Ground-based synthetic aperture radar (GBSAR) interferometry for deformation monitoring

Ph. D ThesisGround-based synthetic aperture radar (GBSAR), together with interferometry, represents a powerful tool for deformation monitoring. GBSAR has inherent flexibility, allowing data to be collected with adjustable temporal resolutions through either continuous or discontinuous mode. The goal of this research is to develop a framework to effectively utilise GBSAR for deformation monitoring in both modes, with the emphasis on accuracy, robustness, and real-time capability. To achieve this goal, advanced Interferometric SAR (InSAR) processing algorithms have been proposed to address existing issues in conventional interferometry for GBSAR deformation monitoring. The proposed interferometric algorithms include a new non-local method for the accurate estimation of coherence and interferometric phase, a new approach to selecting coherent pixels with the aim of maximising the density of selected pixels and optimizing the reliability of time series analysis, and a rigorous model for the correction of atmospheric and repositioning errors. On the basis of these algorithms, two complete interferometric processing chains have been developed: one for continuous and the other for discontinuous GBSAR deformation monitoring. The continuous chain is able to process infinite incoming images in real time and extract the evolution of surface movements through temporally coherent pixels. The discontinuous chain integrates additional automatic coregistration of images and correction of repositioning errors between different campaigns. Successful deformation monitoring applications have been completed, including three continuous (a dune, a bridge, and a coastal cliff) and one discontinuous (a hillside), which have demonstrated the feasibility and effectiveness of the presented algorithms and chains for high-accuracy GBSAR interferometric measurement. Significant deformation signals were detected from the three continuous applications and no deformation from the discontinuous. The achieved results are justified quantitatively via a defined precision indicator for the time series estimation and validated qualitatively via a priori knowledge of these observing sites.China Scholarship Council (CSC), Newcastle Universit

Fourier Transform Hyperspectral Imaging for Cultural Heritage

Hyperspectral imaging is a technique of analysis that associates to each pixel of the image the spectral content of the radiation coming from the scene. This content can be helpful to recognize the chemical nature of the materials within the scene or to calculate their colours under particular conditions. Different solutions of hyperspectral imager have been realized with different spatial resolution, spectral resolution and range in the electromagnetic spectrum. In particular, improving the spectral resolution allows discriminating smaller features in the spectrum and the unambiguous detection of the absorption bands characteristic of superficial materials. Hyperspectral imagers based on interferometers have the advantage of having a spectral resolution that can be varied according to the needs by changing the optical path delay of the interferometer. A spectrum for each pixel is obtained with an algorithm based on the Fourier transform of the calibrated interferogram. We present the results of the application of a hyperspectral imager based on Fabry‐Perot interferometers to the field of cultural heritage. On different artworks, the hyperspectral imager has been used for pigment recognition, for colour rendering elaborations of the image with different light sources or standard illuminants and for calculating the chromatic coordinates useful for specific purposes

Rice Plant Height Monitoring from Space with Bistatic Interferometry

This chapter provides an overview of the possibility to derive paddy rice plant heights with spaceborne bistatic SAR interferometry (InSAR). By using the only available interferometer in space, TanDEM-X, an investigation of rice crops located in Turkey is performed. Before analyzing the main outcomes, an introduction to the generation of elevation models with InSAR is provided, with a special focus on the agricultural land cover. The processing chain and the modifications foreseen to properly produce plant elevations and a roadmap for the quality assessment are described. The results obtained, with a very high interferometric coherence supporting an accurate estimation due to a limited electromagnetic wave penetration into the canopy, support a temporal change analysis on a field-by-field basis. For the purpose, an automatic approach to segment the fields without external auxiliary data is also provided. The study is concluded with an analysis of the impact of the wave polarization in the results

Differential interferometry for multi-frequency ground-based SAR systems

Ground-Based Synthetic Aperture Radars (GB-SAR) are a type of Synthetic Aperture Radars (SAR) with a higher flexibility and lower cost than their satellite counterparts. The effectiveness of GB-SAR terrain displacement monitoring has already been proven with bare soils. There is interest in studying their effectiveness in areas covered by vegetation. The Multi-Frequency GB-SAR system developed by Balamis is able to operate in bands X, C, L and P. A measurement campaign took place in the "Castell de Subirats" to study an area covered by vegetation. Two Trihedral Corner Reflectors (TCR) were placed in the area, one of them surrounded by vegetation. Can we consider the reflectors stable? A big Rock is present in the area but is covered by vegetation. Can it be used as natural reflector for low bands? Is it possible to monitor displacements in the reflectors in all bands? In X-band the reflectors are not stable because of the surrounding vegetation. In C-band the TCRs are stable. In L-band one of the TCR and the Rock are stable. In P-band only the Rock is stable. The Rock can be used as a reflector and a calibrator for bands L and P as those frequencies can penetrate vegetation. It is possible to monitor the displacements, except in band X because of the phase shifts caused by the vegetation.Los Radares de Apertura Sintética Terrestres (GB-SAR en inglés) son un tipo de Radares de Apertura Sintética (SAR) con una mayor flexibilidad y un coste más bajo que sus homólogos de satélite. La eficacia de la monitorización de desplazamientos del terreno con sistemas GB-SAR ya se ha demostrado con suelos desnudos. Hay interés por estudiar su efectividad en zonas cubiertas de vegetación. El sistema Multi-Frecuencia GB-SAR desarrollado por Balamis es capaz de operar en las bandas X, C, L y P. Se realizó una campaña de medida en el "Castillo de Subirats" para estudiar una zona cubierta de vegetación. Se colocaron en la zona dos Reflectores de Esquinas Triédricas (TCR), uno de ellos rodeado de vegetación. Podemos considerar los reflectores estables? Hay una gran Piedra presente en la zona pero está cubierta de vegetación. Se puede utilizar como reflector natural para bandas bajas? Es posible medir los desplazamientos en los reflectores en todas las bandas? En banda X, los reflectores no son estables debido a la vegetación circundante. En banda C, los TCR son estables. En banda L, uno de los TCR y la Piedra son estables. En banda P, sólo la Piedra es estable. La Piedra se puede utilizar como reflector y calibrador para las bandas L y P, ya que estas frecuencias pueden penetrar en la vegetación. Es posible medir los desplazamientos, excepto en banda X debido a las inestabilidades de fase provocadas por la vegetación.Els Radars d'Obertura Sintètica Terrestres (GB-SAR en anglès) són un tipus de Radars d'Obertura Sintètica (SAR) amb una major flexibilitat i un cost més baix que els seus homòlegs de satèl·lit. L'eficàcia de la monitorització de desplaçaments del terreny amb sistemes GB-SAR ja s'ha demostrat amb sòls nus. Hi ha interès per estudiar la seva efectivitat en zones cobertes de vegetació. El sistema Multi-Freqüència GB-SAR desenvolupat per Balamis és capaç d'operar a les bandes X, C, L i P. Es va realitzar una campanya de mesura al "Castell de Subirats" per estudiar una zona coberta de vegetació. Es van col·locar a la zona dos Reflectors de Cantonades Trièdriques (TCR), un d'ells envoltat de vegetació. Podem considerar els reflectors estables? Hi ha una gran Pedra present a la zona però està coberta de vegetació. Es pot utilitzar com a reflector natural per a bandes baixes? És possible mesurar els desplaçaments als reflectors a totes les bandes? A banda X, els reflectors no són estables a causa de la vegetació circumdant. A banda C, els TCR són estables. A banda L, un dels TCR i la Pedra són estables. A banda P, només la Pedra és estable. La Pedra es pot utilitzar com a reflector i calibrador per a les bandes L i P, ja que aquestes freqüències poden penetrar la vegetació. És possible mesurar els desplaçaments, excepte a la banda X a causa de les inestabilitats de fase provocades per la vegetació

Geodetic monitoring of complex shaped infrastructures using Ground-Based InSAR

In the context of climate change, alternatives to fossil energies need to be used as much as possible to produce electricity. Hydroelectric power generation through the utilisation of dams stands out as an exemplar of highly effective methodologies in this endeavour. Various monitoring sensors can be installed with different characteristics w.r.t. spatial resolution, temporal resolution and accuracy to assess their safe usage. Among the array of techniques available, it is noteworthy that ground-based synthetic aperture radar (GB-SAR) has not yet been widely adopted for this purpose. Despite its remarkable equilibrium between the aforementioned attributes, its sensitivity to atmospheric disruptions, specific acquisition geometry, and the requisite for phase unwrapping collectively contribute to constraining its usage. Several processing strategies are developed in this thesis to capitalise on all the opportunities of GB-SAR systems, such as continuous, flexible and autonomous observation combined with high resolutions and accuracy. The first challenge that needs to be solved is to accurately localise and estimate the azimuth of the GB-SAR to improve the geocoding of the image in the subsequent step. A ray tracing algorithm and tomographic techniques are used to recover these external parameters of the sensors. The introduction of corner reflectors for validation purposes confirms a significant error reduction. However, for the subsequent geocoding, challenges persist in scenarios involving vertical structures due to foreshortening and layover, which notably compromise the geocoding quality of the observed points. These issues arise when multiple points at varying elevations are encapsulated within a singular resolution cell, posing difficulties in pinpointing the precise location of the scattering point responsible for signal return. To surmount these hurdles, a Bayesian approach grounded in intensity models is formulated, offering a tool to enhance the accuracy of the geocoding process. The validation is assessed on a dam in the black forest in Germany, characterised by a very specific structure. The second part of this thesis is focused on the feasibility of using GB-SAR systems for long-term geodetic monitoring of large structures. A first assessment is made by testing large temporal baselines between acquisitions for epoch-wise monitoring. Due to large displacements, the phase unwrapping can not recover all the information. An improvement is made by adapting the geometry of the signal processing with the principal component analysis. The main case study consists of several campaigns from different stations at Enguri Dam in Georgia. The consistency of the estimated displacement map is assessed by comparing it to a numerical model calibrated on the plumblines data. It exhibits a strong agreement between the two results and comforts the usage of GB-SAR for epoch-wise monitoring, as it can measure several thousand points on the dam. It also exhibits the possibility of detecting local anomalies in the numerical model. Finally, the instrument has been installed for continuous monitoring for over two years at Enguri Dam. An adequate flowchart is developed to eliminate the drift happening with classical interferometric algorithms to achieve the accuracy required for geodetic monitoring. The analysis of the obtained time series confirms a very plausible result with classical parametric models of dam deformations. Moreover, the results of this processing strategy are also confronted with the numerical model and demonstrate a high consistency. The final comforting result is the comparison of the GB-SAR time series with the output from four GNSS stations installed on the dam crest. The developed algorithms and methods increase the capabilities of the GB-SAR for dam monitoring in different configurations. It can be a valuable and precious supplement to other classical sensors for long-term geodetic observation purposes as well as short-term monitoring in cases of particular dam operations

Measuring thermal expansion using X-band Persistent Scatterer Interferometry

This paper is focused on the estimation of the thermal expansion of buildings and infrastructures using X-band Persistent Scatterer Interferometry (PSI) observations. For this purpose an extended PSI model is used, which allows separating the thermal expansion from the total observed deformation thus generating a new PSI product: the map of the thermal expansion parameter, named thermal map. The core of the paper is devoted to the exploitation of the information contained in the thermal maps: three examples are discussed in detail, which concern a viaduct, a set of industrial buildings and two skyscrapers. The thermal maps can be used to derive the thermal expansion coefficient of the observed objects and information on their static structure. In addition, the paper illustrates the distortions in the PSI deformation products that occur if the thermal expansion is not explicitly modelled. Finally, an inter-comparison exercise is described, where the thermal expansion coefficients estimated by PSI are compared with those derived by a Ku-band ground-based SAR campaign