Interferometric orbit determination system for geosynchronous SAR missions: experimental proof of concept

Future Geosynchronous Synthetic Aperture Radar (GEOSAR) missions will provide permanent monitoring of continental areas of the planet with revisit times of less than 24 h. Several GEOSAR missions have been studied in the USA, Europe, and China with different applications, including water cycle monitoring and early warning of disasters. GEOSAR missions require unprecedented orbit determination precision in order to form focused Synthetic Aperture Radar (SAR) images from Geosynchronous Orbit (GEO). A precise orbit determination technique based on interferometry is proposed, including a proof of concept based on an experimental interferometer using three antennas separated 10–15 m. They provide continuous orbit observations of present communication satellites operating at GEO as illuminators of opportunity. The relative phases measured between the receivers are used to estimate the satellite position. The experimental results prove the interferometer is able to track GEOSAR satellites based on the transmitted signals. This communication demonstrates the consistency and feasibility of the technique in order to foster further research with longer interferometric baselines that provide observables delivering higher orbital precision.This work has been supported by the Spanish Science, Research and Innovation Plan (MICINN) with Project Codes TEC2017-85244-C2-2-P and PID2020-117303GB-C21 and by Unidad de Excelencia Maria de Maeztu MDM-2016-0600 financed by the Agencia Estatal de Investigación, Spain.Peer ReviewedPostprint (published version

Research progress on geosynchronous synthetic aperture radar

Based on its ability to obtain two-dimensional (2D) high-resolution images in all-time and all-weather conditions, spaceborne synthetic aperture radar (SAR) has become an important remote sensing technique and the study of such systems has entered a period of vigorous development. Advanced imaging modes such as radar interferometry, tomography, and multi-static imaging, have been demonstrated. However, current in-orbit spaceborne SARs, which all operate in low Earth orbits, have relatively long revisit times ranging from several days to dozens of days, restricting their temporal sampling rate. Geosynchronous SAR (GEO SAR) is an active research area because it provides significant new capability, especially its much-improved temporal sampling. This paper reviews the research progress of GEO SAR technologies in detail. Two typical orbit schemes are presented, followed by the corresponding key issues, including system design, echo focusing, main disturbance factors, repeat-track interferometry, etc, inherent to these schemes. Both analysis and solution research of the above key issues are described. GEO SAR concepts involving multiple platforms are described, including the GEO SAR constellation, GEO-LEO/airborne/unmanned aerial vehicle bistatic SAR, and formation flying GEO SAR (FF-GEO SAR). Due to the high potential of FF-GEO SAR for three-dimensional (3D) deformation retrieval and coherence-based SAR tomography (TomoSAR), we have recently carried out some research related to FF-GEO SAR. This research, which is also discussed in this paper, includes developing a formation design method and an improved TomoSAR processing algorithm. It is found that GEO SAR will continue to be an active topic in the aspect of data processing and multi-platform concept in the near future

Analysis of Background Ionospheric Effects on Geosynchronous SAR Imaging

Background ionospheric propagation effects are adverse to the performance of Geosynchronous Synthetic Aperture Radar (GEO SAR) system. This paper focuses on the background ionospheric phase advance, which can be modelled as a function of Slant Total Electron Content (STEC). The dispersive feature of the phase advance caused by the background ionosphere could be able to distort the GEO SAR range-imaging. Furthermore, for GEO SAR, the integration time is ultra-long and the coverage is ultra-large, thus temporal and spatial distributions of the background ionosphere have to be taken into account. The resultant ionospheric phase variations might decorrelate the azimuth signal and then lead to azimuth-imaging deteriorations. In this paper, the theoretical model of the background ionospheric effects on GEO SAR imaging is established and in-depth analyses are presented. Finally, theoretical analyses are validated by the signal-level simulation

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

Geosynchronous synthetic aperture radar : design and applications

Synthetic Aperture Radar (SAR) imaging from geosynchronous orbit has significant potential advantages over conventional low-Earth orbit (LEO) radars, but also challenges to overcome. This thesis investigates both active and passive geosynchronous SAR configurations, presenting their different features and advantages. Following a system design trade-off that involved phase uncertainties, link budget, frequency and integration time, an L band bi-static configuration with 8-hour integration time that reuses the signal from a non-cooperative transmitter has been presented as a suitable solution. Cranfield Space Research Centre looked into this configuration and proposed the GeoSAR concept, an L band bi-static SAR based on the concept by Prati et al. (1998). It flies along a circular ground track orbit, reuses the signal coming from a noncooperative transmitter in GEO and achieves a spatial resolution of about 100 m. The present research contributes to the GeoSAR concept exploring the implications due to the 8-hour integration time and providing insights about its performance and its possible fields of application. Targets such as canopies change their backscattered phase on timescales of seconds due to their motion. On longer time scales, changes in dielectric properties of targets, Earth tides and perturbations in the structure of the atmosphere contribute to generate phase fluctuations in the collected signals. These phenomena bring temporal decorrelation and cause a reduction in SAR coherent integration gain. They have to be compensated for if useful images are to be provided. A SAR azimuth simulator has been developed to study the influence of temporal decorrelation on GeoSAR point spread function. The analysis shows that ionospheric delay is the major source of decorrelation; other effects, such as tropospheric delay and Earth tides, have to be dealt with but appear to be easier to handle. Two different options for GeoSAR interferometry have been discussed. The system is well suited to differential interferometry, due to the short perpendicular baseline induced by the geometry. A GeoSAR has advantages over a Low Earth Orbit (LEO) SAR system to monitor processes with significant variability over daily or shorter timescales (e.g. soil moisture variation). This potential justifies further study of the concept.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Geosynchronous synthetic aperture radar : design and applications

Synthetic Aperture Radar (SAR) imaging from geosynchronous orbit has significant potential advantages over conventional low-Earth orbit (LEO) radars, but also challenges to overcome. This thesis investigates both active and passive geosynchronous SAR configurations, presenting their different features and advantages. Following a system design trade-off that involved phase uncertainties, link budget, frequency and integration time, an L band bi-static configuration with 8-hour integration time that reuses the signal from a non-cooperative transmitter has been presented as a suitable solution. Cranfield Space Research Centre looked into this configuration and proposed the GeoSAR concept, an L band bi-static SAR based on the concept by Prati et al. (1998). It flies along a circular ground track orbit, reuses the signal coming from a noncooperative transmitter in GEO and achieves a spatial resolution of about 100 m. The present research contributes to the GeoSAR concept exploring the implications due to the 8-hour integration time and providing insights about its performance and its possible fields of application. Targets such as canopies change their backscattered phase on timescales of seconds due to their motion. On longer time scales, changes in dielectric properties of targets, Earth tides and perturbations in the structure of the atmosphere contribute to generate phase fluctuations in the collected signals. These phenomena bring temporal decorrelation and cause a reduction in SAR coherent integration gain. They have to be compensated for if useful images are to be provided. A SAR azimuth simulator has been developed to study the influence of temporal decorrelation on GeoSAR point spread function. The analysis shows that ionospheric delay is the major source of decorrelation; other effects, such as tropospheric delay and Earth tides, have to be dealt with but appear to be easier to handle. Two different options for GeoSAR interferometry have been discussed. The system is well suited to differential interferometry, due to the short perpendicular baseline induced by the geometry. A GeoSAR has advantages over a Low Earth Orbit (LEO) SAR system to monitor processes with significant variability over daily or shorter timescales (e.g. soil moisture variation). This potential justifies further study of the concept.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Measuring and modelling the impact of the ionosphere on space based synthetic aperture radars

Synthetic aperture radar (SAR) is a technique widely used in applications that require all-weather imaging. The ionosphere affects the operation of these radars, with those operating at L-band (1-2 GHz) and below at risk of being seriously compromised by the ionosphere. A method of using Global Positioning System (GPS) data to synthesize the impact of the ionosphere on SAR systems has been presented. The technique was used to assess the viability of using a signal phase correction derived from a reference location in a SAR image to correct ionospheric effects across the image. A dataset of SAR images and GPS measurements collected simultaneously on Ascension Island were used to test two techniques for deriving ionospheric strength of turbulence (C$_k$L) from SAR images – one using measurements of trihedral corner reflectors (CR) and the other measurements of natural clutter. The CR C$_k$L values showed a correlation of 0.69 with GPS estimates of C$_k$L, whilst the clutter measurements showed a correlation of up to 0.91 with the CR values. Finally, a study of using the effects of intensity scintillation on SAR images to measure the S$_4$ index was performed. The study was not able to reproduce previous results, but produced significant practical conclusions

Europe's Space capabilities for the benefit of the Arctic

In recent years, the Arctic region has acquired an increasing environmental, social, economic and strategic importance. The Arctic’s fragile environment is both a direct and key indicator of the climate change and requires specific mitigation and adaptation actions. The EU has a clear strategic interest in playing a key role and is actively responding to the impacts of climate change safeguarding the Arctic’s fragile ecosystem, ensuring a sustainable development, particularly in the European part of the Arctic. The European Commission’s Joint Research Centre has recently completed a study aimed at identifying the capabilities and relevant synergies across the four domains of the EU Space Programme: earth observation, satellite navigation, satellite communications, and space situational awareness (SSA). These synergies are expected to be key enablers of new services that will have a high societal impact in the region, which could be developed in a more cost-efficient and rapid manner. Similarly, synergies will also help exploit to its full extent operational services that are already deployed in the Arctic (e.g., the Copernicus emergency service or the Galileo Search and rescue service could greatly benefit from improved satellite communications connectivity in the region).JRC.E.2-Technology Innovation in Securit

Excess path delays from sentinel interferometry to improve weather forecasts

A synthetic aperture radar can offer not only an accurate monitoring of the earth surface deformation, but also information on the troposphere, such as the total path delay or the columnar water vapor at high horizontal resolution. This can be achieved by proper interferometric processing and postprocessing of the radar interferograms. The fine and unprecedented horizontal resolution of the tropospheric products can offer otherwise unattainable information to be assimilated into numerical weather prediction models, which are progressively increasing their resolving capabilities. A number of tricks on the most effective processing approaches, as well as a novel method to pass from multipass differential interferometry products to absolute tropospheric columnar quantities are discussed. The proposed products and methods are assessed using real Sentinel-1 data. The experiment aims at evaluating the accuracy of the derived information and its impact on the weather prediction skill for two meteorological events in Italy. The main perspective of the study is linked to the possibility of exploiting interferometric products from a geosynchronous platform, thus complementing the inherent high resolution of SAR sensors with the required frequent revisit needed for meteorological applications