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

Deriving VTEC Maps from SMOS Radiometric Data

Special Issue Ten Years of Remote Sensing at Barcelona Expert Center.-- 18 pages,14 figures, 2 tablesIn this work, a new methodology is proposed in order to derive vertical total electron content (VTEC) maps from the radiometric measurements of the Soil Moisture and Ocean Salinity (SMOS) mission as an alternative approach to those based on external databases and models. This approach uses spatiotemporal filtering techniques with optimized filters to be robust against the thermal noise and image reconstruction artifacts present in SMOS images. It is also possible to retrieve the Faraday rotation angle from the recovered VTEC maps in order to correct the effect that it causes in the SMOS brightness temperaturesThis research was supported by the European Space Agency and Deimos Engenharia (Portugal), SMOS P7 Subcontract DME CP12 no. 2015-005; ERDF (European Regional Development Fund); by the Spanish public funds, projects TEC2017-88850-R and ESP2015-67549-C3-1-R; and through the award “Unidad de Excelencia María de Maeztu” MDM-2016-0600, financed by the “Agencia Estatal de Investigación” (Spain) and by the European Regional Development Fund (ERDF)With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)Peer reviewe

Performance Of Sea Surface Salinity And Soil Moisture Retrieval Algorithms With Different Auxiliary Datasets In 2-D L-Band Aperture Synthesis Interferometric Radiometers

The Soil Moisture and Ocean Salinity (SMOS) Earth Explorer Opportunity Mission was selected in May 1999 by the European Space Agency Earth Observation Programme Board to provide global and frequent soil moisture (SM) and sea surface salinity (SSS) maps. SMOS' single payload is the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS) sensor, an L-band two-dimensional aperture synthesis interferometric radiometer with multiangular and polarimetric imaging capabilities. The definition of the SMOS Level 2 Processor requires the selection of the optimum operation mode (dual-polarization or full-polarimetric) for each application, the specification of the required auxiliary data, and the optimum retrieval algorithms. Using the SMOS simulator and based on the experience gained in previous works, this paper presents a study of the SM and SSS retrieval capabilities over homogeneous pixels, in the two modes of operation with different auxiliary data. It is found that SSS retrievals using the first Stokes parameter measured in the dual-polarization mode perform somewhat worse than using the vertical (T/sub vv/) and horizontal (T/sub hh/) brightness temperatures measured in the full-polarimetric mode, and the performance degrades for cold waters due to the lower sensitivity of the brightness temperature to SSS at low sea surface temperature (SST). Due to the larger angular variation of T/sub hh/ and T/sub vv/, SM retrievals using T/sub hh/ and T/sub vv/ measured in the full-polarimetric mode exhibit a significant better performance over bare soils than over vegetation-covered soils. Over vegetation-covered soils vegetation parameters (opacity and albedo) can be inferred over a 550-km swath width in the full-polarimetric mode. However, since the first Stokes parameter is independent of both geometric and Faraday rotations, it is very robust in the presence of instrumental and geophysical errors. In the SSS retrieval problem and in the SM retrieval problem (with T/sub hh/ and T/sub vv/ measured in the full-polarimetric mode), the performance of the retrieval algorithms tested is not significantly altered if the model parameters are not exactly known, but are left as adjustable parameters in the optimization process.Peer Reviewe

Performance of sea surface salinity and soil moisture retrieval algorithms with different auxiliary datasets in 2-D L-band aperture synthesis interferometric radiometers

The Soil Moisture and Ocean Salinity (SMOS) Earth Explorer Opportunity Mission was selected in May 1999 by the European Space Agency Earth Observation Programme Board to provide global and frequent soil moisture (SM) and sea surface salinity (SSS) maps. SMOS' single payload is the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS) sensor, an L-band two-dimensional aperture synthesis interferometric radiometer with multiangular and polarimetric imaging capabilities. The definition of the SMOS Level 2 Processor requires the selection of the optimum operation mode (dual-polarization or full-polarimetric) for each application, the specification of the required auxiliary data, and the optimum retrieval algorithms. Using the SMOS simulator and based on the experience gained in previous works, this paper presents a study of the SM and SSS retrieval capabilities over homogeneous pixels, in the two modes of operation with different auxiliary data. It is found that SSS retrievals using the first Stokes parameter measured in the dual-polarization mode perform somewhat worse than using the vertical (T/sub vv/) and horizontal (T/sub hh/) brightness temperatures measured in the full-polarimetric mode, and the performance degrades for cold waters due to the lower sensitivity of the brightness temperature to SSS at low sea surface temperature (SST). Due to the larger angular variation of T/sub hh/ and T/sub vv/, SM retrievals using T/sub hh/ and T/sub vv/ measured in the full-polarimetric mode exhibit a significant better performance over bare soils than over vegetation-covered soils. Over vegetation-covered soils vegetation parameters (opacity and albedo) can be inferred over a 550-km swath width in the full-polarimetric mode. However, since the first Stokes parameter is independent of both geometric and Faraday rotations, it is very robust in the presence of instrumental and geophysical errors. In the SSS retrieval problem and in the SM retrieval problem (with T/sub hh/ and T/sub vv/ measured in the full-polarimetric mode), the performance of the retrieval algorithms tested is not significantly altered if the model parameters are not exactly known, but are left as adjustable parameters in the optimization process.Peer Reviewe

Correcting the FRA systematic error in VTEC maps from SMOS radiometric data

The Faraday rotation (FR) is a nonnegligible effect at the L-band, which is the operation frequency of the Soil Moisture and Ocean Salinity (SMOS) mission. This effect introduces a rotation in the electromagnetic field polarization when propagating through the ionosphere that must be compensated. Recently, a methodology was developed in order to retrieve the vertical total electron content (VTEC) from SMOS radiometric data with the aim to better correct the FR effect [1] . In that work, systematic patterns in the retrieved FR angle (FRA) were detected. In this article, these systematic patterns are characterized and corrected to improve the quality of the retrieved VTEC maps. These maps can be then reused in the SMOS level 2 processor for the correction of the FRA in the mission. The impact of using the SMOS-derived VTEC maps instead of the VTEC data from global positioning system (GPS) measurements on the ocean brightness temperatures (TB) measurement has also been analyzed. Results of this analysis show that the usage of those maps allows a significant enhancement in the quality of the TB, which will lead to an improvement on salinity retrievals.This work was supported in part by the European Space Agency, Soil Moisture and Ocean Salinity (SMOS) Expert Support Laboratories (ESL) for SMOS Level 1 and Level 2 over Land, Ocean and Ice Project under Grant RFQ/3-16138/19/I-BG; in part by the SMOS P7 under Contract DME CP12 no. 2015-005 (in joint with Deimos Engenharia, Portugal); in part by the Spanish Public Funds under Project TEC2017-88850-R and Project ESP2015-67549-C3-1-R through the Award “Unidad de Excelencia María de Maeztu” MDM-2016-0600, financed by the “Agencia Estatal de Investigación” (Spain); in part by the European Regional Development (ERDF); in part by the SMOS ESL for SMOS Level 1 and Level 2 over Land, Ocean and Ice Project under Grant ARG/003-032/0315/ICMCSIC; in part by the Spanish Research and Development Project INTERACT under Grant PID2020-114623RB-C31; and in part by the Spanish Government through the “Severo Ochoa Centre of Excellence” accreditation under Grant CEX2019-000928-S.Peer ReviewedPostprint (author's final draft

Ocean salinity observations with SMOS mission

The purpose of this paper is to present the capabilities of SMOS (Soil Moisture and Ocean Salinity mission) for the global mapping of ocean salinity from space. SMOS has been selected by the European Space Agency as the second Earth Explorer Opportunity with a launch date in June 2005. The sensor embarked on SMOS is MIRAS, a Microwave Imaging Radiometer with Aperture Synthesis. MIRAS works at L-band, in the two-polarisations, and has full polarimetric capability. The measurement of sea surface salinity (SSS) is one of the challenges of SMOS. This paper presents first the scientific requirements for a number of oceanographic applications. The scientific requirements are then translated into instrument accuracy, sensitivity, stability and spatial resolution. Major sources of error in the retrieval of ocean salinity will be addressed through an experimental campaign which is described.Peer ReviewedPostprint (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

The Determination of Surface Salinity with the European SMOS Space Mission

The European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission aims at obtaining global maps of soil moisture and sea surface salinity from space for large-scale and climatic studies. It uses an L-band (1400–1427 MHz) Microwave Interferometric Radiometer by Aperture Synthesis to measure brightness temperature of the earth’s surface at horizontal and vertical polarizations ( h and v). These two parameters will be used together to retrieve the geophysical parameters. The retrieval of salinity is a complex process that requires the knowledge of other environmental information and an accurate processing of the radiometer measurements. Here, we present recent results obtained from several studies and field experiments that were part of the SMOS mission, and highlight the issues still to be solved

Potential synergetic use of GNSS-R signals to improve the sea-state correction in the sea surface salinity estimation: Application to the SMOS mission

It is accepted that the best way to monitor sea surface salinity (SSS) on a global basis is by means of L-band radiometry. However, the measured sea surface brightness temperature (TB) depends not only on the SSS but also on the sea surface temperature (SST) and, more importantly, on the sea state, which is usually parameterized in terms of the 10-m-height wind speed (U10) or the significant wave height. It has been recently proposed that the mean-square slope (mss) derived from global navigation satellite system (GNSS) signals reflected by the sea surface could be a potentially appropriate sea-state descriptor and could be used to make the necessary sea state TB corrections to improve the SSS estimates. This paper presents a preliminary error analysis of the use of reflected GNSS signals for the sea roughness correction and was performed to support the European Space Agency’s Soil Moisture and Ocean Salinity (SMOS) mission; the orbit and parameters for the SMOS instrument were assumed. The accuracy requirement for the retrieved SSS is 0.1 practical salinity units after monthly averaging over 2◦ × 2◦ boxes. In this paper, potential improvements in salinity estimation are hampered mainly by the coarse sampling and by the requirements of the retrieval algorithm, particularly the need for a semiempirical model that relates TB and mss.Postprint (published version