Abstract This paper summarizes an evaluation by experts of how coordination of Earth‐observing Synthetic Aperture Radar (SAR) missions among the world's space agencies could advance toward game‐changing scientific discoveries and fully realizing SAR's practical capability to address many issues facing society. We consider key science disciplines for which spaceborne SAR sensors are routinely used, with an emphasis on SAR imaging instruments. We outline the current state of the science and identify critical information gaps for 10 disciplines: Ice Sheets and Glaciers, Solid Earth Science, Hazards, Forests and Biomass, Wetlands, Agriculture and Crop Monitoring, Soil Moisture, Sea Ice, Permafrost, and Oceans. We provide recommendations on how these gaps can be addressed by coordination of missions currently operating or in development, then look forward to the next decade during which as‐yet‐unplanned coordinated SAR constellations could be game‐changing. We identify synergies and conflicts between the optimal SAR configurations required for individual disciplines to achieve transformational science advancement. Finally, we provide summary recommendations for beneficial coordination that consider SAR‐enabled Earth science studies both as a whole and within the context of multiple individual disciplines that have benefited from a common observational strategy. Overall, there are clear benefits that can be derived from coordinated utilization of spaceborne SAR assets based on their individual capabilities and availability, and through coordinated and shared data and observation strategies.

Knowledge of Antarctic glacier grounding lines, which mark the transition between grounded and floating ice, is a vital parameter in determining the stability of major ice shelves and hence the ice sheet. Rapid grounding line retreat and associated mass loss has been documented at numerous Antarctic glaciers, particularly in the Amundsen Sea Embayment. However, few comprehensive grounding line mappings exist, particularly from recent years. Here, we utilize a unique record of Sentinel-1 Synthetic Aperture Radar 1 d repeat-pass imagery to generate a comprehensive retrieval of grounding line location in the Amundsen Sea Embayment in 2025 and evaluate recent changes.

The Grounding Line (GL)-the transition from ice grounded on the continent and ice afloat in the ocean-is a sensitive indicator of glacier stability and mass balance. Using differential synthetic aperture radar interferometry from ERS-1/2, Sentinel-1, RADARSAT-1/2, RADARSAT Constellation Mission, ALOS PALSAR-2, COSMO-SkyMed, and ICEYE, we assemble a continental scale record of grounding line migration from 1992 to 2025. Over 77 ± 10% of Antarctic coastal length, we detect no GL migration. Retreat is concentrated in i) the Antarctic Peninsula-2 to 18 km along Larsen A-B and 2 to 6 km along parts of George VI; ii) Wilkes and George V lands-6 to 10 km on Denman, Totten, Moscow, Frost, Holmes, Mertz, Ninnis, and Cook, and 26 km on Vanderford; and iii) West Antarctica-5 to 7-km on Ferrigno, Fox, and Venable, with extreme retreat in the Amundsen and Getz sectors (Pine Island 33 km, Thwaites 26 km, Haynes 20 km, Pope 23 km, Smith 42 km, Kohler 12 km, East Getz 9 km toward Berry 18 km, Hull 14 km, and Land 5 km). The ice sheet lost 12,820 ± 1,873 km2 of grounded ice in 1996-2025, or 442 ± 64 km2/y, with 62% from West Antarctica and 28% from East Antarctica. Retreat clusters in areas where bathymetry channelizes warm Circumpolar Deep Water toward deep grounding zones where beds are retrograde, except in the northeastern Antarctic Peninsula. The results provide a harmonized benchmark for ice grounding zone-based ice sheet models and identifies gateways where future retreat is likely to accelerate.

In response to the 2017 Decadal Survey, NASA conducted a five-year study on the Surface Deformation and Change (SDC) designated observable to study potential mission concepts. As part of the SDC mission study, the Commercial Synthetic Aperture Radar (ComSAR) subgroup was tasked with evaluating the current landscape of the SAR and interferometric SAR (InSAR) industry to assess whether NASA could leverage commercial smallsat products to meet the needs of the SDC science mission. The assessment found that although the commercial SAR industry is growing rapidly, off-the-shelf products can currently only make a small-albeit distinct-contribution to SDC mission goals. This gap is due to different design goals between current commercial systems (which prioritize targeted high-resolution, non-interferometric observations at short wavelengths with a daily or faster revisit) and a future SDC architecture (which focuses on broad, moderate-resolution, and interferometric observations at long wavelengths). Even by 2030, planned commercial constellations are expected to only cover similar to ${\sim} $65% of the area needed to match NISAR coverage. Still, high-resolution and rapid-repeat capabilities can augment scientific findings from a future SDC mission, as demonstrated by recent contributions from commercial data to applied sciences, cryosphere, and volcanology. Future innovations on smallsat constellation concepts could further contribute to SDC science and applications. Although current constellation designs are not fully able to satisfy desired SDC science capabilities, initial positive feedback to a request for information indicates a potential future path for a customized SDC commercial architecture; more studies will be needed to determine the feasibility of these approaches.

Ice shelves restrain grounded ice discharge into the ocean, and their break-up contributes significantly to Antarctica's sea level rise. Using aerial imagery from the 1960s and modern satellite data, we construct a long-term record of Wordie Ice Shelf's disintegration and its effects on tributary glaciers. Early changes in pinning points and ocean warming in Marguerite Bay since the 1960s strongly suggest increasing basal melt as the primary driver of the ice shelf disintegration. Some glaciers responded immediately to the ice shelf break-up, with surface velocities tripling, thinning up to 160 m, and grounding line retreat of 7.5 km, while others reacted decades later due to buttressing from remnant parts of the ice shelf. Our findings emphasize the importance of long-term observations to understand ice shelf disintegration and its impacts, offering crucial insights for assessments of future ice loss from the Antarctic Ice Sheet.

The contribution of Thwaites Glacier, Antarctica, to sea level rise is influenced by how quickly warm salty seawater of Circumpolar Deep Water origin melts basal ice near its grounding line. Satellite observations reveal tidally forced kilometer-scale seawater intrusions beneath grounded ice that form an "Ice Grounding Zone" (IGZ) where ice melts vigorously. Although melt rates have been measured at selected sites using Automated phase-sensitive Radar Echo Sounders (ApRES) and Automated Underwater Vehicle (AUV) instruments, their spatial distribution and total magnitude within the IGZ remain uncertain. Here, we present 2D high-resolution simulations of the melt regime of Thwaites Glacier using the Massachusetts Institute of Technology global circulation model ocean model. The model predicts high melt at the entrance of the IGZ, with a quadratic decay inside the IGZ, a linear increase with ocean thermal forcing and a sublinear increase with cavity length. On the slow-moving Thwaites Eastern Ice Shelf, the modeled melt rate is low (10 m/y) due to a flat, shallow ice base, in agreement with in situ observations from AUV and ApRES instruments. On the Thwaites Glacier Tongue (TGT), the modeled melt rate is high (50 m/y) due to a steeper and deeper ice base. The modeled melt rates are consistent with the estimates derived from the satellite. Hence, multiple lines of evidence indicate high melt in the IGZ of TGT, which controls 80% of the glacier mass balance. An updated representation of grounding zones in ice sheet models including seawater intrusions will increase their sensitivity to ocean warming and will revise sea-level projections upward.

We employ a time series of ERS-1/2, ALOS-1/2 PALSAR, Sentinel-1, COSMO-SkyMed, and RCM differential synthetic-aperture radar interferometry data from 1996 to 2023 to document the short and long-term migrations of the grounding line (GL) of Berry Glacier, West Antarctica, a tributary of Getz Ice Shelf that controls 10% of its ice discharge. In 2019-2021, we detected a short-term GL migration of 18.0 ± 0.9 km, which is exceptionally long and implies that the glacier bed is up to 1300 m deeper than previously known. On short time scales, the GL migrates between three states controlled by bed topography. The observed flexing of the glacier suggests that seawater is trapped in the newly formed ice shelf cavity in an irregular fashion during the tidal cycle. From 1996 to 2021, the most inland position of the GL retreated by 18.1 ± 0.4 km, or 0.7 km/year, the ice thickness decreased by 11 ± 1 m/year, the ice sped up by 64 ± 5%, and the glacier lost a total mass of 131 ± 23 Gt. We attribute the rapid retreat to an enhanced ocean heat flux from warm Circumpolar Deep Water (CDW) reaching the grounding line through favorable bathymetry channels, combined with km-sized seawater intrusions beneath the glacier that cause rapid melting of basal ice.

The NASA-ISRO Synthetic Aperture Radar (NISAR) Mission experienced some technical issues in observatory level testing that required mitigations to be carried out on the reflector system, preventing a launch in 2024 as previously planned. The reflector has been reconditioned to address these issues, and NISAR is now on target for launch in early 2025. After launch, the spacecraft is planned to undergo commissioning for period of 90 days, after which science operations will begin. NISAR has two radar instruments - an L-band (24 cm wavelength) radar provided by NASA, and an S-band (9.4 cm wavelength) radar provided by ISRO - each of which can be operated individually or simultaneously. Each radar has a swath width of greater than 240 km for all modes at a variety of resolutions and polarimetric states. Due to precise orbit control and pointing, each radar also will produce repeat-pass interferometric measurements over all science targets. During the science phase, NISAR will collect about 35 Terabits of L-band radar image data each day, observing all land and ice-covered surfaces of Earth on the ascending and descending portions of each orbit every 12 days, and collecting about 5 Terabits of S-band radar image data each day over India and surrounding areas, Antarctica, and distributed global scientific areas of interest. Nearly all S- band acquisitions are collected simultaneously with L-band acquisitions, creating a unique globally distributed time- series data set. The commissioning plan calls for early engineering mode acquisitions around one month after launch, some of which may be usable to form images, followed by a period of orbit adjustment and system timing and pointing calibration. To prepare for science operations, the NISAR project has worked with the science team to develop a list of observational areas where early data can be acquired to demonstrate the preliminary quality of the data and to illustrate the science themes NISAR is addressing: solid Earth sciences, ecosystems sciences including global soil moisture, and cryosphere sciences, as well as many applications. In addition, cloud-based tools for image processing and diagnostic analysis, usable by the project and science team members alike, will be available to examine these early data sets.

Ice shelves restrain the discharge of grounded ice into the ocean, and their break-up plays a major role in Antarctica’s contribution to sea level rise. By combining aerial images from 1966 with modern satellite data, we produce one of the longest and most complete time series of an ice shelf collapse in Antarctica, specifically the Wordie Ice Shelf. Our results indicate that hydrofracturing and rising air temperatures played a minimal role in the collapse. Instead, we detect signs of substantial basal melt since the 1970s, with ice shelf changes correlated with increasing ocean temperatures from 1960-1990. Changes at tributary glaciers occurred at different intervals, with a tripling of surface velocities and grounding line retreats of up to 13 km since 1966. The long record of ice shelf change also demonstrates a heterogeneity in the timing of glacier retreat, mainly correlated with the loss of pinning points and the geomorphology of the Bay. Our results demonstrate that long-term observations of ice shelf collapses are critical for understanding the driving processes and consequences for tributary glaciers.

Warm water from the Southern Ocean has a dominant impact on the evolution of Antarctic glaciers and in turn on their contribution to sea level rise. Using a continuous time series of daily-repeat satellite synthetic-aperture radar interferometry data from the ICEYE constellation collected in March-June 2023, we document an ice grounding zone, or region of tidally controlled migration of the transition boundary between grounded ice and ice afloat in the ocean, at the main trunk of Thwaites Glacier, West Antarctica, a strong contributor to sea level rise with an ice volume equivalent to a 0.6-m global sea level rise. The ice grounding zone is 6 km wide in the central part of Thwaites with shallow bed slopes, and 2 km wide along its flanks with steep basal slopes. We additionally detect irregular seawater intrusions, 5 to 10 cm in thickness, extending another 6 km upstream, at high tide, in a bed depression located beyond a bedrock ridge that impedes the glacier retreat. Seawater intrusions align well with regions predicted by the GlaDS subglacial water model to host a high-pressure distributed subglacial hydrology system in between lower-pressure subglacial channels. Pressurized seawater intrusions will induce vigorous melt of grounded ice over kilometers, making the glacier more vulnerable to ocean warming, and increasing the projections of ice mass loss. Kilometer-wide, widespread seawater intrusion beneath grounded ice may be the missing link between the rapid, past, and present changes in ice sheet mass and the slower changes replicated by ice sheet models.

Warming of the ocean waters surrounding Greenland plays a major role in driving glacier retreat and the contribution of glaciers to sea level rise. The melt rate at the junction of the ocean with grounded ice-or grounding line-is, however, not well known. Here, we employ a time series of satellite radar interferometry data from the German TanDEM-X mission, the Italian COSMO-SkyMed constellation, and the Finnish ICEYE constellation to document the grounding line migration and basal melt rates of Petermann Glacier, a major marine-based glacier of Northwest Greenland. We find that the grounding line migrates at tidal frequencies over a kilometer-wide (2 to 6 km) grounding zone, which is one order of magnitude larger than expected for grounding lines on a rigid bed. The highest ice shelf melt rates are recorded within the grounding zone with values from 60 ± 13 to 80 ± 15 m/y along laterally confined channels. As the grounding line retreated by 3.8 km in 2016 to 2022, it carved a cavity about 204 m in height where melt rates increased from 40 ± 11 m/y in 2016 to 2019 to 60 ± 15 m/y in 2020 to 2021. In 2022, the cavity remained open during the entire tidal cycle. Such high melt rates concentrated in kilometer-wide grounding zones contrast with the traditional plume model of grounding line melt which predicts zero melt. High rates of simulated basal melting in grounded glacier ice in numerical models will increase the glacier sensitivity to ocean warming and potentially double projections of sea level rise.

We employ a time series of Sentinel-1 differential radar interferometry data from 2018 to detect the variability in grounding line position of the Fisher, Mellor, and Lambert glaciers, which drain about 47 billion tons of ice per year from East Antarctica. We observe kilometer-scale tidal migration, two orders of magnitude larger than expected for ice flowing over a hard bed. The migration is not in phase with changes in oceanic tide. In two estuaries underlaid by subglacial channels, we observe two states of migration that switch on and off over time scales of several weeks. The range of vertical motion reveals a water column thickness of 2-20 cm. Such intrusions of seawater over wide grounding zones are not accounted for in physical models. Including them will add vigorous melting of grounded ice that will enhance the sensitivity of glaciers to ocean warming and increase projections of mass loss.

<p>Petermann Glacier (80.75N, 60.75W) terminates in one of the most extensive remaining ice tongues of the Greenland Ice Sheet. The glacier is grounded 600 meters below sea level on a downsloping bed and could significantly contribute to sea level rise during the 21st century. Recent observations showed an ongoing acceleration in ice flow and kilometric-scale grounding line retreat after decades of stable dynamic conditions. Warming of the ocean waters surrounding Greenland has been indicated as the main driver of this process. However, the melting regime of the glacier at the interface between ocean waters and grounded ice is not well known and needs to be investigated.</p> <p>In this study, we achieve this goal by employing a time series of satellite radar interferometry data available between 2011 and 2022. We document grounding line migration using high-frequency observations from the Italian COSMO-Skymed constellation and the Finnish ICEYE constellation. Furthermore, we use time-tagged digital elevation models from the German TanDEM-X mission to assess the ice shelf basal melt regime in a Lagrangian framework.</p> <p>InSAR observations reveal kilometer-size grounding line migrations - (2-6 km) grounding zones - during tidal cycles, with preferential seawater intrusions of 6 km along pre-existing subglacial channels. In addition, results from the Lagrangian approach indicate that the highest ice shelf melt rates occur at these locations, with values reaching peaks ranging from 60 to 80 meters per year.</p> <p>Such high melt rates concentrated in kilometer-wide grounding zones contrast with the traditional plume model adopted by physical models with zero melt at a fixed grounding line. Their inclusion in physical models will increase the glacier's sensitivity to ocean warming and double the projections of sea level rise from the glacier.</p> <p>This work was supported by a grant from NASA.</p>

The Surface Deformation and Change (SDC) mission study is investigating a synthetic aperture radar (SAR) mission that is expected to launch in the next decade, building on the foundation established by the NASA ISRO Synthetic Aperture Radar (NISAR) mission. Since 2019, the SDC study team has updated the observation needs identified by the 2017 Earth Science Decadal Survey, developing a Science and Applications Traceability Matrix (SATM) that includes an expanded set of geophysical observables (GOs). These needs were further refined by a team of discipline experts, resulting in 48 GOs. For each GO, imaging characteristics such as revisit, accuracy, resolution, polarisation, data latency, are defined in the SATM. This paper describes the benefit assessment methodology, provides an example to generate current commercial feasibility scores for each GO in the SATM, even though the SDC mission will not be launched until the next decade. This methodology generates a quantitative assessment of commercial SAR data in meeting the measurement needs of a GO defined in SDC's SATM. Our assessment suggests that current commercial SAR data are particularly useful for constraining geophysical processes that benefit from short-repeat acquisition times and high spatial resolution.

Ice losses from the Greenland and Antarctic ice sheets have accelerated since the 1990s, accounting for a significant increase in the global mean sea level. Here, we present a new 29-year record of ice sheet mass balance from 1992 to 2020 from the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE). We compare and combine 50 independent estimates of ice sheet mass balance derived from satellite observations of temporal changes in ice sheet flow, in ice sheet volume, and in Earth's gravity field. Between 1992 and 2020, the ice sheets contributed 21.0±1.9 mm to global mean sea level, with the rate of mass loss rising from 105 Gt yr−1 between 1992 and 1996 to 372 Gt yr−1 between 2016 and 2020. In Greenland, the rate of mass loss is 169±9 Gt yr−1 between 1992 and 2020, but there are large inter-annual variations in mass balance, with mass loss ranging from 86 Gt yr−1 in 2017 to 444 Gt yr−1 in 2019 due to large variability in surface mass balance. In Antarctica, ice losses continue to be dominated by mass loss from West Antarctica (82±9 Gt yr−1) and, to a lesser extent, from the Antarctic Peninsula (13±5 Gt yr−1). East Antarctica remains close to a state of balance, with a small gain of 3±15 Gt yr−1, but is the most uncertain component of Antarctica's mass balance. The dataset is publicly available at https://doi.org/10.5285/77B64C55-7166-4A06-9DEF-2E400398E452 (IMBIE Team, 2021).

Ice motion and boundaries are critical information for ice sheet models that project ice evolution in a warming climate. We present four historical, continent-wide, maps of Antarctic-wide ice motion and boundaries for the time period 1995-2022. The results reveal no change in the interior region of East Antarctica, iceberg detachments at ice shelf fronts, and widespread glacier speedup that propagates 100 km's inland in West Antarctica and the Antarctic Peninsula. Speedup affects the entire drainage of the Amundsen Sea Embayment sector; the entire west coast of the Antarctic Peninsula down to GeorgeVI Ice Shelf; the east coast down to Larsen C Ice Shelf; Getz Ice Shelf, Hull and Land glaciers in West Antarctica; Matusevitch, Ninnis, Mertz and Denman glaciers, glaciers in Porpoise and Vincennes Bay; and Robert, Wilma and Rayner glaciers in Enderby Land, in East Antarctica. We attribute the observed glacier changes to increased melting by warmer ocean waters.

The Pope, Smith and Kohler glaciers, in the Amundsen Sea Embayment of West Antarctica, have experienced enhanced ocean-induced ice-shelf melt, glacier acceleration, ice thinning and grounding-line retreat in the past 30 years. Here we present observations of the grounding-line retreat of these glaciers since 2014 using a constellation of interferometric radar satellites combined with precision surface elevation data. We find that the grounding lines develop spatially variable, kilometre-scale, tidally induced migration zones. After correction for tidal effects, we detect a sustained pattern of retreat coincident with high melt rates of ungrounded ice, marked by episodes of more rapid retreat. In 2017, Pope Glacier retreated 3.5 km in 3.6 months, or 11.7 km yr –1 . In 2016–2018, Smith West retreated at 2 km yr –1 and Kohler at 1.3 km yr –1 . While the retreat slowed in 2018–2020, these retreat rates are faster than anticipated by numerical models on yearly timescales. We hypothesize that the rapid retreat is caused by unrepresented, vigorous ice–ocean interactions acting within newly formed cavities at the ice–ocean boundary.