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This study presents the feasibility and performance evaluation of a developed ground-based synthetic aperture radar (GBSAR) system for monitoring vertical displacement in bridge structures. Bridges, as critical transportation infrastructures, require regular assessment to ensure safety. In recent years, ground-based radar and synthetic aperture radar (SAR) systems have demonstrated their effectiveness as non-contact displacement monitoring tools for structural displacement monitoring. This study utilizes a prototype GBSAR system consisting of a W-band radar, TI-AWR1642BOOST, with a maximum signal bandwidth of 4 GHz, incorporated with a 15 cm horizontal mechanical rail to enhance cross-range resolution. To evaluate the performance of the GBSAR, two types of experiments were conducted: laboratory experiments in a controlled situation and outdoor simulations by creating a lightweight bridge and inducing displacements through the addition or removal of loads on its surface. The results obtained from the laboratory experiments showcased a measurement accuracy ranging from 50 to 80 micrometers in vertical displacement, while the results from the simulated bridge showed a final accumulated vertical displacement error below 1 mm. These overall results validate the applicability of the developed GBSAR system for vertical displacement monitoring of structures with sub-millimeter accuracy. [ABSTRACT FROM AUTHOR]

The accuracy of surface displacements measured by differential radar interferometry is significantly degraded by the atmospheric phase screen (APS). This article presents a practical and efficient approach for APS mitigation based on the coherent pixels technique (CPT) displacement velocity estimation algorithm. In the proposed approach, all motionless coherent pixels closest to the moving area are defined as seeds surrounding the moving area at the integration step of the CPT. This arrangement consequently minimizes the integration path and the APS effect in the final velocity result. It is designed for terrestrial radar interferometry (TRI) applications. A piecewise processing chain is further introduced as a continuous operational mode processing framework to derive arbitrary temporal displacement patterns in this work. Three-day datasets measured by Ku-band TRI over a mountainous region in the canton of Valais, Switzerland, were used for validation. Through this validation, a comparative study of five algorithms was carried out. This evaluation showed the efficiency of the proposed approach. The proposed approach does not require phase unwrapping, kriging interpolation, and spatio-temporal covariance inference for APS mitigation, which is appropriate for continuous TRI operation.

Monitoring glacier flow speed and calving rates is of interest for climate research, global sea-level studies and Arctic ship traffic. The research station in Ny-Ålesund, Svalbard, offers a unique location close to multiple glacier fronts. In this study, we explore the possibilities of permanent monitoring of glaciers in the Ny-Ålesund area using a ground-based interferometric radar with a significantly higher temporal resolution than what is achievable from satellites or mechanical scanning ground-based radars. Measurements were made from two different locations—Pynten and the Ny-Ålesund research station—located 5 and 15 km from Kronebreen glacier, respectively. The temporal resolution of the radar is flexible, and in this experiment is limited to five images per minute, providing data with high temporal resolution of glacier flow. We calculated a geo-located two-dimensional flow map of the glacier from the radar data, extracted glacier speed profiles and identified major calving onsets. This type of near real-time data may well be used with machine-learning techniques for more advanced monitoring systems. The radar measurements agree well with previous satellite measurements at lower temporal resolution. The observed mean flow of the Kronebreen glacier front varies across the glacier from around 0.4 m/day at the edges to 3 m/day in the central part. We identify and discuss possible improvements to the radar system and conclude that Ny-Ålesund is a well-suited location for radar monitoring of glacier flow velocities.

Measurements of short-interval variations in glacier surface velocity, which contribute to our understanding of ice motion mechanisms, remain scarce on the Tibetan Plateau. Here we present sub-hourly measurements of glacier surface motion variations at the terminus region of Laohugou No. 12 Glacier. Field observations were collected over 4 d in July 2015 from terrestrial radar interferometry. The observed glacier displacement time series are generally in agreement with the results measured by differential GPS and highlight that glacier surface velocity is characterized by clear diurnal fluctuations in the study period. During day-time hours, glacier flow speeds were higher than 3.0 mm h−1, whereas they were below 1.0 mm h−1 during night-time hours. The large diurnal fluctuations of glacier surface velocity indicate that variations in basal slip are the dominant motion mechanism. Moreover, a positive correlation (R = 0.82, P < 0.001) between air temperature and glacier surface velocity suggests that glacier motion variations are probably affected by changes in air temperature during the ablation season.

Jakobshavn Isbræ, a tidewater glacier that produces some of Greenland's largest icebergs and highest speeds, reached record-high flow rates in 2012 (Joughin and others, 2014). We use terrestrial radar interferometric observations from August 2012 to characterize the events that led to record-high flow. We find that the highest speeds occurred in response to a small calving retreat, while several larger calving events produced negligible changes in glacier speed. This non-linear response to calving events suggests the terminus was close to flotation and therefore highly sensitive to terminus position. Our observations indicate that a glacier's response to calving is a consequence of two competing feedbacks: (1) an increase in strain rates that leads to dynamic thinning and faster flow, thereby promoting destabilization, and (2) an increase in flow rates that advects thick ice toward the terminus and promotes restabilization. The competition between these feedbacks depends on temporal and spatial variations in the glacier's proximity to flotation. This study highlights the importance of dynamic thinning and advective processes on tidewater glacier stability, and further suggests the latter may be limiting the current retreat due to the thick ice that occupies Jakobshavn Isbræ’s retrograde bed.

Measurements of short-interval variations in glacier surface velocity, which contribute to our understanding of ice motion mechanisms, remain scarce on the Tibetan Plateau. Here we present sub-hourly measurements of glacier surface motion variations at the terminus region of Laohugou No. 12 Glacier. Field observations were collected over 4 d in July 2015 from terrestrial radar interferometry. The observed glacier displacement time series are generally in agreement with the results measured by differential GPS and highlight that glacier surface velocity is characterized by clear diurnal fluctuations in the study period. During day-time hours, glacier flow speeds were higher than 3.0 mm h−1, whereas they were below 1.0 mm h−1 during night-time hours. The large diurnal fluctuations of glacier surface velocity indicate that variations in basal slip are the dominant motion mechanism. Moreover, a positive correlation (R = 0.82, P < 0.001) between air temperature and glacier surface velocity suggests that glacier motion variations are probably affected by changes in air temperature during the ablation season. [ABSTRACT FROM AUTHOR]

Time-varying elevations near the calving front of Jakobshavn Isbræ, Greenland were observed with a terrestrial radar interferometer (TRI) in June 2015. An ice block with surface dimensions of 1370 m × 290 m calved on 10 June. TRI-generated time series show that ice elevation near the calving front began to increase 65 h prior to the event, and can be fit with a simple block rotation model. We hypothesize that subsurface melting at the base of the floating terminus breaks the gravity-buoyancy equilibrium, leading to slow subsidence and rotation of the block, and its eventual failure.

Jakobshavn Isbræ, a tidewater glacier that produces some of Greenland's largest icebergs and highest speeds, reached record-high flow rates in 2012 (Joughin and others, 2014). We use terrestrial radar interferometric observations from August 2012 to characterize the events that led to record-high flow. We find that the highest speeds occurred in response to a small calving retreat, while several larger calving events produced negligible changes in glacier speed. This non-linear response to calving events suggests the terminus was close to flotation and therefore highly sensitive to terminus position. Our observations indicate that a glacier's response to calving is a consequence of two competing feedbacks: (1) an increase in strain rates that leads to dynamic thinning and faster flow, thereby promoting destabilization, and (2) an increase in flow rates that advects thick ice toward the terminus and promotes restabilization. The competition between these feedbacks depends on temporal and spatial variations in the glacier's proximity to flotation. This study highlights the importance of dynamic thinning and advective processes on tidewater glacier stability, and further suggests the latter may be limiting the current retreat due to the thick ice that occupies Jakobshavn Isbræ's retrograde bed. [ABSTRACT FROM AUTHOR]

Monitoring glacier flow speed and calving rates is of interest for climate research, global sea-level studies and Arctic ship traffic. The research station in Ny-Ålesund, Svalbard, offers a unique location close to multiple glacier fronts. In this study, we explore the possibilities of permanent monitoring of glaciers in the Ny-Ålesund area using a ground-based interferometric radar with a significantly higher temporal resolution than what is achievable from satellites or mechanical scanning ground-based radars. Measurements were made from two different locations—Pynten and the Ny-Ålesund research station—located 5 and 15 km from Kronebreen glacier, respectively. The temporal resolution of the radar is flexible, and in this experiment is limited to five images per minute, providing data with high temporal resolution of glacier flow. We calculated a geo-located two-dimensional flow map of the glacier from the radar data, extracted glacier speed profiles and identified major calving onsets. This type of near real-time data may well be used with machine-learning techniques for more advanced monitoring systems. The radar measurements agree well with previous satellite measurements at lower temporal resolution. The observed mean flow of the Kronebreen glacier front varies across the glacier from around 0.4 m/day at the edges to 3 m/day in the central part. We identify and discuss possible improvements to the radar system and conclude that Ny-Ålesund is a well-suited location for radar monitoring of glacier flow velocities. [ABSTRACT FROM AUTHOR]

Rockfalls are major natural hazards in mountainous regions and as such monitored if they pose a high risk to people or infrastructure. Ground-based radar interferometry is a relatively new technique suitable for such monitoring. It offers the potential for determining sub-mm- to mm-level displacements by remote measurements under various weather conditions. To avoid damage from smaller rocks and debris, critical surfaces are often protected by rockfall catch fences. We present an experimental investigation proving that the radar measurements are indeed significantly affected by a catch fence made of steel even if its mesh size is larger compared to that of the wavelength of the radar. A stable rock wall in a quarry was monitored by means of a ground-based synthetic aperture radar for 2 days. Different fences varying in shape, size, and density of mesh and in various geometrical configurations were erected at different locations for short periods of time during the experiment. We show that for surfaces observed through the fence, the reflected power can be reduced by 20 dB and thus the signal-to-noise ratio is significantly deteriorated. We also observed spurious interferometric phase shifts. Even parts of the rock wall not covered by the fences are affected. Side lobes and mixed pixels result, e.g., in severe loss of coherence and thus potentially mask actual displacements. [ABSTRACT FROM AUTHOR]

The benefits of terrestrial radar interferometry (TRI) for deformation monitoring are restricted by the influence of changing meteorological conditions contaminating the potentially highly precise measurements with spurious deformations. This is especially the case when the measurement setup includes long distances between instrument and objects of interest and the topography affecting atmospheric refraction is complex. These situations are typically encountered with geo-monitoring in mountainous regions, e.g. with glaciers, landslides or volcanoes. We propose and explain an approach for the mitigation of atmospheric influences based on the theory of intrinsic random functions of order k (IRF-k) generalizing existing approaches based on ordinary least squares estimation of trend functions. This class of random functions retains convenient computational properties allowing for rigorous statistical inference while still permitting to model stochastic spatial phenomena which are non-stationary in mean and variance. We explore the correspondence between the properties of the IRF-k and the properties of the measurement process. In an exemplary case study, we find that our method reduces the time needed to obtain reliable estimates of glacial movements from 12 h down to 0.5 h compared to simple temporal averaging procedures. [ABSTRACT FROM AUTHOR]

Time-varying elevations near the calving front of Jakobshavn Isbræ, Greenland were observed with a terrestrial radar interferometer (TRI) in June 2015. An ice block with surface dimensions of 1370 m × 290 m calved on 10 June. TRI-generated time series show that ice elevation near the calving front began to increase 65 h prior to the event, and can be fit with a simple block rotation model. We hypothesize that subsurface melting at the base of the floating terminus breaks the gravity-buoyancy equilibrium, leading to slow subsidence and rotation of the block, and its eventual failure. [ABSTRACT FROM AUTHOR]

Warming ocean currents are considered to be a contributing factor to the retreat of marine-terminating glaciers worldwide, but direct observations near the ice–ocean interface are challenging. We use radar intensity imagery and an iceberg tracking algorithm to produce half-hourly current maps within an imaged portion of Jökulsárlón, a proglacial lagoon in southeastern Iceland. Over our 43.5-h observation period, the lagoon has clockwise circulation with current speeds of order 3–8 cm/s and occasional strong glacier outflows of up to ∼15 cm/s. The currents driven by the glacial outflows appear to be dominantly inertial. [ABSTRACT FROM AUTHOR]

This paper presents a review of the current state of the art in the use of terrestrial radar interferometry for the detection of surface changes related to mass movement. Different hardware-types and acquisition concepts are described, which use either real or synthetic aperture for radar image formation. We present approaches for data processing procedures, paying special attention to the separation of high resolution displacement information from atmospheric phase variations. Recent case studies are used to illustrate applications in terrestrial radar interferometry for change detection. Applications range from detection and quantification of very slow moving (millimeters to centimeters per year) displacements in rock walls from repeat monitoring, to rapid processes resulting in fast displacements (~50 m/yr) acquired during single measurement campaigns with durations of only a few hours. Fast and episodic acting processes such as rockfall and snow avalanches can be assessed qualitatively in the spatial domain by mapping decorrelation caused by those processes. A concluding guide to best practice outlines the necessary preconditions that have to be fulfilled for successful application of the technique, as well as in areas characterized by rapid decorrelation. Empirical data from a Ku-band sensor show the range of temporal decorrelation of different surfaces after more than two years for rock-surfaces and after a few seconds to minutes in vegetated areas during windy conditions. The examples show that the displacement field can be measured for landslides in dense grassland, ice surfaces on flowing glaciers and snowpack creep. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We describe a method for rapid identification and precise quantification of slope deformation using a portable radar interferometer. A rockslide with creep-like behavior was identified in the rugged and inaccessible headwaters of the Illgraben debris-flow catchment, located in the Central Swiss Alps. The estimated volume of the moving rock mass was approximately 0.5 \times 10^6\ \m^3 with a maximum daily (3-D) displacement rate of 3 mm. Fast scene acquisition in the order of 6 s/scene led to uniquely precise mapping of spatial and temporal variability of atmospheric phase delay. Observations led to a simple qualitative model for prediction of atmospheric disturbances using a simple model for solar radiation, which can be used for advanced campaign planning for short observation periods (hours to days). [ABSTRACT FROM PUBLISHER]

Volcanoes with rugged terrain remain a challenging target for generating high-resolution digital elevation models (DEMs), especially in tropical areas with frequent cloud cover. Using Nevado del Ruiz volcano as an example, we combined DEMs from the TanDEM-X (TDX) satellite mission, terrestrial radar interferometry (TRI), and Structure from Motion (SfM), to generate a new DEM with 10-m spatial resolution. This is the first study combining satellite radar, ground-based radar, photography, and freely available global DEMs to generate a high-resolution DEM without data gaps. TDX data from ascending and descending orbits were combined to generate the base DEM. Instead of using a raster format to fuse DEMs generated from different data sets with different resolutions, we developed a methodology based on 3-D point clouds: 1) re-georeference the 5-m TRI and ~1-m SfM DEMs to the 10-m TDX DEM using the iterative closest point (ICP) algorithm to minimize the horizontal and vertical discrepancy between DEMs; then 2) merge the multiple point clouds to generate a final DEM without data gaps using an adaptive algorithm that uses two search distances to smooth the transition at the edges of different data sets. We assess the new 10-m DEM by comparing simulated inundation zones obtained with two volcano flow models, LaharZ (for lahars) and VolcFlow (for pyroclastic flows), and find significant differences with respect to the 30-m SRTM DEM. Our LaharZ simulation over the new DEM shows a longer lahar run-out distance. For pyroclastic flows, the VolcFlow simulation over the new DEM produces highly channelized flows over the steep portions of a river channel and gives a larger extent of thicker deposits compared to those obtained with the 30-m SRTM DEM. Quantitative and qualitative geomorphic analysis suggests that up-to-date DEMs with high spatial resolution (~ 10 m or even better) need to be generated to improve volcano hazard assessment for active volcanoes. • Ground-based TRI and SfM data fill data gaps in the satellite TDX DEMs. • A method is developed for fusing DEMs with different resolutions based on 3-D point clouds. • A 10-m DEM of Nevado del Ruiz volcano was generated and is available for download. • The new 10-m DEM improves the results of volcanic flow models. [ABSTRACT FROM AUTHOR]