Search

This article introduces the extended timing annotation dataset (ETAD) product for Sentinel-1 (S-1) which was developed in a joint effort of German Aerospace Center (DLR) and European Space Agency (ESA). It allows to correct range and azimuth timing of S-1 images for geophysical effects and for inaccuracies in synthetic aperture radar (SAR) image focusing. In combination with the precise orbit solution, these effects determine the absolute geolocation accuracy of S-1 SAR images and the relative collocation accuracy of repeat pass image stacks. ETAD contains the gridded timing corrections for the tropospheric and ionospheric path delays, the tidal-based surface displacements, and the SAR processing effects, all of which are computed for each data taken using standard models from geodesy and auxiliary atmospheric data. The ETAD product helps S-1 users to significantly improve the geolocation accuracy of the S-1 SAR products to better than 0.2 m and offers a potential solution for correcting large-scale interferometric phase variations. The product layout and product generation are described schematically. This article also reports first the results for different SAR techniques: first, the improvement in geolocation accuracy down to a few centimeters by verification of accurately surveyed corner reflector positions in the range–azimuth plane; second, the well-established offset-tracking technique, which is used for systematic ice velocity monitoring of ice sheets and glaciers, where ETAD can reduce velocity biases down to subcentimetric values; and third, the correction of atmospheric phase contributions in wide-area interferograms used for national and European ground motion services. These early results proof the added value of the ETAD corrections and that the product design is well-suited to be integrated into the processing flows of established SAR applications such as absolute ranging of targets, speckle/feature tracking, and interferometry. [ABSTRACT FROM AUTHOR]

Glacier calving front position (CFP) is an important glaciological variable. Traditionally, delineating the CFPs has been carried out manually, which was subjective, tedious, and expensive. Automating this process is crucial for continuously monitoring the evolution and status of glaciers. Recently, deep learning approaches have been investigated for this application. However, the current methods get challenged by a severe class imbalance problem. In this work, we propose to mitigate the class imbalance between the calving front class and the noncalving front class by reformulating the segmentation problem into a pixelwise regression task. A convolutional neural network (CNN) gets optimized to predict the distance values to the glacier front for each pixel in the image. The resulting distance map localizes the CFP and is further postprocessed to extract the calving front line. We propose three postprocessing methods, one method based on statistical thresholding, a second method based on conditional random fields (CRFs), and finally the use of a second U-Net. The experimental results confirm that our approach significantly outperforms the state-of-the-art methods and produces accurate delineation. The second U-Net obtains the best performance results, resulting in an average improvement of about 21% Dice coefficient enhancement. [ABSTRACT FROM AUTHOR]

Since the 1960s, remote sensing (as an innovative, comprehensive, and interdisciplinary academic area) has been adopted in a wide range of disciplines related to Earth observation, including hydrology, ecology, oceanography, glaciology, geology, military, intelligence, business, economy, and planning –. The constant development of the remote sensing image acquisition technology now allows for the collection of a wide variety of images with different characteristics and resolutions, obtained by remote sensing instruments mounted on spacecraft or aircraft platforms. These images record some type of signal or energy measured from the Earth’s surface, which depends on the type of sensor used. [ABSTRACT FROM AUTHOR]

Multichannel radar depth sounding systems are able to produce two-dimensional (2D) and three-dimensional (3D) imagery of the internal structure of polar ice sheets. Information such as ice thickness and surface elevation is extracted from these data and applied to research in ice flow modeling and ice mass balance calculations. Due to a large amount of data collected, we seek to automate the ice-bottom layer tracking and allow for efficient manual corrections when errors occur in the automated method. We present improvements made to previous implementations of the Viterbi and sequential tree-reweighted message passing (TRW-S) algorithms for ice-bottom extraction in 2D and 3D radar imagery. These improvements are in the form of novel cost functions that allow for the integration of further domain-specific knowledge into the cost calculations and provide additional evidence of the characteristics of the ice sheets surveyed. Along with an explanation of our modifications, we demonstrate the results obtained by our modified implementations of the two algorithms and by previously proposed solutions to this problem, when compared to manually corrected ground truth data. Furthermore, we perform a self-assessment of tracking results by analyzing differences in the estimated ice-bottom for surveyed locations where flight paths have crossed and, thus, two separate measurements have been made at the same location. Using our modified cost functions and preprocessing routines, we obtain significantly decreased mean error measurements from both algorithms, such as a 47% reduction in average tracking error in the case of 3D imagery between the original and our proposed implementation of TRW-S. [ABSTRACT FROM AUTHOR]

Over the last 30 years, the use of airborne and satellite remote sensing techniques has revolutionized glaciology through unequaled improvements in the scale and in the temporal and spatial resolutions of cryospheric observations. In this sense, the use of synthetic aperture radar (SAR) configurations likely was the greatest advance of the past two decades in the monitoring of the Earth's cryosphere. This paper describes a novel polarimetric scattering model to interpret polarimetric SAR (PolSAR) measurements of glaciers and ice sheets. Different scattering components are considered according to the specific glacier facies, in order to infer physical parameters of the ice volume. Total backscattering is modeled as the incoherent sum of possible surface and volume contributions. Surface scattering is described using the X-Bragg model in order to account for roughness, whereas clouds of spheroidal scatterers (either prolates or oblates) are considered to model volume scattering. The model includes differential propagation effects related to firn anisotropy and is therefore able to interpret polarimetric phase differences often present in PolSAR data acquired over glaciers. The developed model is used to interpret different scenarios imaged at L-band in fully polarimetric mode by the E-SAR sensor of the German Aerospace Center (DLR) over test sites located on the Austfonna ice cap, Svalbard, Norway, as part of the ICESAR2007 campaign. The obtained results are consistent with the reference information provided by the available ground measurements about the subsurface structure of the study area and encourage the use of dedicated scattering models for the different glacier zones. [ABSTRACT FROM PUBLISHER]

Geophysical surveys have played a fundamental role in glaciology, climatology, and geology studies conducted at and around the site of the EPICA Dome C Ice Core (Antarctica). Analysis of radio-echo sounding (RES) data collected during the 2009 and 2011 Italian Antarctic Expeditions, in the immediate vicinity (2 km) of the Core site, reveals the presence of small-scale bedrock structures. Data acquired during the 2011 campaign, which applied the latest updates of our instrument, show double echoes from the ice bottom separated by very small time intervals. These reflections are distributed close to the topographic dome and do not always correspond to particular features of the bedrock structure, but instead may correspond to small-scale subglacial water storage. The unprecedented spatial resolution of the basal environment for Dome C from these recent surveys allows us to identify this and other features that have been missed by previous larger scale RES surveys of this area. [ABSTRACT FROM PUBLISHER]

Historical variations in the extent of mountain glaciers give insight into natural and forced changes of these bellwethers of the climate. Because of the limited number of ground observations relative to the number of glaciers, it is useful to develop techniques that permit for the monitoring of glacier systems using satellite imagery. Here, we propose a new approach for identifying the glacier terminus over time from Landsat images. The proposed method permits for detecting inflection points in multispectral satellite imagery taken along a glacier's flow path in order to identify candidate terminus locations. A gated tracking algorithm is then applied to identify the best candidate for the glacier terminus location through time. Finally, the long-term trend of the terminus position is estimated with uncertainty bounds. This is achieved by applying nonparametric regression to the temporal sequence of estimated terminus locations. The method is shown to give results consistent with ground-based observations for the Franz Josef and Gorner glaciers and is further applied to estimate the retreat of Viedma, a glacier with no available ground measurements. [ABSTRACT FROM PUBLISHER]

The automated generation of a digital elevation model over the Antarctic using stereo matching high-resolution satellite images is a challenging task. Moreover, the homogeneous radiometry in the icy environment and the strong geometric dissimilarity between stereo pairs over the steep terrain limit the use of area-based matching techniques. To overcome this issue, we propose template matching with image transformation in order to reduce the geometric dissimilarity. First, we generated epipolar resampled images to ensure the ease of estimation and handling of image dissimilarities from various viewing directions. We then utilized the normalized cross-correlation (NCC) and transformed the image patches within the matching window along the sample, line, and diagonal directions in order to improve the match rates within the steep areas. Furthermore, we tested the proposed method using Antarctic IKONOS stereo images and found that the overall matching success rate improved from 93.5% to 97.0% for all image pixels. We then computed the success rates over an area in which the NCC produced a low elevation point density and observed a more significant improvement from 58.7% to 79.26%. When compared to the manually generated elevation, the maximum vertical difference improved from 11.4 to 4.7 m. With these improvements, we can build a 1-m resolution elevation model over the glaciated high relief terrain. [ABSTRACT FROM PUBLISHER]

We investigate the degree to which ultrahigh frequency radio emission can be used to estimate subsurface physical temperature in the polar ice sheets. We combine electromagnetic emission forward models with plausible models of depth-dependent physical properties in the ice sheet. Temperature models are parameterized with variables including accumulation rate, geothermal heat flux, and surface temperature. Scattering is parameterized using empirical observations of grain growth combined with measured densities. Electromagnetic absorption is modeled using dielectric dispersion processes and semiempirical models based on observations. Our models illustrate that information about East Antarctic ice sheet temperature from near the surface to near the base can be gleaned from ultrawideband radiometer data. Based on our modeling study, we illustrate an instrument concept to measure ice sheet temperature profiles comprising a novel ultrawideband radiometer. [ABSTRACT FROM PUBLISHER]

Snow accumulation over an ice sheet is the sole mass input, making it a primary measurement for understanding the past, present, and future mass balance. Near-surface frequency-modulated continuous-wave (FMCW) radars image isochronous firn layers recording accumulation histories. The Semiautomated Multilayer Picking Algorithm (SAMPA) was designed and developed to trace annual accumulation layers in polar firn from both airborne and ground-based radars. The SAMPA algorithm is based on the Radon transform (RT) computed by blocks and angular orientations over a radar echogram. For each echogram's block, the RT maps firn segmented-layer features into peaks, which are picked using amplitude and width threshold parameters of peaks. A backward RT is then computed for each corresponding block, mapping the peaks back into picked segmented-layers. The segmented layers are then connected and smoothed to achieve a final layer pick across the echogram. Once input parameters are trained, SAMPA operates autonomously and can process hundreds of kilometers of radar data picking more than 40 layers. SAMPA final pick results and layer numbering still require a cursory manual adjustment to correct noncontinuous picks, which are likely not annual, and to correct for inconsistency in layer numbering. Despite the manual effort to train and check SAMPA results, it is an efficient tool for picking multiple accumulation layers in polar firn, reducing time over manual digitizing efforts. The trackability of good detected layers is greater than 90%. [ABSTRACT FROM PUBLISHER]

Automatic weather stations (AWSs) are widely used for an environmental sensing in harsh environments such as Antarctica, high mountains, and deserts. As these systems are often deployed far from mains power sources, they are usually equipped with rechargeable batteries and energy harvesting systems. Predeployment configuration of an AWS is a challenging task, as designers have to face with contrasting energy-related choices, the correct tradeoff of which determines the success of the AWS's mission and its survivability. Among them, the most effective are the energy harvesting technology, size of the battery, and frequency of sensing and communication. In this paper, we describe AENEAS, an energy-aware simulator of AWSs that allows designers to assess the impact of hardware and software choices on the energy evolution of the system. The tool is extensively configurable, thus enabling the simulation of a large number of hardware configurations as well as of the sensing and communication applications running on the AWS. The simulator has been validated by comparing results computed by AENEAS with data collected from two real-world AWSs installed on an alpine glacier and in a urban environment, obtaining a high accuracy in both cases. A number of use cases are discussed to demonstrate how AENEAS can be used to assess the impact on the energy behavior of the AWS of different batteries, energy harvesters, and application behaviors. [ABSTRACT FROM PUBLISHER]

This paper provides an overview of a wide area wireless sensor network that was deployed on the calving front of the Helheim Glacier in Greenland during the summer of 2013. The purpose of the network was to measure the flow rate of the glacier using accurate satellite positioning data. The challenge in this extreme environment was to collect data in real time at the calving edge of the glacier. This was achieved using a solar powered 2.4-GHz Zigbee wireless sensor network operated in a novel hybrid cellular/mesh access architecture consisting of ice nodes communicating with base stations placed on the rock adjacent to the glacier. This highly challenging transmission environment created substantial signal outage conditions, which were successfully mitigated by a radio network diversity scheme. The network development and measurement campaign were highly successful yielding significant results on glacial dynamics associated with climate change. [ABSTRACT FROM PUBLISHER]

Passive microwave remote sensing is extensively used in polar regions to study the cryosphere. To better understand the measured signal above continental ice-covered areas, our objective is to estimate the microwave emission of bubbly-ice surfaces using a physically based multilayer electromagnetic model, i.e., the dense media radiative transfer-multilayer model (DMRT-ML). This model accounts for ice layers with variable amounts of bubbles. Each layer is fully described by its temperature, density, thickness, and air bubble radius. Simulations are performed using in situ data from two distinct sites: one in Antarctica on a coastal Blue Ice Area near the Cap Prud'Homme (CPH) station in Adelie Land and the other on the Barnes Ice Cap (BIC) located on Baffin Island in the Arctic. On this ice cap, superimposed ice with seasonal snow cover about 1 m thick was observed. In both cases, several ice parameters were measured or estimated, and the others were optimized. Results of the DMRT-ML simulations are compared with in situ surface-based radiometer (SBR) measurements at 11, 19, and 37 GHz at both horizontal and vertical polarizations. Results show that DMRT-ML is able to reproduce the microwave emission of different ice types with good accuracy when accounting for ice bubbles: final RMSE = 7.37 K and 8.42 K, for CPH and BIC, respectively, compared with RMSE ranging from 15 K to 40 K without bubbles. Comparisons between SBR measurements and satellite data for the BIC also show good agreement (RMSE = 4.1 K for 19 and 37 GHz, both polarizations). [ABSTRACT FROM PUBLISHER]

We describe a lightweight wideband C-band radar sounder, which is designed for use in a small unmanned aerial vehicle, primarily for measuring snow and ice. The waveform used is a gated frequency modulated continuous wave, which enables transmission at low power with relatively high energy in the compressed pulse. Gating allows use of a single antenna, reducing the influence of the direct wave on the systems' dynamic range. The radar operates at 5.3-GHz center frequency and has a bandwidth of 1 GHz, for a nominal range resolution of 15 cm in air or 12 cm in snow. Laboratory testing and airborne acquisition of data over the glacier Hardangerjøkulen show that the radar appears to work well for the intended purpose of measuring snow layer thickness. [ABSTRACT FROM PUBLISHER]

Differential synthetic aperture radar interferometry (D-InSAR) is a powerful method for measuring surface deformation, such as in studies of the earthquake cycle, volcano deformation monitoring, land subsidence monitoring, and glaciological studies. However, its application to glaciological studies is limited by the lack of accurate digital elevation models (DEMs), particularly over the Antarctic ice sheet. Previous studies on ice motion using D-InSAR are mostly based on short-baseline interferograms because these data sets are insensitive to DEM errors. Unfortunately, short-baseline interferograms are often unavailable. In this paper, we refine the InSAR technique by using a combination of two interferograms to make accurate ice-flow velocity measurements. The refined technique is tested in the Grove Mountains area, East Antarctica. Ice-flow velocities from the baseline-combination method are in good agreement with those measured by short-baseline interferograms. This method is also capable of reducing phase errors by combining the appropriate data sets. The reliability of the data sets is assessed by defining a baseline-combination parameter and ensuring that it is less than or equal to 1.0. With this method, we are able to extend the usefulness of D-InSAR for glaciological studies. [ABSTRACT FROM PUBLISHER]

In this paper, 3 years of surface displacement measurements obtained by space-borne synthetic aperture radar (SAR) observations are presented over the Argentière glacier in the Mont-Blanc massif, France. This temperate glacier is instrumented by a network of four Global Positioning System (GPS) stations used as ground truth. Thirty-eight pairs of descending and ascending high-resolution TerraSAR-X (TSX) acquisitions covering the study region are used to derive displacement fields at 11-day intervals in spring and summer 2009 and summer 2011. The combination of ascending and descending pairs acquired over the same period allows 3-D displacement fields to be inverted. Our SAR analysis quantifies displacement rates from 10 cm/day at the altitude of 2600 m to 30 cm/day at the altitude of 1800 m. Time series of SAR displacement results are compared with in situ GPS measurements of a continuous station set up at the altitude of 2441 m. Both data sources present displacement of the same order of magnitude with an average value of 20 cm/day in 3-D and show intra-seasonal variabilities, with fast accelerations over short time intervals. [ABSTRACT FROM PUBLISHER]

It has been well known that a few areas on the Earth's surface have a relatively constant backscattering coefficient and can serve as radar calibration targets. Examples include the Amazon rain forest, the Greenland ice sheet, parts of Antarctica, etc. However, there has not been any extensive investigation and quantitative evaluation of these targets in terms of their time variation, isotropy, and spatial variation. Here, we have analyzed a consistent set of Ku-band radar measurements for more than ten years from the QuikSCAT mission. This valuable set of data provides an unprecedented opportunity for us to study the long-term variability of observed backscattering from the Earth's surface at Ku-band. In this paper, we performed a global survey of potential constant land targets and evaluated their variability. Quantitative measurements of temporal and spatial variabilities (homogeneity) and isotropy are used to identify the locations of the best natural calibration targets. We also discuss annual and long-term trends in the data and offer potential explanations for these trends. We examine small regions with the least overall variation. By concentrating on low-variation areas, we can identify useful calibration targets for future radar missions and for intercalibration between existing radars. At the same time, by focusing on regions with little spatial or temporal heterogeneity, we can analyze the temporal variation on diurnal, seasonal, and decadal scales in homogenous natural terrain types including rain forest, dry brushy areas, and ice sheets. We found that rain forest targets in the Amazon and Congo are very stable in time and homogeneous. However, they are subjected to diurnal difference. On the other hand, the Antarctica ice sheet is another good candidate for stable target, but it has seasonal variability. The Greenland ice sheet shows a significant trend in backscatter in recent years, and therefore, may not be a suitable calibration site anymore. Another location for a good stable target is a dry brushy area in the Sahara, which shows comparable stability and isotropy with those of the Amazon, Congo, and Antarctica. [ABSTRACT FROM PUBLISHER]

This paper presents a processing chain for the change detection of Arctic glaciers from multitemporal multipolarization synthetic aperture radar (SAR) images. We produce terrain-corrected multilook complex covariance data by including the effects of topography on both geolocation and SAR radiometry as well as azimuth slope variations on polarization signature. An unsupervised contextual non-Gaussian clustering algorithm is employed for the segmentation of each terrain-corrected polarimetric SAR image and subsequently labeled with the aid of ground-truth data into glacier facies. We demonstrate the consistency of the segmentation algorithm by characterizing the expected random error level for different SAR acquisition conditions. This allows us to determine whether an observed variation is statistically significant and therefore can be used for the postclassification change detection of Arctic glaciers. Subsequently, the average classified images of succeeding years are compared, and changes are identified as the detected differences in the location of boundaries between glacier facies. In the current analysis, a series of dual-polarization C-band ENVISAT ASAR images over the Kongsvegen glacier, Svalbard, is used for demonstration. [ABSTRACT FROM PUBLISHER]

This paper presents a radar sensor package specifically developed for wide-coverage sounding and imaging of polar ice sheets from a variety of aircraft. Our instruments address the need for a reliable remote sensing solution well-suited for extensive surveys at low and high altitudes and capable of making measurements with fine spatial and temporal resolution. The sensor package that we are presenting consists of four primary instruments and ancillary systems with all the associated antennas integrated into the aircraft to maintain aerodynamic performance. The instruments operate simultaneously over different frequency bands within the 160 MHz-18 GHz range. The sensor package has allowed us to sound the most challenging areas of the polar ice sheets, ice sheet margins, and outlet glaciers; to map near-surface internal layers with fine resolution; and to detect the snow-air and snow-ice interfaces of snow cover over sea ice to generate estimates of snow thickness. In this paper, we provide a succinct description of each radar and associated antenna structures and present sample results to document their performance. We also give a brief overview of our field measurement programs and demonstrate the unique capability of the sensor package to perform multifrequency coincidental measurements from a single airborne platform. Finally, we illustrate the relevance of using multispectral radar data as a tool to characterize the entire ice column and to reveal important subglacial features. [ABSTRACT FROM AUTHOR]

A large archive of digital elevation model (DEM) data spanning more than a decade is now available for a wide range of earth surface process studies. Precise coregistration is a critical step for obtaining accurate elevation change measurements from repeat DEMs. We present an algorithm for coregistering stereoscopic gridded DEMs that includes an automatic detection of control surfaces, a reliable estimation technique for defining a geometric relationship between DEMs through a 3-D conformal transformation, and a quality check (QC) for the confidence of the coregistration. For the control surface detection, we use a geometric constraint approach that uses similarities in slope, aspect, and height undulation as observation weights in an iterative transformation model solution. The QC is possible through the evaluation of the geometric similarities and the amount of detected relatively unchanged surfaces. We test our algorithm using a sample of repeat stereoscopic gridded DEMs from multiple sources over a rapidly changing glacier in Greenland. The algorithm results, on average, in a 24% decrease in the standard error, as obtained from elevation over stationary surfaces. The result is not sensitive to the distribution of the control surface. The QC metric provides a simple and effective criterion for parsing result quality in large data sets. Surface height decreases of ~ 110 and 200 m at the up-glacier and down-glacier ends of Kangerdlugssuaq Glacier in Greenland are observed between April 2001 and June 2010, respectively. [ABSTRACT FROM PUBLISHER]

The C-band EUMETSAT Advanced Scatterometer (ASCAT) was launched in 2007, and provides backscatter measurements with excellent azimuth and incidence angle diversity in polar regions. C-band backscatter measurements can be used to retrieve near-surface snow/firn properties such as grain size and accumulation rate, however the Antarctic Ice Sheet (AIS) exhibits strong microwave backscatter anisotropy as a function of both incidence and azimuth angles, and this anisotropy must be well understood before such physical parameter retrieval can be accurately performed. This paper presents a detailed comparison of several different parameterizations of both the azimuth and the incidence anisotropy. Parameterizations used here result in residuals similar to the radiometric performance of ASCAT, indicating accurate characterization of the anisotropy. The analysis of these parameterizations provides an insight into the physical mechanisms taking place near the surface of the AIS. 30-day composite maps of several parameters are presented, and discussed in the context of near-surface glaciology. This anisotropy characterization will form the basis of a technique for extraction of physical parameters of the AIS. [ABSTRACT FROM PUBLISHER]

This paper describes the calibration of the CryoSat-2 interferometer, whose principal purpose is to accurately measure the height of the Antarctic and Greenland ice sheets. A sequence of CryoSat-2 data acquisitions over the tropical and midlatitude oceans were obtained between June and September 2010, from the SIRAL “A” and redundant SIRAL “B” radars operating in their “SARIN” mode, during a sequence of satellite rolls between -0.6° and 0.4°. Using the arrival angle of the echo relative to the interferometer baseline, the attitude of the satellite determined by the star trackers, and estimates of the ocean surface across-track slope from the EGM08 geoid, we determined the errors in the interferometer estimate of surface slope as functions of the roll angle and ocean surface waveheight. These were found to be in close agreement with the theoretical description. The scale factor of the interferometric measurement of angle was determined to be 0.973 ± 0.002. We estimate the accuracy of the across-track slope measurement of the interferometer by applying this scale factor to the measured phase. In applying this scale factor to the measurements, the across-track slope of the marine geoid was obtained with an accuracy of 26 μrad at 10 km and 10 μrad at 1000 km. We conclude that the instrument performance considerably exceeds that needed for the accurate determination of height over the sloping surfaces of the continental ice sheets. The results also demonstrate that CryoSat-2 provides the first observations of the instantaneous vector gradient of the ocean surface, and that the normal-incidence interferometric configuration has a greater potential for the measurement of the ocean across-track slope than has been previously recognized. [ABSTRACT FROM AUTHOR]

A suite of sensors has been assembled to map surface elevation and topography with fine resolution from small unmanned aircraft systems. The sensor package consists of a light detecting and ranging (LIDAR) instrument, an inertial measurement unit (IMU), a Global Positioning System (GPS) module, and digital still and video cameras. The system has been utilized to map ice sheet topography in Greenland and to measure sea ice freeboard and roughness in Fram Strait off the coast of Svalbard and in the Southern Ocean near McMurdo, Antarctica. The elevation measurement accuracy is found to be <; 10 cm (1σ) when short-baseline differential GPS processing is used to position the aircraft, and IMU data are used to correct for off-nadir pointing of the LIDAR. The system is optimized to provide coincident surface topography measurements and imagery of ice sheets, glaciers, and sea ice, and it has the potential to become a widely distributed observational resource to complement manned-aircraft and satellite missions. [ABSTRACT FROM AUTHOR]

The main phase of the moderately sized November 2004 eruption of the Grímsvötn volcano, located in the center of the 8100 km2 Vatnajökull glacier, was monitored by the Icelandic Meteorological Office C-band weather radar in Keflavík, 260 km west of the volcano. The eruption plume reached a height of 6-10 km relative to the vent. The distribution of the most distal tephra was measured in the autumn of 2004, while the deposition on the glacier was mapped in the summers of 2005 and 2006. The tephra formed a well-defined layer on the glacier in the region north and northeast of the craters. The total mass of the tephra layer is quantitatively compared with the retrieved values, obtained from an improved version of the volcanic ash radar retrieval (VARR) algorithm. VARR was statistically calibrated with ground-based ash size distribution samples, taken at Vatnajökull, and by taking into account both antenna beam occlusion and wind-driven plume advection. The latter was implemented by using a space-time image phase-based cross-correlation technique. Accuracy of the weather radar records was also reviewed, noting that a large variability in the plume height estimation may be obtained using different approaches. The comparisons suggest that, at least for this subglacial eruption, the surface tephra mass, estimated by using the VARR inversion approach, is in a fairly good agreement with in situ measurements in terms of spatial extension, distribution, and amount. [ABSTRACT FROM AUTHOR]

Ice sheets in Greenland and Antarctica are based on continental rock and are coupled strongly to changes in global sea level. Their beds may be frozen or thawed, but it has not been known how much or where basal melt occurs, except in special cases where a borehole has reached the bed or where airborne radar has revealed subglacial lakes and reflective ice stream beds in Antarctica. We have used a previously published technique to detect subglacial melt water in the general case and have here applied it to radar-sounder data collected over northern and central Greenland. We have found extensive subglacial water along between 13% and 20% of the flight paths. This paper provides maps of the measured locations and probable extent of subglacial water. [ABSTRACT FROM AUTHOR]

We describe a prototype ultrawideband radar. We show how the system was designed and how the hardware was developed for the radar prototype. Waveform generation, radar parameters, and signal processing for the stepped frequency waveform are discussed. The radar operates from 500 MHz to 3 GHz with a nominal resolution of 6 cm in air. The advantage of the stepped frequency approach over an impulse radar is better matching between the transmitted waveform and the receiver. We use range gating to improve the system dynamic range. The advantages are illustrated with laboratory measurements and field measurements from glacial ice and permafrost in Svalbard, showing penetration depths of 11 m. Antennas which do not require contact with the ground were developed and used in the experiments. [ABSTRACT FROM AUTHOR]

The radio echo sounding (RES) system is one of the most widely used active remote sensing techniques for polar ice sheet exploration, including bedrock morphology studies and subglacial lake investigations. Recently, bedrock characterization has been improved through the analysis of radar echo strength. In this paper, the analysis of the RES signal amplitude has been used to collect information about the controversial problem of electromagnetic ice absorption to highlight areas of high reflectivity variation, ascribable to wet ice-bedrock interfaces. A method to distinguish a wet or dry bedrock-ice interface using a model to describe the internal ice absorption is proposed and discussed. Moreover, the comparison between the ice absorption rates from RES measurements and from European Project for Ice Coring in Antarctica Dome C (Antarctica) ice core conductivity data, the signal amplitude contributions of internal ice layers, and different kinds of rock interface is evaluated. Encouraged by the results, the data analysis led to obtaining a bedrock reflectivity variation map of the Dome C area. This map outlined a wide dispersion of wet/dry rock interfaces in the studied area, indicating the possibility of flowing water along both sides of the Concordia Trench. [ABSTRACT FROM AUTHOR]

In recent years, there has been increased interest in using synthetic aperture radar (SAR) to study and monitor glaciers and ice sheets for glaciological and climate change research. However, due to the medium's complexity, SAR backscattering from ice remains poorly understood, including the relative importance of scattering from surface and volume layers and also dependences on frequency and glacier zone. Extreme weather conditions can result in quickly changing surface conditions influencing backscatter signatures while leaving the underlying volume of interest unchanged. Surface and volume components must thus be separated in order to infer information regarding the properties of the ice volume. This paper describes a three-component scattering model to decompose polarimetric SAR (PolSAR) images of glacier ice. Total backscatter is modeled as the incoherent summation of surface, volume, and sastrugi (wind-induced feature) components. The proposed model adapts and extends the Freeman and Durden decomposition for an ice volume scenario in which the volume is a dielectric medium. Forms of the model for both random and oriented volumes are considered, and a new oriented sastrugi component is introduced which is able to explain backscatter behavior between different winter scenes. Validation is performed with airborne PolSAR data at L- and P-band collected using the E-SAR system of the German Aerospace Center over the Austfonna ice cap in Svalbard, Norway, as part of the ICESAR campaign. [ABSTRACT FROM AUTHOR]

Currently, one of the major methodological gaps in the observation of glaciers from space is the measurement of volume changes of mountain glaciers and ice caps. In this paper, we present a case study of comparing a digital elevation model derived from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite optical stereo, elevation data derived from Ice, Cloud, and land Elevation Satellite Geoscience Laser Altimeter System (GLAS) laser altimetry, and contour lines from a topographic map from the 1970s. For two ice caps in Eastern Svalbard, Kvalpyntfonna and Digerfonna, we obtain an overall elevation change of -0.55 or -0.61 m/year between 1970 and 2002 (ASTER) or GLAS (2006), respectively. From comparison of different methods and from different quality checks, we estimate the error of this numbers to be on the order of 5%. This paper demonstrates that and on how long-term glacier volume changes can be observed from space over a large number of ice caps and glaciers. [ABSTRACT FROM AUTHOR]

This letter extends the simultaneous least squares adjustment method to the calibration and merging of velocity measurements, which are obtained by applying interferometric and speckle-tracking methods to multiple frames of repeat-pass interferometric synthetic aperture radar (InSAR) data. The underlying observation equations have been derived for velocity control points and tie points. By exploiting a set of tie points, individual frames within a strip or a block can be correlated with each other, and the parameters of calibration models for all individual frames can be simultaneously determined. With Radarsat InSAR data over the Recovery Glacier, Antarctica, we demonstrate that our simultaneous least squares adjustment method has effectively removed velocity discontinuities between adjacent frames and greatly reduced the velocity-control-point requirement. [ABSTRACT FROM PUBLISHER]

Spaceborne differential synthetic aperture radar (SAR) interferometry has been proven to be a powerful tool in monitoring environmental phenomena and, in particular, in observing glaciers and retrieving information about their surface topography and dynamics. In the last decade, the use of this technique has been successfully extended from space to ground-based observations as a tool for monitoring, on a smaller scale, single landslides, unstable slopes, and more recently, areas covered by snow but not yet glaciers. In this letter, the results of an experimental activity carried out to evaluate the potential of ground-based microwave interferometry to estimate the velocity of an unstable area belonging to a glacier is reported. This experiment demonstrated the possibility of remotely monitoring surface displacements of the monitored glacier up to a distance of about 3 km even if, due to the lack of ground truths on the observed area, the data interpretation must be carefully worked out. [ABSTRACT FROM PUBLISHER]

This letter presents a technique to adjust and unify disconnected interferogram fringe regions for the derivation of accurate surface-velocity measurements. The interferogram from repeat-pass interferometric synthetic aperture radar data is often partitioned by shear margins of ice streams and other low-coherence zones into many small disconnected fringe regions. Although these isolated fringe regions can be unwrapped separately, the unwrapped phase for each region is referenced to a different seed point. Our technique exploits absolute range-offset measurements from the speckle-tracking method to bridge the isolated fringe regions in the interferogram. In this way, the unwrapped phases in these regions can be adjusted into consistent surface-displacement measurements with a common reference point. Using Radarsat interferometric data in Antarctica, we demonstrated that the synergetic fusion of the measurements from the interferometric and speckle-tracking methods can produce a highly accurate two-dimensional velocity field [ABSTRACT FROM PUBLISHER]

Signs of long-term glaciological processes and past ice-sheet structure are preserved in the internal layer manifestations of the Greenland ice sheet. Internal layer data have been collected over a substantial portion of the Greenland ice sheet using the University of Kansas ice-sounding radar. In this letter, these layers are traced along thousands of kilometers of flight lines from the ice divide toward Jakobshavn, which is the most active glacier in Greenland. The authors determine the traced radar layers' age at the Greenland Ice Core Project (GRIP) site using the GRIP core age-depth relationship. Inasmuch as the depth varies spatially for a layer of a specific dated age, an age-depth relationship for each location along the flight lines of this letter can be found using the traced layers. Thirty-one points where flight lines cross over one another were analyzed. From the flight line crossover analysis, a 9-m maximum difference (< 1%) was found [ABSTRACT FROM PUBLISHER]

This study combines two satellite radar techniques, low-resolution C-/Ku-band scatterometer and high-resolution C-band synthetic aperture radar (SAR) for glaciological studies, in particular mass-balance estimations. Three parameters expressing the mean backscattering and its dependency on azimuth and incidence angle are used to describe and classify the Antarctic ice sheets backscattering behavior. Simple linear regression analyses are carried out between ground truth accumulation data and the SAR backscattering coefficient along continuous profile lines. From this we parameterize the accumulation rate separately for certain snow pack regimes. We find that SAR data can be used to map mass-balance changes, however only within limited areas. Applying this method therefore generally requires accurate ground truth for regional calibration together with additional information regarding mean air temperature or elevation. This investigation focuses on the area of Dronning Maud Land, Antarctica. We present the first high-resolution accumulation map based on SAR data for the surrounding area of the EPICA deep ice core drilling site Kohnen, which is compared to reliable ground truth records as well as to a surface-mass-balance map interpolated from these at low resolution. [ABSTRACT FROM AUTHOR]

We present a digital elevation model (DEM) of King Edward VII Peninsula, Sulzberger Bay, West Antarctica, developed using 12 European Remote Sensing (ERS) synthetic aperture radar (SAR) scenes and 24 Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry profiles. We employ differential interferograms from the ERS tandem mission SAR scenes acquired in the austral fall of 1996, and four selected ICESat laser altimetry profiles acquired in the austral fall of 2004, as ground control points (GCPs) to construct an improved geocentric 60-m resolution DEM over the grounded ice region. We then extend the DEM to include two ice shelves using ICESat profiles via Kriging. Twenty additional ICESat profiles acquired in 2003-2004 are used to assess the accuracy of the DEM. After accounting for radar penetration depth and predicted surface changes, including effects due to ice mass balance, solid Earth tides, and glacial isostatic adjustment, in part to account for the eight-year data acquisition discrepancy, the resulting difference between the DEM and ICESat profiles is -0.57±5.88 m. After removing the discrepancy between the DEM and ICESat profiles for a final combined DEM using a bicubic spline, the overall difference is 0.05±1.35 m. [ABSTRACT FROM PUBLISHER]

In the frame of the Swiss Alpine Airborne SAR Experiment 2003 (SASARE), multitemporal/multibaseline polarimetric synthetic aperture radar (SAR) interferometry (Pol-InSAR) datasets were acquired with the German Experimental SAR (E-SAR) platform at L- and P-band in the region of the Aletsch glacier in Switzerland. For SASARE, several sub-test-sites had been selected below and significantly above the equilibrium line. We provide results from the analysis of the multibaseline Pol-InSAR L- and P-band measurements of the polarization-dependent complex interferometric coherence of an Alpine glacier in the cold infiltration zone. Interferometric volume decorrelation is demonstrated as a function of polarization. Conventional polarimetric decomposition techniques confirm the polarization-dependent backscattering properties of the analyzed snow and ice surfaces. [ABSTRACT FROM PUBLISHER]

We are developing a multifrequency multistatic synthetic aperture radar (SAR) for determining polar ice sheet basal conditions. To obtain data for designing and optimizing radar performance, we performed field measurements with a network-analyzer-based system during the 2003 field season at the North Greenland Ice Core Project camp (75.1 N and 42.3 W). From the measurements, we determine the ice sheet complex transfer function over the frequency range from 110-500 MHz by deconvolving out the system transfer function. Over this frequency range, we observe an increase in total loss of 8±2.5 dB using a linear regression to the log-scale data. With the ice sheet transfer function and an ice extinction model, we estimate the return loss from the basal surface to be approximately 37 dB. These measurements have broad applicability to interpreting radar-sounding data, which are widely used in glaciological studies of the polar ice sheets. These data have also been used in the link budget for the design considerations of the multifrequency multistatic SAR system. [ABSTRACT FROM PUBLISHER]

We describe an investigation that combines space-based observations of glacier parameters with historical glaciological data derived by traditional means to predict changes in ice extent and volume for the Ak-shirak Range in the interior Tien Shan of Central Asia. A variety of geographic information systems and photogrammetric tools are used to extract glacier outlines, derive of a digital elevation model, and compute area versus elevation distribution functions from data acquit-s by the Advanced Spaceborne Thermal Emission and Reflection radiometer. These products are then used in a glaciological methodology that can predict the response of glacier systems to changes in local climate. [ABSTRACT FROM AUTHOR]

This letter proposes the use of solar background radiation scattered by ice clouds for validating space lidar depolarization calibration. The method takes advantage of the fact that the background light scattered by optically thick ice clouds is almost entirely unpolarized. The theory is examined with background light measurements acquired by the Cloud Physics Lidar. [ABSTRACT FROM PUBLISHER]

We developed a hardware target simulator for measuring the system response and testing of an airborne wideband radar that operates over the frequency range 600-900 MHz to map the near-surface internal layers in glacial ice. It uses optical and microwave delay lines for evaluating and optimizing the performance of the wideband radar in the continental United States without expensive field trips to polar regions and can be used to test the radar without interfering with commercial wireless devices and television stations. The target simulator replicates the feed-through signal between the transmit and receive antenna, as well as reflections from the air-firn interface and the internal layers of the ice sheet, which are spaced about 50 cm apart. This is because the measured radar resolution is about 60 cm in free space and can map layers with about 50-cm resolution in firn. We simulated the internal layers by incorporating a feedback loop with a short delay line. The target simulator is being used for testing and evaluation of the radar in the laboratory and for in-flight testing of the radar. [ABSTRACT FROM PUBLISHER]

Geosat Follow On (GFO) radar altimeter data are processed with previous altimeter datasets to measure long-term elevation change of higher elevation portions of the southern Greenland ice sheet over time periods from 1978-1988, 1985-2002, and 1978-2002. Average elevation change results indicate approximately zero overall elevation change for all time periods. The results also indicate that upper-elevation thinning in southeast Greenland has been widespread and has existed for several decades. [ABSTRACT FROM PUBLISHER]