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.

Interferometric Synthetic Aperture Radar (InSAR) provides constraints on lithospheric kinematics at high spatial resolution. Interpreting InSAR-derived deformation maps at continental scales is challenged by long-wavelength correlated noise and the inherent limitation of measuring relative displacements within the data footprint. We address these issues by applying corrections to InSAR time series to estimate ground velocity fields with millimeter-per-year precision over hundreds of kilometers. We use these velocity fields to determine the angular velocity of the local tectonic plate, assuming negligible long-wavelength vertical and intra-plate deformation. The uncertainty of the angular velocity is primarily influenced by observational errors and the limited imaging geometries available. Using the Arabian plate as an example, this work demonstrates the potential to improve plate motion models and evaluate intra-plate deformation in regions with sparse ground-based instrumentation.

The National Aeronautics and Space Administration (NASA) in the United States and the Indian Space Research Organisation (ISRO) have developed the NASA-ISRO Synthetic Aperture Radar (NISAR) mission, planned for launch in 2025. The mission will use SAR to map Earth's solid surfaces every 12 days, persistently on ascending and descending portions of the orbit, over all land and ice. The mission's primary objectives will be to study Earth's land and ice deformation and ecosystems in areas of common interest to the U.S. and Indian science communities. This single observatory solution with L-band (24-cm wavelength) and S-band (9.4-cm wavelength) imaging radars has a swath of more than 240 km at 5-10-m resolution, using full polarimetry where needed. The data will be processed into a suite of products in radar-specific and geographic coordinates tailored to the needs of each science discipline. The product suite is designed to be analysis ready and will be freely and openly available. To achieve these unprecedented capabilities, both radars use a reflector-feed system whereby the feed aperture elements are individually sampled to allow a scan-on-receive capability at both the L band and S band. The project is preparing for launch at the integration and test facilities in India. The launch will take place at the Satish Dhawan Space Center in India on ISRO's Geosynchronous Launch Vehicle (GSLV) Mark II. NISAR will be launched into a sun-synchronous polar orbit at a 748-km altitude with an exact 12-day repeat cycle. This article summarizes the mission, the science, the measurements, and plans for commissioning and early operations.

Constraining the effective rheology of major faults contributes to improving our understanding of the physics of plate boundary deformation. Geodetic observations over the earthquake cycle are often used to estimate key rheological parameters, assuming specific laboratory-informed classes of viscous or frictional rheological models. However, differentiating between various rheological model classes using only observations of a single earthquake (coseismic and postseismic deformation) is difficult—especially in the presence of coarse spatiotemporal sampling, inherent observational noise, and non-uniqueness of the inverted properties. In this study, we present a framework to estimate key rheological parameters of a subduction zone plate interface using simulations of sequences of earthquakes and aseismic slip, constrained by pre- and postseismic surface displacement timeseries. Our simplified forward model consists of a two-dimensional subduction zone, represented by a discretized planar fault or narrow shear zone, divided into a locked, shallow region (“asperity”) experiencing periodically imposed coseismic events, and a stress-driven creeping section governed by power-law viscoelasticity or rate-dependent friction. Our inverse model fits the rheological parameters of the interface to surface displacement timeseries in a Bayesian probabilistic way. We validate that our proposed framework can successfully recover depth-dependent profiles of effective viscosity using a synthetic dataset of pre- and postseismic observations. Our first set of numerical experiments show that our framework is only mildly sensitive to uncertainties in the rupture history or assumed coseismic slip, making it robust enough to be applied to real observations of subduction zones. Our second set of tests considers the similarities of surface displacement timeseries between synthetic models that model the plate interface either as a shear zone described by power-law viscosity, or a surface described by rate-dependent friction. Here, we find that the ability to fit surface observations using functional or mechanical models assuming frictional behavior does not constitute sufficient evidence to actually infer frictional behavior at depth, as the surface expressions are virtually indistinguishable from deformation generated from models with depth-variable power-law viscous behavior. Based on our numerical experiments, we conclude that studies that aim to infer the mechanical behavior and rheological properties at depth in subduction zones should consider the surface expression from time periods representative of the entire seismic cycle.

Measurements of both horizontal and vertical surface displacements allow for rigorous estimation of the moment deficit and the fault locking along subduction zones, including continental megathrusts. Previous measurements in the Himalayas were restricted to horizontal velocities from Global Navigational Satellite Systems, so the locking and the width of transition from apparent locking to interseismic creep were not well constrained. We present new observations of surface deformation from interferometric synthetic aperture radar for approximately 800 km along Himalaya. The interseismic velocity field along arc-perpendicular transects suggests a 5-8 mm/yr uplift in the higher Himalayas. We infer that the megathrust accommodates 20-22 mm/yr convergence over a width of similar to 115 km from the frontal thrust followed by a similar to 40 km transition zone. Sufficient strain has accumulated over the past 5-7 centuries in the central seismic gap that could be released by two Mw 8.8 earthquakes.

The relationship between stress and deformation of the solid Earth at various temporal and spatial scales can be described using a variety of constitutive relations. Although our knowledge of the elastic component of these constitutive relations is relatively well-constrained by seismological observations, there are limited opportunities to probe the inelastic component of plausible constitutive relations. Focusing on viscoelastic rheologies, the exact formulation of the viscous component of the rheology of the crust–mantle system is not uniquely constrained, i.e. multiple formulations (linear Burgers and power-law) are able to recreate geodetic observations of the earthquake cycle. Here, we show that it is possible to discriminate between these commonly adopted rheological models under certain conditions, even with the limited observational time span of geodetic networks. We first run a set of numerical simulations of periodic earthquake cycles as well as 2-event sequences for a two-dimensional strike–slip plate boundary, assuming a 20-km-thick frictional fault in an elastic layer overlying a 30-km-thick viscoelastic channel, to predict the resulting surface displacement time series over a 20-year time window. We use a rate-dependent friction law for the fault and a combined diffusion and dislocation creep viscous flow law with laboratory-derived rheological parameters to describe the non-elastic properties of the medium. We invert the synthetic surface displacements to obtain best-fit parameters for a simplified boundary element representation of each rheological model, assuming all viscous strain is localized beneath the fault, and compare the misfits. Linear Burgers and power-law rheologies are nearly indistinguishable when considering periodic events, but they can be distinguished using data from earthquake sequences when events are sufficiently different in magnitude (at least 0.2 units in magnitude) and occur with adequate temporal separation ( $$\sim $$ ∼ 1–10 years). However, when the true parameters represent a power-law rheology, it is not possible to uniquely recover all the rheological parameters. Graphical abstract

A secondary zone of surface uplift (SZU), located from 200 to 400 kilometers landward of the trench, has been measured after several megathrust earthquakes. The SZU reached a few centimeters hours to days after the 2011 Mw 9.1 Tohoku (Japan) and 2010 Mw 8.8 Maule (Chile) earthquakes. One interpretation is that this SZU is universal, driven by volume deformation around the slab interface (van Dinther et al. 2019). Indeed, published coseismic finite-fault models for these events do not reproduce the measured SZU.Here, we build on the case of the SZU to understand if, and how, our prior assumptions on the forward model can prevent us (or allow us) to make the most out of our dataset. In particular, we investigate under which assumptions the SZU can, or cannot, be predicted with fault slip. We show the SZU cannot be reproduced with coseismic finite-fault models that neglect 3D elastic heterogeneities in lithospheric structure. In contrast, we can recover the SZU with fault slip if elastic heterogeneities associated with the subducting slab are accounted for, as opposed to assuming homogeneous or layered elastic lithospheric structures. The SZU may therefore result from slip on the slab interface, downdip of the main coseismic patch. We suggest SZU might be caused by rapid afterslip, but a deformation of the volume around the fault cannot be ruled out.Reference: van Dinther, Y., Preiswerk, L. E., & Gerya, T. V. (2019). A Secondary Zone of Uplift Due to Megathrust Earthquakes. Pure and Applied Geophysics, 176(9), 4043–4068. https://doi.org/10.1007/s00024-019-02250-z

We demonstrate the effectiveness of long-term continuous interferometric Synthetic Aperture Radar (InSAR) monitoring for local resource management. Sustainable yield is a key concept in groundwater management to ensure sustainable, low-impact groundwater extraction. This study proposes to estimate sustainable yield using InSAR, combined with local groundwater production data. We apply this method to the Hollywood Basin in Los Angeles, California, leveraging nearly 30 years of InSAR data (1992-2023) to investigate ground deformation linked to groundwater extraction. High spatial InSAR measurements reveal deforming regions linked to anthropogenic activities previously not well-characterized by in situ observation networks. By integrating InSAR data with production records, we estimate the sustainable yield for the basin to be 1.44 to 1.67 million cubic meters per year, significantly lower than the current operating safe yield of 3.70 million cubic meters per year. Utilizing Independent Component Analysis, we are able to distinguish hydrological signals originating from the deep and shallow aquifers in the Hollywood Basin. Our findings demonstrate the effectiveness of InSAR for long-term monitoring of anthropogenic deformation and for supporting urban planning and resource management.

Enceladus is among the most intriguing bodies in the solar system due to its astrobiological potential. Determining the extent and duration of habitability (i.e., sustained habitability ) requires characterizing the interior properties and the level and distribution of tidal heating in Enceladus. Inferring the intensity of geophysical activity in the core has direct implications for the potential hydrothermal activity and supply of chemical species important for habitability to the ocean. We build a statistical framework to constrain the interior using estimates of libration, shape, heat flux, gravity, and total mass. We use this framework to examine the extent that geodetic measurements can improve our understanding of the interior structure, with an emphasis on partitioning of dissipation between the shell and the core. We quantify plausible ranges of gravitational ( k _2 ) and displacement ( h _2 , l _2 ) tidal Love numbers consistent with existing observations. We demonstrate that measuring k _2 alone can only constrain the total tidally dissipated energy, not its radial distribution. However, measuring the amplitude and phase of h _2 or l _2 facilitates determining the extent of tidal dissipation in the shell and the core. We provide the precisions required for measuring k _2 , h _2 , and l _2 that enable distinguishing between the main tidal heating scenarios, i.e., in the shell versus the core. We also explore the effect of the structural heterogeneities of the shell on the tidal response. Lastly, we evaluate the efficacy of future geodetic measurements to constrain key interior properties essential to understand the present-day ( instantaneous ) and long-term ( sustained ) habitability at Enceladus.

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.

In order to improve our understanding of the interior structure of Saturn's small moon Enceladus, we reanalyze radiometric tracking and onboard imaging data acquired by the Cassini spacecraft during close encounters with the moon. We compute the global shape, gravity field, and rotational parameters of Enceladus in a reference frame consistent with the International Astronomical Union's definition, where the center of the Salih crater is located at -5 degrees East longitude. We recover a quadrupole gravity field with J3 and a forced libration amplitude of 0.091 degrees +/- 0.009 degrees (3-sigma). We also compute a global shape model using a stereo-photoclinometry technique with a global resolution of 500 m, although some local maps have higher resolutions ranging from 25 to 100 m. While our overall results are generally consistent with previous studies, we infer a thicker 27-33 km mean ice shell, a thinner 21-26 km mean ocean thickness, and a mean core density range of 2,270-2,330 kg/m3. Geodetic data, such as shape, gravity, and rotation, provide important constraints for probing a planetary body's interior structure. We analyze radiometric tracking and onboard imaging data acquired during close encounters of Enceladus by the Cassini spacecraft to compute geodetic products including topographic and gravitational fields in a common reference frame. The recovered Enceladus topography has a global resolution of 500 m, with some local regions having 25-100 m resolution. Our study suggests that Enceladus has a 27-33 km mean ice shell thickness, a 21-26 km mean ocean thickness, and a mean core density range of 2,270-2,330 kg/m3. A full quadrupole gravity field with J3 and the forced libration amplitude of 0.091 degrees +/- 0.009 degrees are recoveredA 500-m resolution global topography model was computed, with some local regions having 25-100 m resolutionThe results suggest that Enceladus has a 27-33 km mean ice shell thickness, a 21-26 km ocean thickness, and a mean core density range of 2,270-2,330 kg/m3

The NASA ISRO Synthetic Aperture Radar (NISAR) is scheduled for launch early in 2024 from the Satish Dhawan Space Centre (SDSC), at Sriharikota, near Chennai, India. This mission is the result of a collaboration between NASA and Indian Space Research Organization (ISRO), where NASA has contributed elements of the mission such as an L-band SAR, and ISRO has contributed other elements, such as an S-band SAR. After successful launch, the NISAR mission will collect left-looking L-band SAR data over most of the Earth's land areas twice during every 12-day exact repeat orbit. (once while in an ascending orbit direction and once while in a descending orbit direction). NASA and ISRO have individual and joint requirements on the mission that include the performance of the imaging radars onboard the spacecraft. For example, NASA must demonstrate that this L-band SAR will achieve a set of identified science measurement accuracy requirements that span Ecosystem science, Solid Earth science, and Cryosphere science disciplines. Likewise, ISRO has several applications objectives on both the L-band and S-band data from NISAR that the ISRO science team and project will be developing and testing. Pre-launch and post-launch activities have been planned to validate that these requirements are met. Here, we will discuss how the NASA plans are being executed and will present any initial results at the conference.

Understanding the buildup of stress in space and time at tectonic plate interfaces is a key component of explaining sequences of earthquakes and aseismic slip (SEAS) at plate boundaries. In turn, successful modeling of coseismic slip and post- and interseismic fault creep can provide insights into mechanical properties of the fault zone material, as well as into potential seismic hazards. In the past, incomplete modeling of geodetic data to infer plate interface coupling has led to false assumptions of relative seismic quiescence. The issue of stress shadows in particular has led to the realization that improving our knowledge of the stress deficit on any given plate interface requires modeling the influence of past earthquakes (e.g., Hetland & Simons, 2010).This study aims to model the known historical sequence of earthquakes along the Northern Japan subduction zone in terms of post- and interseismic fault creep, at least to the extent it affects observed GNSS surface displacement timeseries of the last decades. We start from the framework presented by Kanda et al. (2013), who performed endmember simulations of multi-cycle SEAS on the Japanese megathrust, assuming varying rheological properties and fully-locked patches on the interface (“asperities”) in order to match GNSS-derived interseismic plate velocities. We extend this model by (1) taking into account the entire GNSS network timeseries available in Northern Japan (containing co-, post-, and interseismic periods) and (2) integrating it into a Markov chain Monte Carlo (MCMC) solver to solve for the most probable rheological properties of the plate interface.We present the results of selected forward simulations, including timeseries of slip rate on the fault and surface displacement, and compare them to GNSS observations. We furthermore present preliminary results of the inferred best-fitting rheological parameters (assuming rate-dependent friction).

Although we have a reasonable understanding of the physical and chemical conditions required to support the growth and reproduction of organisms, we still have only a rudimentary grasp of the geophysical conditions required to sustain those conditions over geological timescales. We propose that a strengthening of the interface between geophysics and biology is required to quantify sustained habitability and ultimately to mature the science of comparative habitability. Comparative habitability will inform our understanding of the common principles that allow habitability to be sustained on different planetary bodies, and whether habitability is predictable or contingent for a given set of planetary body characteristics. These developments are enabled by missions in our Solar System, including those to icy bodies such as Europa, Enceladus and Titan, in combination with telescopes allowing us to study habitability on exoplanets, thus providing essential insights into processes that can enable sustained habitability. Comparative habitability will help to determine whether Earth is a rare outpost of conditions suitable for a multi-billion-year biosphere, or whether the conditions that allowed for sustained habitability here are common in the Universe. The habitability of a planet is defined at a fixed time. A bigger challenge is to understand how that habitability is sustained over geological timescales, and how the underlying processes compare across different planetary bodies.

Saturn's icy moon Enceladus is tidally locked in an eccentric orbit, and experiences periodic tidal forcing every 33 hours as it orbits the planet. These tidal forces deform the icy shell, creating active cryovolcanic jet activity localized along major fault structures referred to as the 'Tiger Stripes" located within the South Polar Terrain. This activity and associated tectonic motions transform the surface, making Enceladus a natural laboratory for studying the geophysics of an ocean world using geodetic techniques, if one could measure its dynamic shape. Furthermore, it has been shown that geodetic observations of surface strain can directly constrain spatial variations in ice shell thickness. Repeat Pass Interferometric Synthetic Aperture Radar (lnSAR) is a technique that has been used in Earth orbiting missions to create wide-area, fineresolution maps of surface displacement, transforming the field of geodesy. The technique requires that the satellite be placed in an exact repeat orbit around target body to ensure coherence of the surface over time. Recent analysis suggests that a satellite can be placed in a stable repeating orbit around Enceladus, leading to the possibility of developing a repeatpass lnSAR mission to measure the dynamics and structure of Enceladus. Understandjng the geophysical environment of Enceladus, what leads to its surface features and changes over time, can provide insights into the energy budget of the body, its thermal state, and internal structure, all essential elements in understanding habitability of the moon. An InSAR-capable radar instrument design must balance wavelength, bandwidth, power, and other system parameters to address the observational uncertainties associated with the unusual radar scattering behavior of Enceladus, as well as orbit control limitations. This paper describes some of these design trades.

This paper presents an image formation algorithm for the focusing of SAR data with space-variant impulse response functions caused by eccentric orbits around small high-curvature surfaces, such as is encountered in stable orbits around Saturn’s moon Enceladus, a potential target for future SAR missions such as the Nightingale mission concept under development at JPL. Due to the extreme geometry, the range history shows a significant dependence on the target’s azimuth position within time scales significantly shorter than the synthetic aperture duration. Therefore, additional steps are needed in order to compensate for this effect and minimize image degradation. In this context, the present contribution evaluates the space variance of the geometry for SAR surveys over Enceladus and proposes a processing flow to account for it. Point target simulations using the proposed processing algorithm are shown to verify the approach.

We document one of several methodologies used to validate the NASA-ISRO Synthetic Aperture Radar (NISAR) mission requirements for solid earth deformation. NISAR’s deformation requirements cover steady-state, coseismic, and transient deformation processes and were designed to confirm that the mission is able to meet its solid earth science goals. We use independent observations of earth surface deformation from continuous Global Navigation Satellite System (GNSS) stations as ground truth for NISAR-observed deformation, and we provide a statistical framework to assess the quality of the associated NISAR data products. Our validation workflows have been developed as Jupyter Notebooks and are publicly available via GitHub/GitLab.

Launching in early 2024, the NASA-ISRO SAR (NISAR) mission will provide global data freely accessible enabling large scale surface deformation monitoring with synthetic aperture radar (SAR) acquired at L-band and S-band radar wavelengths. In preparation for calibration and validation of the NISAR L-band data, the NISAR Solid earth science team is systematically processing over 450 Japan Aerospace Exploration Agency (JAXA) ALOS-2 PalSAR-2 wide-swath (ScanSAR) L-band acquisitions covering the West Coast of the United States for measuring co-seismic, secular and transient displacements. The area spans California, Washington and Oregon.

The Central Andes subduction has been the theater of numerous large earthquakes since the beginning of the 21st Century, notably the 2001 Mw = 8.4 Arequipa, 2007 Mw = 8.0 Pisco and 2014 Mw = 8.1 Iquique earthquakes. We present an analysis of 47 permanent and 26 survey global navigation satellite system (GNSS) measurements acquired in Central-South Peru between 2007 and 2022 to better understand the frictional properties of the megathrust interface. Using a trajectory model that mimics the different phases of the cycle, we extract a coherent interseismic GNSS field at the scale of the Central Andes from Lima to Arica (12-18.5 degrees S). Interseismic models on a 3D slab geometry indicate that the locking level is relatively high and concentrated between 20 and 40-km depth. Locking distributions indicate a high spatial variability of the coupling along the trench, with the presence of many locked patches that spatially correlate with the seismotectonic segmentation. Our study confirms the presence of a creeping segment where the Nazca Ridge is subducting; we also observe a lighter apparent decrease of coupling related to the Nazca Fracture Zone (NFZ). However, since the Nazca Ridge appears to behave as a strong barrier, the NFZ is less efficient to arrest seismic rupture propagation. Considering various uncertainty factors, we discuss the implication of our coupling estimates with size and timing of large megathrust earthquakes considering both deterministic and probabilistic approaches. We estimate that the South Peru segment could have a Mw = 8.4-9.0 earthquake potential depending principally on the considered seismic catalog and the seismic/aseismic slip ratio. Using dense global navigation satellite system (GNSS) data collected in the South-Central Peru, we extracted a large scale interseismic velocity (surface velocity between two earthquakes) field at the scale of the Central Andes of Peru, where the oceanic Nazca plate goes under the continental South America plate at a velocity of about 6 cm/yr. This area has been the theater of several great subduction earthquakes and tsunamis, then estimating the stress build-up on the subduction interface is key to better anticipate future large earthquakes. Through a modeling of the GNSS velocities on a 3D slab geometry, we were able to obtain useful informations on the location, size, magnitude and return period of future great earthquakes in South Peru. Thereby, we obtained a very heterogeneous spatial distribution of interseismic coupling (degree of locking between the two tectonic plates), with low-coupled areas where the Nazca Ridge and the Nazca Fracture Zone are subducting, but highly-coupled areas close to the coasts of Lima and Arequipa. Finally, we estimate that the South Peru segment between the Nazca Ridge and the Arica band could have the potential to host a Mw = 8.4 to Mw = 9.0 earthquake, with a one century and one millennial recurrence time respectively. We present a dense interseismic velocity field at the scale of the South Peruvian Andes, from new decadal global navigation satellite system data at 73 locations Low locking (similar to 0.4) is estimated along the Nazca Ridge and the Nazca Fracture Zone, delimiting wide patches of high locking (similar to 0.9) Moment budget analysis shows that the South Peru segment could host a Mw = 8.4-9.0 earthquake with a 100 to 1,000 years recurrence time