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Detection of marine litter is an urgent task given the large amount of plastics that end up in all water systems. This manuscript provides the description and results obtained from a ground-based Global Navigation Satellite System - Reflectometry (GNSS-R) system built in a controlled indoor environment intended to test the capabilities of GNSS-R techniques for marine litter detection, when different wave conditions and plastic litter were used. Results show that it may be possible to detect the damping of waves produced by the accumulations of some types of plastic., This article was part of the project ”GENESIS: GNSS Environmental and Societal Missions – Subproject UPC”, Grant PID2021-126436OB-C21 funded by the Ministerio de Ciencia e Investigación (MCIN)/Agencia Estatal de Investigación (AEI)/10.13039/501100011033 and EU FEDER “Una manera de hacer Europa”. Amadeu Gonga (FI2022/809) and Adrian Perez-Portero (FISDUR2020/105) were supported by grants for recruitment of early-stage research funded by the Agencia Gestió d’Ajuts Universitaris i de Recerca (AGAUR) Generalitat de Catalunya, Spain., Peer Reviewed, Postprint (author's final draft)

Over 14 million tons of plastic end up yearly in the ocean, originating from polluted waterways such as rivers, sewers, or storm waters, or from industrial and nautical activities with improper garbage disposal methods. Due to its long decomposition time, and the presence of currents and gyres, most of plastic debris freely floating in the ocean end up forming large extensions of marine litter known as garbage patches. An example is the Great Pacific garbage patch located in the central North Pacific Ocean covering an area larger than 1.6 million squared kilometers. Some works have recently reported the potential of GNSS-Reflectometry to detect marine plastic litter from space. This study presents the results of a controlled field experiment conducted under the auspices of ESA at the "Atlantic Basin" at the Deltares research institute (Delft, The Netherlands)., Research funded by ESA project GLobal MonItoring of MicroPlastics using GNSS-Reflectometry (GLIMPS) 40001324 88/20/NL/GLC through a subcontract with Deimos Space UK Ltd “GLIMPS Subcontract DMU-UPC Ref. DMU-CP51 no. 2021-015; project “GENESIS: GNSS Environmental and Societal Missions—Subproject UPC”, Grant PID2021-126436OB-C21, sponsored by MCIN/AEI/10.13039/501100 011033/ and EU ERDF “A way to do Europe”, and grant for recruitment of early stage research staff of Agència Gestió d’Ajuts Universitaris i de Recerca (AGAUR) Generalitat de Catalunya, Spain (FISDUR2020/105 and FI2022/809)., Peer Reviewed, Postprint (author's final draft)

Altimetric performance of Global Navigation Satellite System - Reflectometry (GNSS-R) instruments depends on receiver’s bandwidth and signal-to-noise ratio (SNR). The altimetric delay is usually computed from the time difference between the peak of the direct signal waveform and the maximum of the derivative of the reflected signal waveform. Dual-frequency data gathered by the airborne Microwave Interferometric Reflectometer (MIR) in the Bass Strait, between Australia and Tasmania, suggest that this approach is only valid for flat surfaces and large bandwidth receivers. This work analyses different methods to compute the altimetric observables using GNSS-R. A proposed novel method, This work was funded by the Spanish Ministry of Science, Innovation and Universities, “Sensing with Pioneering Opportunistic Techniques”, grant RTI2018-099008-B-C21/AEI/10.13039/ 501100011033, and the grant for recruitment of early-stage research staff FI-DGR 2015 of the AGAUR— Generalitat de Catalunya (FEDER), Spain, and the grant for recruitment of early-stage research staff FI 2018 of the AGAUR—Generalitat de Catalunya (FEDER), Spain, and Unidad de Excelencia María de Maeztu MDM-2016-060, Postprint (published version)

Vegetation canopy height (CH) is one of the important remote-sensing parameters related to forests’ structure, and it can be related to the biomass and the carbon stock. Global navigation satellite system-reflectometry (GNSS-R) has proved capable to retrieve vegetation information at a moderate resolution from space (20–65 km) using L1 C/A signals. In this study, data retrieved by the airborne microwave interferometric reflectometer (MIR) GNSS-R instrument at L1 and L5 are compared to the Global Forest CH product, with a spatial resolution of 30 m. This work analyzes the waveforms (WFs) measured at both bands, and the correlation of the waveform width and the reflectivity values to the CH product. A neural network algorithm is used for the retrieval, showing that the combination of the reflectivity and the waveform width allows to estimate the CH information at a very high resolution, with a root-mean-square error (RMSE) of 4.25 and 4.07 m at L1 and L5, respectively, which is an error about 14% of the actual CH., Postprint (updated version)

The Microwave Interferometric Reflectometer (MIR) is an airborne GNSS-R instrument developed by Universitat Politècnica de Catalunya. In 2018, it was flown twice over the agricultural Yanco area, New South Wales, Australia, once after a very dry period, and a further time the day after a strong rain event. This rain event resulted in many crop fields being entirely flooded, producing a saturation in the GNSS-R reflectivity value. In this work, the received data set is processed to identify the optimum integration time with the goal to minimize pixel blurring. This issue is assessed for airborne conditions, and then extra-polated to the spaceborne case. The presented results show that the blurring of the GNSS waveform is produced even from an airborne sensor with short integration times. Following the determination of an optimal integration time for the platform in use, the surface roughness term in the reflectivity equation can be isolated due to the signal saturation during very wet surface conditions. The final results from the two channels (L1 C/A and L5) are subsequently presented. In this case, it is shown that most reflectivity variations in GNSS-R measurements are linked to surface roughness and Speckle noise fluctuations rather than soil moisture changes., Postprint (updated version)

At L-band (1–2 GHz), and particularly in microwave radiometry (1.413 GHz), vegetation has been traditionally modeled with the τ-ω model. This model has also been used to compensate for vegetation effects in Global Navigation Satellite Systems-Reflectometry (GNSS-R) with modest success. This manuscript presents an analysis of the vegetation impact on GPS L1 C/A (coarse acquisition code) signals in terms of attenuation and depolarization. A dual polarized instrument with commercial off-the-shelf (COTS) GPS receivers as back-ends was installed for more than a year under a beech forest collecting carrier-to-noise (C/N0) data. These data were compared to different ground-truth datasets (greenness, blueness, and redness indices, sky cover index, rain data, leaf area index or LAI, and normalized difference vegetation index (NDVI)). The highest correlation observed is between C/N0 and NDVI data, obtaining R2 coefficients larger than 0.85 independently from the elevation angle, suggesting that for beech forest, NDVI is a good descriptor of signal attenuation at L-band, which is known to be related to the vegetation optical depth (VOD). Depolarization effects were also studied, and were found to be significant at elevation angles as large as ~50°. Data were also fit to a simple τ-ω model to estimate a single scattering albedo parameter (ω) to try to compensate for vegetation scattering effects in soil moisture retrieval algorithms using GNSS-R. It is found that, even including dependence on the elevation angle (ω(θe)), at elevation angles smaller than ~67°, the ω(θe) model is not related to the NDVI. This limits the range of elevation angles that can be used for soil moisture retrievals using GNSS-R. Finally, errors of the GPS-derived position were computed over time to assess vegetation impact on the accuracy of the positioning., This work has been funded by the Spanish MCIU and EU ERDF project (RTI2018-099008-B-C21) “Sensing with pioneering opportunistic techniques” and grant to “CommSensLab-UPC” Excellence Research Unit Maria de Maeztu (MINECO grant MDM-2016-600)., Peer Reviewed, Postprint (published version)

© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., As compared to the using L1C/A signals, L5/E5a Global Navigation Satellite System - Reflectometry (GNSS-R), gives improved resolution over the Earth's surface due to the sharper auto-correlation function. Furthermore, the larger transmitted power (+3dB with respect to L1 C/A), and correlation gain (+40dB) allows the reception of weaker reflected signals. If high directivity antennas are used, very short incoherent integration times are needed to have enough signal-to-noise (SNR) ratios, allowing the reception of multiple specular reflection points such as crest of consecutive waves without the blurring induced by long incoherent integration times. This study presents for the first time experimental evidence of the wind and swell waves signatures in the GNSS-R waveforms, and compares them with models., Postprint (author's final draft)

© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Global Navigation Satellite Systems Reflected (GNSS-R) signals exhibit an incoherent and a coherent components [1], [2]. Current models assume that one or the other are dominant, and the calibration, and geophysical parameter retrieval (eg. wind speed, soil moisture ...) are developed accordingly. Even the presence itself of the coherent component of a GNSS reflected signal has been a matter of discussion in the last years. In this work, the method used in [3] to separate the leakage of the direct signal from the reflected one is applied to a set of GNSS signals reflected collected over the ocean by the MIR [4], [5], an airborne dual-band (L1/E1 and L5/E5a), multi-constellation (GPS and Galileo) GNSS-R instrument with two 19-elements array with 4 beam-steered each. The results presented demonstrate the feasibility of the proposed technique to untangle the coherent and incoherent components in GNSS reflected signals. This technique allows the processing of these components separately, which will increase the calibration accuracy (as today both are mixed together), and allows high resolution applications since the spatial resolution of the coherent component is determined by the size of the first Fresnel zone [6] (300-500 meters from a LEO satellite), and not by the size of the glistening zone (~25 km from a LEO satellite)., This work was supported by the Spanish Ministry of Science, Innovation and Universities, “Sensing with Pio- neering Opportunistic Techniques”, grant RTI2018-099008- B-C21, and the grant for recruitment of early-stage research staff FI-DGR 2015 and 2018 of the AGAUR - Generalitat de Catalunya (FEDER), Postprint (author's final draft)

As compared to GPS L1C/A signals, L5/E5a Global Navigation Satellite System-Reflectometry (GNSS-R) improves the spatial resolution due to the narrower auto-correlation function. Furthermore, the larger transmitted power (+3 dB), and correlation gain (+10 dB) allow the reception of weaker reflected signals. If directive antennas are used, very short incoherent integration times are enough to achieve good signal-to-noise ratios, allowing the reception of multiple specular reflection points without the blurring induced by long incoherent integration times. This study presents for the first time experimental evidence of the wind and swell waves signatures in the GNSS-R waveforms, and it performs a statistical analysis, a time-domain analysis, and a frequency-domain analysis for a unique data set of waveforms collected by the UPC MIR instrument during a series of flights over the Bass Strait, Australia, This work was founded by the Spanish Ministry of Science, Innovation and Universities, “Sensing with Pioneering Opportunistic Techniques”, grant RTI2018-099008-B-C21, and the grant for recruitment of early-stage research staff FI-DGR 2015 of the AGAUR - Generalitat de Catalunya (FEDER), Spain, and the grant for recruitment of early-stage research staff FI 2018 of the AGAUR - Generalitat de Catalunya (FEDER), Spain, and Unidad de Excelencia María de Maeztu MDM-2016-060., Peer Reviewed, Postprint (published version)

Premi a la Millor tesi doctoral en teledetecció, defensada a Espanya durant l'any 2020, atorgat pel Capítulo español de IEEE-Geoscience and Remote Sensing Society (GRSS), GNSS reflectometry (GNSS-R) is a set of techniques that uses the reflected GNSS signals over the Earth’s surface as opportunity signals for remote sensing applications. In 1993, the European Space Agency (ESA) suggested to use those signals as a tool to estimate sea height anomalies in an attempt to detect tsunamis before they reach the shore. Since then, GNSS-R has been a subject of topical interest, and has proven its feasibility for a diverse number of applications in land, ice and water bodies, from ground-based, airborne and space-borne instruments. This Ph.D. thesis has its roots in the ESA project PARIS-IoD, which aimed to study the feasibility of GNSS-R for sea altimetry from a space-borne instrument. Unfortunately, the project was canceled shortly after starting this thesis. However, some of the questions that have arisen during the initial phase of the project, have been the goals of this Ph.D. The thesis has two objectives. The first one is to solve some theoretical issues required for the development of next generation of GNSS-R instruments. The second one is to participate in the development of an airborne GNSS-R instrument mimicking the one intended for the PARIS-IoD, and to use it in field campaigns. The main feature of this instrument is that it has two dual-band (L1/L5) array antennas, with two beams per band that are analog steered towards the desired satellite and reflection point. As for the theoretical objectives; this thesis investigates five topics. Firstly, the thesis gives closed-form expressions of the so-called Woodward Ambiguity Functions (WAFs) of the modern GNSS signals as function of the receiver bandwidth. The motivation behind this work is that these equations can be later used in simulators in order to create reference models based on different physical magnitudes. Secondly, the thesis finds the optimum receiver bandwidth for each GNSS signal in terms of altimetric precision. The idea behind this study is that using a large bandwidt, La reflectometria GNSS (GNSS-R) és un conjunt de tècniques que utilitzen els senyals GNSS reflectits sobre la superfície terrestre com a senyals d’oportunitat per a aplicacions de teledetecció. El 1993, l'Agència Espacial Europea (ESA) va suggerir utilitzar aquests senyals com a eina per estimar anomalies de l'alçada del mar en un intent de detectar tsunamis abans que aquests arribin a la costa. Des de llavors, GNSS-R ha estat un tema de gran d’interès i ha demostrat la seva viabilitat per a un nombre divers d’aplicacions terrestres, de gel i de mar, des d’instruments terrestres, aerotransportats i espacials. Aquesta tesis doctoral té les seves arrels en el projecte de la ESA PARIS-IoD, que tenia com a objectiu estudiar la viabilitat de GNSS-R per a l’altimetria des de l’espai. Malauradament, el projecte es va cancel·lar poc després d’iniciar aquesta tesi. No obstant això, algunes de les qüestions que van sorgir durant la fase inicial del projecte han estat els objectius d’aquest doctorat. La tesi té dos objectius. El primer és resoldre alguns problemes teòrics necessaris per al desenvolupament de la nova generació d'instruments GNSS-R. El segon consisteix a participar en el desenvolupament d’un instrument GNSS-R aerotransportat que imita el originalment planejat per PARIS-IoD i utilitzar-lo en campanyes de mesures. La característica principal d’aquest instrument és que compta amb dues antenes doble banda (L1 / L5), amb dos feixos per banda dirigits analògicament cap al satèl·lit i el punt de reflexió desitjat. En quant als objectius teòrics, aquesta tesi investiga cinc temes. En primer lloc, la tesi dóna expressions analítiques de les anomenades funcions d’ambigüitat (WAF) dels senyals GNSS moderns en funció de l’amplada de banda del receptor. La motivació d’aquest treball és que aquestes equacions es poden utilitzar posteriorment en simuladors per tal de crear models de referència basats en diferents magnituds físiques. En segon lloc, la tesi troba l’ample de band, Award-winning, Postprint (published version)

As opposed to monostatic radars where incoherent backscattering dominates, in bistatic radars, such as Global Navigation Satellite Systems Reflectometry (GNSS-R), the forward scattered signals exhibit both an incoherent and a coherent component. Current models assume that either one or the other are dominant, and the calibration and geophysical parameter retrieval (e.g., wind speed, soil moisture, etc.) are developed accordingly. Even the presence of the coherent component of a GNSS reflected signal itself has been a matter of discussion in the last years. In this work, a method developed to separate the leakage of the direct signal in the reflected one is applied to a data set of GNSS-R signals collected over the ocean by the Microwave Interferometer Reflectometer (MIR) instrument, an airborne dual-band (L1/E1 and L5/E5a), multi-constellation (GPS and Galileo) GNSS-R instrument with two 19-elements antenna arrays with 4 beam-steered each. The presented results demonstrate the feasibility of the proposed technique to untangle the coherent and incoherent components from the total power waveform in GNSS reflected signals. This technique allows the processing of these components separately, which increases the calibration accuracy (as today both are mixed and processed together), allowing higher resolution applications since the spatial resolution of the coherent component is determined by the size of the first Fresnel zone (300–500 meters from a LEO satellite), and not by the size of the glistening zone (25 km from a LEO satellite). The identification of the coherent component enhances also the location of the specular reflection point by determining the peak maximum from this coherent component rather than the point of maximum derivative of the incoherent one, which is normally noisy and it is blurred by all the glistening zone contributions, This work was founded by the Spanish Ministry of Science, Innovation and Universities, “Sensing withPioneering Opportunistic Techniques”, grant RTI2018-099008-B-C21, and the grant for recruitment of early-stageresearch staff FI-DGR 2015 of the AGAUR—Generalitat de Catalunya (FEDER), Spain, and Unidad de ExcelenciaMaría de Maeztu MDM-2016-060., Peer Reviewed, Postprint (published version)

Global navigation satellite system reflectometry (GNSS-R) is a group of techniques that uses satellite navigation signals as signals of opportunity for remote sensing applications. In GNSS-R, large amounts of data are acquired and have to be processed. Computation time is typically the bottleneck for ground and airborne experiments. This article presents an efficient solution for off-line GNSS-R processing data taking advantage of graphics processing units (GPUs). After comparing to the typically used CPU languages, such as MATLAB and C++, the advantage of using parallel processing on the GPU is clear. GPU-based computation can reduce the processing time by as much as 95% of the acquisition time of the data. An implementation taking advantage of a home-use GPU is proposed for the data processing units. Thanks to its efficiency, even real-time processing experiments are feasible., This work was supported in part bythe Spanish Ministry of Economy and Competitiveness and FEDER EU underthe project “AGORA: Técnicas Avanzadas en Teledetección Aplicada UsandoSeñales GNSS y Otras Señales de Oportunidad” (MINECO/FEDER) ESP2015-70014-C2-1-R by the Agencia Estatal de Investigación; in part by the SpanishMinistry of Science, Innovation, and Universities, “Sensing with Pioneering Opportunistic Techniques,” under Grant RTI2018-099008-B-C21; in part by the Collaboration under Grant 998142 by the Spanish Ministry of Education; inpart by Unidad de Excelencia María de Maeztu MDM-2016-0600; and in partby Prof. Camps ICREA Academia 2015 award of the Generalitat de Catalunya., Peer Reviewed, Postprint (published version)

This manuscript describes the Microwave Interferometric Reflectometer (MIR) instrument, a multi-beam dual-band GNSS-Reflectometer with beam-steering capabilities built to assess the performance of a PAssive Reflectrometry and Interferometry System—In Orbit Demonstrator (PARIS-IoD) like instrument and to compare the performance of different GNSS-R techniques and signals. The instrument is capable of tracking up to 4 different GNSS satellites, two at L1/E1 band, and two at L5/E5 band. The calibration procedure of the up- and down-looking arrays is presented, the calibration performance is evaluated, and the results of the validation experiments carried out before the field experiments are shown in this paper., Peer Reviewed, Postprint (author's final draft)

This work analyzes the satellite cross-talk observed by the microwave interferometric reflectometer (MIR), a new global navigation satellite system (GNSS) reflectometer, during an airborne field campaign in Victoria and New South Wales, Australia. MIR is a GNSS reflectometer with two 19-element, dual-band arrays, each of them having four steerable beams. The data collected during the experiment, the characterization of the arrays, and the global positioning system (GPS) and Galileo ephemeris were used to compute the expected delays and power levels of all incoming signals, and the probability of cross-talk was then evaluated. Despite the MIR highly directive arrays, the largest ever for a GNSS-R instrument, one of the flights was found to be contaminated by cross-talk almost half of the time at the L1/E1 frequency band, and all four flights were contaminated ~5–10% of the time at the L5/E5a frequency band. The cross-talk introduces an error of up to 40 cm of standard deviation for altimetric applications and about 0.24 dB for scatterometric applications, Peer Reviewed, Postprint (published version)

As the TDS-1 and CYGNSS spaceborne GNSS-R missions progress, a large number of Delay-Doppler Maps (DDM) is being collected, from the land scattered as well as from the ocean. Based on those data, the feasibility study on the retrieval of land geophysical parameters such as soil moisture and vegetation is being studied. This work presents the E2E simulator that generates the synthesized DDM reflected over land. For the efficient development, the existing simulator for ocean scattering has been upgraded to accommodate the specific functionalities for land scattering: topography, surface roughness, vegetation effects, in addition to surface soil moisture, obviously. The developed end-to-end (E2E) simulator is used to conceive the retrieval algorithms and assess their performance. In this work, the upgraded items are described and illustrated with some simulation results., Peer Reviewed, Postprint (published version)

Radio-Frequency Interference (RFI) is one of the most concerning topics in Global Navigation Satellite Systems (GNSS) and GNSS - Reflectometry (GNSS-R) because any undesired signal present at the antenna can corrupt the received signal and the eventual geophysical measurements. The purpose of this paper is two-fold. First, an RFI analysis is performed to the raw data obtained from an airborne dual-band GNSS reflectometer. Finally, the Front-End GNSS Interference eXcisor (FENIX) mitigation algorithm is applied to the raw data, and then the Signal-to-Noise Ratio (SNR) improvement at the Delay-Doppler Map (DDM) is obtained. From the analysis of one of the datasets, two RFI signals were detected and characterized. Moreover, FENIX output signal was revealed to be much cleaner than before. The mean SNR improvement with coherent DDMs of 1 ms is 0.72 dB, whereas the SNR improvement increases to 1.49 dB when considering 50 incoherent averages. This improvement represents a reduction in the geophysical parameter estimation uncertainty., Peer Reviewed, Postprint (published version)

©2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., The maximum coherent integration time of GNSS reflected (GNSS-R) signals over sea depends on the sea state, as well as on the geometry formed by the receiver, the GNSS satellite and the reflection point. In this paper, real data obtained with a dedicated airborne instrument named MALYGNSS (Multi-band Airborne L-Band reflectometrY with GNSS) is used to acquire GPS L1 C/A and L2C, and Galileo E1OS signals. The direct signals are used as a reference to detect any phase shift due to data bits changes and the presence of a residual Doppler frequency. These phase transitions are removed in the reflected signals, and coherent correlations of different lengths are performed in order to study sea correlation time., Peer Reviewed, Postprint (published version)

The 3 Cat-3/MOTS mission is a joint initiative between the Institut Cartogràfic i Geològic de Catalunya (ICGC) and the Universitat Politècnica de Catalunya-BarcelonaTech (UPC) to foster new and innovative technologies on Earth Observation (EO) based on the data fusion of Global Navigation Satellite Systems Reflectometry (GNSS-R) and optical payloads on a 6U CubeSat platform. The final objective of the mission is to provide soil moisture mapping through the GNSS-R payload and improve its resolution by data fusion with the multi-spectral optical data acquired. This technique has been successfully applied to SMOS, but never performed from a 6U CubeSat carrying both payloads. An airborne optical and GNSS-R field experiment has been carried out to test/develop the algorithm and to analyze the results both separately (GNSS-R + optical data) and as a soil moisture product, once performed the data fusion., Peer Reviewed, Postprint (published version)

In order to provide high resolution soil moisture maps, the Institut Cartografic i Geológic de Catalunya (ICGC) in colab-oration with Universitat Politecnica de Catalunya are planing to deploy the 3 Cat-3 picosatellite, which aims to estimate soil moisture from space using Global Navigation Satellite Systems - Reflectometry (GNSS-R) and to downscale it using data fusion with a multi-spectral camera. As a proof of concept, a field campaign is being carried out in Lleida, Catalunya. MALYGNSS is a GNSS reflectometer flying as an opportunity sensor that will validate the main reflectometer of the project: the CORTO instrument., Peer Reviewed, Postprint (published version)

Global Navigation Satellite Systems – Reflectometry (GNSS-R) is a relatively new remote sensing technique that benefits from GNSS signals as signals of opportunity as a bistatic radar. These signals can be processed as a scatterometer by measuring the relative scattered power, as an altimeter by measuring the relative delay between the direct and the scattered signals, and as an unfocused Synthetic Aperture Radar (SAR) with very short coherent integration times (typically a few millisecons). Wider bandwidth signals provide an opportunity to increase the performance of some GNSS reflectometry applications (i.e. altimetry). The Microwave Interferometric Reflectometer (MIR) is an instrument developed at UPC to compare different GNSS-R techniques, notably the conventional and interferometric ones, and assess the capabilities of these new signals and bands of the GPS and Galileo systems (L1/E1 and L5/E5). This paper describes the MIR beamforming system., Peer Reviewed, Postprint (published version)

Peer Reviewed, Postprint (published version)

This work addresses the accuracy of the Global Navigation Satellite Systems (GNSS)-Reflectometry (GNSS-R) scatterometric measurements considering the presence of both coherent and incoherent scattered components, for both conventional GNSS-R (cGNSS-R) and interferometric GNSS-R (iGNSS-R) techniques. The coherent component is present for some type of surfaces, and it has been neglected until now because it vanishes for the sea surface scattering case. Taking into account the presence of both scattering components, the estimated Signal-to-Noise Ratio (SNR) for both techniques is computed based on the detectability criterion, as it is done in conventional GNSS applications. The non-coherent averaging operation is considered from a general point of view, taking into account that thermal noise contributions can be reduced by an extra factor of 0.88 dB when using partially overlapped or partially correlated samples. After the SNRs are derived, the received waveform’s peak variability is computed, which determines the system’s capability to measure geophysical parameters. This theoretical derivations are applied to the United Kingdom (UK) TechDemoSat-1 (UK TDS-1) and to the future GNSS REflectometry, Radio Occultation and Scatterometry on board the International Space Station (ISS) (GEROS-ISS) scenarios, in order to estimate the expected scatterometric performance of both missions., Peer Reviewed, Postprint (published version)

©2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., In the past decade Global Navigation Satellites System Reflectometry (GNSS-R) has emerged as a new technique for earth remote sensing for various applications, such as ocean altimetry and sea state monitoring. After the success of the GNSS-R demonstrator payloads aboard the UK-DMC or TDS-1 satellites; at present, there are several missions planned to carry GNSS reflectometers. The GNSS rEflectometry, Radio Occultation, and Scatterometry onboard International Space Station (GEROS-ISS) is an innovative ISS experiment exploiting GNSS-R technique to measure key parameters of ocean, land, and ice surfaces. For GEROS-ISS mission, the European Space Agency (ESA) supported the study of GNSS-R assessment of requirements and consolidation of retrieval algorithms (GARCA). For this, it was required to accurately simulate the GEROS-ISS measurements including the whole range of parameters affecting the observation conditions and the instrument, which is called GEROS-SIM. To meet these requirements, the PAU/PARIS end-to-end performance simulator (P$^{2}$EPS) previously developed by UPC BarcelonaTech was used as the baseline building blocks for the level 1 (L1) processor of GEROS-SIM. P$^{2}$EPS is a flexible tool, and is capable of systematically simulating the GNSS-R observations for spaceborne GNSS-R missions. Thanks to the completeness and flexibility, the instrument-to-L1 data module of GEROS-SIM could be implemented by proper modification and update of P$^{2}$EPS. The developed GEROS-SIM was verified and validated in the GARCA study as comparing to the TDS-1 measurements. This paper presents the design, implementation, and results of the GEROS-SIM L1 module in a generic way to be applied to GNSS-R instruments., Peer Reviewed, Postprint (author's final draft)

©2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Radio–frequency interference (RFI) signals are a well-known threat for microwave radiometry (MWR) applications. In order to alleviate this problem, different approaches for RFI detection and mitigation are currently under development. Since RFI signals are man made, they tend to have their power more concentrated in the time–frequency (TF) space as compared to naturally emitted noise. The aim of this paper is to perform an assessment of different TF RFI mitigation techniques in terms of probability of detection, resolution loss (RL), and mitigation performance. In this assessment, six different kinds of RFI signals have been considered: a glitch, a burst of pulses, a wide-band chirp, a narrow-band chirp, a continuous wave, and a wide-band modulation. The results show that the best performance occurs when the transform basis has a similar shape as compared to the RFI signal. For the best case performance, the maximum residual RFI temperature is 14.8 K, and the worst RL is 8.4%. Moreover, the multiresolution Fourier transform technique appears as a good tradeoff solution among all other techniques since it can mitigate all RFI signals under evaluation with a maximum residual RFI temperature of 21 K, and a worst RL of 26.3%. Although the obtained results are still far from an acceptable bias Misplaced < 1 K for MWR applications, there is still work to do in a combined test using the information gathered simultaneously by all mitigation techniques, which could improve the overall performance of RFI mitigation., Peer Reviewed, Postprint (author's final draft)

The demonstration of the feasibility of navigation signals signals for remote sensing has lead to a significant increase of new GNSS-R instruments in the recent years. Despite of the type instrument, the observables are based on the cross-correlations of the received GNSS signals. Most of these instruments combine the signals received by multiple chains, in such a way that they need a proper calibration. This work proposes a calibration procedure for GNSS-R instruments by using a calibration signal with similar statistics to the GNSS signals. The method is tested and validated with the Microwave Interferometric Reflectometer (MIR) instrument, which contains 152 chains. The test results will be presented at the conference., Peer Reviewed, Postprint (published version)

Global Navigation Satellite Systems - Reflectometry (GNSS-R) has opened promising results for remote sensing applications. The broadcast of new signals with wider bandwidth provides an opportunity to increase the performance of GNSS reflectometry applications. The Microwave Interferometric Reflectometer (MIR) is an instrument being developed to compare different GNSS-R techniques, namely the conventional and interferometric ones, and assess the capabilities of these new signals and bands of the GPS and Galileo systems. This work describes the tests to validate the beamforming system., Peer Reviewed, Postprint (published version)

Radio-Frequency Interference (RFI) is a growing problem for applications based on the Global Navigation Satellite Systems (GNSS), including Earth observation using GNSS - Reflectometry (GNSS-R). Nowadays, many efforts are being concentrated to develop RFI signal detectors with high sensitivity, which can help to control the proliferation of RFI generators or jammers. This work aims at designing and testing an instrument capable of detecting and localizing RFI signals at GNSS bands using a microwave radiometer architecture, and a combination of statistical and time-frequency digital detection techniques. The instrument has been tested using different kinds of RFI signals, and its performance has been evaluated as a function of the RFI power. A sensitivity of -135 dBm for continuous wave and chirp signals is reported., Peer Reviewed, Postprint (published version)

This work compares microwave radiometry and global navigation satellite systems-reflectometry (GNSS-R) observations using data gathered from airborne flights conducted for three different soil moisture conditions. Two different regions are analyzed, a crops region and a grassland region. For the crops region, the correlation with the I/2 (first Stokes parameter divided by two) was between 0.74 and 0.8 for large incidence angle reflectivity data (30°-50°), while it was between 0.51 and 0.61 for the grassland region and the same incidence angle conditions. For the crops region, the correlation with the I/2 was between 0.64 and 0.69 for lower incidence angle reflectivity data (<;30°), while it was between 0.41 and 0.6 for the grassland region. This indicates that for large incidence angles the coherent scattering mechanism is dominant, while the lower incidence angles are more affected by incoherent scattering. Also a relationship between the reflectivity and the polarization index (PI) is observed. The PI has been used to remove surface roughness effects, but due to its dependence on the incidence angle only the large incidence angle observations were useful. The difference in ground resolution between microwave radiometry and GNSS-R and their strong correlation suggests that they might be combined to improve the spatial resolution of microwave radiometry measurements in terms of brightness temperature and consequently soil moisture retrievals., This work was supported in part by the Spanish Ministry of Science and Innovation, “AROSA-Advanced Radio Ocultations and Scatterometry Applications using GNSS and other opportunity signals,” under Grant AYA2011-29183-C02-01/ESP and “AGORA: Tecnicas Avanzadas en Teledetección Aplicada Usando Señales GNSS y Otras Señales de Oportunidad,” under Grant ESP2015-70014-C2-1-R (MINECO/FEDER), in part by the Monash University Faculty of Engineering 2013 Seed Grant, and in part by the Advanced Remote Sensing Ground-Truth Demo and Test Facilities and Terrestrial Environmental Observatories funded by the German Helmholtz-Association. The work of A. A.-Arroyo was supported by the Fulbright Commission in Spain through a Fulbright grant., Peer Reviewed, Postprint (author's final draft)

Global Navigation Satellite Systems Reflectometry (GNSSR) is now being recognized as a powerfull tool for remote sensing. With the addition of new frequency bands and codes, it is possible to increase the resolution and accuracy of the measurements. GPS L5 and Galileo E5 bands are becoming popular because of the open codes, with wider bandwidths than the L1 band ones. However, these bands are shared with two radio navigation systems (DME and TACAN) that transmit with high powers and might interfere them. The use of mitigation techniques may affect the power of the received signal, which will affect the GNSS-R observables. This work presents several the mitigation techniques, and evaluates the performance and impact of the selected ones for ESA's GEROS experiment onboard the International Space Station., Peer Reviewed, Postprint (published version)

©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., This paper presents a statistical analysis of the crosstalk phenomenon in interferometric Global Navigation Satellite Systems Reflectometry (iGNSS-R). Crosstalk occurs when the Delay-Doppler Map (DDM) of a tracked satellite overlaps others fromundesired satellites. This study is performed for ground-based and airborne receivers and for a receiver onboard the International Space Station (ISS) such as the upcoming GNSS Reflectometry, Radio Occultation and Scatterometry onboard ISS experiment. Its impact on ocean altimetry retrievals is analyzed for different antenna arrays. Results show that for elevation angles higher than 60 degrees, crosstalk can be almost permanent from ground, up to 61% from airborne receivers at 2-km height, and up to similar to 10% at the ISS. Crosstalk can only be mitigated using highly directive antennas with narrow beamwidths. Crosstalk impact using a seven-element hexagonal array still induces large errors on ground, but reduces to centimeter level on airborne receivers, and is negligible from the ISS., Peer Reviewed, Postprint (author's final draft)

The number of applications based on Global Navigation Satellite Systems (GNSS) has been increasing in last years. In the passive remote sensing field, GNSS-Reflectometry (GNSS-R) systems have been revealed as cost-effective solutions to retrieve a number of geophysical parameters. However, GNSS-based devices are prone to suffer from Radio-Frequency Interference (RFI) effects due to the extreme low power of involved signals. This paper shows the preliminary results of the Front-End GNSS Interference eXcisor (FENIX), a system designed to mitigate the RFI problem in GNSS-based systems. FENIX is composed by two stages: the RF stage and the SP stage. The former conditions the signal from the antenna, throw the SP stage, and to the receiver; and the latter excises the RFI from the GNSS signal combining thresholding with the Multiresolution Fourier Transform (MFT). Preliminary results show that FENIX can mitigate up to 30 dB a chirp RFI signal generated from commercial jammers., Peer Reviewed, Postprint (published version)

Radio-Frequency Interference (RFI) is a well-known problem for Microwave Radiometers (MWR). A number of RFI mitigation techniques have been studied to deal with RFI from the receiver's side. In this study, a comparison in terms of performance among several real-time time-frequency RFI mitigation techniques is presented. Time-frequency techniques are of high importance because the vast majority of RFI signals are well-localized in the time-frequency space. The five techniques compared under study are: Pulse Blanking (PB), Frequency Blanking (FB), Spectrogram Blanking (SB), Wavelet Denoising (WD), and Multiresolution Fourier Transform (MFT). Results show that the best performance depends on the particular type of RFI, and occurs when the transform basis is similar to the RFI, that is, the projection on that particular basis is maximized. However, there is not a single technique that is effective for all RFI signals, and future approaches may be based on a combination of several techniques., Peer Reviewed, Postprint (published version)

©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Global navigation satellite system reflectometry (GNSS-R) is becoming a widely accepted technique for remote sensing. The interferometric technique (iGNSS-R) correlates the direct signal received from a satellite and the same signal reflected on the Earth's surface, whereas the conventional technique (cGNSS-R) correlates the reflected signal with a locally generated replica of the transmitted code. As GNSS signals are received below the noise level, this technique is extremely sensitive to radio frequency interference. The distance measurement equipment, and the TACtical air navigation systems are two radio navigation systems that transmit in the GPS L5, and Galileo E5 bands with powers up to 3.5 kW. This work studies in depth the impact of these systems on iGNSS-R, and cGNSS-R instruments. This study is then applied to a hypothetical reflectometer that will be placed in the international space station: The GEROS experiment. It is shown that the received power in space will be strong enough to degrade the system's performance by increasing the noise floor, but the sea altimetry precision will still be accurate enough for scientific studies., Peer Reviewed, Postprint (author's final draft)

This work presents an improvement of the P2EPS end-to-end simulator to assess the performances of Global Navigation Satellite System Reflectometry (GNSS-R) space missions over land. Beyond the single specular reflection model, topography effects are included. In order to avoid the computational burden of facet approach based on the bi-static radar model, the Phong reflection model is employed. By calculating the delay, Doppler frequency, and relative intensity of scattered points, the topography effects can be accounted for as a sort of impulse response in the delay, and Doppler domains to be convolved with the Woodward Ambiguity function., Peer Reviewed, Postprint (published version)

The GNSS rEflectometry, Radio Occultation and Scatterometry onboard the International Space Station (GEROS-ISS) experiment consists of the installation of a GNSS reflectometry (GNSS-R) instrument on the International Space Station (ISS). Due to the complex structure of ISS, the GEROS-ISS instrument suffers from multipath effects. The collected signals are not only from received directly, but also after multiple reflections in the ISS structure. This work analyzes the multipath effect of the GEROS-ISS instrument based on the ISS structure. Using the ray tracing simulation, all rays reaching the instrument after one or two reflections are computed, as well as the corresponding additional delays. This information, together with the direction of arrival to the antenna arrays (up and down-looking), and the co- and cross-polar antenna patterns is used to assess the impact of multipath on the waveforms are demonstrated., Peer Reviewed, Postprint (published version)

This work shows a technique to estimate the vegetation opacity and infer the vegetation water content using a single GPS receiver located under the vegetation layer being analyzed. It is based on the measurement of the received signal power when the GPS satellite is at the zenith, and when the GPS satellite is with an elevation of 30 degrees. Assuming a horizontally stratified vegetation layer, the attenuation on the GPS signal due to vegetation doubles at 30° elevation angle compared to the zenith. This attenuation is directly related to the vegetation opacity. Then, the vegetation opacity is related to the vegetation water content by a linear parameter. Initial results relating the measured GPS signal power to canopy parameters obtained from hemispheric photos are shown., Peer Reviewed, Postprint (published version)

©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Radio-frequency interference (RFI) is a serious threat for systems working with low power signals such as those coming from the global navigation satellite systems (GNSS). The spectral separation coefficient (SSC) is the standard figure of merit to evaluate the signal-to-noise ratio (SNR) degradation due to the RFI. However, an in-depth assessment in the field of GNSS-Reflectometry (GNSS-R) has not been performed yet, and particularly, about which is the influence of the RFI on the so-called delay-Doppler map (DDM). This paper develops a model that evaluates the contribution of intra-/inter-GNSS and external RFI effects to the degradation of the SNR in the DDM for both conventional and interferometric GNSS-R techniques. Moreover, a generalized SSC is defined to account for the effects of nonstationary RFI signals. The results show that highly directive antennas are necessary to avoid interference from other GNSS satellites, whereas mitigation techniques are essential to keep GNSS-R instruments working under external RFI degradation., Peer Reviewed, Postprint (author's final draft)

The use of Global Navigation Satellite System (GNSS) signals for mesoscale altimetry was first proposed in [1]. Since then, many experiments and a couple of space missions have been carried out to validate the concept in multiple fields including not only ocean scenarios, but also land and ice [2]. The ensemble of these techniques is referred to as GNSS Reflectometry (GNSS-R). The most complete observable of these techniques is the so-called Delay Doppler Map (DDM), which is the 2D cross-correlation between the reflected signal and a reference one at different Doppler frequencies. In the conventional method (cGNSS-R), the reference signal is a clean replica of the transmitted one, whereas in the interferometric technique (iGNSS-R), the reference signal is the direct onesignal itself., Peer Reviewed, Postprint (published version)

This work presents two field campaigns performed between December 2014 and June 2015 in order to better understand the behavior of Global Navigation Satellite Systems (GNSS) reflected signals over land. The instruments used in both field campaigns are briefly introduced. One of them is focused on circular polarization, whereas the other is focused on linear polarization (horizontal and vertical). In one field experiment there is evidence of multipath due to the antenna performance. A method to mitigate it is proposed in this work. Finally, some preliminary soil moisture estimations using vertical polarization are shown., Peer Reviewed, Postprint (published version)

Radio-Frequency Interference (RFI) is a growing problem specially for those systems that work with low power signals such as passive remote sensing instruments. Consequently, RFI mitigation techniques are currently under development. This works aims at evaluating back-end mitigation algorithms in terms of their probability of detection and mitigation performance. Results show that Wavelet Denoising (WD), and Multiresolution Fourier Transform (MFT) are the best techniques in most scenarios, specially for GNSS-based instruments, Peer Reviewed, Postprint (author's final draft)

The MIR (Microwave Interferometric Reflectometer) is a new dual frequency Global Navigation Satellite Systems Re- flectometry (GNSS-R) instrument that implements conventional and interferometric reflectometry, both with GPS and Galileo signals. This paper presents the instrument development progress, mainly in the control part, where a methodology to determine how much the platform attitude will affect the beam steering and a satellite selection algorithm have been developed., Peer Reviewed, Postprint (published version)

El equipo RSLAB está construyendo un instrumento para estudiar el estado del mar mediante las señales de posicionamiento global (GNSS) y su reflexión sobre la superficie (técnica conocida como GNSS-R). El instrumento incluye dos agrupaciones de antenas con 4 haces cada una, dirigibles para apuntar a los satélites GNSS y a sus respectivas señales reflejadas. La cadena receptora requiere una calibración periódica para asegurar un buen apuntamiento., [ANGLÈS] This project describes the design and implementation of a calibration network for the Microwave Interferometric Reflectometer (MIR), a reflectometer that the Remote Sensing Lab (RSLAB) is developing at the Department of Signal Theory and Communications (TSC) at the Catalonia Polytechnic University BarcelonaTech (UPC). This particular instrument requires periodic calibration to ensure good targeting of its 2 antenna arrays’ beams using a pseudorandom noise (PRN) sequence at L1 and L5 center frequencies. This calibration network is capable of retrieving the amplitude and phase distortion of both channels after the cross-correlation of the received signal and a clean replica of the PRN., [CASTELLÀ] Este proyecto describe el diseño e implementación de una red de calibración para el Microwaves Interferometric Reflectometer (MIR), un reflectómetro que el Remote Sensing Lab (RSLAB) está desarrollando en el Departamento de Teoría de la Señal y Comunicaciones (TSC) de la Universidad Politécnica de Cataluña BarcelonaTech (UPC). Este instrumento en particular requiere una calibración periódica para asegurar una buen apuntamiento de las haces de sus 2 conjuntos de antenas utilizando una secuencia de ruido pseudoaleatorio (PRN) en frecuencias centrales de las bandas L1 y L5. Esta red de calibración es capaz de recuperar las distorsiones de amplitud y la fase de ambos canales después de realizar la correlación cruzada de la señal recibida y una réplica limpia del PRN., [CATALÀ] Aquest projecte descriu el disseny i implementació d'una xarxa de calibratge pel Microwaves Interferometric Reflectometer (MIR), un reflectòmetre que el Remote Sensing Lab (RSLAB) està desenvolupant en el Departament de Teoria del Senyal i Comunicacions (TSC) de la Universitat Politècnica de Catalunya BarcelonaTech (UPC). Aquest instrument en particular requereix un calibratge periòdic per assegurar una bon apuntant de les fas dels seus 2 conjunts d'antenes utilitzant una seqüència de soroll pseudoaleatori (PRN) en freqüències centrals de les bandes L1 i L5. Aquesta xarxa de calibratge és capaç de recuperar les distorsions d'amplitud i la fase de tots dos canals després de realitzar la correlació creuada del senyal rebut i una rèplica neta del PRN.

This work shows a state-of-the-art GNSS reflectometry endto-end performance simulator, selected for the ESA’s GNSS rEflectometry, Radio Occultation and Scatterometry onboard International Space Station (GEROS-ISS). For the efficient development process, the PAU/PARIS End-to-end Performance Simulator (P2EPS) are used as main building blocks for the GEROS-Simulator. To accommodate the GEROS specific situation of platform and instrument, P2EPS is modified. Additionally, several considerations in spaceborne GNSS-R techniques are newly updated in GEROS-SIM, improving the performance in terms of accuracy and completeness. GEROS-SIM will be delivered as a web application, i.e., executable via web browser connecting GEROS-SIM webpage hosted by UPCBarcelonaTech., Peer Reviewed, Postprint (author's final draft)

©2014 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Since 1993, when the European Space Agency (ESA) proposed the use of Global Navigation Satellite Systems reflected signals for sea mesoscale altimetry, a wide range of applications have appeared. This paper focuses on the retrieval of significant wave height (SWH) and the mean sea surface level (MSSL) from a ground-based experiment using the interference pattern technique (IPT). Two different observables on the IPT are analyzed: the oscillation frequency and the angle where coherency is lost. The point where coherency in the reflection process is lost can be related to the Rayleigh criterion for smooth surfaces and helps to determine the SWH. Spectral analysis on the interference pattern helps to determine the MSSL. A three-month field campaign was performed on the, Peer Reviewed, Postprint (author's final draft)