Gamma Remote Sensing (Switzerland)

<strong class="journal-contentHeaderColor">Abstract.</strong> This study presents the first systematic field evaluation of dock-based UAV (Uncrewed Aerial Vehicle) systems for geohazard monitoring in mountainous terrain. We tested their potential across three different environments: (1) a fast-moving glacier icefall (Supphellebreen, Norway), (2) an unstable rock slope (Skj&oslash;ld, Norway), and (3) a post-failure landscape resulting from a catastrophic rock-ice avalanche (Blatten, Switzerland). Effective hazard management requires timely detection of displacement patterns and terrain change. To address these issues, we introduce an automated workflow integrating multitemporal UAV dock data acquisition with an end-to-end processing pipeline for displacement field generation and change detection. The results show that this workflow has the potential to provide data at centimetre-level accuracy before, during, and after hazard events, supporting both precautionary risk assessments and timely decision-making in critical phases of potential hazard evolution. Wider adoption will depend on supportive regulatory frameworks, reliable power and communication infrastructure, and sufficient expertise to ensure effective operation, maintenance, data interpretation and risk management. Overall, dock-based UAV systems represent a significant technological advancement in efficient geohazard monitoring, facilitating rapid response in critical situations, thereby contributing to increased resilience of communities living in vulnerable mountain environments.

Abstract. Europe's forests store nearly 40 PgC and provide a critical carbon sink of ~0.2 PgC yr-1, yet climate-driven disturbances increasingly threaten this capacity. Although disturbance rates from windthrow and bark beetle outbreaks have risen in recent decades, it remains unclear whether these events increasingly affect the oldest and largest trees, which store a disproportionate share of carbon. Here, we combine three decades of satellite-derived disturbance maps with spatially explicit data on forest age, biomass, and species composition to reveal patterns of structural selectivity across Europe. We show that natural disturbances have shifted toward older, carbon-rich stands, with disturbed forest area &gt; 60 years old nearly tripling since 2010 (from 0.38 to 1.06 Mha). This structural shift is most pronounced in spruce-dominated regions of Central Europe (effect size = 1), where compound heat and drought events have amplified susceptibility to bark beetles. Biomass losses from natural disturbances in spruce forests increased eightfold between the early (2011–2016) and recent (2017–2023) periods. Trend-based projections indicate that, if current patterns of structural selectivity persist, natural disturbances could expose biomass carbon stocks equivalent to approximately 20 % of Europe’s contemporary forest carbon sink by 2040 (~0.05 PgC yr -1 or ~0.7 PgC cumulative). Our findings reveal a previously unquantified vulnerability: climate-driven disturbances increasingly affect forest structures with high per-hectare carbon stocks, amplifying disturbance-related carbon exposure and weakening the long-term effectiveness of Europe’s forest carbon sink. Adaptive management strategies that promote structural and compositional diversification in high-risk regions will be critical to stabilise forest carbon storage under continued climate change.

Abstract. Retrogressive Thaw Slumps (RTS) are slope failures triggered by permafrost thaw that occur in ground-ice-rich regions of the Arctic and the Qinghai-Tibet Plateau (QTP). A strong warming trend has amplified RTS activity on the QTP in recent years. Although the region currently acts as a carbon sink, its permafrost-covered area (40 %) contains substantial soil organic carbon (SOC) stocks. Intensifying thaw-driven mass wasting may transform the QTP into a net carbon source by mobilising previously frozen SOC and enhancing decomposition. Yet, regional remote sensing studies have not yet quantified RTS mass wasting, including material erosion volumes and associated SOC mobilisation. Analysing time-series data from digital elevation models (DEMs) enables direct observation of RTS activity by measuring changes in active area, eroded material volume, and the overall magnitude of surface change. However, most available DEM sources lack the spatial resolution and temporal frequency required for comprehensive RTS monitoring. In contrast, optical data provide higher spatial resolution and more frequent observations, but lack elevation information. Here, we evaluated RTS mass wasting across the QTP from 2011 to 2020 by combining DEMs derived from bistatic Interferometric Synthetic Aperture Radar (InSAR) observations of the TanDEM-X mission with annual RTS inventories generated from high-resolution optical satellite imagery and geophysical soil property data to estimate erosion volume, ground ice loss, and SOC mobilisation. We estimated that RTS activity on the QTP during 2011–2020 relocated 5.020.7525.35×107 m3 previously frozen material, resulting in a loss of 3.580.2828.20×106 m3 of ground ice, and mobilised 2.780.117.98×108 kg C of organic carbon. We found a reliable power-law scaling relationship between RTS area in the optical RTS inventory and calculated volume change, with α values ranging from 1.20±0.01 to 1.30±0.01 (R2=0.87, p&lt;0.001) depending on the regression model used, which may readily transform planimetric RTS area into volume estimates at scale on the QTP. Despite the relatively recent initiation and smaller size of RTSs on the QTP, material erosion and SOC mobilisation over the past decade exceeded levels in some Siberian Arctic regions, but remained up to 10 times lower than hotspots in the Canadian High Arctic. While current RTS impacts on the QTP are relatively modest, affecting &lt; 0.01 % of the total permafrost area and contributing approximately 0.1 % to the regional carbon budget, the accelerating rates of RTS activity indicate that this phenomenon could become increasingly significant in the future. Our findings highlight the importance of regional studies in advancing our understanding of permafrost thaw-driven changes to the carbon dynamics of rapidly changing permafrost ecosystems.

Abstract. We present findings from radar calibration experiments involving three radars operated by the Colorado State University (CSU) in the US and by the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. The experiments were based on the comparison between measured radar variables and the known properties of artificial point targets electronically generated with a polarimetric radar target simulator (RTS) from Palindrome Remote Sensing. Radars under test included the two magnetron-based radars CHILL and its mobile version SPLASH from CSU and EPFL's new solid state radar StXPol. For the CHILL and SPLASH calibration measurements in Colorado, a mobile lifting platform was employed that elevated the target simulator instrument to approximately 15 m above ground. The creation of virtual targets with polarimetric signatures allowed for a direct calibration of polarimetric variables. While the SPLASH radar exhibited good Zdr and sufficient Zh accuracy, a remarkable precision and stability was found in CHILL's reflectivity data time series, where the reflectivity bias compared to the virtual target was less than 0.2 dB over a one-hour time series. Calibration issues that arise with solid state radar systems were investigated with experiments conducted with EPFL StXPol radar. This pulse compression system transmits a linear frequency-modulated long pulse as well as a non-modulated short pulse for observations at close ranges. The two pulses are separated in frequency by 50 MHz and consequently calibration targets were generated independently for the two channels. An excellent stability and accuracy was found for Zdr in both channels. While Zh stability was also very high, a large reflectivity bias in both the long and the short pulse channel was detected. For the first time, the article introduces and analyses a weather radar calibration procedure that is based on electronically generated radar targets. Experimental data suggests that precise absolute and differential calibrations can be achieved if data is obtained in an environment free from multi-paths and if the generated targets are precisely located in the center of the radar's range gate. Experimental shortcomings associated with limited sampling resolution of the radar scan over the targets are also investigated.

<strong class="journal-contentHeaderColor">Abstract.</strong> Rock glaciers, composed of debris and ice, are widely distributed across cold mountain regions worldwide. Although research on rock glaciers is gaining momentum, the distinct behaviour of rock glaciers in the marginal periglacial environments remains poorly understood. In this study, we combine remote sensing and in situ methods to gain insights into the characteristics of transitional rock glaciers in the Carpathian Mountains. We applied Persistent Scatterer Interferometry (PSInSAR) to Sentinel-1 images from 2015 to 2020 to identify areas with slope movements associated with rock glaciers and differential GNSS measurements (2019&ndash;2021) to detect the horizontal movement of 25 survey markers. Continuous ground temperature monitoring and measurements of the bottom temperature of the winter snow cover were used to examine the energy exchange fluxes characteristics of transitional rock glaciers in the Carpathians. The subsurface of one transitional rock glacier was investigated using geophysical measurements (electrical resistivity tomography and refraction seismic tomography), while petrophysical joint inversion was used to quantify the ice content. The PSInSAR methodology identified 110 moving areas (MAs) with low displacement rates (&lt; 5 cm yr^-1). These MAs are generally located between 2000 and 2300 meters where solar radiation is minimal. Late winter ground surface temperature data from slow-moving rock glaciers point to permafrost conditions. Geophysical investigations reveal remnants of ice-rich permafrost within the Galeșu rock glacier, while petrophysical joint inversion modelling indicates a low ground ice content (~ 20 %) in its upper sector. The slow surface movement of rock glaciers in the marginal periglacial mountains is driven by the deformation of thin, frozen layers. Regarding activity status, the majority of rock glaciers in the Retezat Mountains are categorized as relict, with only 21 % classified as transitional. The results of our study emphasize the benefit of combining Sentinel-1 SAR data with comprehensive field investigations, particularly in regions with slow-moving rock glaciers.

<strong class="journal-contentHeaderColor">Abstract.</strong> Rock glaciers, composed of debris and ice, are widely distributed across cold mountain regions worldwide. Although research on rock glaciers is gaining momentum, the distinct behaviour of rock glaciers in the marginal periglacial environments remains poorly understood. In this study, we combine remote sensing and in situ methods to gain insights into the characteristics of transitional rock glaciers in the Carpathian Mountains. We applied Persistent Scatterer Interferometry (PSInSAR) to Sentinel-1 images from 2015 to 2020 to identify areas with slope movements associated with rock glaciers and differential GNSS measurements (2019&ndash;2021) to detect the horizontal movement of 25 survey markers. Continuous ground temperature monitoring and measurements of the bottom temperature of the winter snow cover were used to examine the energy exchange fluxes characteristics of transitional rock glaciers in the Carpathians. The subsurface of one transitional rock glacier was investigated using geophysical measurements (electrical resistivity tomography and refraction seismic tomography), while petrophysical joint inversion was used to quantify the ice content. The PSInSAR methodology identified 110 moving areas (MAs) with low displacement rates (&lt; 5 cm yr^-1). These MAs are generally located between 2000 and 2300 meters where solar radiation is minimal. Late winter ground surface temperature data from slow-moving rock glaciers point to permafrost conditions. Geophysical investigations reveal remnants of ice-rich permafrost within the Galeșu rock glacier, while petrophysical joint inversion modelling indicates a low ground ice content (~ 20 %) in its upper sector. The slow surface movement of rock glaciers in the marginal periglacial mountains is driven by the deformation of thin, frozen layers. Regarding activity status, the majority of rock glaciers in the Retezat Mountains are categorized as relict, with only 21 % classified as transitional. The results of our study emphasize the benefit of combining Sentinel-1 SAR data with comprehensive field investigations, particularly in regions with slow-moving rock glaciers.

The 6 February 2023 Kahramanmaraş Earthquakes (Mw 7.8 and Mw 7.6) caused extensive deformation along the East Anatolian Fault Zone, producing significant north-south displacements. Standard InSAR methods face significant challenges in resolving near-field deformation due to phase decorrelation caused by high displacement gradients and heavy snowfall. This study demonstrates the capability of Subswath and Burst Overlap Interferometry (SBOI) which leverages all possible overlap regions of Sentinel-1 TOPS-mode to retrieve along-track deformation, providing improved resolution of 3D displacement fields. We develop a novel methodology to calculate pixel-based scaling factors for spectral separation, enabling accurate deformation measurements in both burst and subswath overlap regions. The SBOI dataset is combined with azimuth and range offset tracking outputs to generate 3D displacement fields and perform coseismic slip inversion using an unsmoothed Bayesian approach. Results reveal significant improvements in constraining north-south deformation, reducing residuals, and decreasing uncertainties in the slip inversion model compared to solutions without SBOI. Linear regression analysis between solutions with and without SBOI demonstrate $\sim$40$\%$ reduction in RMS error for the north-south component of 3D displacement field. Near-field SBOI profiles also capture detailed deformation gradients along fault ruptures, offering insights into fault kinematics and rupture dynamics. This study highlights SBOI as a powerful tool for resolving high-gradient deformations and enhancing 3D displacement fields and geodetic slip models, particularly in regions experiencing large coseismic deformation with significant north-south components, where conventional InSAR and SAR measurements are insufficient.

The identification of spatial soil moisture patterns is of high importance for various applications in high latitude permafrost regions, but challenging with common remote sensing approaches due to high landscape heterogeneity. Seasonal thawing and freezing of near-surface soil lead to subsidence-heave cycles in the presence of ground ice, which can exhibit magnitudes of several centimeters. Our investigations document higher Sentinel-1 InSAR seasonal subsidence rates for locations with higher near-surface soil moisture compared to dryer ones. Based on this, we demonstrate that the relationship of thawing degree days -a measure of seasonal heating -and subsidence signals can be interpreted to assess spatial variations of near-surface soil moisture. A range of challenges, however, need to be addressed. We discuss the implications of using different sources of temperature data for deriving thawing degree days on the results. Atmospheric effects must be considered, as simple spatial filtering can suppress large-scale permafrost-related subsidence signals and lead to the underestimation of displacement values, making GACOS-corrected results preferable for the tested sites. Seasonal subsidence rate retrieval which considers these aspects provides a valuable tool for distinguishing between wet and dry landscape features, which is relevant for permafrost degradation monitoring in Arctic lowland permafrost regions. Spatial resolution constraints, however, remain for smaller typical permafrost features which drive wet versus dry conditions such as high and low centred polygons.

Spatial and temporal variation of the free electron concentration in the ionosphere affects SAR interferograms, in particular at low radar frequencies. In this work the identification, estimation and compensation of ionospheric path delay phases in PALSAR-3 and NISAR-L interferograms is discussed. Both these L-band sensors acquire simultaneously SAR data in a main spectral band and in an additional, spectrally separated, narrower second band to support the mitigation of ionospheric path delays. The methods presented permit separating the dispersive and the non-dispersive phase terms based on the double-difference interferogram between the two available spectral bands and the differential interferogram of the main band. The applicability of the proposed methods is demonstrated using PALSAR-3-like data that was simulated based on PALSAR-2 SM1 mode data.