Konstantinos Papathanassiou

• German Aerospace Center (DLR)

The main goal of the TerraSAR-X Add-On for Digital Elevation Measurements (TanDEM-X) mission is the generation of a global digital elevation model (DEM) of unprecedented accuracy and coverage. The global TanDEM-X DEM product became available in 2016, surpassed all expectations, and became a reference for a wide range of Earth science, commercial, a...

A R T I C L E I N F O Editor: Jing M. Chen Keywords: Forest height Fusion Lidar GEDI InSAR TanDEM-X Model-based inference A B S T R A C T NASA's Global Ecosystem Dynamic Investigation (GEDI) mission provides billions of lidar samples of canopy structure over the Earth's temperate and pantropical forests. Using the GEDI sample data alone, gridded he...

Tomographic Synthetic Aperture Radar (SAR) allows the reconstruction of the 3D radar reflectivity of forests from a large(r) number of multi-angular acquisitions. However, in most practical implementations it suffers from limited vertical resolution and/or reconstruction artefacts as the result of non-ideal acquisition setups. Polarisation Coherenc...

In this letter, we discuss the estimation of the location of ionospheric irregularities exploiting the appearance of intensity scintillations in ALOS-2 images as they are semi-focused at different heights. The intensity scintillations (stripes) are not always visible in the Single-Look Complex (SLC) images. However, they start to be visible as the...

Model-based forest height inversion from single-and multi-baseline Pol-InSAR data is today well-established. One of the critical performance points is the parameterisation of the vertical reflectivity profile. In this paper, the parameterisation of the vertical reflectivity profile using a tomographic reconstruction is proposed. To mitigate the lim...

Combining machine learning with physical models can significantly impact retrieval algorithms designed to invert geophysical parameters from remote sensing data. Such hybrid models integrate physical knowledge with domain expertise through a joint architecture, potentially enhancing performance by increasing the efficiency and flexibility of the ph...

The present study addresses the development, implementation and validation of a forest height mapping scheme based on the combination of TanDEM-X interferometric coherence and GEDI waveform measurements. The very general case where only a single polarisation TanDEM-X interferogram, a set of spatially discrete GEDI waveform measurements and no DTM a...

This paper addresses the implementation of an above ground biomass (AGB) estimation scheme relying on the height-to-biomass allometry at stand level in the context of the synergistic use of continuous TanDEM-X (bistatic) interferometric synthetic aperture radar acquisitions and spatial discrete GEDI waveform lidar measurements. The estimation of fo...

ESA Forest Carbon Monitoring project (FCM) is developing Earth Observation based, user-centric approaches for forest carbon monitoring. Forest carbon accounting based on forest inventory requires precise and timely estimation of forest variables at various spatial levels accompanied by verifiable uncertainty information. In this paper, we present t...

The combination of TanDEM-X interferometric measurements with GEDI lidar full waveform measurements can provide continuous high-resolution forest height maps at global scale with sufficient accuracy without using external information about the underlyingtopography. In previous studies, the GEDI lidar full waveforms have been used to provide an appr...

Recently, the German TanDEM-X mission provided valuable acquisitions for developing models for the single-pass single-pol X-band forest height inversion. In this paper, the assessment of the two machine learning approaches to estimate forest height from the interferometric coherence are investigated and compared to the state-of-art physical models...

Different approaches have been proposed in recent years to map glacier zones using Synthetic Aperture Radar (SAR). Most of them rely on empirical or statistical approaches for the interpretation of backscatter intensity measurements and their seasonal variations related to the melting regime of the different glacier zones. This study focuses on the...

The knowledge of ice zones extent and their temporal variations is fundamental for the retrieval of surface mass balance of glaciers and ice sheets. In this context, a key parameter is the firn line (FL), the lower boundary of the firn zone, whose location is an indicator of time-integrated mass balance changes. Several approaches have been develop...

Allometric relations that link forest above ground biomass to top forest (i.e. canopy) height are of particular significance in the context of lidar and interferometric Synthetic Aperture Radar (SAR) remote sensing, as both techniques allow accurate height measurements at ecologically relevant spatial scales. Besides the often unknown allometry its...

Synthetic Aperture Radar (SAR) measurements are unique for mapping forest 3D structure and its changes in time. Tomographic SAR (TomoSAR) configurations exploit this potential by reconstructing the 3D radar reflectivity. The frequency of the SAR measurements is one of the main parameters determining the information content of the reconstructed refl...

The Total Electron Content (TEC) is probably the most important single parameter to characterise the ionospheric state and its impact on Synthetic Aperture Radar (SAR) data. Its accurate estimation is therefore essential when it comes to the correction and calibration of SAR data distorted by the ionosphere. In recent years, the estimation of Farad...

Polarimetric and (multi-baseline) interferometric techniques are promising tools to investigate the subsurface properties of glaciers and ice sheets, due to the signal penetration of up to several tens of meters into dry snow, firn, and ice. Two different lines of research were addressed in recent years. The first is based on PolSAR, which provides...

The influence of the subsurface properties of ice sheets on polarimetric synthetic aperture radar interferometry (Pol-InSAR) measurements is well known. In order to invert this relationship for the extraction of geophysical parameters from Pol-InSAR data, models of the subsurface scattering structure are required. One potential application is the e...

Model-based forest height inversion from Pol-InSAR data relies on the realistic parameterization of the underlying (vertical) radar reflectivity function. In the context of interferometric TanDEM-X measurements - especially in the global single pol DEM mode - this is not possible due to the limited dimensionality of the observation space. In order...

Synthetic Aperture Radar (SAR) waves at low frequency can penetrate through the different forest layers down to the ground. The received signal contains information not only on the 3-D distribution of the vegetation scatterers, but also on the underlying ground. Using multiple acquisitions, SAR tomographic (TomoSAR) techniques estimate vertical re-...

The ionosphere effects on microwave propagation and degrades the image quality of space-borne SAR acquisitions. Despite of its reduced effect at high frequencies, phase distortions on the interferograms are often visible on X-band TSX/TDX data. In this paper, the ionospheric phase distortions associated with the small-scale irregularities in TSX/TD...

Polarization Coherence Tomography addresses the TomoSAR inversion by approximating the vertical reflectivity profile by a weighting functions of a basis. The individual weights can be estimated from the available interferometric coherences. This concept becomes particularly suitable for space borne TomoSAR implementations over forest volumes in whi...

The estimation and monitoring of 3D forest structure at large scales strongly relies on the use of remote sensing techniques. Today, two of them are capable to provide 3D forest structure estimates: lidar and Synthetic Aperture Radar (SAR) configurations. The differences in wavelength, imaging geometry, and technical implementation make the measure...

Synthetic aperture radar tomography (TomoSAR) at lower frequencies allows the reconstruction of the 3-D radar reflectivity of volume scatterers allowing access to their physical 3-D structure by means of multiangular SAR acquisitions. The performance of the reconstruction critically depends on the number and (spatial) distribution of the tomographi...

TanDEM-X experiments have shown that forest height can be estimated with single polarization X-band interferometric coherences. An external digital terrain model (DTM) not only allows to use both coherence magnitude and phase information, but also to overcome X-band penetration limitations. However, DTM information is not available for large areas....

The application of polarimetric Synthetic Aperture Radar (SAR) to forest observation for mapping, classification and parameter estimation (especially biomass) has a relatively long history. The radar penetration through forest volume, and hence the multi-layer nature of scattering models, make fully polarimetric data the observation space enabling...

The retrieval of snow and firn properties on large scales is essential for a wide range of cryosphere applications and research questions, implying the necessity to employ remote sensing. Among the existing remote sensing techniques, synthetic aperture radars (SARs) allow monitoring polar regions independently of sun illumination and in (nearly) al...

Synthetic Aperture Radar Interferometry (InSAR) is able to provide important information for the characterization of the surface topography of glaciers and ice sheets. However, due to the inherent penetration of microwaves into dry snow, firn, and ice, InSAR elevation models are affected by a penetration bias. The fact that this bias depends on the...

This article proposes a change detection and analysis technique for monitoring the phenological development of agricultural vegetation by means of multitemporal Polarimetric Synthetic Aperture Radar (PolSAR) acquisitions. The technique relies on the generalized eigendecomposition of the polarimetric covariance matrices of the individual acquisition...

Recent TanDEM-X experiments have shown that forest height can be estimated with the single polarization X-band interferometric coherences. However, an external digital terrain model (DTM) is necessary to retrieve canopy height. Forest height derived from coherence model inversion is known as to be only accurate in certain condition like boreal fore...

The penetration of microwave signals into snow and ice, especially in dry conditions, introduces a bias in digital elevation models generated by means of synthetic aperture radar (SAR) interferometry. This bias depends directly on the vertical backscattering distribution in the subsurface. At the same time, the sensitivity of interferometric SAR me...

The full waveform GEDI lidar mission will map forest height and biomass at 1 km resolution globally in temperate and tropical forests. In order to obtain biomass maps with higher resolution and overcome the limitations of the footprint sampling on ground, the possibility to combine GEDI waveforms and TanDEM-X coherences is currently considered usin...

Knowledge of the spatial patterns of successional stages (i.e., primary and secondary forest) in tropical forests allows to monitor forest preservation, mortality and regeneration in relation to natural and anthropogenic disturbances. Different successional stages have also different capabilities of re-establishing carbon stocks. Therefore, a succe...

At lower frequencies, synthetic aperture radar images can be significantly affected by ionospheric scintillations. In thispaper an approach that combines the Faraday rotation and the autofocus estimation method is proposed, which achieves asuperior performance when compared to using these techniques independently. This improvement is demonstrated w...

Forest structure is a key parameter for forest applications, but it is difficult to be estimated at the required spatial and temporal scales. In this context, synthetic aperture radar Tomography (TomoSAR) that allows, at lower frequencies, the 3-D imaging of natural volume scatterers with high spatial and temporal resolution may be a game changer....

Compact polarimetry is a dual-polarization hybrid imaging mode and provides partially backscatter characteristics of scatterers. The compact polarimetric (CP) synthetic aperture radar (SAR) measures the combinations of fully polarimetric (FP) scattering coefficients, which is dependent on the transmit-wave polarization state. The backscattered wave...

Digital elevation models generated with SAR interferometry (InSAR) are an important information source for glacier and ice sheet mass balance. However, the measured elevations suffer from a penetration bias due to the interferometric phase center being up to several tens of meters below the surface. The penetration of the microwave signals depends...

The need of large scale observations with high temporal frequency has promoted airborne and satellite remote sensing techniques for glaciological applications. In particular, active microwave sensors, such as synthetic aperture radars (SARs), offer all-weather and daylight independent operability which is of great advantage at high latitudes, where...

This work introduces a method to separate the ground and volume contributions from Polarimetric SAR Interferometric acquisitions. Based on the employed two layer model, the radar response may be decomposed into these two main contributions and the full rank polarimetric covariance matrices of the corresponding ground and volume layers may be extrac...

NASA’s Global Ecosystem Dynamics Investigation (GEDI) waveform lidar is expected to provide unprecedented measurements of forest structure and biomass in tropical and temperate environments. In order to bridge the limitations induced by the ground sampling of the GEDI waveforms, and to obtain enhanced forest structure estimates, the potential of co...

The objective of this paper is to address the penetration capabilities of different synthetic aperture radar (SAR) frequencies in forest scenarios in terms of “visibility” of the ground. First, the ratio between the ground and the volume scattering powers is estimated from multi-baseline polarimetric interferometric (Pol-InSAR) data across differen...

The estimation and monitoring of 3D forest structure at large scales strongly rely on the use of remote sensing techniques. Today, two of them are able to provide 3D forest structure estimates: lidar and synthetic aperture radar (SAR) configurations. The differences in wavelength, imaging geometry, and technical implementation make the measurements...

The wavelength of synthetic aperture radar (SAR) waves affects the sensitivity to scatterers depending on their size and the penetration through forest volumes. Therefore, forest backscattering at different wavelengths (from P- up to X-band) contains different structural information about the geometry of vegetation elements and their distribution i...

There is a common agreement on the relevance of L-band wavelengths (around 20 cm) for forest observation. For instance, the larger backscatter dynamic range increases the sensitivity to larger biomass gradients with respect to C- and X-band. Furthermore, L-band allows the penetration into and through dense forest canopies in all forest ecosystems,...

Advanced Multi-Parametric Synthetic Aperture Radar Techniques for Environmental Applications

Ionosphere imposes critical difficulties in the calibration of space-borne SAR systems operating at low frequencies. It degrades the focusing quality, induces positioning error, adds phase screen, and often undulates amplitude at low latitudes. The spatial inhomogeneity of the ionosphere imposes additional complicacy into the system calibration. Mo...

The analysis of data from an airborne synthetic aperture radar (SAR) campaign in the percolation zone of Greenland revealed an interferometric coherence undulation behavior with respect to vertical wavenumber, which cannot be explained with existing models. We propose a model extension that accounts for scattering from distinct layers below the sur...

Polarimetric SAR data are well known to provide a better characterization of a scattering scene compared to single-polarization measurements. In order to further extend the Observation space, multi-frequency (polarimetric) measurements can be considered. The joint use of different wavelengths allows to gain sensitivity about scatterers at different...

Synthetic aperture radar (SAR) remote sensing configurations are able to provide continuous measurements on global scales sensitive to the vertical structure of forests with a high spatial and temporal resolution. Furthermore, the development of tomographic SAR techniques allows the reconstruction of the three-dimensional (3-D) radar reflectivity o...

Synthetic Aperture Radar (SAR) Tomography (TomoSAR) allows reconstructing a vertical profile of the power backscattered by natural volumes by combining more than two SAR images acquired with slightly different incidence angles. Being an electromagnetic quantity, the profile depends on the radar frequency, polarization, and acquisition geometry, but...

The aim of this paper is to compare L- and P-band vertical backscattering profiles estimated by means of Synthetic Aperture Radar (SAR) Tomography (TomoSAR) and full LIght Detection And Ranging (lidar) waveforms in terms of their ability to distinguish different tropical forest structure types. The comparison relies on the unique DLR F-SAR and NASA...

Polarimetric SAR Interferometry (Pol-InSAR) is a SAR remote sensing discipline with unique and powerful applications related to the vertical structure of natural and man-made volume scatterers. The coherent combination of single- or multi-baseline interferograms acquired at different polarisations provides sensitivity to the vertical distribution o...

Synthetic Aperture Radar Tomography techniques provide 3D information of the forest due to the ability of microwaves to penetrate through vegetation. Recent studies link the radar 3D information to forest 3D structure in order to translate the tomographic results to an ecological interpretation. However, due to the undersampled nature of tomographi...

Recent TanDEM-X experiments have shown that the limited penetration capability at X-band in forest volumes allow the estimation of the height variability of the top canopy layer, which can be used as a proxy to the horizontal structure (i.e. heterogeneity), by using high resolution digital elevation models (DEMs). However, the use of an external di...

Ionospheric parameters e.g., ionospheric height, TEC and the level of disturbances, are estimated using L-band SAR data acquired by ALOS-2 PALSAR-2 over Fairbanks, Alaska. These parameters are compared with coinciding ground-based monitoring data form the Poker Flat Incoherent Scatterer Radar (PFISR) and keograms measured by the University of Alask...

Understanding the impact of soil and plant parameter changes in agriculture on Synthetic Aperture Radar (SAR) measurements is of great interest when it comes to monitor the temporal evolution of agricultural crops by means of SAR. In this regard, specific transitions between phenological stages in corn, barley, and wheat have been identified associ...

This paper presents an overview of the Ground Processor Prototype (GPP) being developed in the frame of ESA's Earth Explorer BIOMASS mission. In the first part of the paper, the main challenges to be addressed by the GPP are addressed, namely, the ionosphere, the radio-frequency interferences and the system and data calibration, together with the e...

All radar frequencies (from P- up to X-band) can image horizontal and/or vertical forest structure, although with different structure information content and sensitivity. The assessment of both is crucial for instance in understanding synergies and complementarities of multiple SAR bands towards enhanced forest structure products. In this work, the...

Recently a framework for forest structure based on TomoSAR data has been defined. Based on the spatial distribution of the reflectivity peaks, two complementary descriptors are evaluated at a given scale. This paper high- lights the dependence of the structure estimate on scale and shows, through analysis on real and simulated data that the restric...

Monitoring of changes in forest biomass requires accurate transfer functions between remote sensing-derived changes in canopy height (ΔH) and the actual changes in aboveground biomass (ΔAGB). Different approaches can be used to accomplish this task: direct approaches link ΔH directly to ΔAGB, while indirect approaches are based on deriving AGB stoc...

The aim of this paper is to interpret and characterize the changes of the 3-D polarimetric scattering signatures of agricultural crops at C-band and to relate them to temporal changes of the soil and plant parameters. For this, a time series of multibaseline (MB) synthetic aperture radar (SAR) data acquired at C-band by the airborne F-SAR system of...

Synthetic aperture radar (SAR) has proven to be a key remote sensing technique for Earth observation. However, conventional SAR systems are limited in that a wide coverage can only be achieved at the expense of a degraded resolution. Staggered SAR overcomes this limitation by means of digital beamforming and continuous variation of the pulse repeti...

Remote sensing systems have been demonstrated to be a powerful source of information for monitoring tropical forest ecosystems. In particular, Synthetic Aperture Radar (SAR) and LIght Detection And Ranging (lidar) systems provide measurements sensitive to 3-D forest structure parameters at high spatial and temporal resolutions. This is of critical...

Space borne implementations of SAR Tomography (TomoSAR) for forest volumes are particularly challenging even in a single-pass case, as the estimation of 3D reflectivity has to be carried out relying on a small number of acquisitions (or interferometric coherences) and/or non-uniform baseline distributions. A way to overcome this shortcoming is to a...

The penetration capability of microwaves into and through vegetation layers allows scattering interactions across the whole vertical vegetation extend. This, combined with the ability of multi-baseline Synthetic Aperture Radar (SAR) techniques to reconstruct the 3D radar reflectivity opens the door to the use of air- and space-borne SAR configurati...

Synthetic Aperture Radar Tomography (TomoSAR) allows the reconstruction of the 3D reflectivity of natural volume scatterers such as forests, thus providing an opportunity to infer structure information in 3D. In this paper, the potential of TomoSAR data at L-band to monitor temporal variations of forest structure is addressed using simulated and ex...