<p>A common way to measure height in aerial vehicles is to use a radar height altimeter (RHM). Since the RHM transmits radar pulses that can be detected, a passive alternative would be desirable in military applications. The idea to use reflected signals from the Global Positioning System (GPS) as a bistatic radar, has been established over the last years. The GPS signals are already present and would not reveal aeroplanes in covert operations. </p><p>In this thesis, the use of reflected GPS signals as a bistatic, passive altimeter is examined. A simulation model has been developed and implemented, and simulations using the model have been done. Different types of ground cover have been investigated, both water and land types, with varying reflectivity and scattering behaviour. For larger terrain variations, e.g. mountains and valleys, a ground elevation database has been used. Furthermore, several parameters, like the antenna coverage and the satellite elevation angle, have been varied and the result of this examined. </p><p>The results of these simulations show that measuring height is possible for bothsea and land surfaces. The accuracy depends on several error factors, like a bias originating from surface roughness and measurement errors due to noise in the receiver. The simulations also show that the most important design parameter is the antenna, which must be designed to give a sufficiently large SNR, capture the specular reflection and avoid unwanted reflections.</p>

A common way to measure height in aerial vehicles is to use a radar height altimeter (RHM). Since the RHM transmits radar pulses that can be detected, a passive alternative would be desirable in military applications. The idea to use reflected signals from the Global Positioning System (GPS) as a bistatic radar, has been established over the last years. The GPS signals are already present and would not reveal aeroplanes in covert operations. In this thesis, the use of reflected GPS signals as a bistatic, passive altimeter is examined. A simulation model has been developed and implemented, and simulations using the model have been done. Different types of ground cover have been investigated, both water and land types, with varying reflectivity and scattering behaviour. For larger terrain variations, e.g. mountains and valleys, a ground elevation database has been used. Furthermore, several parameters, like the antenna coverage and the satellite elevation angle, have been varied and the result of this examined. The results of these simulations show that measuring height is possible for bothsea and land surfaces. The accuracy depends on several error factors, like a bias originating from surface roughness and measurement errors due to noise in the receiver. The simulations also show that the most important design parameter is the antenna, which must be designed to give a sufficiently large SNR, capture the specular reflection and avoid unwanted reflections.

<div>Surface soil moisture is one of the few direct hydrological variables which can be measured. It plays a crucial part in the water cycle, agriculture, drought development, runoff generation, and many other phenomena. Satellite observations from active and passive microwave radiometers are best suited for the retrieval of soil moisture. The relationship between soil dielectric constant and water content is direct and is used to determine the surface soil moisture levels. Active microwave remote sensing techniques measure the energy reflected from target surfaces (ocean, soil, biomass) after transmitting a pulse of microwave energy, whereas passive microwave sensors measure the self-emissions of the target surfaces. The passive missions by ESA's SMOS and NASA's SMAP have demonstrated this technology for remote sensing on a global scale. Global Navigation Satellite System-Reflectometry (GNSS-R) is an alternative approach to the remote sensing of soil moisture, as demonstrated through several airborne and ground-based experiments. The new technique of Signals of Opportunity (SoOp) uses a bistatic radar configuration in which the non-cooperative transmitter already transmits signals designed for communication or navigation. The receiver reuses the reflected energy from the target surface (ocean, soil, biomass), thereby making the digital communication and navigation signal spectrum useful to the remote sensing science community. Several airborne and ground-based experiments have been conducted on the use of digital communication signals, a range of frequencies from P-band to Ku-band, for measurement of ocean surface roughness, wind speed, and soil moisture. </div><div> </div><div>This thesis presents the preliminary results obtained for reflectivity retrievals for the 2017 and 2018 S-band tower-based SoOp field experiment conducted at Purdue's Agronomy Center for Research and Education (ACRE). XM signals were recorded by a sky-facing antenna and an Earth-facing antenna mounted atop a tower. The line-of-sight (direct) signal is captured by the sky-facing antenna and reflected signal from the soil captured by the Earth-facing antenna was used for the ambiguity function of XM transmission. A link budget was used to determine the received signal to noise ratio (SNR). The cross-correlation between the direct and the reflected XM signals was used to estimate reflectivity values. The reflectivity retrievals were compared with the in-situ soil moisture content at 5 cm depth obtained by the HydraProbes. The reflectivity values were also verified by a Signals of Opportunity (SoOp) Coherent Bistatic (SCoBi) simulated model.</div>

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The agricultural sector is the backbone which supports the livelihoods and the economic development of nations across the globe. In consequence, the need for robust and continuous monitoring of agricultural crops is primordial to face the interlinked challenges of growth rate population, food security and climate change. Synthetic Aperture Radar (SAR) sensors have the powerful imaging capability of operating at almost all weather conditions, independent of day and night illumination. By penetrating through clouds and into the vegetation canopy, the incident radar signal interacts with the structural and dielectric properties of the vegetation and soil, thus providing critical information of the crop state, such as height, biomass, crop yield or leaf structure, which can help devise sustainable agricultural management practices. This is achieved by means of the Polarimetric SAR Interferometry (PolInSAR) technique, which by coherently combining interferometric SAR acquisitions at different polarization states allows for the retrieval of biophysical parameters of the vegetation. In this framework, this thesis focuses on the development of crop monitoring techniques that properly exploit satellite-based PolInSAR data. All the known InSAR and PolInSAR methodologies for this purpose have been analysed. The sensitivity of these data provided by the TanDEM-X bistatic system to both the physical parameters of the scene (height and structure of the plants, moisture and roughness of the soil) and the sensor configuration (polarization modes and observation geometry) is evaluated. The effect of different simplifications made in the physical model of the scene on the crop estimates is assessed. The interferometric sensitivity requirements to monitor a crop scenario are more demanding than others, such as forests. Steep incidences associated with the largest spatial baselines provided by the available data set lead to the most accurate estimates under all the different model assumptions. Shallower incidences, on the other hand, generally yield important errors due to their characteristic shorter spatial baselines. Through the methodologies proposed in this thesis, PolInSAR data have shown potential to refine current methods for the quantitative estimation of crop parameters. Results encourage to continue further research towards the objective of achieving operational crop monitoring applications. / Work supported by the Spanish Ministry of Science and Innovation, the State Agency of Research (AEI) and the European Funds for Regional Development (EFRD) under Project TEC2017-85244-C2-1-P. Noelia Romero-Puig received a grant from the Generalitat Valenciana and the European Social Fund (ESF) [ACIF/2018/204].

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L’étude traite de la détection de cibles mobiles dans un contexte de radar passif bistatique utilisant les émetteursde télévision numérique TNT (DVB-T) comme émetteurs d’opportunité. Outre leur présence généralisée sur leterritoire, l’intérêt de ces émissions réside dans leur relative largeur de bande permettant une bonne précisiond’estimation. Le principal inconvénient de ce type d’approche réside dans l’éblouissement par le signal en trajetdirect, des échos de très faible intensité des cibles d’intérêt. Après un rappel du principe du radar bistatique etde la norme OFDM utilisée par les signaux TNT, une première étude donne une construction originale du signalde référence dans le cas multi-capteurs : le signal de référence est construit par un traitement d’antenne de typeCAPON où le balayage des paramètres optimaux est remplacé par la connaissance de signaux pilotes inséré dansles symboles OFDM. Ensuite le rapport se focalise sur l’estimation d’un filtre de canal multitrajet à partir dela connaissance de la modulation OFDM utilisée. Ce filtre, d’abord étudié comme réjecteur de fouillis originalavant détection par la fonction d’ambiguïté, donne des résultats semblables aux méthodes classiques de réjectionde fouillis standard. Étendu à toutes les fréquences Doppler, son module au carré est utilisé comme un nouveaudétecteur présentant un très faible niveau de clutter, surpassant ainsi la fonction d’ambiguïté. Une interprétationen terme de traitement d’antennes du nouveau détecteur ouvre la voie à des variantes haute-résolution de celui-ci.La validité du nouveau détecteur est illustrée par des résultats sur données réelles. / The study focuses on moving target detection from passive bistatic radar with DVB-T transmitters used asopportunity transmitters. In addition to their widespread geographical coverage, they allow a good estimationaccuracy due to their quite large bandpass. Nevertheless the continuous powerful direct path masks the verylow intensity echoes of targets of interest. The passive bistatic radar principle and the CP-OFDM standardused by DVB-T are briefly reminded, then, a new first study of reference signal retrieving in multiple sensorsconfiguration is given : the reference signal is built using a Capon receptor where the parameters scan is replacedby the knowledge of pilot signals inserted in the OFDM symbols. Next, the report addresses the multipath channelestimate by using the OFDM signal structure. This channel is firstly studied for clutter rejection before detectionfrom the cross ambiguity function (CAF). We obtain similar results than those of the classical rejection methods.This channel is extended to the whole Doppler shift, and its squared modulus acts as a new low clutter detectorthat outperforms classical CAF. A virtual beamforming interpretation of the channel estimation opens a new pathtowards high resolution array processing. Results given on real data illustrate the validity of this new channeldetector (CHAD).

Achieving acceptable plane wave uniformity throughout an expanded volume is necessary to conduct scattering measurements on a large target in a controlled environment. An expanded volume is large relative to the size of the nearfield array configuration used to produce plane wave uniformity. The optimum set of shading coefficients for a nearfield array may not produce acceptable plane wave uniformity as the volume and frequency domain are expanded for a given array configuration. Choosing the frequency domain as a single frequency for an optimum set of coefficients will produce plane wave uniformity throughout the largest possible volume for a given array configuration. This study determines the acceptability of uniformity results produced by an optimum set of frequency dependent coefficients throughout an expanded volume for two array configurations that comprise a system for measuring bistatic target strength in the nearfield. Minimizing the frequency domain chosen for an optimum set of coefficients will produce plane wave uniformity for the largest possible volume for a given array configuration. This study determines the acceptability of uniformity results produced by an optimum set of frequency dependent coefficients throughout an optimistic volume for two array configurations that comprise a bi-static array.