Exploiting Sentinel-1 amplitude data for glacier surface velocity field measurements. Feasibility demonstration on baltoro glacier

The leading idea of this work is to continuously retrieve glaciers surface velocity through SAR imagery, in particular using the amplitude data from the new ESA satellite sensor Sentinel-1 imagery. These imagery key aspects are the free access policy, the very short revisit time (down to 6 days with the launch of the Sentinel-1B satellite) and the high amplitude resolution (up to 5 m). In order to verify the reliability of the proposed approach, a first experiment has been performed using Sentinel-1 imagery acquired over the Karakoram mountain range (North Pakistan) and Baltoro and other three glaciers have been investigated. During this study, a stack of 11 images acquired in the period from October 2014 to September 2015 has been used in order to investigate the potentialities of the Sentinel-1 SAR sensor to retrieve the glacier surface velocity every month. The aim of this test was to measure the glacier surface velocity between each subsequent pair, in order to produce a time series of the surface velocity fields along the investigated period. The necessary co-registration procedure between the images has been performed and subsequently the glaciers areas have been sampled using a regular grid with a 250 × 250 meters posting. Finally the surface velocity field has been estimated, for each image pair, using a template matching procedure, and an outlier filtering procedure based on the signal to noise ratio values has been applied, in order to exclude from the analysis unreliable points. The achieved velocity values range from 10 to 25 meters/month and they are coherent to those obtained in previous studies carried out on the same glaciers and the results highlight that it is possible to have a continuous update of the glacier surface velocity field through free Sentinel-1 imagery, that could be very useful to investigate the seasonal effects on the glaciers fluid-dynamics

Monitoraggio dei campi di velocità superficiali dei ghiacciai tramite i dati Sentinel-1 Sar: definizione di una metodologia e implementazione del software

I ghiacciai, oltre ad essere una risorsa preziosa per l'umanità, costituiscono uno dei principali indicatori dei cambiamenti climatici su scala locale e globale, essendo influenzati dalle variazioni di temperatura e dall'entità delle precipitazioni. Numerose osservazioni sui cambiamenti di massa, volume, area e lunghezza dei ghiacciai sono state raccolte dal World Glacier Monitoring Service in collaborazione con GLIMS (Global Land Ice Measurements from Space), i cui dati rappresentano uno strumento fondamentale per migliorare la comprensione delle dinamiche e dei comportamenti di queste formazioni. Tra i numerosi parametri osservati, le velocità di scorrimento superficiale dei ghiacciai è uno dei più importanti, in quanto influenzate dagli eventi connessi a cambiamenti nella sua morfologia e struttura dei ghiacciai stessi. Lo scopo di questo lavoro è di presentare un software, sviluppato in ambiente Free and Open Source presso l'Area di Geodesia e Geomatica dell'Università di Roma “La Sapienza”, basato su un'innovativa metodologia per il calcolo dei campi di velocità superficiale di scorrimento dei ghiacciai ottenuta dall'elaborazione di una serie di immagini in ampiezza SAR Sentinel-1A. Diversi test sono stati eseguiti utilizzando immagini acquisite sulle aree della catena montuosa del Karakorum (Pakistan del nord) e sullo Hielo Patagonico Nord (Cile), conseguendo risultati coerenti con gli studi esistenti e mettendo in luce i diversi comportamenti fluidodinamici dei ghiacciai nonché le variazioni stagionali dei flussi. Per stimare gli spostamenti della superficie si è utilizzato il metodo dell'Intensity Offset-Tracking, basato sulla tecnica del template matching. I risultati presentati in questo lavoro confermano l'affidabilità del metodo di Offset-Tracking nel determinare la velocità superficiale e la possibilità di avere un continuo aggiornamento dei campi di velocità attraverso l'impiego delle immagini gratuite Sentinel-1A, offrendo in prospettiva la possibilità dell'istituzione di un servizio sistematico globale finalizzato al monitoraggio dei ghiacciai.Glaciers are among the most reliable indicators of climate change at local and global scale and many studies on the glaciers dynamics have been carried out. Nowadays, the World Glacier Monitoring Service in collaboration with the Global Land Ice Measurements from Space, collects observations on time changes in mass, volume, area and length of glaciers. Among the parameters used for glacier monitoring, the glaciers surface velocity plays a crucial role, since it influences the events connected to glaciers changes. The aim of this work was to present a new Free and Open Source software developed at the Geodesy and Geomatic Division of the University of Roma Sapienza, based on a innovative methodology for glacier surface velocity fields computing, using Sentinel-1A SAR amplitude imagery. Several tests have been performed using Sentinel-1 imagery acquired over the Karakoram mountain (North Pakistan) and the Northen Patagonian Ice Field (Chile) in order to highlight the behaviour of different glaciers also the seasonal fluctuations. To estimate the glacier surface displacement, it has chosen to implement Intensity Offset-tracking technique, well tested and reliable method as testified by the numerous existing applications and based on template matching procedure obtaining results coherent with previous studies. The results presented in this work are coherent to those obtained in previous studies and confirm that SAR Off-set tracking is an effective method to obtain glacier surface velocity and highlight that it is possible to have a continuous update of the glacier surface velocity field through free Sentinel-1 imagery

Dynamic changes on the Wilkins Ice Shelf during the 2006–2009 retreat derived from satellite observations

The vast ice shelves around Antarctica provide significant restraint to the outflow from adjacent tributary glaciers. This important buttressing effect became apparent in the last decades, when outlet glaciers accelerated considerably after several ice shelves were lost around the Antarctic Peninsula (AP). The present study aims to assess dynamic changes on the Wilkins Ice Shelf (WIS) during different stages of ice-front retreat and partial collapse between early 2008 and 2009. The total ice-shelf area lost in these events was 2135 ± 75 km2 ( ∼  15 % of the ice-shelf area relative to 2007). Here, we use time series of synthetic aperture radar (SAR) satellite observations (1994–1996, 2006–2010) in order to derive variations in surface-flow speed from intensity-offset tracking. Spatial patterns of horizontal strain-rate, stress and stress-flow angle distributions are determined during different ice-front retreat stages. Prior to the final break up of an ice bridge in 2008, a strong speed up is observed, which is also discernible from other derived quantities. We identify areas that are important for buttressing and areas prone to fracturing using in-flow and first principal strain rates as well as principal stress components. Further propagation of fractures can be explained as the first principal components of strain rates and stresses exceed documented threshold values. Positive second principal stresses are another scale-free indicator for ice-shelf areas, where fractures preferentially open. Second principal strain rates are found to be insensitive to ice-front retreat or fracturing. Changes in stress-flow angles highlight similar areas as the in-flow strain rates but are difficult to interpret. Our study reveals the large potential of modern SAR satellite time series to better understand dynamic and structural changes during ice-shelf retreat but also points to uncertainties introduced by the methods applied

The glaciers climate change initiative: Methods for creating glacier area, elevation change and velocity products

Glaciers and their changes through time are increasingly obtained from a wide range of satellite sensors. Due to the often remote location of glaciers in inaccessible and high-mountain terrain, satellite observations frequently provide the only available measurements. Furthermore, satellite data provide observations of glacier character- istics that are difficult to monitor using ground-based measurements, thus complementing the latter. In the Glaciers_cci project of the European Space Agency (ESA), three of these characteristics are investigated in detail: glacier area, elevation change and surface velocity. We use (a) data from optical sensors to derive glacier outlines, (b) digital elevation models from at least two points in time, (c) repeat altimetry for determining elevation changes, and (d) data from repeat optical and microwave sensors for calculating surface velocity. For the latter, the two sensor types provide complementary information in terms of spatio-temporal coverage. While (c) and (d) can be generated mostly automatically, (a) and (b) require the intervention of an analyst. Largely based on the results of various round robin experiments (multi-analyst benchmark studies) for each of the products, we suggest and describe the most suitable algorithms for product creation and provide recommendations concerning their practical implementation and the required post-processing. For some of the products (area, velocity) post-processing can influence product quality more than the main-processing algorithm

Area and mass changes of Siachen Glacier (East Karakoram)

The authors thank the European Space Agency for providing the Envisat data under the AOE 668 project. T. Bolch and T. Strozzi acknowledge funding by the European Space Agency (ESA) within the Glaciers_cci project (code 4000109873/14/I-NB).Here, we present a comprehensive assessment of Siachen Glacier (East Karakoram), in terms of its area and elevation change, velocity variations and mass budget, utilizing different satellite datasets including Landsat, Hexagon, Cartosat-I, Shuttle Radar Topography Mission, Envisat Advanced Synthetic Aperture Radar and Japanese Advanced Land Observing Satellite Phased Array-type L-band SAR. The total areal extent of Siachen Glacier did not change significantly between 1980 and 2014; however the exposed-ice area decreased during that period. The terminus of the glacier has experienced substantial downwasting (on average 30 m) over the period of 1999-2007, followed by a retreat of the transition between exposed and debris-covered ice by a distance of 1.3 km during the short span 2007-14. The spatial patterns of the elevation difference and velocity are heterogeneous over the large areal extent of Siachen Glacier. The average velocity of the entire glacier, as computed between 11 December 2008 and 26 January 2009, was 12.3 ± 0.4 cm d -1 , while those estimated separately for the accumulation and ablation regions were 9.7 ± 0.4 cm d -1 and 20.4 ± 0.4 cm d -1 , respectively. The mass budget of Siachen Glacier is estimated to be -0.03 ± 0.21 m w.e. a -1 for the period of 1999-2007.Publisher PDFPeer reviewe

Remote Sensing of Mountain Glaciers and Related Hazards

Mountain glaciers are highly sensitive to temperature and precipitation fluctuations and active geomorphic agents in shaping the landforms of glaciated regions which are direct imprints of past glaciations, providing reliable evidence of the evolution of the past Cryosphere and contain important information on climatic variables. But most importantly, glaciers have aroused a lot of concern in terms of glacier area changes, thickness change, mass balance and their consequences on water resources as well as related hazards. The contribution of glacier mass loss to global sea-level rise and increasing number of glacier-related hazards are the most important and current socioeconomic concerns. Therefore, understanding the dynamics of the changes and constant monitoring of glaciers are essential for studying climate, water resource management and hydropower and also to predict and evade glacier-related hazards. The recent advances in the techniques of earth observations have proved as a boon for investigating glaciers and glacier-related hazards. Remote sensing technology enables extraction of glacier parameters such as albedo/reflectance/scattering, glacier area, glacier zones and facies, equilibrium line, glacier thickness, volume, mass balance, velocity and glacier topography. The present chapter explores the prospective of remote sensing technology for understanding and surveying glaciers formed at high, inaccessible mountains and glacier-induced hazards

Multi-Temporal X-Band Radar Interferometry Using Corner Reflectors: Application and Validation at the Corvara Landslide (Dolomites, Italy)

From the wide range of methods available to landslide researchers and practitioners for monitoring ground displacements, remote sensing techniques have increased in popularity. Radar interferometry methods with their ability to record movements in the order of millimeters have been more frequently applied in recent years. Multi-temporal interferometry can assist in monitoring landslides on the regional and slope scale and thereby assist in assessing related hazards and risks. Our study focuses on the Corvara landslides in the Italian Alps, a complex earthflow with spatially varying displacement patterns. We used radar imagery provided by the COSMO-SkyMed constellation and carried out a validation of the derived time-series data with differential GPS data. Movement rates were assessed using the Permanent Scatterers based Multi-Temporal Interferometry applied to 16 artificial Corner Reflectors installed on the source, track and accumulation zones of the landslide. The overall movement trends were well covered by Permanent Scatterers based Multi-Temporal Interferometry, however, fast acceleration phases and movements along the satellite track could not be assessed with adequate accuracy due to intrinsic limitations of the technique. Overall, despite the intrinsic limitations, Multi-Temporal Interferometry proved to be a promising method to monitor landslides characterized by a linear and relatively slow movement rates

Deriving glacier surface velocities from repeat optical images

The velocity of glaciers is important for many aspects in glaciology. Mass accumulated in the accumulation area is transported down to the ablation area by deformation and sliding due to the gravitational force, and hence gla­cier velocity is connected to the mass balance of glaciers. It also contributes directly to the mass balance of calving glaciers because it is an important control of the ice discharge rate for such glaciers. Changing glacier velocities is an indicator of instable glaciers, and monitoring velocity over time can make people aware of possible hazards that may arise from instable glaciers. The movement of glaciers is also important for transporting material and for eroding the landscape. The focus of this thesis is to further develop image matching within glaciology. In image matching, images from two di.erent times are compared us­ing correlation techniques to derive glacier displacement over the time period. Most studies have concentrated on using image matching to derive glacier velocities instead of developing this method further. To be able to derive the densest possible velocity grids for all glaciers in the world, image matching methods over glacier surfaces have to be explored further. So far all images that have been used to derive velocity in glaciology have been high or medium spatial resolution images. Low resolution images cover large sections in one image, and this makes them suited for investigating the velocity of large areas such as Antarctic ice shelves. We derive velocities for Antarctic ice shelves using MODIS images with a spatial resolution of 250 m to test whether these images are suited for deriving ice shelf velocity. Because the accuracy is about one fourth of a pixel, and it is possible to use images acquired several years apart due to the low surface transformation, MODIS images are well suited for deriving velocity of Antarctic ice shelves and also to monitor their changes over time. We found when comparing di.erent image matching methods over dif­ferent glacier surfaces that the most commonly used method, normalized cross-correlation, generally performs worse compared to orientation correla­tion and the matching part of the program COSI-Corr. The only situation where normalized cross-correlation outperforms the two other methods are on narrow glaciers where small window sizes are needed. COSI-Corr per­forms best overall, but orientation correlation performs almost as well. In addition orientation correlation is the only method that manages to match striped Landsat images after the failure of the Scan Line Corrector. Both orientation correlation and COSI-Corr are considered to be methods well suited for global glacier velocity mapping. Normalized cross-correlation can supplement these two methods on narrow glaciers. The effort that has been put into developing image matching in glaciology since the start of this study, both in this study and in other studies, makes it possible to derive glacier velocities over large regions, and only computer processing time hinders automatic matching of glacier velocities worldwide. Global glacier velocities can give valuable insights. We show in this thesis that it can give information about how glaciers respond to climate change. Glacier velocity of .ve regions of the world with negative mass balance is derived, and in all regions the general glacier speed is decreasing over the last decades. In addition global glacier velocities can be used to understand glacier dynamics, and predict glacier hazards. It can be tested against gla­cier inventory parameters, and it can be used to estimate erosion rates and transport times

Calibration of DART Radiative Transfer Model with Satellite Images for Simulating Albedo and Thermal Irradiance Images and 3D Radiative Budget of Urban Environment

Remote sensing is increasingly used for managing urban environment. In this context, the H2020 project URBANFLUXES aims to improve our knowledge on urban anthropogenic heat fluxes, with the specific study of three cities: London, Basel and Heraklion. Usually, one expects to derive directly 2 major urban parameters from remote sensing: the albedo and thermal irradiance. However, the determination of these two parameters is seriously hampered by complexity of urban architecture. For example, urban reflectance and brightness temperature are far from isotropic and are spatially heterogeneous. Hence, radiative transfer models that consider the complexity of urban architecture when simulating remote sensing signals are essential tools. Even for these sophisticated models, there is a major constraint for an operational use of remote sensing: the complex 3D distribution of optical properties and temperatures in urban environments. Here, the work is conducted with the DART (Discrete Anisotropic Radiative Transfer) model. It is a comprehensive physically based 3D radiative transfer model that simulates optical signals at the entrance of imaging spectro-radiometers and LiDAR scanners on board of satellites and airplanes, as well as the 3D radiative budget, of urban and natural landscapes for any experimental (atmosphere, topography,…) and instrumental (sensor altitude, spatial resolution, UV to thermal infrared,…) configuration. Paul Sabatier University distributes free licenses for research activities. This paper presents the calibration of DART model with high spatial resolution satellite images (Landsat 8, Sentinel 2, etc.) that are acquired in the visible (VIS) / near infrared (NIR) domain and in the thermal infrared (TIR) domain. Here, the work is conducted with an atmospherically corrected Landsat 8 image and Bale city, with its urban database. The calibration approach in the VIS/IR domain encompasses 5 steps for computing the 2D distribution (image) of urban albedo at satellite spatial resolution. (1) DART simulation of satellite image at very high spatial resolution (e.g., 50cm) per satellite spectral band. Atmosphere conditions are specific to the satellite image acquisition. (2) Spatial resampling of DART image at the coarser spatial resolution of the available satellite image, per spectral band. (3) Iterative derivation of the urban surfaces (roofs, walls, streets, vegetation,…) optical properties as derived from pixel-wise comparison of DART and satellite images, independently per spectral band. (4) Computation of the band albedo image of the city, per spectral band. (5) Computation of the image of the city albedo and VIS/NIR exitance, as an integral over all satellite spectral bands. In order to get a time series of albedo and VIS/NIR exitance, even in the absence of satellite images, ECMWF information about local irradiance and atmosphere conditions are used. A similar approach is used for calculating the city thermal exitance using satellite images acquired in the thermal infrared domain. Finally, DART simulations that are conducted with the optical properties derived from remote sensing images give also the 3D radiative budget of the city at any date including the date of the satellite image acquisition