This thesis investigates whether the scale-dependent bias observed in Sentinel-1 SAR CMOD5.N wind retrievals is primarily a mathematical consequence of nonlinear aggregation or evidence of physical scale-dependence in the backscatter--wind relationship. The study analyzes 2020–2021 Sentinel-1 Wave Mode imagettes at two incidence-angle geometries (WV1: ~23.5°; WV2: ~36.5°), aggregating retrieved fields from 100 m native resolution to scales of 200–2500 m under unstable atmospheric conditions. A second-order Taylor expansion of the CMOD5.N forward model is derived and reformulated into an operational prediction that uses only observable retrieval statistics — retrieved wind variance, incidence angle variance, and GMF curvature. A five-step diagnostic chain tests: whether retrieved variance follows atmospheric turbulence scaling (SQ1), whether the Taylor expansion predicts the empirical bias (SQ2), whether GMF curvature remains constant across scales (SQ3), whether bias depends on spectral structure beyond total variance (SQ4), and whether WV1–WV2 differences are consistent with incidence-angle geometry (SQ5). Results across 24 strata show that retrieved wind variance scales with domain size consistently with atmospheric boundary layer turbulence (β in [0.10, 0.20]). The wind-variance curvature term accounts for more than 98% of the Taylor prediction in every stratum. Spectral independence holds universally (mean partial correlations below 0.15), confirming that bias depends on total variance rather than on its spectral distribution. GMF curvature is constant across scales in all WV2 strata and in WV1 strata away from a curvature zero-crossing. Two WV2 strata (9–11 m/s crosswind) achieve the strongest agreement (r = 0.95), classified as pure mathematical aggregation (S1). The majority of strata fall into contaminated aggregation (S2): the mathematical mechanism is correct, but directional contamination from coarse auxiliary wind direction, residual noise, and second-order truncation error degrade prediction quality. WV1 crosswind strata at 9–11 m/s are classified as S3* — a curvature zero-crossing edge case that renders the Taylor prediction ill-conditioned without implying physical scale-dependence. No stratum exhibits evidence of physical scale-dependence (S3). The practical implication is that aggregation bias at SAR resolution can, in principle, be corrected analytically using local variance estimates and GMF curvature, without requiring reformulation of the geophysical model function. ... Read more

Synthetic Aperture Radar (SAR) satellites produce vast archives of high-dimensional ocean imagery, capturing complex multi-scale surface patterns induced by sea-air interaction processes. To estimate geophysical parameters such as turbulence fluxes, scientists traditionally apply domain knowledge to manually select physically meaningful features, an implicit form of dimensionality reduction. This thesis explores whether Variational Autoencoders (VAEs) can automate this process of dimensionality reduction by learning compressed latent representations directly from raw SAR ocean imagery.
Using 220,000 Sentinel-1 Wave Mode images co-located with ERA5 reanalysis data, VAE architectures were trained across four latent dimensionalities (32, 64, 128 and 256). The multi-scale complexity of SAR scenes introduced a strong frequency bias: standard pixel-wise losses such as Mean Squared Error failed to capture fine-scale detail, and conventional metrics such as PSNR and SSIM proved insufficient to measure this. Frequency Focal Loss (FFL) was incorporated to address reconstruction in the spectral domain, alongside a dynamic weight matrix that refocuses the loss on difficult-to-learn features. Dynamically annealing loss term weights had a striking effect on reconstruction quality, and subsequent hyperparameter optimisation using Optuna further confirmed that loss function tuning dominates over architectural choices. This sensitivity to loss function design is a central finding of this work.
VAEs successfully reconstructed large- and intermediate-scale ocean patterns at latent dimensions of 128 and 256. For air-sea flux estimation, three configurations were compared: a direct CNN regressor, a frozen VAE encoder with regression head, and a jointly trained VAE. All three underperform the physics-informed approach of O'Driscoll et al.\cite{o2023obukhov}, suggesting unsupervised deep learning alone cannot extract flux-relevant information, though task objectives incorporate ... Read more

The concept of closure phases is introduced in Interferometric Synthetic Aperture Radar (InSAR) as a circular summation of three multilooked interferometric phases. Since the beginning of InSAR theory, the interferometric phase is considered conservative, in other words, the expected value of the interferometric phases is redundant, given their circular summation, the closure phase, is equal to zero. In the 2010s, approximately two decades after the widely recognized crucial role of InSAR technique in monitoring Earth's surface change, real data examples showing non-zero closure phases are reported. The wide existence of non-zero closure phases challenges the validity of the implicit phase consistency assumption, and thus fosters extensive discussion within the InSAR community regarding its origin, where the main debate lies in whether there is a geophysical signal within the quantity or it is purely phase noise. Not long after the first report of the non-zero closure phases, following up research clearly points out the non-zero quantity stems from a combination of phase noise and geophysical signal. The evident existence of geophysical components in closure phases stimulates the development of new InSAR stack processing strategies to minimize their effect in estimating geometric changes. Meanwhile, researchers started exploring the possible application of this new quantity could offer, and main efforts have been devoted in building interferometric soil moisture models and retrieving soil moisture from interferometric phases.

Since the coexistence of a geophysical component and phase noise is acknowledged, predicting the contribution of the phase noise and subsequently quantifying the significance of the geophysical closure phase help justify its non-negligibility and assess the feasibility of further investigating the underlying scattering mechanisms. Following this rationale, this research begins by characterizing the significance of closure phases through a statistical approach. The analysis is conducted over a large spatiotemporal scale using Sentinel-1 acquisitions, thereby providing a general perspective on the prevalence of significant closure phases.

With the aid of the current analytical understanding of the geophysical origins of closure phases, soil moisture variations, and volumetric scattering in combination with perpendicular baselines, we investigate the correlation between closure phases and these geophysical factors. Combined with the statistical significance, this approach allows us to quantify the explainable variance in the observations. The results indicate that a substantial portion of the observed signals remains unexplained by existing models, highlighting the limitations of current knowledge in explaining the observed signals.

The correlation between the unexplained variance in the observed closure phase and land use provides additional insights into their potential physical origins, particularly the high correlation with agriculture fields. This finding motivates a focused investigation into the geophysical closure phases related to vegetation. To this end, we selected croplands in southern Spain to examine vegetation-induced closure phases by qualitatively comparing Sentinel-1 observations with vegetation parameters, such as crop calendar and Leaf Area Index (LAI). The analysis reveals a clear signal associated with vegetation development. We proposed two models: one accounting for variations in vegetation water content and another considering line-of-sight (LoS) motion introduced by vegetation growth. Both models offer plausible explanations for the observed signatures and extend the analytical understanding of the individual scattering mechanisms contributing to geophysical closure phases. Building on these insights, we develop a composite model to integrate as many known mechanisms as possible, including soil moisture variations, ground surface scattering, and vegetation water content changes. Although the model predictions demonstrate an improved understanding and provide a partial analytical representation of the relevant processes, we remain in search of closure regarding the origins of geophysical closure phases. Future physical modeling efforts should therefore aim to capture the coupled effects arising from multiple scattering mechanisms.

Overall, this dissertation presents the prevalence of statistically significant closure phases, quantifies the extent to which the observed signals can be explained by current knowledge, identifies clear closure phase signatures associated with vegetation development, provides analytical representations of vegetation-related scattering mechanisms, integrates known processes within a composite analytical framework, and highlights that the coupled effects of multiple scattering mechanisms should be considered in future studies. ... Read more

The launch of NASA’s SEASAT satellite in 1978 ushered in the era of radar oceanography. So treasured were the insights provided by this mission that its untimely demise, only three months after launch, did not preclude a legacy of success. In fact, rumors of sabotage sprang up, claiming that the instrument’s remote sensing prowess had jeopardized military assets. Scuttling SEASAT had surely been a matter of national security.

Like plenty of rumors, these, too, were grounded in some truth: SEASAT had shed light on many a mystery, though primarily in the field of physical sciences rather than Cold War conspiracies. Nonetheless, the value of spaceborne oceanography was now clear, and space agencies around the world did not stand idly by. Europe quickly followed suit with its own missions, launching in succession a series of high-resolution imagers, including ERS-1, ERS-2, ENVISAT, and, since 2014, the Sentinel-1 constellation.

Aided by these instruments and many others, the previous decades have been marked by continued scientific advances in the ever-growing field of radar oceanography. Building on that rich heritage, this dissertation aims to advance two distinct (but entwined) subdomains relating to either side of the air-sea interface.

In the first half of this dissertation, Chapters 2 and 3, we investigate the ubiquitous yet often overlooked atmospheric features present in most high-resolution radar images of the ocean. It may seem counterintuitive to study the atmosphere using ocean imagery, but much of the ocean’s surface variability is wind-driven, such that the ocean surface acts as a mirror for the wind field aloft. Till recently, the only systematic use for these atmospheric features had been in studies aiming to relate (an)isotropy to wind directions. But the information content is rich, and much more can be done.

In Chapter 2, we show that significant quantities of atmospheric information remain unexploited. And by using a domain-inspired encoding scheme, we transform high-dimensional radar images into a much lower-dimensional latent space, from which we extract substantially more atmospheric information than preexisting analytical methods.

Then in Chapter 3 we leverage the ever-increasing catalog of Sentinel-1 observations to go one step further. We illustrate that radar-derived atmospheric variables are complementary to state-of-the-art and reference atmospheric estimates. The challenge here is one of persuasion: how to argue that new estimates can complement state-of-the-art estimates when there are insufficient “ground truth” measurements to reconcile discrepancies between them? We aim to address this difficulty through a series of qualitative and quantitative assessments.

In the second half of the dissertation, Chapters 4 and 5, we switch to the other side of the air-sea interface, to investigate different methods of retrieving surface currents—a long-standing challenge in spaceborne oceanography. First, we investigate surface current retrieval for a proposed satellite modification, DopSCA, in Chapter 4. This system could be uniquely capable of retrieving surface current information, but its unorthodox design makes it susceptible to several additional noise sources. One of these noise sources, which we refer to as leakage—the undesired interaction between geometric Doppler, azimuthal backscatter gradients, and the antenna pattern—is investigated to assess its mission-compromising potential. Fortunately, this seems to be a comparatively minor issue.

Then, using a very different instrument in Chapter 5, we consider an alternative surface-current retrieval that incorporates polarimetry. Here we rely on the cross-correlation between wave motion, its radar cross section, and polarization to disentangle the convolved mixture of signals received by the satellite. Simulations and limited real observations illustrate that this approach may be a fruitful avenue towards the long-standing challenge of distinguishing between wave- and current motions. Future systems with additional polarimetric capabilities, such as Harmony or Sentinel-1 Next Generation, stand to gain from this approach in particular.

To summarize, this dissertation investigates radar-based retrieval of heat and momentum transport (from atmospheric convection and surface currents, respectively) by exploiting unique observational properties of a variety of radar satellites. We’ve made progress in both domains: providing the first quantitative and SAR-retrieved air-sea heat-flux estimates, and introducing a new method for surface-current retrieval. But fear not, enough work remains to entertain many radar oceanographers to come ;)
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Floods are natural hazards with severe impacts, and their frequency and intensity are increasing due to climate change. Synthetic Aperture Radar (SAR) satellites are widely used for flood mapping, as they operate independently of weather and time of day. This thesis examines the potential of SAR-based flood monitoring through a case study of the July 2021 flood in Limburg, the Netherlands. Comparing Capella Space on-demand X-Band imagery (sub-meter resolution) with Sentinel-1 open-source C-Band imagery (5 m x 20 m resolution). The Water-Land Boundary was determined by estimating flood extent and water levels, with multiple methods evaluated for the distinct products from both data sources. Capella Space provided a single image acquisition at the flood’s peak, with a thresholding method used to classify flooded pixels. For Sentinel-1, an Amplitude Time Series Analysis (ATSA) was applied to data from 2017 to 2024 to identify flood-related outliers. The water levels are estimated from flood extent edges with the national LiDAR DEM (AHN4).
Evaluation of the modeled results using an error matrix at the acquisition time showed that 67% and 68% of the pixels were correctly classified from the flood extents derived from Capella Space and Sentinel-1, respectively. The maximum flood extent from Sentinel-1 data decreased to 45% correct classification when compared to the modeled results at the peak of the flood. This is consistent with the acquisition times, which were taken before and two days after the flood peak, missing the peak flood moment. SAR-based water levels showed an overall precision of 0.141 m for Capella Space and 0.156 m for Sentinel-1. Agreement with water level gauge measurements was better in flatter, less vegetated areas and lower in steep, vegetated areas. Achieving consistent 20 cm water level accuracy (as required by the Dutch Ministry of Infrastructure and Water Management), across the study area remains complex. Both methods are prone to false positives and negatives, especially in areas with steep slopes, narrow canals, high vegetation, or roads. False classifications result in inaccurate flood extents, thus decreasing water level accuracy. Higher resolution of Capella Space images provided better alignment with the maximum flood extent, while Sentinel-1 images have wider coverage but missed the timing of the flood peak. In the end, the choice of SAR-system depends on timing, surface characteristics, and mapping extent needs. ... Read more

Ocean surface topography (OST), the hills and valleys of the ocean surface, provides information on several physical phenomena at different spatial and temporal scales. At scales between 100 km and 500 km, the mesoscales, the currents of the ocean flow between the topographic highs and lows. OST measurements from nadir radar altimeters have facilitated the study of the mean oceanic flow and substantially improved oceanographic knowledge. At the smaller submesoscales, 10 km to 100 km, variations of the OST are related to eddies and fronts. Additionally, tropical and extratropical cyclones produce strong disturbances of the surface topography. At the lower end of the submesoscales, with wavelengths of only a few kilometers, internal waves produced by the interaction of tidal currents and bottom topography leave perturbations on the height of the surface.

Synthetic-aperture radar (SAR) is a unique remote sensing instrument, particularly at C-band, capable of sensing the ocean surface at the submesoscales, with a wide swath, and in nearly all weather conditions. Harmony, the European Space Agency’s 10th Earth Explorer, features two SAR companion satellites. Two, out of a total of five, years of the mission’s life will be spent in a formation where the system will operate as a cross-track interferometer. Cross-track interferometry (XTI) is a technique that estimates the relative height of the surface from two SAR images of the same scene. Thus, Harmony could theoretically retrieve variations of the OST. In other words, the system could operate as a bistatic wide-swath ocean altimeter (WSOA). At the same time, Harmony will retrieve stress-equivalent wind fields, and instantaneous surface currents. Therefore, Harmony has the potential of providing an unprecedented wealth of co-located simultaneous data related to the ocean and the atmosphere. The aim of this thesis is to devise a method to estimate submesoscale ocean surface topography with a bistatic SAR interferometer, such as Harmony.

Assessing the design of a bistatic WSOA requires knowledge of the interferometric sensitivity, and temporal lag. The first obstacle that we encountered was that there is no model or analytical expression for these parameters that apply to a bistatic SAR with a squinted line of sight. The established relations found in the literature assume a zero-squint geometry. Hence, we use the Fourier Diffraction Slice Theorem to derive an analytical expression for the interferometric sensitivity, and the temporal lag. We show that forming an interferogram aligns the regions of support of the two images in the Fourier domain at each resolution cell, and that the temporal lag is the time offset that aligns the two regions. The sensitivity is equal to the vertical component of the aligned wave vectors projected on the elevation direction. We verify our results using simulations and confirm that our analytical expressions agree with the well-established relations for sensitivity and temporal lag for zero-squint systems.

We use the analytical expressions of sensitivity and temporal lag to build an interferometric performance model that computes the standard error of the height estimate for a formation-flying cross-track interferometer. The model considers the following random error sources: temporal decorrelation, thermal noise, spectral shift, volumetric decorrelation, and the effect of removing the phase due to motion of the surface using the individual phase centers of the instruments. Additionally, we derive a relation between the formation parameters that, when satisfied, minimizes the effective temporal lag, while maximizing the interferometric sensitivity. We then proceed to assess the performance over an orbit and along the 250 km-swath of an optimized formation.

Finally, we propose a data-driven algorithm to synchronize the signals of the independent SAR receivers. The algorithm achieves an unbiased root mean square error of 0.010◦ , reducing the phase synchronization error to within the error budget allocation.

Overall, the thesis presents how one can design, analyze, and retrieve relative ocean topography at the submesoscales with a bistatic SAR interferometer. It sets the foundations for an experimental OST product for the Harmony mission. Adding such a product to the mission would offer the first simultaneously acquired observations of wind field, current field, directional wave spectrum, and relative sea-surface height at high resolution and over a 250 km-wide swath. ... Read more

Spaceborne sensors, particularly Synthetic Aperture Radar (SAR), provide valuable tools for monitoring agricultural resources, improving yield predictions, and ensuring sustainable farming practices. In this research, we explore several venues to advance the use of SAR observation time series for agricultural and vegetation monitoring applications.
The first part of this research evaluates the added value of Sentinel-1 InSAR coherence time series for land cover classification, using an agricultural region in São Paulo, Brazil, as a case study. This region is characterized by a mixture of crops, pastures, and sugarcane plantations, all managed asynchronously. The findings demonstrate that incorporating InSAR coherence alongside SAR backscatter improves classification accuracy, particularly during the dry season when the distinctions between vegetation and bare soil are more pronounced. The research employed machine learning approaches to analyze pixel-level and field-level classifications using different sampling schemes. It highlights how multi-looking strategies can be adjusted to improve the accuracy of the classification outcomes in agricultural settings. This research highlights the usefulness of coherence data for the detection of events such as harvesting, offering valuable insights for more dynamic agricultural monitoring. The sensitivity of the coherence to agricultural changes leads to the observed improvement in Land Use Land Cover (LULC) mapping.
Forward models, or observation operators, are essential for the interpretation of radar observations and for the development of assimilation frameworks. In particular, in this research, we are interested in forward modeling the relation between crop bio-geophysical parameters, such as Leaf Area Index (LAI), Above Ground Biomass (AGB), and soil moisture, the inputs to our data-driven model, and radar observables, the outputs.
In the second part of this research, we integrate an existing crop growth model, the Decision Support System for Agrotechnology Transfer (DSSAT), with machine learning techniques to train a forward model to predict SAR observables over silage maize fields in The Netherlands across multiple years. Using crop growth models circumvents the dependency on limitedly available field measurements. When we use training and validation data from the same growth season, we obtain accurate predictions, with a mean absolute error (MAE) of less than 1.23 dB. Some of the field-to-field variability is accounted for by including the mean backscatter intensity during a few acquisitions before crop emergence. The obtained performance suggests the potential of using this approach to generate observation operators for data assimilation frameworks or for anomaly detection, supporting large-scale agricultural monitoring. However, the results also highlight one of the main challenges: the resulting data-driven model fails to generalize when presented with input bio-geophysical parameters that fall outside the regions of the parameter space spanned by the training dataset, as can happen, for example, during a drought period.
In the final part, we develop a physics-guided machine learning approach to address the limitations of data-driven models: lack of generalizability, tendency to overfitting, and reliance on extensive training data sets. We introduce physical constraints in an artificial neural network (ANN) in two ways. First, by modifying the loss function, used to train the ANN, by including a penalty for unphysical behavior, in particular by penalizing negative values of the partial derivative of the predicted backscatter intensity with respect to the surface soil moisture, since we assume this should always be positive. Second, by mirroring the architecture of the widely used Water Cloud Model (WCM) in the network topology. The added physical term to the loss function improves the ANN performance in all cases considered, with an R2 increase of 3 percentage points (p.p). The WCM-inspired model performs slightly worse when trained and tested with data from the same year, but it generalizes better, producing significantly better results for unseen conditions. In addition, the WCM-inspired model also produces individual contributions to the observed intensity, such as the surface-scattering component and the vegetation backscatter component. ... Read more

Over the past three decades, synthetic aperture radar (SAR) interferometry (InSAR) has become one of the most important Earth observation technologies in the world, and its use has become common in applications such as topographic mapping, monitoring earthquakes and volcanoes, as well as the built environment. Despite these advances, many technical and scientific challenges remain unsolved in the field, which prevent its use across diverse regions and biomes. One such type of region are wetlands and peatlands, which are notoriously challenging to monitor remotely due to poor signal quality and rapidly changing conditions between SAR acquisitions. This problem is particularly relevant in the Netherlands, because a significant portion of the country is composed of drained peat and clay soils which lie below sea level. These “soft soils” exhibit highly dynamic temporal behaviour that is closely linked to the phreatic groundwater system. In addition, they also exhibit a slow, irreversible subsidence caused by compaction and oxidation, the latter of which is a greenhouse gas (GHG) emitting process. It is this slow, irreversible subsidence component which scientists, governments, farmers and other stakeholders are trying to better understand, and evaluate the risks it poses. Previous efforts in monitoring the cultivated soft soil regions of the Netherlands by InSAR have been hampered by two main problems, which in this work are referred to as “cycle slips” and “loss-of-lock”. The former refers to consistent errors made in ambiguity resolution due to signals which exhibit such highly dynamic behaviour that standard algorithms cannot correctly interpret the wrapped phase data. The latter term refers to a permanent and irreparable loss of coherence in an interferometric SAR data stack. It is common in peatland regions for coherence levels to rise and fall seasonally, and in general, no coherent interferometric combination exists between the coherent periods. This condition means that the interferometric time series is severed during these incoherent periods, and only intermittent, disconnected temporal subsets of data are useable... ... Read more

In the current generation, most land surface models (LSMs) do not explicitly model the plant hydraulic states or fluxes, which limits the ability of LSMs to model evapotranspiration [1], stomatal conductance [2], and monitor and predict drought [3]. Therefore it is necessary to constrain the canopy water dynamics in LSMs. Advanced SCATterometer (ASCAT) provides a long record of C-band backscatter since 2007. A key advantage of the ASCAT instrument is the ability to obtain measurements of the Earth’s surface from different incidence angles. The dependence of ASCAT backscattering coefficient (hereafter referred to as backscatter) on incidence angle provides valuable information about vegetation water dynamics via normalized backscatter (σo 40), and vegetation parameters (slope (σ′), and curvature (σ′′)) of the Taylor expansions of backscatter to incidence angle [4–7]. In this thesis, the ASCAT normalized backscatter and slope are assimilated into the "Interactions between soil, biosphere and atmosphere" (ISBA-A-gs, hereafter referred to as ISBA) LSM.
In order to assimilate ASCAT observables, an observation operator is needed to link between LSM states to radar observations. Radiative transfer models (RTMs) are often used to assimilate radar backscatter into LSM [8, 9]. However, RTMs require moisture content or dielectric properties of soil and vegetation cover which are not simulated by the LSM. Therefore, to directly link land surface states and ASCAT observables, a Deep Neural Network (DNN) was trained and validated in Chapter 3. The performances and sensitivity of theDNNwere evaluated tomake sure the observation operator is physically plausible... ... Read more

Fire both shapes and destroys forests. Forest fires are therefore essential to ecological processes and vegetation as we know them. With climate change, forest fires are expected to increase in severity and frequency. To maintain the functioning of the forests, it is important to understand the vegetation response to and recovery from forest fires. Within this field of study, remote sensing techniques are common, and optical indices such as the NDVI are most prevalent.

With recent developments, the ASCAT variables slope and curvature have become of increasing interest in structural vegetation monitoring. These variables are a second-order Taylor polynomial’s first and second derivatives used to normalise the ASCAT backscatter-incidence angle relationship. This study explored the possibility to use these novel variables in forest fire research.
The focus was to discover to what extent the variables responded to a major forest fire.

To do so, grid points affected during the 2009 Australian Black Saturday Fires are compared to unaffected control grid points utilizing Z-scores. These control grid points have been selected based on time series similarity in the two years before the fire. Time series from 2007 to 2021 are used to investigate fire impact and recovery. The ASCAT variables are compared to a similar
NDVI time series to aid in interpreting the results.

The findings show that both slope and curvature are sensitive to the major forest fire. Both variables show an impact shortly after the fire, which can be explained by the loss of scattering elements in the vegetation due to the fire. In the following years, there is a notable recovery which can be explained by the vegetation regrowth forming new scatterers. The NDVI showed similar
behaviour but the recovery was differently timed, suggesting that the signal recovery is driven by something else or that the regrowth of leaves is different from the regrowth of the structural elements that the ASCAT variables represent.

The results help in understanding the ASCAT variables and their interpretation in terms of vegetation scatterers. Especially for the curvature, the clear change in signal deflects the discussion of whether the variable has information potential. Nevertheless, for the ASCAT variables to be applicable in forest fire research, they need to be better understood and additional research is necessary. Suggestions are for example a ground validation study or a global forest fire study, which would suit the coarse resolution of the ASCAT variables better and improve the understanding of the interaction between forest fires and the variables.

Although using the ASCAT variable for forest fire research is in its infancy, the results about its suitability are promising, both for ASCAT product development, as well as for the forest fire research field. Exploring the possibilities further is worthwhile, especially considering the fact that the slope and curvature time series cover a long continuous period starting as early as 1991. This long time series makes the ASCAT variables suitable for long-term forest fire monitoring, and the daily nature of the data might also make them interesting for short-term monitoring. ... Read more

This thesis developed a forward model for Sentinel-1 C-band co-pol and cross-pol backscatter and coherence using crop biophysical variables including leaf area index, tops weight, surface soil moisture and root zone soil moisture as inputs for sugarbeet. These input variables are simulated using a crop model called Decision Support System for Agrotechnology Transfer (DSSAT). The prediction of SAR signals is conducted using random forest regression model across all the sugarbeet fields in Noord-Brabant, the Netherlands. The correlation between simulated variables and the C-band SAR observables is investigated, as well as an evaluation of the effect of different feature combinations.
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How to deal with the presence of weather affected data is an unavoidable topic in the processing of optical imagery. Clouds and cloud shadows significantly alter the spectral signatures obtained from satellite data, which often leads to problems for any kind of scientific analysis. In this research there has been elaborated on two different kind of problems: The detection of clouds and cloud shadows and the mitigation of the effect caused by cloud shadows. Most of existing operational cloud detection algorithms are so-called rule-based. Their performance is highly variable and they have their limitations. A new promising research was done by Mohajerani and Parvaneh (2019), where a convolutional neural network (CNN) named ’Cloud-Net’ was developed. In this study we have elaborated on this CNN, by converting the analysis to Sentinel-2 data and making significant modifications on the model setup. The results have been compared to the ESA Scene Classification Map (SEN2COR algorithm). It was found that for the detection of clouds the overall CNN accuracy outperforms the ESA Scene Map (95.6% vs. 92.0% respectively). For the detection of cloud shadows the modified Cloud-Net model also gave better results (90.4% vs. 84.4%). Previous work on cloud shadow correction algorithms show rather complex and inconvenient methods, where the only goal was to remove the effect of the shadow. If one is interested to also correct for illumination effects, to make it more aligned to a predetermined ground truth, new possibilities arise which allows for simpler and more direct methods. Two proposed methods have been investigated in this study. The first method, called ’decomposition of components’, investigated the use of a single formula. The affected cloud shadow pixel is corrected based on the RGB difference with a ground truth image, and a single correction factor that was determined based under the assumption that cloud shadows cause a homogeneous alteration effect in a small area. The second method, called the ’CNN based method’, presents a totally new idea by changing the Cloud-Net model to a regression model, in order to correctly alter cloud shadow affected pixels. The performance of both methods was quantified by the structural similarity index measure (SSIM). It was found that the decomposition of components method has the most potential, showing significant improvements on the correction of cloud shadow affected areas. ... Read more

From three coherent SAR images it is possible to estimate three interferograms. Combined in a circular way, the sum of the three interferometric phases is called the closure phase which necessarily adds up to zero on a single pixel level. However, if the interferograms are spatially averaged, phase consistency is not guaranteed. In most of the interferometric studies, those mismatches were assumed to be caused by decorrelation noise alone, and were either not considered or deemed negligible, eluding further investigations of its origin. However, recent publications have confirmed that inconsistent phase closures are systematic and not the exception, pointing to an underlying geophysical cause. Comparisons of the spatial signatures of phase closures with land cover maps suggest a spatial and temporal correlation that is related to the characteristics of different land cover types. Since interferometric measurements are sensitive to variations of the dielectric constant, those similarities have been attributed to dynamics in vegetation and soil moisture. A closure phase significance test developed at the Geoscience and Remote Sensing department at TU Delft aimed to increase the signal-to-noise ratio of this geophysical signal component by providing a significance ratio for phase closures. However, the sensitivity of (significant) phase closures to dynamics in vegetation and soil over different land cover types has not been assessed yet. Here we show that with enough averaging of the interferometric phase, the spatial and temporal characteristics of closure phase can be used to distinguish between different land cover types. We found that the degree of spatial averaging has a significant impact on both the phase closure values and its spatial and temporal consistency. The magnitudes of significant phase closures generally increased over low-vegetated land covers, suggesting that closure phases are most sensitive to soil moisture dynamics, whereas vegetation cover was associated with decreasing phase closure magnitudes and spatial inconsistency. Besides spatial averaging, significant differences were observed between closure phases from different polarizations. Furthermore, we found that amplitude backscatter and closure phase are spatially and temporally correlated, pointing to similar influencing mechanisms. Our results demonstrate the importance of applying a closure phase significance test and describe the effect of spatial averaging on the characteristics of phase closures with respect to different land cover types. We anticipate this study to provide useful steps towards using the closure phase for soil and vegetation monitoring in the future. For example, the findings could be used to further exploit potential synergies with amplitude backscatter for soil moisture retrieval from closure phase or develop more sophisticated methods for land cover mapping using InSAR. If not used for applications linked to land cover, vegetation or soil, being able to better predict the effect of those parameters on the interferometric phase and coherence, eventually enables to separate their contribution from other signals, such as deformation estimates. Additional research is needed to relate significant phase closures to moisture changes in vegetation. ... Read more

Oceans play a vital role in the regulation of Earth's intricate climate system. The majority of gas and energy exchanges occurring at the ocean-atmosphere interface are driven by small-scale, O(1 km), coupled processes. Despite its importance, too few observations have been able to capture the ocean-atmosphere coupling at scales smaller than O(10 km). This has led to poor parameterisation in climate models. In an effort to enhance our understanding of the ocean-atmosphere coupling, this study aims to test and improve upon two separate Marine Atmospheric Boundary Layer (MABL) characterisation methodologies (called algorithm 2A and algorithm 2B) put forward by Young et al., (2000). Algorithms 2A and 2B relate processed Synthetic Aperture Radar (SAR) image properties to a specific atmospheric state through use of surface-layer similarity theory and a combination of surface-layer and mixed-layer similarity theory respectively. Results of both methods indicate significant inherent limitations. Algorithm 2A's utility is curbed by uncertainty introduced during the estimation of Convective Boundary Layer depth Zi and spectral power-law extrapolation. Algorithm 2B suffers from uncertainty introduced by the dimensionless energy dissipation rate psi. As a result of these (and other) uncertainties, the estimated atmospheric instability can be off by a factor 2 or more. Further analyses suggest shortcomings in the applicability of the Geophysical Model Function (GMF). It is hypothesised that both employed GMFs underestimate the horizontal wind-field variance at scales relevant to turbulent convection, which subsequently manifests itself as (part of) the observed average 50% overestimation of absolute Obukhov length and subsequent 33% underestimation of the atmospheric instability. Additional research is required to support and quantify the GMF-induced underestimation hypothesis. Inspired by algorithm 2B, a third method (algorithm 2C) is developed which circumvents major uncertainties inherent to both algorithm 2A and 2B. However, due to limitations of its own, algorithm 2C is incapable of replacing algorithms 2A or 2B for a large range of atmospheric instabilities. If improved upon and successfully employed, spaceborne characterisation of the MABL could benefit climate studies by providing a wealth of continuous and global atmospheric-state measurements on scales previously unavailable. ... Read more

Ice jam events can be devastating for the environment, human infrastructure, and local population. During breakup season, it is of great importance to be informed about the river ice cover condition in order to mitigate breakup flood risk. The Athabasca River near FortMcMurray, located in Alberta, is particularly prone to ice jam events and subsequent floodings. Satellite remote sensing techniques provide the necessary means to monitor the ice cover. Because of the wide availability most research and operational services for SAR river ice classification are based on single- or dual-polarized images. However, such imagery is limited in its ability to distinguish certain river ice types and open water states. The research presented examines how SAR polarimetry influences the detecting possibilities of specific ice types. Sentinel-1 (dual-polarization), RADARSAT-2 (quad-polarization) and RADARSAT Constellation Mission (compact-polarization) data were used to classify river ice during breakup. This study was about analysing a stretch of the Athabasca River which is prone to ice jam formation. First, SAR images from the 2018-2019 and 2019-2020 breakup were studied to find the temporal and spatial patterns of the radar backscatter. Next, sample areas with known ice stage (sheet ice, ice jam or open water) were selected. The sample areas of each ice stage were compared to assess the influence of SAR characteristics, as incidence angle and overpass time. In the last part of this study, a Random Forest classification was implemented in which intensity, texture and polarimetric features were used. Results show that classification accuracies increase with the inclusion of polarimetric decomposition features and GLCM mean texture features by enhancing between class separability and reducing the misclassification. Accuracies of 85.6% (Kappa = 0.78), 91.2% (Kappa = 0.87) and 91.0% (Kappa = 0.87) were obtained for Sentinel-1, RADARSAT-2 and RCM, respectively. The majority of the confusion between classes was due to similarities at backscatter signatures in very small incidence angles, mainly between open water and sheet ice under melting conditions. Also sheet ice early in the breakup season was confused with ice jams. To reduce the likelihood of misclassification, it is recommended to only use images with incidence angles higher than 30º and to include polarimetric and texture features in a classifier. Additional improvements can be achieved when using expert knowledge for tracking, since previous SAR images can provide added information when one understands the temporal patterns of river ice breakup. Further research should be directed at the development of an automatic classification approach that should be able to detect ice jams during the entire ice covered season. Having more knowledge about river ice breakup may help to eventually develop a river ice forecasting system, which may significantly reduce flood risk. ... Read more

Oceans cover a significant part of the Earth's surface. The coupling between the upper ocean and the atmosphere is very complicated with defied theoretical understanding, while it is essential for climate studies, weather prediction, and marine ecosystems. With the advent of spaceborne Synthetic Aperture Radar (SAR) systems, surface signatures of ocean and atmospheric processes have been revealed. As winds blowing over the ocean excite the wind waves, all undulations of the ocean surface are assumed as waves in this study. The primary sources for ocean surface signatures in SAR images are waves that are created by the exertion of the local wind stress. Wind waves cause changes in the backscattered power due to three mechanisms: specular reflections, Bragg scattering, and a contribution from wave breaking. A statistical multi-static normalized radar cross-section (NRCS) background model in terms of the directional wave spectrum is developed, considering both Bragg and non-Bragg mechanisms for various polarization states. As the qualitative comparison between optical and SAR data reveals a significant correlation in sea surface signatures, a synthetic attempt is made to estimate the SAR signals from optical signatures. This is realized with the transformation of the wave spectrum in a nonuniform medium, as a consequence of surface currents, and varying near-surface wind fields. A comparison between modeled NRCS and observations is presented. This modulated NRCS model advances the quantitative interpretation of the upper ocean dynamics from satellite measurements. ... Read more

The Netherlands has for long been witnessing problems due to subsidence. While urban infrastructure is mostly safeguarded from differential settlement by deep pile foundations, the same cannot be said about greenhouses. The greenhouses of the Netherlands has enabled the country to become the largest exporter of vegetables, second only to the United States of America. It is imperative that these greenhouses which form the backbone of the agriculture infrastructure of the country, are monitored continuously to minimize unprecedented damage.

This thesis aims to study the feasibility of using time series Interferometric Synthetic Aperture Radar (InSAR) as a means to monitor differential settlement in greenhouse structures. The analysis was primarily done using RADARSAT-2 data. In case of translucent surfaces of greenhouses, it was important to firstly identify the physical targets that are associated to scattering centres. This was done by analysing the statistics of the heights of the scatterers which helps in ascertaining where the radar signal is getting back-scattered from. It was inferred that the persistent and distributed scatterers are primarily identified from objects on the roof and outer walls of the greenhouses.

Moreover, the magnitude of deformation estimated from the scatterers have been corroborated with geotechnical data. It was seen that higher magnitudes of deformation was seen in locations with compressible soil types such as clay and peat close to the ground surface. It was also seen that greenhouse structures are prone to differential settlement when the depths of the piles are insufficient in areas with varying soil types. The effect of thermal contributions has also been studied and it was found that the estimation of thermal expansion does not significantly affect the estimated deformations.

From the study, it is evident that time series InSAR offers an effective means to monitor differential settlements in greenhouses. In order to check for differential settlement in individual greenhouses, it is proposed that a persistent scatter interferometric analysis be done initially and if it is seen that the density of these scatterers is insufficient, the analysis can be followed up with time series interferometry of distributed scatterers. Incorporating multiple track directions of radar data increases the number of greenhouses that can be monitored. Moreover, it was also seen that persistent scatterers were identified from additional greenhouses when Sentinel-1 data was used, despite its poorer spatial resolution. For further study, it is recommended that corner reflectors are used to validate the positions of the targets that are identified as persistent and distributed scatterers.
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Natural or man-made disasters can have a drastic impact on social, economic and environmental aspects of an affected population. Specifically, earthquakes are one of the most potent natural hazards, which cause a disproportionate amount of fatalities, primarily due to a) unexpected building collapses, b) restricted or limited access to basic amenities and c) potential hazards following earthquakes such as landslides, tsunamis etc. It is crucial to have an overview of the infrastructural damage caused following a disaster for search and rescue services to assess the extent of the damage. For the purpose of this research, Sentinel 1 imagery is used to map the building damage in an urban area after a disaster. A combination of parameters such as persistent scatterers, pixel amplitude and phase is used with a timeseries of full-resolution and spatially averaged radar images. Points that are stable in amplitude over a long timeseries, also known as Persistent Scatterers, are extracted from a stack of full-resolution images. The amplitudes of persistent scatterers, along with amplitude and coherence of pixels derived from a stack of spatially-averaged images, are statistically analysed to check the trends of the parameters pre- and post the disaster. A change detection algorithm is applied to this stack in order to localise the areas of building damage. The results are superimposed on Google Earth for easy interpretation using a graded damage scale. The analysis shows that exploiting the persistent scatterer amplitudes in the manner used in this research provides a novel way of locating building damage. This technique can be used effectively in urban areas. Using a combination of pixel amplitudes and coherence along with the persistent scatterers helps correctly find new and unique points of damage for each parameter used. The results were validated using reference Grading and crowd-sourced maps. The results illustrate that the proposed approach can be used for detecting and producing informative maps on infrastructural damage detection in urban areas. ... Read more

Blue ice areas, are areas in Antarctica where, either due to local heat sources (areas with lower albedo and thus more absorption of shortwave radiation - i.e. Nunataks) or high windspeed, all the snow is melted or eroded away and the underlying (blue) ice is visible. This occurs often around the grounding line between the ice sheet and ice shelf. At this grounding line area, a micro climate exists above the blue ice, which increase surface melt, due to a combination of decreased albedo and warming due to the mixing of cold and warm air. Detection of surface melt on this blue ice is important because this warmer surface melt water results in the increase of hydrofracturing and as a result, the decrease of ice shelf stability. Radar imagery above snow areas is a effective method to detect surface melt, which also ensures a continuous data record. Above blue ice, this is continuous data record of surface melt is also desired, but not done yet and therefore the focus of this thesis is surface melt detection on blue ice with radar imagery. By using the method of Hui et al., 2014 to classify blue ice areas, it is shown that the blue ice area extent (non-stable blue ice) is increasing over the years in the peak of the melt season. However, the extent is slightly decreasing during the non-melt season (stable blue ice). The data of Sentinel-1B is used during the austral summer of 2017/2018, to detect
surface melt on blue ice. This is done via interferometry (and the corresponding coherence) and with the backscatter coefficient. Coherence turns out the be an unreliable method to detect surface melt, since the influence of wind and precipitation on the decrease of coherence is dominant. Thus, surface melt detection via this method is difficult. Backscatter showed some potential to detect surface melt on blue ice, but due to the larger standard deviation than the actual decrease of backscatter (assumed due to surface melt), a clear distinction between blue ice and surface melt can not be made. Melt features, such as rivers, lakes and ponds are detectable with the backscatter, due to their distinctive shape. Since these melt features are linked to surface melt, backscatter can indirectly be used to detect surface melt on blue ice. ... Read more