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Earth Observation (EO) is key for climate and environmental monitoring at global level, and in specific regions where the effects of global warming are more noticeable, such as in polar regions, where ice melt is also opening new commercial maritime routes. Soil moisture is also useful for agriculture and monitoring the advance of desertification, as well as biomass and carbon storage. Global Navigation Satellite System - Reflectometry (GNSS-R) and L-band microwave Radiometry are passive microwave remote sensing techniques that can be used to perform these types of measurements regardless of the illumination and cloud conditions, and -since they are passive- they are well suited for small satellites, where power availability is a limiting factor. GNSS-R was tested from space onboard the UK-DMC and the UK TechDemoSat-1, and several missions have been launched using GNSS-R as main instrument, as CyGNSS, BuFeng-1, or the FSSCAT [1] mission. These missions aim at providing soil moisture [2], ocean wind speed [3], and flooding mapping of the Earth. L-band microwave radiometry data has also been retrieved from space with SMOS and SMAP missions, obtaining sea ice thickness, soil moisture, and ocean salinity data [4]. The 3Cat-4 mission was selected by the ESA Academy "Fly your Satellite" program in 2017. It aims at combining both GNSS-R and L-band Microwave Radiometry at in a low-power and cost-effective 1-Unit (1U) satellite. Moreover, the 3Cat-4 can also detect Automatic Identification System (AIS) signals from vessels. The single payload is the Flexible Microwave Payload 1 (FMPL-1) [5] that performs the signal conditioning and signal processing for GNSS-R, L-Band microwave radiometry and AIS experiments. The spacecraft has three payload antennas: (1) a VHF monopole for AIS signals; (2) an uplooking antenna for the direct GPS signals; (3) a downlooking antenna that captures reflected GPS signals, and for the Microwave Radiometer. The downlooking antenna is a deployable h

The Federated Satellite Systems/ 3 Cat-5 (FSSCat) mission was the winner of the European Space Agency (ESA) Sentinel Small Satellite (S 3 ) Challenge and overall winner of the 2017 Copernicus Masters competition. It consisted of two six-unit CubeSats. The Earth observation payloads were 1) the Flexible Microwave Payload 2 (FMPL-2) onboard 3 Cat-5/A, an L-band microwave radiometer and GNSS reflectometer (GNSS-R) implemented using a software-defined radio (SDR), and 2) the HyperScout-2 onboard 3 Cat-5/B, a hyperspectral camera, with the first experiment using artificial intelligence to discard cloudy images. FSSCat was launched on 3 September 2020 and injected into a 535-km synchronous orbit. 3 Cat-5/A was operated for three months until the payload was probably damaged by a solar flare and coronal mass ejection. During this time, all scientific requirements were met, including the generation of coarse-resolution and downscaled soil moisture (SM) maps, sea ice extent (SIE) maps, concentration and thickness maps, and even wind speed (WS) and sea surface salinity (SSS) maps, which were not originally foreseen. 3 Cat-5/B was operated a few more months until the number of images acquired met the requirements. This article briefly describes the FSSCat mission and the FMPL-2 payload and summarizes the main scientific results., This work was supported by the 2017 ESA S3 challenge and Copernicus Masters overall winner award (FSSCat project) and ESA project FSSCat Validation Experiment in MOSAIC; Spanish Ministry of Science, Innovation, and Universities Sensing With Pioneering Opportunistic Techniques grant RTI2018-099008-BC21/AEI/10.13039/501100011033; GNSS Environmental and Societal Missions grant PID2021- 126436OB-C21; and Unidad de Excelencia Maria de Maeztu grant MDM-2016-0600., Peer Reviewed, Article signat per 30 autors/es: Adriano Camps, Universitat Politècnica de Catalunya, Barcelona, Spain, Institut d’Estudis Espacials de Catalunya, Barcelona, Spain, United Arab Emirates University, Abu Dhabi, United Arab Emirates; Joan Francesc Munoz-Martin, Universitat Politècnica de Catalunya, Barcelona, Spain, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA; Joan Adrià Ruiz-de-Azua, Universitat Politècnica de Catalunya, Barcelona, Spain, i2Cat, The Internet Research Center, Barcelona, Spain; Lara Fernandez, Universitat Politècnica de Catalunya, Barcelona, Spain; Adrian Perez-Portero, Universitat Politècnica de Catalunya, Barcelona, Spain; David Llavería, Universitat Politècnica de Catalunya, Barcelona, Spain; Christoph Herbert, Universitat Politècnica de Catalunya, Barcelona, Spain; Miriam Pablos, Institut de Ciències del Mar, Barcelona, Spain, Barcelona Expert Center on Remote Sensing, Barcelona, Spain; Alessandro Golkar, Universitat Politècnica de Catalunya, Barcelona, Spain, Department of Aerospace and Geodesy, Technical University of Munich, Munich, Germany; Antonio Gutiérrez, Deimos Engenharia, Lisbon, Portugal; Carlos António, Deimos Engenharia, Lisbon, Portugal; Jorge Bandeiras, Deimos Engenharia, Lisbon, Portugal; Jorge Andrade, Deimos Engenharia, Lisbon, Portugal; David Cordeiro, Deimos Engenharia, Lisbon, Portugal; Simone Briatore, Golbriak Space, Tallin, Estonia; Nicola Garzaniti, Golbriak Space, Tallin, Estonia, School of Aerospace, Transport, and Manufacturing, Cranfield University, Cranfield, Bedfordshire, U.K.; Fabio Nichele, Tyvak International, Torino, Italy; Raffaele Mozzillo, Tyvak International, Torino, Italy; Alessio Piumatti, Tyvak International, Torino, Italy; Margherita Cardi, Tyvak International, Torino, Italy; Marco Esposito, Cosine, Sassenheim, The Netherlands; Chris van Dijk, Cosine, Sassenheim, The Netherlands; Nathan Vercruyssen, Cosine, Sassenheim, The Netherlands; Joao Barbosa, Cosine, Sassenheim, T, Postprint (author's final draft)

This work presents the first scientific results over land from the Flexible Microwave Payload -2 (FMPL-2), onboard the FSSCat mission. FMPL-2 is composed of an L-band microwave radiometer and a Global Navigation Satellite System - Reflectometer (GNSS-R). Two separate ANNs models are trained using the first three months of collected data of both observations, with the objective to retrieve global soil moisture maps. The first network addresses the coarsely-resolved FMPL-2 antenna footprint in a downscaling approach. Predicted values resulted in good agreement with those obtain from the SMAP mission, with an error smaller than 9.6%, and a bias smaller than 0.001 m 3 /m 3 . The second network is implemented to estimate soil moisture exclusively on GNSS-R data. In this second case, the combination of multiple GNSS-R measurements in a single track allows to retrieve soil moisture data with an error standard deviation with respect to SMAP lower than 0.056 m 3 /m 3 , with a bias smaller than 0.0007 m 3 /m 3 ., This work was supported by 2017 ESA S3 challenge and Copernicus Masters overall winner award (“FSSCat” project). This work has been (partially) sponsored by project SPOT: Sensing with Pioneering Opportunistic Techniques grant RTI2018-099008-B-C21 / AEI / 10.13039/501100011033, and by the Unidad de Excelencia Maria de Maeztu MDM2016-0600, and by the Spanish Ministry of Science and Innovation project ESP2017-89463-C3, the Centro de Excelencia Severo Ochoa CEX2019-000928-S and the CSIC Plataforma Tematica Interdisciplinar de Teledetección (PTI-Teledetect)., Peer Reviewed, Postprint (published version)

FSSCat, the “Federated Satellite Systems/ 3 Cat-5” mission was the winner of the 2017 ESA S^3 (Sentinel Small Satellite) Challenge and overall winner of the Copernicus Masters competition. FSSCat consists of two 6 unit cubesats carrying on board UPC's Flexible Microwave Payload - 2 (FMPL-2), an L-band microwave radiometer and GNSS-Reflectometer implemented in a software defined radio, and Cosine's HyperScout-2 visible and near infrared + thermal infrared hyperspectral imager, enhanced with PhiSat-1, a on board Artificial intelligence experiment for cloud detection. Both spacecrafts include optical and UHF inter-satellite links technology demonstrators, provided by Golbriak Space and UPC, respectively. This paper describes the mission, and the main scientific results of the FMPL-2 obtained during the first three months of the mission, notably the sea ice concentration and thickness, and the downscaled soil moisture products over the Northern hemisphere., This work was supported by 2017 ESA S 3 challenge and Copernicus Masters overall winner award (“FSSCat” project) and ESA project “FSSCat Validation Experiment in MOSAIC”, by the Spanish Ministry of Science, Innovation and Universities, "Sensing with Pioneering Opportunistic Techniques" SPOT, grant RTI2018-099008- BC21/AEI/10.13039/501100011033, and by the Unidad de Excelencia Maria de Maeztu MDM-2016-0600., Peer Reviewed, Article signat per 25 autors/es: A. Camps 1,2; J.F. Munoz‐Martin 1; J.A. Ruiz‐de‐Azua 1,2; L. Fernandez 1; A. Perez-Portero 1; D. Llavería 1; C. Herbert 1; M. Pablos 3; A. Golkar 4,1; A. Gutiérrrez 5; C. António 5; J. Bandeiras 5; J. Andrade 5; D. Cordeiro 5; S. Briatore 4,6; N. Garzaniti 4,6; F. Nichele 7; R. Mozzillo 7; A. Piumatti 7; M. Cardi 7; M. Esposito 8; B. Carnicero Dominguez 9; M. Pastena 9; G. Filippazzo 10; A. Reagan 10 // 1. Universitat Politècnica de Catalunya, Barcelona, Spain; 2. Institut d’Estudis Espacials de Catalunya, Barcelona, Spain; 3. Institut de Ciències del Mar (ICM-CSIC) & Barcelona Expert Center (BEC) on Remote Sensing, Barcelona, Spain; 4. Skolkovo Institute of Science and Technology, Moscow, Russia; 5. Deimos Eng., Lisbon, Portugal; 6. Golbriak Space, Tallin, Estonia; 7. Tyvak International, Torino, Italy; 8. Cosine, Oosteinde, The Netherlands; 9. ESA ESTEC, Noordwijk, The Netherlands; 10. ESA ESRIN, Frascati, Italy, Postprint (author's final draft)

CubeSat-based Earth Observation missions have emerged in recent times, achieving scientifically valuable data at a moderate cost. FSSCat is a two 6U CubeSats mission, winner of the ESA S3 challenge and overall winner of the 2017 Copernicus Masters Competition, that was launched in September 2020. The first satellite, 3Cat-5/A, carries the FMPL-2 instrument, an L-band microwave radiometer and a GNSS-Reflectometer. This work presents a neural network approach for retrieving sea ice concentration and sea ice extent maps on the Arctic and the Antarctic oceans using FMPL-2 data. The results from the first months of operations are presented and analyzed, and the quality of the retrieved maps is assessed by comparing them with other existing sea ice concentration maps. As compared to OSI SAF products, the overall accuracy for the sea ice extent maps is greater than 97% using MWR data, and up to 99% when using combined GNSS-R and MWR data. In the case of Sea ice concentration, the absolute errors are lower than 5%, with MWR and lower than 3% combining it with the GNSS-R. The total extent area computed using this methodology is close, with 2.5% difference, to those computed by other well consolidated algorithms, such as OSI SAF or NSIDC. The approach presented for estimating sea ice extent and concentration maps is a cost-effective alternative, and using a constellation of CubeSats, it can be further improved., This work was supported by 2017 ESA S3 challenge and Copernicus Masters overall winner award (“FSSCat” project). this work has been (partially) sponsored by project SPOT: Sensing with Pioneering Opportunistic Techniques grant RTI2018-099008-B-C21/AEI/10.13039/501100011033, and by the Unidad de Excelencia Maria de Maeztu MDM-2016-0600. This work has also been (partially) sponsored by the Spanish Ministry of Science and Innovation through the project ESP2017-89463-C3, and by the Centro de Excelencia Severo Ochoa (CEX2019-000928-S), and by the CSIC Plataforma Temática Interdisciplinar de Teledetección (PTI-Teledetect). David Llavería receives support from a FPU fellowship from the Spanish Ministry of Education FPU18/06107.; Joan Francesc Munoz-Martin receives support from the grant for recruitment of early stage research staff FI-DGR 2018 of the AGAUR—Generalitat de Catalunya (FEDER), Spain; C.H. receives support of a fellowship from “la Caixa” Foundation (ID 100010434) with the fellowship code LCF/BQ/DI18/11660050, and funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 713673., Peer Reviewed, Postprint (published version)

The Federated Satellite System mission (FSSCat), winner of the 2017 Copernicus Masters Competition and the first ESA third-party mission based on CubeSats, aimed to provide coarse-resolution soil moisture estimations and sea ice concentration maps by means of the passive microwave measurements collected by the Flexible Microwave Payload-2 (FMPL-2). The mission was successfully launched on 3 September 2020. In addition to the primary scientific objectives, FMPL-2 data are used in this study to estimate sea surface salinity (SSS), correcting for the sea surface roughness using a wind speed estimate from the L-band microwave radiometer and GNSS-R data themselves. FMPL-2 was executed over the Arctic and Antarctic oceans on a weekly schedule. Different artificial neural network algorithms have been implemented, combining FMPL-2 data with the sea surface temperature, showing a root-mean-square error (RMSE) down to 1.68 m/s in the case of the wind speed (WS) retrieval algorithms, and RMSE down to 0.43 psu for the sea surface salinity algorithm in one single pass., This work was supported by the 2017 ESA S3 Challenge and Copernicus Masters Overall Winner award (“FSSCat” project). This work was (partially) sponsored by project SPOT: Sensing with Pioneering Opportunistic Techniques grant RTI2018-099008-B-C21/AEI/10.13039/501100011033, and by the Unidad de Excelencia Maria de Maeztu MDM-2016-0600. Joan Francesc Munoz-Martin received support in the form of a grant for the recruitment of early-stage research staff FI-DGR 2018 from the AGAUR—Generalitat de Catalunya (FEDER), Spain., Peer Reviewed, Postprint (published version)

Several methods have been developed to provide polar maps of sea ice thickness (SIT) from L-band brightness temperature (TB) and altimetry data. Current process-based inversion methods to yield SIT fail to address the complex surface characteristics because sea ice is subject to strong seasonal dynamics and ice-physical properties are often non-linearly related. Neural networks can be trained to find hidden links among large datasets and often perform better on convoluted problems for which traditional approaches miss out important relationships between the observations. The FSSCat mission launched on 3 September 2020, carries the Flexible Microwave Payload-2 (FMPL-2), which contains the first Reflected Global Navigation Satellite System (GNSS-R) and L-band radiometer on board a CubeSat—designed to provide TB data on global coverage for soil moisture retrieval, and sea ice applications. This work investigates a predictive regression neural network approach with the goal to infer SIT using FMPL-2 TB and ancillary data (sea ice concentration, surface temperature, and sea ice freeboard). Two models—covering thin ice up to 0.6 m and full-range thickness—were separately trained on Arctic data in a two-month period from mid-October to the beginning of December 2020, while using ground truth data derived from the Soil Moisture and Ocean Salinity (SMOS) and Cryosat-2 missions. The thin ice and the full-range models resulted in a mean absolute error of 6.5 cm and 23 cm, respectively. Both of the models allowed for one to produce weekly composites of Arctic maps, and monthly composites of Antarctic SIT were predicted based on the Arctic full-range model. This work presents the first results of the FSSCat mission over the polar regions. It reveals the benefits of neural networks for sea ice retrievals and demonstrates that moderate-cost CubeSat missions can provide valuable data for applications in Earth observation., This work was supported by 2017 ESA S3 challenge and Copernicus Masters overall winner award (“FSSCat” project), and has been (partially) sponsored by the project SPOT: Sensing with Pioneering Opportunistic Techniques grant RTI2018-099008-B-C21/AEI/10.13039/501100011033, and by the Unidad de Excelencia Maria de Maeztu MDM-2016-0600. This work has also been (partially) sponsored by the Spanish Ministry of Science and Innovation through the project ESP2017-89463-C3, and by the Centro de Excelencia Severo Ochoa (CEX2019-000928-S), and by the CSIC Plataforma Temática Interdisciplinar de Teledetección (PTI-Teledetect). Christoph Herbert receives support from “la Caixa” Foundation (ID 100010434) with the fellowship code LCF/BQ/DI18/11660050, and funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 713673; Joan Francesc Munoz-Martin receives support from the grant for recruitment of early-stage research staff FI-DGR 2018 of the AGAUR—Generalitat de Catalunya (FEDER), Spain; David Llavería receives support from a FPU fellowship from the Spanish Ministry of Education FPU18/06107., Peer Reviewed, Postprint (published version)

Earth Observation using satellites has helped to better model our planet. The goal of such systems is to provide large data sets to help to understand the effects of our actions on the climate, to predict storms in near real-time, to prevent forest fires, or to prevent desertification, among others. In the last years, thanks to the miniaturization of electronics a new type of satellite have become popular, the CubeSat, a small spacecraft weighing from 1 to 10 kg, which can carry different types of instruments. Thanks to the cost reduction of these new platforms, new techniques can be easily evaluated, and the concept of a mesh of sensors surrounding the Earth is becoming a reality. Besides, the suitability of new techniques is being proven for such small spacecraft, such as GNSS reflectometry (GNSS-R). GNSS-R instruments are typically cheaper, more efficient (in terms of power consumption), and smaller than their traditional equivalent ones. This Ph.D. thesis is devoted to the development of passive microwave remote sensing instruments for CubeSats. However, before entering into the instrument design, several field experiments were proposed to model and validate the use of GNSS-R under different conditions. This Ph.D. thesis is divided into three main parts. The first part is devoted to assessing the capabilities of GNSS-R to retrieve different geophysical parameters through a different set of field experiments. First, the achievable spatial resolution of L1 and L5 GNSS-R signals is analyzed under different conditions. Second, taking into account the incoherent integration limits previously analyzed, the coherency of GNSS-R signals is analyzed at L1 and L5 bands. Third, thanks to the use of short integration times, multiple reflections received in the GPS L5 reflected signal could be identified as multiple peaks, thanks to the narrower auto-correlation function of such signals. Over the ocean, these multiple peaks are analyzed and linked to the sea state. Over the A, iL'observació de la Terra mitjançant satèl·lits ha ajudat a modelar millor el nostre planeta. L'objectiu d'aquests sistemes és proporcionar grans conjunts de dades que ajudin a comprendre els efectes de les nostres accions sobre el clima, a predir tempestes gairebé en temps real, o a prevenir incendis forestals o a prevenir la desertificació, entre d’altres. En els darrers anys, gràcies a la miniaturització de l’electrònica, s’ha popularitzat un nou tipus de satèl·lit, el CubeSat, un petit satèl·lit que pesa menys de 10 kg i que pot transportar diferents tipus d’instruments. Gràcies a la reducció de costos d'aquestes noves plataformes, es poden avaluar fàcilment noves tècniques, com ara la reflectometria GNSS (GNSS-R). Els instruments GNSS-R solen ser més econòmics, més eficients (en termes de consum d’energia) i més petits que els equivalents tradicionals. Aquest doctorat la tesi es dedica al desenvolupament d'instruments de teledetecció passiva per microones per a CubeSats. No obstant això, abans d’entrar en el disseny de l’instrument, es van proposar diversos experiments de camp per modelar i validar l’ús de GNSS-R en diferents condicions. Aquesta tesi es divideix en tres parts principals. La primera part es dedica a avaluar les capacitats de GNSS-R per recuperar diferents paràmetres geofísics mitjançant un conjunt diferent d’experiments de camp. En primer lloc, s’analitza la resolució espacial assolible dels senyals GNSS-R L1 i L5 en diferents condicions. En segon lloc, tenint en compte els límits d’integració incoherents analitzats prèviament, s’analitza la coherència dels senyals GNSS-R a les bandes L1 i L5. En tercer lloc, gràcies a l’ús de curts temps d’integració, les múltiples reflexions rebudes al senyal reflectit GPS L5 s'identifiquen com a múltiples pics. Sobre l’oceà, aquests múltiples pics s’analitzen i es relacionen amb l’estat del mar. A la selva tropical australiana, aquests múltiples pics a L5 es relacionen amb múltiples reflexions a la part super, Postprint (published version)

The Flexible Microwave Payload-2 is the GNSS-R and L-band Microwave Radiometer Payload on board 3Cat-5/A, one of the two 6-unit CubeSats of the FSSCat mission, which were successfully launched on 3 September 2020 on Vega flight VV16. The instrument occupies nearly a single unit of the CubeSat, and its goal is to provide sea-ice extension and thickness over the poles, and soil moisture maps at low-moderate resolution over land, which will be downscaled using data from Cosine Hyperscout-2 on board 3Cat-5/B. The spacecrafts are in a 97.5° inclination Sun-synchronous orbit, and both the reflectometer and the radiometer have been successfully executed and validated over both the North and the South poles. This manuscript presents the results and validation of the first data sets collected by the instrument during the first two months of the mission. The results of the validation are showing a radiometric accuracy better than 2 K, and a sensitivity lower than the Kelvin. For the reflectometer, the results are showing that the sea-ice transition can be estimated even at short integration times (40 ms). The presented results shows the potential for Earth Observation missions based on CubeSats, which temporal and spatial resolution can be further increased by means of CubeSat constellations, This work was supported by 2017 ESA S3 challenge and Copernicus Masters overall winner award (“FSSCat” project). this work has been (partially) sponsored by project SPOT: Sensing with Pioneering Opportunistic Techniques grant RTI2018-099008-B-C21/AEI/10.13039/501100011033, and by the Unidad de Excelencia Maria de Maeztu MDM-2016-0600, and by the ICREA Academia award by the Generalitat de Catalunya, and by the grant for recruitment of early-stage research staff FI-DGR 2018 of the AGAUR—Generalitat de Catalunya (FEDER), Spain., Peer Reviewed, Postprint (published 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)

Altimetric performance of Global Navigation Satellite System - Reflectometry (GNSS-R) instruments is highly dependent on receiver's bandwidth. 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 MIR (Microwave Interferometric Reflectometer) in the Bass Strait suggests that this approach is only valid for flat surfaces, and large bandwidth receivers. This work presents and analyses improved methods to compute the altimetric observables using GNSS-R. They show improved performance when using real data. These novel methods are the Peak-to-Minimum of the 3rd Derivative for narrow-band codes (e.g. L1 C/A), and the Peak-to-Half Power (mid-rise leading-edge) for large bandwidth codes (e.g. L5 or E5a codes). The key difference between these methods is the determination of the delay position in the reflected signal waveform in order to compute the altimetric observable. Airborne experimental results comparing the different methods, bands and GNSS-R processing techniques show that centimeter level accuracy can be achieved.

Weather forecast using GNSS-R relies, to a large extent, on data acquired by airborne and spaceborne platforms due to the extended coverage that can be achieved. GEO satellites can retrieve measurements over large areas but with spatial resolutions on the order of thousands of meters per pixel. LEO satellites with polar orbits can provide measurements over the entire world, but they cannot provide good spatial or high temporal resolutions by themselves. LEO constellations of small satellites have the same coverage with improved spatial resolution and revisit times by using data fusion techniques. High-altitude pseudo-satellites offer an excellent temporal and spatial resolution but only over limited areas. In this article, the potential of using hosted GNSS-R payloads on commercial aircraft is explored as an alternative and cost-effective means to achieve a very high resolution and a very low revisit time for regional environmental applications. Finally, a case study of soil moisture monitoring over Europe is presented, including an analysis of the expected performance., This work was supported in part by the projects of Spanish Ministerio de Ciencia, Innovación y Universidades and EU ERDF project un- der Grant RTI2018-099008-B-C21/AEI/10.13039/501100011033 “SENSING WITH PIONEERING OPPORTUNISTIC TECHNIQUES,” in part by the Grant MDM-2016-0600 to “Maria de Maeztu Excellence Research Units” CommSensLab, from Spanish Ministerio de Economía y Competitividad (MINECO/FEDER), and in part by the Recruitment of Early-Stage Research Staff under Grant FI-SDUR 2020 - 00105 of the Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR)., Peer Reviewed, Postprint (published version)

The Federated Satellite System mission (FSSCat) was the winner of the 2017 Copernicus Masters Competition and the first Copernicus third-party mission based on CubeSats. One of FSSCat’s objectives is to provide coarse Soil Moisture (SM) estimations by means of passive microwave measurements collected by Flexible Microwave Payload-2 (FMPL-2). This payload is a novel CubeSat based instrument combining an L1/E1 Global Navigation Satellite Systems-Reflectometer (GNSS-R) and an L-band Microwave Radiometer (MWR) using software-defined radio. This work presents the first results over land of the first two months of operations after the commissioning phase, from 1 October to 4 December 2020. Four neural network algorithms are implemented and analyzed in terms of different sets of input features to yield maps of SM content over the Northern Hemisphere (latitudes above 45° N). The first algorithm uses the surface skin temperature from the European Centre of Medium-Range Weather Forecast (ECMWF) in conjunction with the 16 day averaged Normalized Difference Vegetation Index (NDVI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) to estimate SM and to use it as a comparison dataset for evaluating the additional models. A second approach is implemented to retrieve SM, which complements the first model using FMPL-2 L-band MWR antenna temperature measurements, showing a better performance than in the first case. The error standard deviation of this model referred to the Soil Moisture and Ocean Salinity (SMOS) SM product gridded at 36 km is 0.074 m3/m3. The third algorithm proposes a new approach to retrieve SM using FMPL-2 GNSS-R data. The mean and standard deviation of the GNSS-R reflectivity are obtained by averaging consecutive observations based on a sliding window and are further included as additional input features to the network. The model output shows an accurate SM estimation compared to a 9 km SMOS SM product, with an error of 0.087 m3/m3. Finally, a fourth, This work was supported by the 2017 ESA S3 challenge and Copernicus Masters overall winner award (“FSSCat” project). This work was (partially) sponsored by project SPOT: Sensing with Pioneering Opportunistic Techniques grant RTI2018-099008-B-C21 / AEI / 10.13039/501100011033, and by the Unidad de Excelencia Maria de Maeztu MDM-2016-0600. This work was also (partially) sponsored by the Spanish Ministry of Science and Innovation through the project ESP2017-89463-C3, by the Centro de Excelencia Severo Ochoa (CEX2019-000928-S), and by the CSIC Plataforma Temática Interdisciplinar de Teledetección (PTI-Teledetect). Joan Francesc Munoz-Martin received support from the grant for the recruitment of early-stage research staff FI-DGR 2018 of the AGAUR - Generalitat de Catalunya (FEDER), Spain; Christoph Herbert received the support of a fellowship from “la Caixa” Foundation (ID 100010434) with the fellowship code LCF/BQ/DI18/11660050 and funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 713673; David Llavería received support from an FPU fellowship from the Spanish Ministry of Education FPU18/06107., Peer Reviewed, Postprint (published version)

The Flexible Microwave Payload -2 is the microwave remote sensing payload of the FSSCat mission. The instrument is the outcome of years of work of the Universitat Politécnica de Catalunya Passive Remote Sensing Laboratory, and the NanoSat-Lab. The FSSCat mission was launched on board the Vega VV16, on September the 3 rd , 2020. The instrument was executed for the very first time on September the 16 th , 2020, and it was commissioned less than two weeks later. This extended abstract presents the geo-located FMPL-2 measurements, which consist of a Global Navigation Satellite System - Reflectometer and an L-band radiometer. The performance of the instrument and preliminary level 1C results are presented, showing that CubeSats can to perform passive microwave remote sensing scientific missions., This work was supported by 2017 ESA S3 challenge and Copernicus Masters overall winner award (“FSSCat” project). This work has been (partially) sponsored by project SPOT: Sensing with Pioneering Opportunistic Techniques grant RTI2018-099008-B-C21 / AEI / 10.13039/501100011033, and by the Unidad de Excelencia Maria de Maeztu MDM- 2016-0600., Peer Reviewed, Postprint (published version)

CubeSats have revolutionized Earth Observation and space science, although their small size severely restricts satellite performance and payload. Antenna deployment from a stowed configuration in these small-satellites remains a great challenge. This paper presents the design, optimization, and testing of an L-band helix antenna deployment system for the 3 Cat-4, a 1U CubeSat developed at the NanoSat Lab (UPC). The 506-mm-long antenna is packed into a 26.8 mm gap together with a tip mass that provides a gravity gradient for nadir-pointing. The 3 Cat-4 Nadir Antenna Deployment Subsystem (NADS) melts dyneema strings to release the antenna in successive steps. PTFE coated fiberglass ensures the helix’s nominal diameter and pitch while a security locking mechanism serves as a redundant system for holding it in place before deploying. Our novel methodology optimizes the number and length of the NADS deployment steps. A slow-motion camera and image recognition software track the velocity and acceleration of the antenna sections by means of tracking dots. Kinematic analysis of the antenna resulted in a final design of four length steps: 90, 300, 420 and 506 mm. Our methodology for calculating these values can be widely applied for measuring many deployment system’s kinematic properties. The NADS performance is tested by characterizing antenna rigidity, analyzing helix behavior after one year in stowed configuration, and by testing the deployment mechanism in a thermal vacuum chamber at -35°C, the most critical temperature stress scenario. All test results are satisfactory. The final design of the NADS deployment mechanism is light, stable, reliable, affordable, highly scalable, and can be used in many antenna configurations and geometries. The 3 Cat-4 mission was selected by the ESA Academy to be launched in Q4 2021., 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)

Weather forecast relies to a large extent on data acquired by satellite. LEO polar satellites provide global coverage, but poor spatial resolution. GEO satellites provide a better coverage and revisit time, at expenses of poorer spatial resolution. LEO constellations of small satellites offer the promise of global coverage with good spatial resolution and revisit time. However, hosted payloads on aircrafts offer the potential of very high resolution and very low revisit time for regional applications. In this work, the idea of equipping commercial airliners with low-cost Global Navigation Satellite System - Reflectometer (GNSS-R) receivers is explored, with emphasis on real-world data and the implications the expected scientific yield would offer., This work has received funding from project Spanish Ministerio de Ciencia, Innovacion y Universidades and EU ERDF project ref. RTI2018-099008-B-C21 ”SENSING WITH PIONEERING OPPORTUNISTIC TECHNIQUES,“ , and support grant MDM-2016-0600 to “María de Maeztu Excellence Units” CommSensLab, from Spanish Ministerio de Economía y Competitividad (MINECO/FEDER)., 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., 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)

The FSSCat mission was the 2017 ESA Sentinel Small Satellite (S^3) Challenge winner and the Copernicus Masters competition overall winner. It was successfully launched on 3 September 2020 onboard the VEGA SSMS PoC (VV16). FSSCat aims to provide coarse and downscaled soil moisture data and over polar regions, sea ice cover, and coarse resolution ice thickness using a combined L-band microwave radiometer and GNSS-Reflectometry payload. As part of the calibration and validation activities of FSSCat, a GNSS-R instrument was deployed as part of the MOSAiC polar expedition. The Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition was an international one-year-long field experiment led by the Alfred Wegener Institute to study the climate system and the impact of climate change in the Arctic Ocean. This paper presents the first results of the PYCARO-2 instrument, focused on the GNSS-R techniques used to measure snow and ice thickness of an ice floe. The Interference Pattern produced by the combination of the GNSS direct and reflected signals over the sea-ice has been modeled using a four-layer model. The different thicknesses of the substrate layers (i.e., snow and ice) are linked to the position of the fringes of the interference pattern. Data collected by MOSAiC GNSS-R instrument between December 2019 and January 2020 for different GNSS constellations and frequencies are presented and analyzed, showing that under general conditions, sea ice and snow thickness can be retrieved using multiangular and multifrequency data, This work was supported by 2017 ESA S3 challenge and Copernicus Masters overall winner award (“FSSCat” project) and ESA project “FSSCat Validation Experiment in MOSAIC” (ESA CN 4000128320/19/NL/FF/ab). This work was also supported by ESA under the PO 5001025474. The PYCARO-2 instrument was developed within the SPOT project: Sensing with Pioneering Opportunistic Techniques grant RTI2018-099008-B-C21/AEI/10.13039/501100011033 and RTI2018-099008-B-C22, and by EU EDRF funds and the Spanish Ministry of Science, Innovation and Universities, and by the Unidad de Excelencia Maria de Maeztu MDM-2016-0600. Data used in this manuscript was produced as part of the international Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) and all their operators and investigators with the following tags: MOSAiC20192020, NSF-1820927, NFR-287871. Project-ID for Polarstern expedition AWI_PS122_00., Peer Reviewed, Postprint (published version)

Global navigation satellite systems reflectometry (GNSS-R) is a relatively novel remote sensing technique, but it can be understood as a multi-static radar using satellite navigation signals as signals of opportunity. The scattered signals over sea ice, flooded areas, and even under dense vegetation show a detectable coherent component that can be separated from the incoherent component and processed accordingly. This work derives an analytical formulation of the response of a GNSS-R instrument to a step function in the reflectivity using well-known principles of electromagnetic theory. The evaluation of the spatial resolution then requires a numerical evaluation of the proposed equations, as the width of the transition depends on the reflectivity values of two regions. However, it is found that results are fairly constant over a wide range of reflectivities, and they only vary faster for very high or very low reflectivity gradients. The predicted step response is then satisfactorily compared to airborne experimental results at L1 (1575.42 MHz) and L5 (1176.45 MHz) bands, acquired over a water reservoir south of Melbourne, in terms of width and ringing, and several examples are provided when the transition occurs from land to a rough ocean surface, where the coherent scattering component is no longer dominant., This work was funded by the Spanish MCIU and EU ERDF project (RTI2018-099008-B-C21/AEI/10.13039/501100011033) “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)

This article presents the FMPL-2 on board the FSSCat mission, the 2017 ESA Small Sentinel Satellite Challenge and overall Copernicus Masters Competition winner. FMPL-2 is a passive microwave instrument based on a software-defined radio that implements a conventional global navigation satellite system-Reflectometer and an L-band radiometer, occupying 1 U of a 6 U CubeSat. The article describes the FSSC at mission context, the payload design and implementation phases, the tests results in a controlled environment, and finally the calibration algorithms applied to the downloaded data in order to extract the appropriate geophysical parameters: sea-ice coverage, sea-ice thickness (SIT), and low-resolution soil moisture. This article covers the overall payload design, from a high-level block diagram down to single-component specifications from both hardware and software points of view. The main block of the instrument is based on the combination of an FPGA, which virtualizes a dual-core ARM processor, where most of the calculus are performed, and a software-defined radio module, in charge of I/Q data demodulation. The article also explains the design and implementation of a signal conditioning board required for the correct operation and calibration of both instruments., The authors are with the Department of Signal Theory and Communications,Universitat Politècnica de Catalunya, Unidad de Excellencia María de Maeztu, Peer Reviewed, Postprint (published version)

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)

Recently, there is a renewed interest in Earth Observation (EO) of the cryosphere as a proxy of global warming, soil moisture for agriculture and desertification studies, and biomass for carbon storage. Global Navigation Satellite System-Reflectometry (GNSS-R) and L-band microwave Radiometry have been used to perform these measurements. However, it is expected that the combination of both can largely improve current observations. Cat-4 mission aims at addressing this technology challenge by integrating a combined GNSS-R and Microwave Radiometer payload into a 1-Unit CubeSat. One of the greatest challenges is the design of an antenna that respects the envelope and stowage requirements of 1-Unit CubeSat, being able to work in the different frequency bands: Global Positioning System (GPS) L1-band (1575 MHz), GPS L2-band (1227 MHz), and microwave radiometry at 1400–1427 MHz. After a trade-off analysis, a helix antenna was found to be the most suitable option. This antenna has 11 turns equally distributed with 68.1 mm of diameter. This design generates an antenna with 506 mm of axial length, providing the maximum radiation gain in the endfire direction. Additionally, a counterweight is added at the tip of the antenna to enhance the directivity, and it is used as gravity gradient technique. The deployment of this antenna in vacuum and extreme temperature conditions is the greatest mechanical challenge that needs to be addressed for the success of the mission. This work presents a mechanical solution that enables to deploy the helix antenna from 25.5 mm (stowed configuration) to the final 506 mm (deployed configuration). By sequentially deploying different parts of the antenna, the final configuration is reached without impacting the attitude pointing of the CubeSat. This is accomplished using dyneema lines that are melted sequentially by commands. In addition, the deployment velocity, acceleration, and waving are presented as part of its characterization. The current test, This work was supported in part by the ‘‘CommSensLab’’ ExcellenceResearch Unit Maria de Maeztu Ministerio de asuntos Económicos y transformación digital (MINECO) under Grant MDM-2016-0600; in part by the Spanish Ministerio de Ciencia e Innovación (MICINN) and European Union - European Regional Development Fund (EUERDF) project ‘‘Sensing with pioneering opportunistic techniques’’ un-der Grant RTI2018-099008-B-C21; and in part from FI-2019 grant from AGAUR-Generalitat de Catalunya., Peer Reviewed, Postprint (author's final draft)

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)

The functional performance of space-borne instruments must be validated on ground before and after satellite integration. The effects of radio-frequency interference are also becoming more important, even in protected bands for earth observation. In this article, a GNSS and GNSS-R signal simulator is developed as part of a testbed of GNSS receivers and GNSS-R payloads’ performance in high dynamics, and to study the effects of RFI in the GNSS-R observables. This article describes the different concepts and key enabling techniques that have been developed to support this project., This work was supported in part by Spanish Ministerio de Ciencia, Innovación y Universidades and EU ERDF under Project RTI2018-099008-B-C21, in part by the Sensing With Pioneering OpportunisticTechniques under Grant MDM-2016-0600, in part by the Maria de Maeztu Excellence Research Units Comm SensLab, and in part by Spanish Ministeriode Economía y Competitividad (MINECO/FEDER)., Peer Reviewed, Postprint (author's final draft)

The FSSCat mission is a two nano-satellite Earth Observation mission based on 6-unit CubeSats. This paper is focused on the description of one of the remote sensing payloads: the Flexible Microwave Payload 2 (FMPL-2), on board the 3 Cat-5/A. The payload design is based on a Software Defined Radio, aiming to demonstrate the capabilities of nano-satellites to provide valuable scientific data. Two different instruments are integrated into a single payload: a multi-constellation (GPS and Galileo) Global Navigation Satellite System Reflectometer (GNSS-R), and a Total Power Radiometer (TPR) with Radio Frequency Interference (RFI) detection and mitigation capabilities. This paper presents the payload design, and the tests conducted during the qualification campaign., This work was supported by the ESA S3 challenge award (FSSCat project) 2017 Copernicus Masters overall winner, by the Spanish Ministry of Economy and Competitiveness, by the Spanish Ministry of Science, Innovation and Universities, ”Sensing with Pioneering Opportunistic Techniques”, grant RTI2018-099008-B-C21, by the Unidad de Excelencia Maria de Maeztu MDM-2016-0600, and by the ICREA Academia award by the Generalitat de Catalunya., Peer Reviewed, Postprint (published version)

During these years, novel Distributed Satellite Systems (DSS) have addressed the new Earth Observation (EO) requirements (e.g. near-real time access to data, or multi-point observations). One of the DSS proposals is the concept of Federated Satellite System (FSS) which has explored the benefits of sharing available and unused resources between satellite to maximize the system utility. The possibility to use additional downlink opportunities thanks to federations is a resource that could improve current EO missions, and thus achieving the required performance. For that reason, the FSS Experiment payload has been implemented as a FSS proof-of-concept between 6-Unit CubeSats in the FSSCAT mission. This article presents the design, and the test results of this payload demonstrating its feasibility for future EO missions., Peer Reviewed, Postprint (published version)

The Flexible Microwave Payload 2 (FMPL-2) is a Remote Sensing payload carried on board 3 Cat-5/A, one of the two 6-unit CubeSats of the FSSCat mission. FMPL-2 is a payload based on a Software Defined Radio (SDR), integrating two instruments in a single platform: a multi-constellation (GPS and Galileo) Global Navigation Satellite System Reflectometer (GNSS-R), and a Total Power Radiometer (TPR) with Radio Frequency Interference (RFI) detection and mitigation capabilities. This paper focuses on the description of the software developed for the platform, with a detailed description on the optimizations used for the two instruments., This work was supported by the ESA S3 challenge award (FSSCat project) 2017 Copernicus Masters overall winner, by the Spanish Ministry of Economy and Competitiveness, by the Spanish Ministry of Science, Innovation and Universities, ’’Sensing with Pioneering Opportunistic Techniques", grant RTI2018-099008-B-C21, by the Unidad de Excelencia Maria de Maeztu MDM-2016-0600, and by the ICREA Academia award by the Generalitat de Catalunya., Peer Reviewed, Postprint (published version)

Global Navigation Satellite System Reflectometry and L-band microwave radiometry have been used for soil moisture, biomass, and cryosphere studies. Combining both technologies in a low-power and cost-effective solution could largely improve current Earth observations. 3 Cat-4 mission is a 1-Unit CubeSat technology demonstrator of the Flexible Microwave Payload - 1, a reduced size payload that combines these two technologies. This work presents the objectives of the 3 Cat-4 mission and the details of the spacecraft architecture and performance. Each spacecraft subsystem is detailed at hardware and software levels. In addition, the presented results indicate that the current spacecraft design will survive the flight conditions (i.e. thermal and structural ones)., Peer Reviewed, Postprint (published version)

In recent years, Earth Observation (EO) technologies have surged in an attempt to better understand the world we live in, and exploit the vast amount of data that can be collected to improve our lives. The field of EO encompasses a broad array of technologies capable of extracting information remotely, in a process called Remote Sensing (RS). CubeSats are causing a revolution in the RS field, and are becoming a really important contribution to it. The lack of testing and preparation are common in CubeSat EO missions due to the low budgets they usually suffer from. A successful CubeSat EO mission must supply the lack of size or funding with properly tested components and environments. In this document, emphasis will be given to preemptive approaches such as studying the performance of Commercial Off-The-Shelf (COTS) Global Positioning System (GPS) receivers and the development of simulators for highly dynamic environments This topic will be expanded upon by introducing the problematic of simulating such signals for testing, and the possible countermeasures to Radio-Frequency Interference (RFI) that threatens the success of the mission. Finally, a new S-Band Ground Station will be built to provide access to this band for future CubeSat missions. All of the above will provide a holistic view on some of the hot challenges that EO faces, and multiple future research paths that open with the recent rise of New Space technologies.

The 3 Cat-4 mission aims to demonstrate the capabilities of nano-satellites plus the versatility of a Software Defined Radio for passive Earth Observation. Three different microwave payloads are integrated into a single unit CubeSat platform: a multi-constellation (GPS and Galileo) and a dual-band (L1 and L2) Global Navigation Satellite System - Reflectometer receiver, a total power radiometer including a novel Radio Frequency Interference (RFI) detection and mitigation technique, and an Automatic Identification System receiver for vessels tracking. Being able to validate these technologies in a CubeSat enables their fast adoption as hosted payloads or in more performing dedicated platforms in the future. This paper shows a novel approach for embedding multiple passive microwave payloads in a single platform., Peer Reviewed, Postprint (published version)

FSSCAT is an innovative mission concept consisting of two federated 6U Cubesats in support of the Copernicus Land and Marine Environment services. The first CubeSat carries the Flexible Microwave Payload-2 (FMPL-2), a dual microwave payload (a GNSS-Reflectometer and an L-band radiometer with interference detection/mitigation), and the second one a hyper-spectral optical payload. The combined use of these payloads will allow to measure soil moisture, ice extent, and ice thickness, and to detect melting ponds over ice. FSSCAT also includes radio and optical inter-satellite links to test some of the techniques and technologies for the upcoming satellite federations. FSSCAT will be the precursor of a constellation of federated small satellites for Earth observation achieving high temporal resolution and moderate spatial resolution in a cost-effective manner., Peer Reviewed, Postprint (published version)

This article introduces 3 Cat-1, the first project of the Technical University of Catalonia to build and launch a nano-satellite. Its main scope is to develop, construct, assemble, test and launch into a low Earth orbit a CubeSat with seven different payloads (mono-atomic oxygen detector, graphene field-effect transistor, self-powered beacon, Geiger radiation counter, wireless power transfer (WPT), new topology solar cells and WPT experiment), all fitted in a single-unit CubeSat. On one hand, this is mainly an educational project in which the development of some of the subsystems is carried out by undergraduate and postgraduate students. The satellite demonstrates its capabilities as a cost-effective platform to perform small scientific experiments and to demonstrate some of the new technologies that it incorporates., Postprint (published version)

Design and implementation of the communications protocols of the 3cat-2 nanosatellite and a remotely controlled receiving ground station consisting of a VHF and S-band down-links, and a UHF up-link.

The first-ever dual-frequency multi-constellation Global Navigation Satellite Systems Reflectometry (GNSS-R) polarimetric measurements over boreal forests and lakes from the stratosphere are presented. Data were collected during the Swedish National Space Board (SNSB)/European Space Agency (ESA) sponsored Balloon Experiments for University Students (BEXUS) 19 stratospheric balloon experiment using the P(Y) and C/A ReflectOmeter (PYCARO) instrument operated in closed-loop mode. Maps of the polarimetric ratio for L1 and L2 Global Positioning System (GPS) and GLObal Navigation Satellite System (GLONASS), and for E1 Galileo signals are derived from the float phase at 27,000 m height, and the specular points are geolocalized on the Earth's surface. Polarimetric ratio (Grl/Grr) maps over boreal forests are shown to be in the range 2-16 dB for the different GNSS codes. This result suggests that the scattering is taking place not only over the soil, but over the different forests elements as well. Additionally to the interpretation of the experimental results a theoretical investigation of the different contributions to the total reflectivity over boreal forests is performed using a bistatic scattering model. The simulated cross- (reflected Left Hand Circular Polarization LHCP) and co-polar (reflected Right Hand Circular Polarization RHCP) reflectivities are evaluated for the soil, the canopy, and the canopy-soil interactions for three different biomass densities: 725 trees/ha, 150 trees/ha and 72 trees/ha. For elevation angles larger than the Brewster angle, it is found that the cross-polar signal is dominant when just single reflections over the forests are evaluated, while in the case of multiple reflections the co-polar signal becomes the largest one., Peer Reviewed, Postprint (published version)

Scientific evaluation of the 3Cat-2 payload (PYCARO reflectometer) has been performed from the BEXUS 19 stratospheric balloon flight with an apogee of ~ 27,000 m over boreal forests and lakes. The payload was configured in closedloop mode during this flight. Results show the first-ever multi-constellation Global Navigation Satellite Systems Reflectometry (GNSS-R) measurements at dual-band and dual-polarization., Peer Reviewed, Postprint (author's final draft)

Cat-2 Assembly, Integration and Verification (AIV) activities of the Engineering Model (EM) and the Flight Model (FM) are being carried out at present. The Attitude Determination and Control System (ADCS) and Flight Software (FSW) validation campaigns will be performed at Universitat Politècnica de Catalunya (UPC) during the incomings months. An analysis and verification of the 3Cat-2 key mission requirements has been performed. The main results are summarized in this work., Peer Reviewed, Postprint (published version)

Universitat Politècnica de Catalunya (UPC) – BarcelonaTech Remote Sensing Laboratory focus on the development of breakthrough concepts. The 3Cat-2 mission is on the synergy of GNSS reflectometry and the CubeSat concept. Scientifically valuable mission data will improve our understanding on the Earth’s environment. In particular, ocean currents need further investigation because of the spatio-temporal evolution of the mesoscale phenomena. The spacecraft development work includes two stratospheric flights in the frame of the REXUS/BEXUS programme coordinated by the European Space Agency (ESA). Coherent and incoherent scattering of Earth-reflected GNSS signals as sensed by the P(Y) & C/A ReflectOmeter (PYCARO) payload will be evaluated using the main state-of-the art Global Navigation Satellite Systems Reflectometry (GNSS-R) methodologies. Future space-borne activities could take advantage of it, including the GNSS REflectometry, Radio Occultation and Scatterometry onboard International Space Station (GEROS-ISS) experiment, Peer Reviewed, Postprint (published version)

Peer Reviewed, Postprint (published version)