Deriving vertical total electron content maps from SMOS full polarimetric data to compensate the Faraday rotation effect

The Faraday rotation is a geophysical effect that causes a rotation of the electromagnetic field components emitted by the Earth when it propagates through the ionosphere. It depends on the vertical total electron content (VTEC) of the ionosphere, the geomagnetic field, and the frequency. For satellite measurements at the L band, this effect is not negligible and must be compensated for. This is the case of the Soil Moisture and Ocean Salinity (SMOS) mission, where the measured polarimetric brightness temperature must be corrected from the Faraday rotation effect before the retrieval of the geophysical parameters. The Faraday rotation angle (FRA) can be estimated using a theoretical formulation that makes use of external sources for the VTEC and the geomagnetic field. Alternatively, it can be continuously retrieved from the SMOS full-polarimetric data. However, this is not straightforward due to the relatively poor radiometric sensitivity (thermal noise) and accuracy (spatial bias) of its payload MIRAS (Microwave Interferometer Radiometer by Aperture Synthesis). In this thesis, a methodology for estimating the total electron content of the ionosphere by using an inversion procedure from the measured rotation angle has been developed. These SMOS VTEC maps are derived from SMOS measurements in the Extended Alias-Free Field of View (EAF-FoV) by applying spatio-temporal filtering techniques to mitigate the radiometric errors present in the full-polarimetric measured brightness temperatures. Systematic error patterns found in the Faraday rotation angle retrieval have been characterized along the mission and corrected. The methodology is independent, not only of external databases and forward models, but also of the target that is being measured. Eventually, these SMOS-derived VTEC maps can then be used in the SMOS level 2 processors to improve the geophysical retrievals. The impact of using these SMOS VTEC maps to correct the FRA in the SMOS mission instead of the commonly used VTEC data from GPS has also been assessed, particularly over ocean, where the ionospheric effect is stronger. This assessment has demonstrated improvements in the spatial biases, in the stability of the brightness temperatures (especially in the third Stokes parameter), and in the reduction of the latitudinal gradient present in the third Stokes parameters. All these quality indicators point to a better quality of the geophysical retrievals.La rotación de Faraday es un efecto geofísico que causa un giro en las componentes del campo electromagnético emitido por la Tierra cuando éste se propaga a través de la ionosfera. Ésta depende del contenido vertical total de electrones (VTEC) en la ionosfera, el campo geomagnético y la frecuencia. En las medidas de los satélites que operan en banda L, este efecto no es despreciable y se debe compensar. Este es el caso de la misión SMOS (Soil Moisture and Ocean Salinity), por lo que el efecto de Faraday se tiene que corregir en las medidas polarimétricas captadas por el instrumento antes de obtener parámetros geofísicos. El ángulo de rotación de Faraday (FRA) se puede estimar con una fórmula teórica que usa bases de datos externas para el VTEC y el campo geomagnético. Alternativamente, se puede obtener de una manera continua a partir de los datos polarimétricos de SMOS. Sin embargo, esto no se logra con un cálculo directo debido a la pobre sensibilidad radiométrica (ruido térmico) y a la baja precisión (sesgos espaciales) que presenta el instrumento MIRAS (Microwave Interferometer Radiometer by apertura Synthesis), que se encuentra a bordo del satélite. En esta tesis, se desarrolla una metodología para estimar el VTEC de la ionosfera usando un proceso inverso a partir del ángulo de rotación medido. Estos mapas de VTEC se derivan de medidas en todo el campo de visión extendido en donde no hay aliasing. Para mitigar los errores radiométricos en las temperaturas de brillo polarimétricas, se aplican técnicas de filtrados temporales y espaciales. En el ángulo de rotación de Faraday recuperado se detectaron errores sistemáticos. Estos se caracterizaron a lo largo de la misión y se corrigieron. La metodología es independiente, no solo de bases de datos externas y modelos de océano, sino también de la superficie medida. Estos mapas de VTEC derivados de los datos SMOS se pueden usar en el procesador de nivel 2 para mejorar las recuperaciones geofísicas. Se ha evaluado el impacto de usar estos mapas para corregir el FRA en la misión, en vez de los datos de VTEC que comúnmente se emplean (mapas provenientes de datos de GPS), particularmente sobre océano, en donde los efectos de la ionosfera son más críticos. Esta verificación ha demostrado mejoras en el sesgo espacial, en la estabilidad de las temperaturas de brillo (especialmente en el tercer parámetro de Stokes) y en la reducción del gradiente latitudinal presente en el tercer parámetro de Stokes. Todos estos indicadores de calidad apuntan a la obtención de parámetros geofísicos de mejor calidad.Postprint (published version

Calculation of Faraday Rotation Angle from SMOS Radiometric Data

The Faraday Rotation (FR) consists of a rotation in the components of the electromagnetic field emitted by the Earth as it propagates through the ionosphere. It depends on the frequency, the geomagnetic field, and the Vertical Total Electron Content (VTEC) of the ionosphere. For the Soil Moisture and Ocean Salinity (SMOS) mission, which operates in the L-band, this effect is not negligible and must be compensated. This project is born from a methodology that consists of the estimation of the ionosphere VTEC of every SMOS overpass through an inversion procedure based on the measured FRA. However, there are some zones where the FRA and VTEC cannot be retrieved due to the presence of Radio Frequency Interferences (RFI) or in zones of dense forest or ice. In order to improve the maps of the recovered VTEC and FRA, these zones where they cannot be recovered have been analyzed. First, the brightness temperature (TB) maps have been reproduced and the FRA formula has been analyzed to observe in detail where the FRA cannot be recovered, focusing on Canada. It will be found that this happens because of an indetermination of the formula. Then, three approaches will be proposed, each one with a different methodology with the aim of improving the recovered VTEC maps. The VTEC cannot have negative values, but in the core methodology, some negative values appear which are then rejected when plotting them on the map, since they correspond to VTEC values that have not been correctly recovered. Therefore, the VTEC recovery maps will be improved by applying one of these approaches, although the statistic will worsen a bit. Finally, more suitable and optimal thresholds are going to be looked for in order to improve the statistics of the maps.La Rotación de Faraday (RF) consiste en una rotación en los componentes del campo electromagnético emitido por la Tierra al propagarse por la ionosfera. Depende de la frecuencia, del campo geomagnético y del contendido total vertical de electrones (VTEC) de la ionosfera. Para la misión Soil Moisture and Ocean Salinity (SMOS), que opera en la banda L, este efecto no es despreciable y debe ser compensado. Este proyecto nace de una metodología que consiste en la estimación del VTEC de la ionosfera de cada pasada del satélite SMOS mediante un procedimiento inverso basado en el FRA medido. Sin embargo, hay algunas zonas en las que el FRA y el VTEC no se pueden recuperar debido a la presencia de interferencias de radiofrecuencia (RFI) o en zonas de bosque o hielo. Para poder mejorar la recuperación de la FRA y el VTEC, se han analizado estas zonas donde no se pueden recuperar. Primero, se han reproducido los mapas de temperatura de brillo (TB) y se ha analizado la fórmula del FRA para poder observar con detalle dónde y porqué no se puede recuperar el FRA, centrándonos en Canadá. Se verá que esto ocurre debido a una indeterminación de la fórmula. Después, se presentarán tres enfoques, cada uno con una metodología diferente con el fin de mejorar los mapas de la recuperación de VTEC. El VTEC no puede tener valores negativos, sin embargo, en la metodología base aparecen algunos valores negativos que luego son rechazados al momento de hacer las gráficas, ya que corresponden a valores de VTEC que no han sido recuperados correctamente. Por lo que, al aplicar uno de estos tres enfoques, los mapas de la recuperación de VTEC mejorarán, aunque a veces empeorando un poco las estadísticas. Por ultimo, se van a buscar umbrales más adecuados y óptimos para mejorar las estadísticas de los mapas.La Rotació de Faraday (RF) consisteix en una rotació en els components del camp electromagnètic emès per la Terra en propagar-se per la ionosfera. Depèn de la freqüència, del camp geomagnètic i del contingut total vertical d'electrons (VTEC) de la ionosfera. Per a la missió Soil Moisture and Ocean Salinity (SMOS), que opera en la banda L, aquest efecte no és menyspreable i ha de ser compensat. Aquest projecte neix d'una metodologia que consisteix en l'estimació del VTEC de la ionosfera de cada passada del SMOS mitjançant un procediment invers basat en el FRA mesurat. No obstant això, hi ha algunes zones en les quals el FRA i el VTEC no es poden recuperar a causa de la presència d'interferències de radiofreqüència (RFI) o en zones de bosc o gel. Per a poder millorar la recuperació de la FRA i el VTEC, s'han analitzat aquestes zones on no es poden recuperar. Primer, s'han reproduït els mapes de temperatura de lluentor (TB) i s'ha analitzat la fórmula del FRA per a poder observar amb detall on i perquè no es pot recuperar el FRA, centrant-nos en el Canadà. Es veurà que això es produeix a causa d'una indeterminació de la fórmula. Després, es presentaran tres enfocaments, cadascun amb una metodologia diferent amb la finalitat de millorar els mapes de la recuperació de VTEC. El VTEC no pot tenir valors negatius, no obstant això, en la metodologia apareixen alguns valors negatius que després són rebutjats al moment de fer les gràfiques, ja que corresponen a valors de VTEC que no han estat recuperats correctament. Pel que, en aplicar un d'aquests tres enfocaments, els mapes de la recuperació de VTEC milloraran però empitjorant una mica les estadístiques. Per últim, es buscaran llindars més adequats i òptims per a millorar les estadístiques dels mapes

Direct faraday rotation angle retrieval in SMOS field of view

2017 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), 23-28 July 2017, Fort Worth, TX, USA.-- 2 pages, 2 figures.-- © 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 worksIt has recently been demonstrated that boresight averaged Faraday rotation angle (FRA) can be retrieved directly from SMOS full-pol radiometric data. However, in order to extend FRA retrievals to the full Alias-Free Field of View (AF-FoV), SMOS relatively poor pixel radiometric sensitivity and accuracy must be compensated by spatial and temporal averaging. This requires some kind of tradeoff to constrain systematic FRA estimation bias both within SMOS AF-FoV and along the orbit. This work presents the first results given by a SMOS end-to-end FRA simulator, currently under development, that is used to trim and assess the performance of several FRA retrieval approachesThis work has been supported by the European Space Agency and Deimos Engenharia (Portugal), SMOS P7 Subcontract DME CP12 no. 2015-005; ERDF (European Regional Development Fund) and by Spanish public funds, projects TEC2014-58582-R and ESP2015-67549-C3-1-RPeer Reviewe

New Methodology To Retrieve the Faraday Rotation Angle Using SMOS Data

European Space Agency’s 2019 Living Planet Symposium, 13-17 May 2019, Milan, ItalyAfter 9 years of operation, the ESA’s SMOS (Soil Moisture and Ocean Salinity) mission continues providing good quality full polarimetric Brightness Temperature (TB) data to generate frequent and global maps of soil moisture over landmasses and surface salinity over the ocean thanks to its unique payload, MIRAS (Microwave Imaging Radiometer using Aperture Synthesis). At its operating frequency (1.41 GHz), there is a non-negligible effect that must be compensated, called Faraday Rotation which rotates the electromagnetic field components coming from the Earth microwave radiation when propagating through the ionosphere. The Faraday rotation angle (FRA) magnitude depends on the frequency, geomagnetic data, and the total electron content on the ionosphere [1] [2]. Currently, the FRA is theoretically estimated in SMOS by using a formulation that depends on two external sources where the first one provides geomagnetic field data and the second one, ionosphere data read from a global VTEC database with an interval of two hours. In order to get improved geophysical parameter retrievals, the FRA must be directly recovered from the SMOS full-pol TB data in a continuous way. Latest advances in image reconstruction led to improving third and fourth Stokes parameters [3] making possible the instantaneous retrieval of the FRA with SMOS full-pol brightness temperature [4]. However, due to the large thermal noise, spatial bias, and image reconstruction artifacts, FRA retrievals for a single snapshot present high errors. A previous work showed that FRA could be directly retrieved at boresight from SMOS full-pol TB with good accuracy by using a smart spatio-temporal filtering strategy [5]. However, averaged boresight FRA estimations are not representative across the complete SMOS field of view (FoV), that is, if the averaged boresight FRA is assigned to all pixels in the FoV, a large systematic bias appears across the FoV [6]. In this work, a new methodology is presented in order to retrieve FRA from SMOS radiometric data in the SMOS overpass using most of the extended alias-free FoV. The method is based on deriving SMOS VTEC maps to use them in the FRA correction instead of the ones from the VTEC database. To make a robust VTEC retrieval despite all the factors that make it challenging, a temporal and spatial filtering strategy has to be applied. The size of the filters has to be thoroughly analyzed and chosen taken into account the tradeoff between the mitigation of the noise effect and the accuracy of the VTEC retrieval. We developed in [6] an end-to-end simulator to assess the performance of the different tested approaches by using simulated TB images. By obtaining the VTEC on the SMOS overpass, the FRA can be retrieved because both variables are directly proportional. This methodology is allowing the recovery of VTEC maps directly from full-pol SMOS data giving promising results. We are currently working on the validation of these retrieved SMOS VTEC maps by comparing them to combined GPS files [7] and with the TEC values provided in the L2OS product [8]. Next step will be focused on correcting the TB by using the retrieved FRA from SMOS following the proposed methodology and evaluating the impact on the quality of the TB images.REFERENCES [1] D. M. Le Vine and S. Abraham, “The effect of the ionosphere on remote sensing of sea surface salinity from space: Absorption and emission at L band,” IEEE Transactions on Geoscience and Remote Sensing, vol. 40, no. 4, pp. 771–782, April 2002. [2] S. H. Yueh, “Estimates of Faraday rotation with passive Microwave polarimetry for microwave remote sensing of earth surfaces,” IEEE Transactions on Geoscience and Remote Sensing, vol. 38, no. 5, pp. 2434–2438, September 2000. [3] L. Wu et al. , “Radiometric performance of SMOS full polarimetric imaging,” IEEE Geoscience and Remote Sensing Letters, vol. 10, no. 6, pp. 1454–1458, November 2013. [4] S. H. Yueh, “Estimates of Faraday rotation with passive Microwave polarimetry for microwave remote sensing of earth surfaces,” IEEE Transactions on Geoscience and Remote. [5] I. Corbella, L. Wu, F. Torres, N. Duffo and M. Martin-Neira. “Faraday Rotation Retrieval Using SMOS Radiometric Data”. IEEE Geoscience & Remote Sensing Letters, Vol.12, iss. 3, pp. 458- 461. 2015. [6] R. Rubino et al., "Direct faraday rotation angle retrieval in SMOS field of view," 2017 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), pp. 697-698, 2017, doi: 10.1109/IGARSS.2017.8127047. [7] Schaer, S., Gurtner, W., & Feltens, J. (1998, February). IONEX: The ionosphere map exchange format version 1. In Proceedings of the IGS AC workshop, Darmstadt, Germany (Vol. 9, No. 11). [8] Vergely, J.-L., P. Waldteufel, J. Boutin, X. Yin, P. Spurgeon, and S. Delwart (2014), New total electron content retrieval improves SMOS sea surface salinity, J. Geophys. Res. Oceans, 119, 7295–7307, doi:10.1002/2014JC010150Peer reviewe