Influence of Surface Roughness Spatial Variability and Temporal Dynamics on the Retrieval of Soil Moisture from SAR Observations

Radar-based surface soil moisture retrieval has been subject of intense research during the last decades. However, several difficulties hamper the operational estimation of soil moisture based on currently available spaceborne sensors. The main difficulty experienced so far results from the strong influence of other surface characteristics, mainly roughness, on the backscattering coefficient, which hinders the soil moisture inversion. This is especially true for single configuration observations where the solution to the surface backscattering problem is ill-posed. Over agricultural areas cultivated with winter cereal crops, roughness can be assumed to remain constant along the growing cycle allowing the use of simplified approaches that facilitate the estimation of the moisture content of soils. However, the field scale spatial variability and temporal variations of roughness can introduce errors in the estimation of soil moisture that are difficult to evaluate. The objective of this study is to assess the impact of roughness spatial variability and roughness temporal variations on the retrieval of soil moisture from radar observations. A series of laser profilometer measurements were performed over several fields in an experimental watershed from September 2004 to March 2005. The influence of the observed roughness variability and its temporal variations on the retrieval of soil moisture is studied using simulations performed with the Integral Equation Model, considering different sensor configurations. Results show that both field scale roughness spatial variability and its temporal variations are aspects that need to be taken into account, since they can introduce large errors on the retrieved soil moisture values

A New Model for Cross-polarization Scattering from Perfect Conducting Random Rough Surfaces in Backscattering Direction

abstract: Scattering from random rough surface has been of interest for decades. Several methods were proposed to solve this problem, and Kirchho approximation (KA) and small perturbation method (SMP) are among the most popular. Both methods provide accurate results on rst order scattering, and the range of validity is limited and cross-polarization scattering coecient is zero for these two methods unless these two methods are carried out for higher orders. Furthermore, it is complicated for higher order formulation and multiple scattering and shadowing are neglected in these classic methods. Extension of these two methods has been made in order to x these problems. However, it is usually complicated and problem specic. While small slope approximation is one of the most widely used methods to bridge KA and SMP, it is not easy to implement in a general form. Two scale model can be employed to solve scattering problems for a tilted perturbation plane, the range of validity is limited. A new model is proposed in this thesis to deal with cross-polarization scattering phenomenon on perfect electric conducting random surfaces. Integral equation is adopted in this model. While integral equation method is often combined with numerical method to solve the scattering coecient, the proposed model solves the integral equation iteratively by analytic approximation. We utilize some approximations on the randomness of the surface, and obtain an explicit expression. It is shown that this expression achieves agreement with SMP method in second order.Dissertation/ThesisMasters Thesis Electrical Engineering 201

FDTD Simulation Techniques for Simulation of Very Large 2D and 3D Domains Applied to Radar Propagation over the Ocean

abstract: A domain decomposition method for analyzing very large FDTD domains, hundreds of thousands of wavelengths long, is demonstrated by application to the problem of radar scattering in the maritime environment. Success depends on the elimination of artificial scattering from the “sky” boundary and is ensured by an ultra-high-performance absorbing termination which eliminates this reflection at angles of incidence as shallow as 0.03 degrees off grazing. The two-dimensional (2D) problem is used to detail the features of the method. The results are cross-validated by comparison to a parabolic equation (PE) method and surface integral equation method on a 1.7km sea surface problem, and to a PE method on propagation through an inhomogeneous atmosphere in a 4km-long space, both at X-band. Additional comparisons are made against boundary integral equation and PE methods from the literature in a 3.6km space containing an inhomogeneous atmosphere above a flat sea at S-band. The applicability of the method to the three-dimensional (3D) problem is shown via comparison of a 2D solution to the 3D solution of a corridor of sea. As a technical proof of the scalability of the problem with computational power, a 5m-wide, 2m-tall, 1050m-long 3D corridor containing 321.8 billion FDTD cells has been simulated at X-band. A plane wave spectrum analysis of the (X-band) scattered fields produced by a 5m-wide, 225m-long realistic 3D sea surface, and the 2D analog surface obtained by extruding a 2D sea along the width of the corridor, reveals the existence of out-of-plane 3D phenomena missed by the traditional 2D analysis. The realistic sea introduces random strong flashes and nulls in addition to a significant amount of cross-polarized field. Spatial integration using a dispersion-corrected Green function is used to reconstruct the scattered fields outside of the computational FDTD space which would impinge on a 3D target at the end of the corridor. The proposed final approach is a hybrid method where 2D FDTD carries the signal for the first tens of kilometers and the last kilometer is analyzed in 3D.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Ultrasonic thickness structural health monitoring of steel pipe for internal corrosion

The naphthenic acid corrosion that can occur in oil refinery process plants at high temperature (400ÃÂðC) due to the corrosive nature of certain crude oils during the refining process can be difficult to predict. Therefore, the development of online ultrasonic thickness (UT) structural health monitoring (SHM) technology for high temperature internal pitting corrosion of steel pipe is of interest. A sensor produced by the sol-gel ceramic fabrication process has the potential to be deployed to monitor such pitting corrosion, and to help investigate the mechanisms causing such corrosion. This thick-film transducer is first characterized using an electric circuit model. The propagating elastic waves generated by the transducer are then experimentally characterized using the dynamic photoelastic visualization method and images of the wave-field are compared with semi-analytical modeling results. Next, the classic elastic wave scattering theory for an embedded spherical cavity is reviewed, results are compared with a newer scattering theory from the seismology community, that has been applied to a hemispherical pit geometry. This hemispherical pit theory is extended so as to describe ultrasonic Non-Destructive Evaluation (NDE) applications, for pitting corrosion, with the derivation of a far-field scattering amplitude term. Data from this new scattering theory is compared with experimental results by applying principals from the Thompson-Gray measurement model. The initial model validation provides the basis for a possible new hemispherical pit geometric reference standard for ultrasonic NDE corrosion applications. Next, UT SHM measurement accuracy, precision, and reliability are described with a new weighted censored relative likelihood methodology to consider the propagation of asymmetric uncertainty in quantifying thickness measurement error. This new statistical method is experimentally demonstrated and applied to thickness measurement data obtained in pulse-echo and pitch-catch configurations for various time-of-flight thickness calculation methods. Finally, the plastic behavior of a corroded steel pipe is modeled with analytical and finite element methods to generate prognosis information

Analysis of Gnss-R Observations for Altimetry and Characterization of Earth Surfaces

Global Navigation Satellite Systems (GNSS) provide abundant, opportunistic signals that can be used to probe the Earth’s environment and surface. Utilizing reflected GNSS signals for remote sensing is called GNSS Reflectometry (GNSS-R). Sensing of the ocean, land, and ice, with potentially dense measurement coverage and rapid revisit times, is possible due to the distributed geometry of GNSS constellations. GNSS-R can provide some advantages over other Earth observation systems, like traditional radar altimeters or microwave radiometers. GNSS signals are well characterized and encoded with precise ranging and timing information. There are multiple transmitters in view at any time, and GNSS signals occupy a protected frequency band (L-band) that penetrates Earth’s atmosphere in all weather conditions.This dissertation focuses on the development of methods and analysis techniques to observe sea surface height and sea ice extent with reflected GNSS signals. A tool-kit is developed to take advantage of experimental data sets from aircraft and spacecraft, and to produce state-of-the-art altimetric retrievals. Algorithms for the re-tracking of altimetric delays are demonstrated. Techniques to characterize and models to correct GNSS-R path delay errors are built through analysis of TechDemoSat-1 (TDS-1) and NASA’s Cyclone Global Navigation Satellite System (CYGNSS) flight data. Neither TDS-1 nor CYGNSS were designed to make precise altimetry observations. Thus, this work evaluates practical performance limitations of these GNSS-R observations, and establishes requirements for future missions. Altimetry results with height retrieval standard deviation of σH = 11 m with 1 sec and σH = 3.8 m with 10 sec observations, are shown.This work creates a foundation of techniques that can support future GNSS-R missions dedicated to ocean surface altimetry by producing results with sufficient accuracy and precision to the ocean science community. These tools are built to inform future mission designs and aid scientific interpretation of GNSS-R measurements

Modélisation cohérente de la diffusion électromagnétique par des surfaces de mer tridimensionnelles en incidence rasante.: Application aux radars HF à ondes de surface.

This thesis deals with the analysis of the electromagnetic interactions between surface waves at HF frequencies, and a time-evolving sea surface. A simulator comprising three specific elements has been developed.The first one enables the user to model a time-varying sea surface in three dimensions, by the application of the linear wave theory. The second one simulates the interaction between the electromagnetic wave at grazing incidence and the sea surface using an exact model based on the method of moments. Lastly, a post-processing tool allows the plotting and the analysis of the resulting Doppler spectra. Validation tests are presented. Various simulations on static and dynamic surfaces (sinusoids and sea surfaces) reveal the Bragg resonance phenomenon and the Doppler effect respectively. The influence of the sea surface and the radar configuration is investigated. The simulation results show a good fit with published data and ONERA measurements. Finally, a film of pollutant on the surface is introduced in the model by the addition of a surface pressure (corresponding here to an attenuation of the heights of the waves). The effect of the presence of the film on the Doppler spectra is analyzed.Cette thèse repose sur l'analyse des interactions électromagnétiques (EM) entre les ondes de surface, aux fréquences HF, et une surface de mer évoluant dans le temps. Un simulateur comprenant trois modules spécifiques est développé. Le premier élément permet de modéliser une surface de mer en 3D variant dans le temps, par application de la théorie linéaire des vagues. Le second fait interagir une onde EM en incidence rasante avec une surface de mer grâce à un modèle exact qui s'appuie sur la méthode des moments. Enfin, un outil de post-traitement offre la possibilité de tracer et d'analyser des spectres Doppler (SD) résultants. Une étude portant sur des surfaces statiques puis dynamiques est menée. Elle fait apparaître respectivement le phénomène de résonance de Bragg et l'effet Doppler. Des données déjà publiées ou mesurées indiquent une bonne adéquation avec les SD simulés. Finalement, l'effet sur les SD, de la présence d'un film de polluant sur la surface de mer, est analysé

Full Wave 2D Modeling of Scattering and Inverse Scattering for Layered Rough Surfaces with Buried Objects.

Efficient and accurate modeling of electromagnetic scattering from layered rough surfaces with buried objects finds applications ranging from detection of landmines to remote sensing of subsurface soil moisture. In this dissertation, the formulation of a hybrid numerical/analytical solution to electromagnetic scattering from layered rough surfaces is first developed. The solution to scattering from each rough interface is sought independently based on the extended boundary condition method (EBCM), where the scattered fields of each rough interface are expressed as a summation of plane waves and then cast into reflection/transmission matrices. To account for interactions between multiple rough boundaries, the scattering matrix method (SMM) is applied to recursively cascade reflection and transmission matrices of each rough interface and obtain the composite reflection matrix from the overall scattering medium. The validation of this method against the Method of Moments (MoM) and Small Perturbation Method (SPM) will be addressed and the numerical results which investigate the potential of low frequency radar systems in estimating deep soil moisture will be presented. Computational efficiency of the proposed method is also addressed. In order to demonstrate the capability of this method in modeling coherent multiple scattering phenomena, the proposed method has been employed to analyze backscattering enhancement and satellite peaks due to surface plasmon waves from layered rough surfaces. Numerical results which show the appearance of enhanced backscattered peaks and satellite peaks are presented. Following the development of the EBCM/SMM technique, a technique which incorporates a buried object in layered rough surfaces is proposed by employing the T-matrix method and the cylindrical-to-spatial harmonics transformation. Validation and numerical results are provided. Finally, a multi-frequency polarimetric inversion algorithm for the retrieval of subsurface soil properties using VHF/UHF band radar measurements is developed. The top soil dielectric constant is first determined using an L-band inversion algorithm. For the retrieval of subsurface properties, a time-domain inversion technique is employed together with a parameter optimization for the pulse shape of time delay echoes from VHF/UHF band radar observations. Some numerical studies to investigate the accuracy of the proposed inversion technique in presence of errors are shown.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/58459/1/kuoch_1.pd