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

Glacier Velocity Monitoring by Maximum Likelihood Texture Tracking

The performance of a tracking algorithm considering remotely sensed data strongly depends on a correct statistical description of the data, i.e. its noise model. The objective of this paper is to introduce a new intensity tracking algorithm for SAR (synthetic aperture radar) data, considering its multiplicative speckle/noise model. The proposed tracking algorithm is discussed regarding the measurement of glacier velocities. Glacier monitoring exhibits complex spatial and temporal dynamics including snowfall, melting and ice flows at a variety of spatial and temporal scales. Due to these complex characteristics, most traditional methods based on SAR suffer from speckle decorrelation that results in a low signal-to-noise ratio (SNR). The proposed tracking technique improves the accuracy of the classical intensity tracking technique by making use of the temporal speckle structure. Even though a new intensity based matching algorithm is proposed especially for incoherent data sets, the analysis of the proposed technique was also performed for correlated data sets. As it is demonstrated, the velocity monitoring can be continuously performed by using the maximum likelihood texture tracking without any assumption concerning the correlation of the dataset. The maximum likelihood texture tracking approach was tested on ENVISAT-ASAR data, acquired during summer 2004 over the Inyltshik glacier in Kyrgyzstan, representing one of the largest alpine glaciers systems of the world. It will be demonstrated that the proposed technique is capable of robustly and precisely detecting the surface velocity field and velocity changes in time