Intercomparison and Validation of SAR-Based Ice Velocity Measurement Techniques within the Greenland Ice Sheet CCI Project

Boncori, John Peter Merryman; Andersen, M.; Dall, Jørgen; Kusk, Anders; Kamstra, Martijn; Andersen, S. B.; Bechor, N.; Bevan, Suzanne; Bignami, Christian; Gourmelen, Noël; Joughin, Ian; Jung, Hyung-Sup; Luckman, Adrian; Mouginot, J.; Neelmeijer, Julia; Rignot, Eric; Scharrer, K.; Nägler, Thomas; Scheuchl, B.; Strozzi, Tazio

doi:10.3390/rs10060929

Cited by 20 publications

(19 citation statements)

References 51 publications

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“…For validation purposes, seven low-cost single-frequency continuous GPS receivers (Wirz et al, 2013;Buchli et al, 2012) were deployed on the glacier using a photovoltaic sys-tem in combination with a battery (see Fig. 1).…”

Section: Gps Velocity Datamentioning

confidence: 99%

Multisensor validation of tidewater glacier flow fields derived from synthetic aperture radar (SAR) intensity tracking

et al. 2019

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Abstract. Following the general warming trend in Greenland, an increase in calving rates, retreat and ice flow has been observed at ocean-terminating outlet glaciers. These changes contribute substantially to the current mass loss of the Greenland Ice Sheet. In order to constrain models of ice dynamics as well as estimates of mass change, detailed knowledge of geometry and ice flow is needed, in particular on the rapidly changing tongues of ocean-terminating outlet glaciers. In this study, we validate velocity estimates and spatial patterns close to the calving terminus of such an outlet derived from an iterative offset-tracking method based on synthetic aperture radar (SAR) intensity data with a collection of three independent reference measurements of glacier flow. These reference datasets are comprised of measurements from differential GPS, a terrestrial radar interferometer (TRI) and repeated unmanned aerial vehicle (UAV) surveys. Our approach to SAR velocity processing aims at achieving a relatively fine grid spacing and a high temporal resolution in order to best resolve the steep velocity gradients in the terminus area and aims to exploit the 12 d repeat interval of the single-satellite Sentinel-1A sensor. Results from images of the medium-sized ocean-terminating outlet glacier Eqip Sermia acquired by Sentinel-1A and RADARSAT-2 exhibit a mean difference of 11.5 % when compared to the corresponding GPS measurements. An areal comparison of our SAR velocity fields with independently generated velocity maps from TRI and UAV surveys showed good agreement in magnitude and spatial patterns, with mean differences smaller than 0.7 m d−1. In comparison with existing operational velocity products, our SAR-derived velocities show an improved spatial velocity pattern near the margins and calving front. There 8 % to 30 % higher surface ice velocities are produced, which has implications on ice fluxes and on mass budget estimates of similarly sized outlet glaciers. Further, we show that offset tracking from SAR intensity data at relatively low spatio-temporal sampling intervals is a valid method to derive glacier flow fields for fast-flowing glacier termini of outlet glaciers and, given the repeat period of 12 d of the Sentinel-1A sensor (6 d with Sentinel-1B), has the potential to be applied operationally in a quasi-continuous mode.

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Section: Gps Velocity Datamentioning

confidence: 99%

Multisensor validation of tidewater glacier flow fields derived from synthetic aperture radar (SAR) intensity tracking

et al. 2019

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“…The maps provide detailed snapshots of contemporary ice flow in Greenland. Quality assessment of the IV retrieval algorithm was performed through internal consistency checks (for example, checking how the algorithm performs on bedrock outcrops that are assumed stationary), and intercomparisons with other algorithms through a dedicated round-robin [24]. The velocity data is validated against independent datasets from ground-based in-situ GPS, higher resolution sensors (TerraSAR-X and COSMO-SkyMed) and available published datasets (MEaSUREs, e.g., [3,25].…”

Section: Ice Velocitymentioning

confidence: 99%

An Integrated View of Greenland Ice Sheet Mass Changes Based on Models and Satellite Observations

Mottram

Simonsen²,

Svendsen³

et al. 2019

Remote Sensing

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The Greenland ice sheet is a major contributor to sea level rise, adding on average 0.47 ± 0.23 mm year − 1 to global mean sea level between 1991 and 2015. The cryosphere as a whole has contributed around 45% of observed global sea level rise since 1993. Understanding the present-day state of the Greenland ice sheet is therefore vital for understanding the processes controlling the modern-day rates of sea level change and for making projections of sea level rise into the future. Here, we provide an overview of the current state of the mass budget of Greenland based on a diverse range of remote sensing observations to produce the essential climate variables (ECVs) of ice velocity, surface elevation change, grounding line location, calving front location, and gravimetric mass balance as well as numerical modelling that together build a consistent picture of a shrinking ice sheet. We also combine these observations with output from a regional climate model and from an ice sheet model to gain insight into existing biases in ice sheet dynamics and surface mass balance processes. Observations show surface lowering across virtually all regions of the ice sheet and at some locations up to −2.65 m year − 1 between 1995 and 2017 based on radar altimetry analysis. In addition, calving fronts at 28 study sites, representing a sample of typical glaciers, have retreated all around Greenland since the 1990s and in only two out of 28 study locations have they remained stable. During the same period, two of five floating ice shelves have collapsed while the locations of grounding lines at the remaining three floating ice shelves have remained stable over the observation period. In a detailed case study with a fracture model at Petermann glacier, we demonstrate the potential sensitivity of these floating ice shelves to future warming. GRACE gravimetrically-derived mass balance (GMB) data shows that overall Greenland has lost 255 ± 15 Gt year − 1 of ice over the period 2003 to 2016, consistent with that shown by IMBIE and a marked increase compared to a rate of loss of 83 ± 63 Gt year − 1 in the 1993–2003 period. Regional climate model and ice sheet model simulations show that surface mass processes dominate the Greenland ice sheet mass budget over most of the interior. However, in areas of high ice velocity there is a significant contribution to mass loss by ice dynamical processes. Marked differences between models and observations indicate that not all processes are captured accurately within models, indicating areas for future research.

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“…Offset tracking has the advantage of producing two-dimensional velocity measurements, namely along the range (satellite line-of-sight) and azimuth (flight path) dimensions, and is applicable even on fast-flowing outlet glaciers, which may reach velocities as high as several km/year. The accuracy and resolution of the amplitude-based offset tracking velocity retrievals, however, are generally substantially poorer than those obtained with DInSAR [6], although the latter technique provides only measurements of the range motion component. A comprehensive review of SAR-based ice velocity measurement techniques applicable to Stripmap imagery is provided in [6].…”

Section: Introductionmentioning

confidence: 96%

“…The accuracy and resolution of the amplitude-based offset tracking velocity retrievals, however, are generally substantially poorer than those obtained with DInSAR [6], although the latter technique provides only measurements of the range motion component. A comprehensive review of SAR-based ice velocity measurement techniques applicable to Stripmap imagery is provided in [6].…”

Section: Introductionmentioning

confidence: 96%

Improved Ice Velocity Measurements with Sentinel-1 TOPS Interferometry

et al. 2020

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In recent years, the Sentinel-1 satellites have provided a data archive of unprecedented volume, delivering C-band Synthetic Aperture Radar (SAR) acquisitions over most of the polar ice sheets with a repeat-pass period of 6–12 days using Interferometric Wide swath (IW) imagery acquired in Terrain Observation by Progressive Scans (TOPS) mode. Due to the added complexity of TOPS-mode interferometric processing, however, Sentinel-1 ice velocity measurements currently rely exclusively on amplitude offset tracking, which generates measurements of substantially lower accuracy and spatial resolution than would be possible with Differential SAR Interferometry (DInSAR). The main difficulty associated with TOPS interferometry lies in the spatially variable azimuth phase contribution arising from along-track motion within the scene. We present a Sentinel-1 interferometric processing chain, which reduces the azimuth coupling to the line-of-sight phase signal through a spatially adaptive coregistration refinement incorporating azimuth velocity measurements. The latter are based on available ice velocity mosaics, optionally supplemented by Burst-Overlap Multi-Aperture Interferometry. The DInSAR processing chain is demonstrated for a large drainage basin in Northeast Greenland, encompassing the Northeast Greenland Ice Stream (NEGIS), and integrated with state-of-the-art offset tracking measurements. In the ice sheet interior, the combined DInSAR and offset tracking ice velocity product provides a spatial resolution of 50 × 50 m and 1-sigma accuracies of 0.18 and 0.44 m/y in the x and y components respectively, compared to GPS.

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Intercomparison and Validation of SAR-Based Ice Velocity Measurement Techniques within the Greenland Ice Sheet CCI Project

Cited by 20 publications

References 51 publications

Multisensor validation of tidewater glacier flow fields derived from synthetic aperture radar (SAR) intensity tracking

Multisensor validation of tidewater glacier flow fields derived from synthetic aperture radar (SAR) intensity tracking

An Integrated View of Greenland Ice Sheet Mass Changes Based on Models and Satellite Observations

Improved Ice Velocity Measurements with Sentinel-1 TOPS Interferometry

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