constructed from simulated SWOT data that additional smoothing will be required for most applications of SWOT estimates of velocity and vorticity. Equations are presented for the swath-averaged standard deviations and wavenumber spectra of residual noise in SSH and geostrophically computed velocity and vorticity after isotropic two-dimensional smoothing for any user-defined smoother and filter cutoff wavelength of the smoothing.