Analysis and evaluation of Terrain Observation by Progressive Scans (TOPSAR) mode in Synthetic Aperture Radar

Synthetic Aperture Radar (SAR) is a technique used in radar (RAdio Detection And Ranging) [1] systems to get high resolution images which are impossible to obtain with a conventional radar. This method allows us to acquire images from the surface of the Earth or other planets from large distances. In SAR [2], a single antenna is used to get information of the targets, and the platform movement, where the antenna is fixed, is used to spread the Doppler history of received echoes improving the resolution of processed images. Remote sensing is a wide area which studies different techniques to acquire information about targets situated at far distances. These techniques can be classified in two different areas according to their basic operation. The first group, called passive remote sensing [3] [4], uses passive sensors to acquire the energy radiated by the targets. This energy can come from an external source, such as Sun radiation, being reflected by the object or it can be emitted by the target itself. On the other hand, active remote sensing systems [4] emit pulses to illuminate the scanned area, providing their own energy. So, although it requires a more complex system, active sensing does not require an external source to operate which is an advantage when the conditions are not favourable. SAR and other radar techniques are examples of active sensors, working at frequencies between 0.3 GHz and 300GHz. These systems send pulses towards the scanned area, the interaction of each pulse with the surface originates an echo which arrives to the receiver. This echo is originated by the energy backscattered by the objects in the scene and it will be dependant of the backscattering profile of the targets (radar cross-section) [5] [6]. The time delay and strength of power received as well as frequency properties of the returns are processed to determine the target locations and characteristics. Synthetic aperture is similar to a conventional real aperture radar (RAR) antenna but it is achieved by signal processing. In a SAR, the antenna, installed in a moving platform, sends pulses to the scene and receives backscattered returns. The movement of the platform makes possible to illuminate the targets at different positions of the satellite trajectory, which is equivalent to have multiple antennas illuminating the scene at the same time. Thus, SAR is a fairly recent acquisition method that has some advantages in comparison with other remote sensing techniques. The most significant are: · Day/Night and all weather condition imaging since it does not depend on external power sources to detect the targets. · Geometric resolution independent of altitude or wavelength. · Signal data characteristic unique to the microwave region of EM spectrum which has suffers less deterioration in atmosphere propagation. The SAR systems started with aero-transported missions and later, first space missions were sent. The SAR beginning dates back to 1951 when C. Wiley postulated the Dopple

Geosynchronous synthetic aperture radar for Earth continuous observation missions

This thesis belongs to the field of remote sensing, particularly Synthetic Aperture Radar (SAR) systems from the space. These systems acquire the signals along the orbital track of one or more satellites where the transmitter and receiver are mounted, and coherently process the echoes in order to form the synthetic aperture. So, high resolution images can be obtained without using large arrays of antennas. The study presented in this thesis is centred in a novel concept in SAR, which is known as Geosynchronous SAR or GEOSAR, where the transmitter and/or receiver are placed in a platform in a geostationary orbit. In this case, the small relative motions between the satellite and the Earth surface are taken to get the necessary motion to form the synthetic aperture and focus the image. The main advantage of these systems with respect to the current technology (where LEO satellites with lower height are considered) is the possibility of permanently acquire images from the same region thanks to the small motion of the platform. Therefore, the different possibilities in the orbital design that offer this novel technology as well as the geometric resolutions obtained in the final image have been firstly studied. However, the use of geosynchronous satellites as illuminators results in slant ranges between 35.000-38.000 Km, which are much higher than the typical values obtained in LEOSAR, under 1.000 Km. Fortunately, the slow motion of the satellite makes possible large integration of pulses during minutes or even hours, reaching Signal-to-Noise Ratio (SNR) levels in the order of LEO acquisitions without using high transmitted power or large antennas. Moreover, such large integration times, increases the length of the synthetic aperture to get the desired geometric resolutions of the image (in the order of a few meters or kilometres depending on the application). On the other hand, the use of long integration time presents some drawbacks such as the scene targets decorrelation, atmospheric artefacts due to the refraction index variations in the tropospheric layer, transmitter and receiver clock jitter, clutter decorrelation or orbital positioning errors; which will affect the correct focusing of the image. For this reason, a detailed theoretical study is presented in the thesis in order to characterize and model these artefacts. Several simulations have been performed in order to see their effects on the final images. Some techniques and algorithms to track and remove these errors from the focused image are presented and the improvement of the final focused image is analysed. Additionally, the real data from a GB-SAR (Ground-Based SAR) have been reused to simulate a long integration time acquisition and see the effects in the image focusing as well as to check the performance of compensation algorithms in the final image. Finally, a ground receiver to reuse signals of opportunity from a broadcasting satellite have been designed and manufactured. This hardware is expected to be an important tool for experimental testing in future GEOSAR analysis.Aquesta tesi s'emmarca dins de l'àmbit de la teledetecció, en particular, en els sistemes coneguts com a radar d'obertura sintètica (SAR en anglès) des de l'espai. Aquests sistemes adquireixen senyal al llarg de l'òrbita d'un o més satèl·lits on estan situats el transmissor i el receptor, i processa els ecos de forma coherent per a formar l'obertura sintètica. D'aquesta manera es poden aconseguir imatge d'alta resolució sense la necessitat d'emprar un array d'antenes molt gran. El treball realitzat en aquest estudi es centra en un nou concepte dins del món SAR que consisteix en l'ús de satèl·lits en òrbita geostacionària per a l'adquisició d'imatges, sistemes coneguts com a Geosynchronous SAR o GEOSAR. En aquest cas, els petits moviments relatius dels satèl·lits respecte de la superfície terrestre s'empren per a aconseguir el desplaçament necessari per a formar l'obertura sintètica i així obtenir la imatge. El principal avantatge d'aquests sistemes respecte a la tecnologia actual (on s'utilitzen satèl·lits en orbites més baixes LEO) és la possibilitat d'adquirir imatges d'una mateixa zona de forma permanent gràcies als petits desplaçaments del satèl·lit. Així doncs, en aquesta tesi s'estudien les diferents possibilitats en el disseny orbital que ofereixen aquests sistemes així com les resolucions d'imatge que s'obtindrien. Tot i així, l'ús de satèl·lits en òrbita geoestacionària, resulta en una distància entre el transmissor/receptor i l'escena entre 35000-38000 Km, molt més gran que les distàncies típiques en els sistemes LEO per sota dels 1000 Km. Tot i així, el moviment lent de les plataformes geostacionàries fa possible la integració de polsos durant minuts o hores, arribant a nivells acceptables de relació senyal a soroll (SNR) sense necessitat d'utilitzar potències transmeses i antenes massa grans. A més a més, aquesta llarga integració també permet assolir unes longituds d'obertura sintètica adients per a arribar a resolucions d'imatge desitjades (de l'ordre de pocs metres o kilòmetres segons l'aplicació). Malgrat això, l'ús de temps d'integració llargs té una sèrie d'inconvenients com poden ser la decorrelació dels blancs de l'escena, l'aparició d'artefactes atmosfèrics deguts als canvis d'índex de refracció en la troposfera, derives dels rellotges del transmissor i receptor, decorrelació del clutter o errors en el posicionament orbital, que poden afectar la correcta focalització de la imatge. Així doncs, en la tesi s'ha fet un detallat estudi teòric d'aquests problemes per tal de modelitzar-los i posteriorment s'han realitzat diverses simulacions per veure els seus efectes en una imatge. Diverses tècniques per a compensar aquests errors i millorar la qualitat de la imatge també s'han estudiat al llarg de la tesi. Per altra banda, dades reals d'un GB-SAR (SAR en una base terrestre) s'han reutilitzat per adaptar-les a una possible adquisició de llarga durada i veure així de forma experimental com afecta la llarga integració en les imatges i com millora l'enfocament després d'aplicar els algoritmes de compensació. Per últim, en la tesi es presenta un sistema receptor terrestre per a poder realitzar un anàlisi experimental del cas GEOSAR utilitzant un il·luminador d'oportunitat. Els primers passos en el disseny i la fabricació del hardware també es presenten en aquesta tes

Radar phase based near surface meteorological data retrievals

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2016, Tutor: Joan Bech RustulletThe aim of this project is to analyze the relation between the meteorological changes in the tropospheric layer and the signal phase history from radar acquisitions. Atmospheric temperature, pressure and relative humidity impact on the atmospheric refraction index is stated which, at the same time, has a critical impact on the radar phase signal. Data from radar GB-SAR campaigns are used to demonstrate the theoretical relatio

Study on the feasibility of geosynchronous satellites for synthetic aperture radar applications

Català: Un complet anàlisi sobre la viabilitat dels satèl.lits geosíncrons per l'adquisició de dades SAR es presenta en aquesta tesis. L'objectiu d'una missió SAR Geosíncrona és la obtenció d'imatges SAR utilitzant el moviment relatiu d'un satèl.lit geosíncron respecte de la superfície terrestre. Aquest moviment es pot aconseguir gracies a petites inclinacions del pla orbital o bé degut a lleugeres pertorbacions en l'excentricitat de l'òrbita. En aquesta tesis, s'estudiaran la formació de l'obertura sintètica, el balanç de potències, l'estudi de la historia doppler dels ecos rebuts, el nivell d'ambigüitats, l'enfocament de la imatge, entre altres característiques del processat GEOSAR.Castellano: Un análisis completo sobre la viabilidad de los satélites geosíncronos para adquisición de datos SAR se presenta en esta tesis. El objetivo de una misión SAR Geosíncrono es la obtención de imágenes SAR utilizando el movimiento relativo de un satélite geosíncrono con respecto de la superficie terrestre. Este movimiento se puede conseguir gracias a órbitas ligeramente inclinadas o bien pequeñas perturbaciones en la excentricidad de la órbita. En esta tesis, se analizarán la formación de la apertura sintética, el balance de potencias, el estudio de la historia doppler de los ecos recibidos, el nivel de ambigüedades, el enfoque de la imagen, entre otras características del procesado GEOSAR.English: A complete analysis on the feasibility of geosynchronous satellites to perform Synthetic Aperture Radar (SAR) acquisition is assessed in this thesis. The goal of a Geosynchronous SAR mission is to obtain SAR images using the relative motion of a geosynchronous satellite with respect to the Earth's surface. This motion can be achieved by a slightly inclined orbital plane relative to the Equator which can be combined with small eccentricities on a satellite non-perfect geostationary orbit. In the thesis, the synthetic aperture formation, power budget and doppler history analysis, ambiguity levels, focusing and other processing issues are presented

Study on the feasibility of geosynchronous satellites for synthetic aperture radar applications

Català: Un complet anàlisi sobre la viabilitat dels satèl.lits geosíncrons per l'adquisició de dades SAR es presenta en aquesta tesis. L'objectiu d'una missió SAR Geosíncrona és la obtenció d'imatges SAR utilitzant el moviment relatiu d'un satèl.lit geosíncron respecte de la superfície terrestre. Aquest moviment es pot aconseguir gracies a petites inclinacions del pla orbital o bé degut a lleugeres pertorbacions en l'excentricitat de l'òrbita. En aquesta tesis, s'estudiaran la formació de l'obertura sintètica, el balanç de potències, l'estudi de la historia doppler dels ecos rebuts, el nivell d'ambigüitats, l'enfocament de la imatge, entre altres característiques del processat GEOSAR.Castellano: Un análisis completo sobre la viabilidad de los satélites geosíncronos para adquisición de datos SAR se presenta en esta tesis. El objetivo de una misión SAR Geosíncrono es la obtención de imágenes SAR utilizando el movimiento relativo de un satélite geosíncrono con respecto de la superficie terrestre. Este movimiento se puede conseguir gracias a órbitas ligeramente inclinadas o bien pequeñas perturbaciones en la excentricidad de la órbita. En esta tesis, se analizarán la formación de la apertura sintética, el balance de potencias, el estudio de la historia doppler de los ecos recibidos, el nivel de ambigüedades, el enfoque de la imagen, entre otras características del procesado GEOSAR.English: A complete analysis on the feasibility of geosynchronous satellites to perform Synthetic Aperture Radar (SAR) acquisition is assessed in this thesis. The goal of a Geosynchronous SAR mission is to obtain SAR images using the relative motion of a geosynchronous satellite with respect to the Earth's surface. This motion can be achieved by a slightly inclined orbital plane relative to the Equator which can be combined with small eccentricities on a satellite non-perfect geostationary orbit. In the thesis, the synthetic aperture formation, power budget and doppler history analysis, ambiguity levels, focusing and other processing issues are presented

Analysis and evaluation of Terrain Observation by Progressive Scans (TOPSAR) mode in Synthetic Aperture Radar

Synthetic Aperture Radar (SAR) is a technique used in radar (RAdio Detection And Ranging) [1] systems to get high resolution images which are impossible to obtain with a conventional radar. This method allows us to acquire images from the surface of the Earth or other planets from large distances. In SAR [2], a single antenna is used to get information of the targets, and the platform movement, where the antenna is fixed, is used to spread the Doppler history of received echoes improving the resolution of processed images. Remote sensing is a wide area which studies different techniques to acquire information about targets situated at far distances. These techniques can be classified in two different areas according to their basic operation. The first group, called passive remote sensing [3] [4], uses passive sensors to acquire the energy radiated by the targets. This energy can come from an external source, such as Sun radiation, being reflected by the object or it can be emitted by the target itself. On the other hand, active remote sensing systems [4] emit pulses to illuminate the scanned area, providing their own energy. So, although it requires a more complex system, active sensing does not require an external source to operate which is an advantage when the conditions are not favourable. SAR and other radar techniques are examples of active sensors, working at frequencies between 0.3 GHz and 300GHz. These systems send pulses towards the scanned area, the interaction of each pulse with the surface originates an echo which arrives to the receiver. This echo is originated by the energy backscattered by the objects in the scene and it will be dependant of the backscattering profile of the targets (radar cross-section) [5] [6]. The time delay and strength of power received as well as frequency properties of the returns are processed to determine the target locations and characteristics. Synthetic aperture is similar to a conventional real aperture radar (RAR) antenna but it is achieved by signal processing. In a SAR, the antenna, installed in a moving platform, sends pulses to the scene and receives backscattered returns. The movement of the platform makes possible to illuminate the targets at different positions of the satellite trajectory, which is equivalent to have multiple antennas illuminating the scene at the same time. Thus, SAR is a fairly recent acquisition method that has some advantages in comparison with other remote sensing techniques. The most significant are: · Day/Night and all weather condition imaging since it does not depend on external power sources to detect the targets. · Geometric resolution independent of altitude or wavelength. · Signal data characteristic unique to the microwave region of EM spectrum which has suffers less deterioration in atmosphere propagation. The SAR systems started with aero-transported missions and later, first space missions were sent. The SAR beginning dates back to 1951 when C. Wiley postulated the Dopple

Ground moving target indication using multi-channel SAR with non-uniform displaced phase centers

This paper presents an evaluation of a post-Doppler spacetime adaptive Processing (STAP) technique for non-uniformly displaced phase center receivers synthetic aperture radar (SAR). Both theoretical and simulation analysis are carried out in order to prove the ground moving target indication (GMTI) capabilities for specific non-uniformly spaced multichannel configurations on-board a single satellite platform. Simulated SAR data in the maritime scenario show improved performance, specially for small and slow targets, compared to current SAR missions, equipped with just two channels.Peer Reviewe

Bistatic geosynchronous SAR for land and atmosphere continuous observation

In this paper the bistatic SAR observation from low-eccentric nearly zero inclined geosynchronous orbits (GEOSAR) is studied. Two operation frequencies have been analysed with different compromises between spatial resolution and acquisition time. The impact of the orbital design in the final SAR radiometric and spatial resolution performance is assessed allowing assessment of appropriate spatial resolution and coverage compromises.Peer Reviewe

Atmospheric phase screen retrieval from GEOSAR long term acquisition

In this paper, a GEOSAR mission with typical LEOSAR parameters but longer integration time to increase the received power is presented. Such systems can provide images of large areas with continuous monitoring. The revisit time is reduced to the synthetic aperture integr ation time to reach the desired resolution and SNR require- ments. However, the Atmospheric Phase Screen (APS) ar tefacts must be characterized to compensate the long term atmospheric phase decorrelation. An APS correc tion methodology from short term periodic acquisitions (sub-apertures) of the whole long term synthetic aperture is analysed. Finally, the first results obtained from Ground-Based SAR (GB-SAR) real data are presented to validate the proposed correction method.Peer ReviewedPostprint (published version