Monitoring the dynamics and surface conditions of lakes is essential for the understanding of climate change dynamics. Lake water level is one of the variables that needs a particular vigilance, requiring a global and consistent monitoring that can be achieved by spaceborne remote sensing. Satellite radar altimetry has been widely used in the monitoring of inland waters in the past decades, with recent great improvements in the spatial resolution through the new generation of Synthetic Aperture Radar (SAR) altimeters. However, current radar altimetry constellations limits the space-time sampling necessary for a systematic and regular mapping of lakes. GNSS-Reflectometry (GNSS-R) could provide complementary observations to densify the spatio-temporal coverage, allowing hydrologists to access to finer details on the lake surface evolution. In particular, grazing GNSS-R broaden the use of opportunistic GNSS signals due to the smaller elevation angles of the reflecting signals. In this letter we address this issue by comparing the altimetry profiles over Great Lakes of grazing GNSS-R data provided by Spire constellation satellites and SAR altimeter data provided by Sentinel-3 constellation satellites, as a first approach in preparation of inter-techniques hydrology lake water level products. This letter confirms the advantages of fusing observations from both types of remote sensing techniques provided that accurate regional geoid model are available. Furthermore, GNSS-R data could also provide an independent lake water level information to validate the radar altimetry measurements over sites which are not equipped with ground means.

Phase altimetry has been recently introduced for high precision surface height measurements in satellite global navigation satellite system reflectometry. In the presence of coherent scattering the effectiveness of the technique has been demonstrated for sea surface at low grazing angle as well as for rivers and lakes. This study develops the concept of differential phase altimetry for land surfaces, provided that the surface roughness is small and the surface undulations are gentle. Differential phase altimetry is more tolerant with respect to atmospheric and systematic errors but its applicability is difficult in the presence of surface slope variations that may generate phase cycle slips that require complex post processing to unwrap the retrieved phase. To mitigate cycle slips an overlapping correlation process is introduced to generate phase values with high spatial resolution. A first evaluation of performance is presented using the CYclone Global Navigation Satellite System raw intermediate frequency acquisitions over land surface with gentle undulations.

In this work we study and demonstrate the performance of a Radio Frequency Interference detection algorithm that is applicable to fully polarized Earth Observation radiometers. The proposed Polarimetric Cross Frequency algorithm is based on analysis of spectral deviation of the 3rdand 4thStokes parameters calculated form the dual polarized input of a radiometer. We define the new algorithm, simulate its performance, compare it with other commonly used detection algorithms, and perform a demonstration with electronics that is designed against space radiometer requirements. The results of the work demonstrate that the detection performance of the proposed algorithm is better than reference algorithms in optimal conditions, however, it has weaknesses for certain signals. Therefore, in future systems our recommendation is to combine the proposed algorithm with other RFI detection algorithms.

Recent applications of passive L-band observations from space are summarized for ocean, land surface and cryosphere applications. The main limitation of the measurements performed by the current generation of sensors is the spatial resolution. The need of a mission ensuring the continuation of L-band measurements from space with high spatial resolution (10-15 km) is discussed.

The Soil Moisture and Ocean Salinity (SMOS) mission of the European Space Agency (ESA), together with NASA's Soil Moisture Active Passive (SMAP) mission, is providing a wealth of information to the user community for a wide range of applications. Although both missions are still operational, they have significantly exceeded their design life time. For this reason, ESA is looking at future mission concepts, which would adequately address the requirements of the passive L-band community beyond SMOS and SMAP. This article proposes one mission concept, TriHex, which has been found capable of achieving high spatial resolution, radiometric resolution, and accuracy, approaching the user needs. This is possible by the combination of aperture synthesis, formation flying, the use of general circular orbits, and alias-free imaging.

ESA's Soil Moisture and Ocean Salinity mission, SMOS, in operation since November 2009, is producing global maps of soil moisture and sea surface salinity with an average resolution of 40 km. In the context of a future L-band mission, it is necessary to address the future needs for a range of applications over land and ocean that call for much enhanced spatial resolution, down to 1–10 km. With today's knowledge, the spatial resolution of a microwave radiometer can be improved only by increasing and reshaping its aperture size. In this context, this paper presents the Formation Flying L-Band Aperture Synthesis (FFLAS) mission concept which focuses on using formation flying in combination with aperture synthesis as a potential way to improve the spatial resolution significantly. Moreover FFLAS makes full use of all lessons learnt from SMOS in-orbit experience.

Since 2004 ESA has supported the Domex experiment to monitor the emission of the East Antarctic Plateau at Dome C to verify and assess the calibration and stability of SMOS L1 products. The region has been monitored from monthly to yearly scale. This work focuses on the absolute calibration methodology developed for the Domex experiment and its improvements along the experiment's life. The calibrated dataset consisting of almost nine years of brightness temperatures is discussed here. Results were compared with the other L-band datasets over Dome-C, such as SMOS, SMAP and DOMECair 2013.

This This paper reviews the choice of polarisation axes in aperture synthesis arrays for remote sensing of the Earth, in particular when using dual polarisation receivers. In this case, as it is shown, there is the freedom of adopting rotated polarisation axes across the array without signal loss. This is not possible in systems having dual polarised antennas connected to single channel receivers, as then, polarisation time multiplexing requires the same antenna polarisation at any time in all elements of the array if signal loss is to be avoided. The flexibility allowed by the first type of instrument leads to a simpler antenna design and system architecture, providing a clear advantage over other polarisation schemes. The review of the choice of the polarisation axes is performed in the frame of the development of an advanced antenna for a future L-band aperture synthesis radiometer, but the analysis is general and applicable to any instrument of this kind. This is a study undertaken by the European Space Agency aiming at a possible future high-resolution follow-on mission to the currently flying Soil Moisture and Ocean Salinity (SMOS) mission.

The European Space Agency (ESA) has recently published the Soil Moisture and Ocean Salinity Mission (SMOS) third mission reprocessing data, which include the changes in calibration and image reconstruction that have been made to the Level 1 Operational Processor (L1OP) version v724 during the past few years. The new L1 processor incorporates several improvements to the calibration and image reconstruction algorithms. The present article proposes a new methodology used by the SMOS team to evaluate the quality and stability of the L1 data. This is done by comparison with an ocean forward model over a well-known, sea-surface salinity stable, region. The L1 reprocessed radiometry accuracy and stability are studied in detail. The results presented here confirm the improvement on data quality expected for the new baseline algorithm. Furthermore, the remaining biases and open points are described and analyzed, establishing the basis for their improvement in following reprocessing campaigns.

The Faraday Rotation (FR) is a non-negligible 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 Faraday rotation effect [1]. In that work, systematic patterns in the retrieved Faraday Rotation Angle (FRA) were detected. In this paper, these systematic patterns are characterized and corrected to improve the quality of the retrieved VTEC maps. These maps can be then re-used 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 GPS measurements on the ocean brightness temperature 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 brightness temperatures, which will lead to an improvement on salinity retrievals.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission [1] has been in orbit for over 11 years, and its Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) in two dimensions keeps being fully operational. This II-year long lifetime of SMOS, so far, has enabled the calibration and Level-1 processor team to improve the calibration procedures and the image reconstruction resulting in a new version of the Level-1 data processor, v724. To present the main performance features of this new version and the improvement in the calibration procedures constitute the main objective and content of this presentation.

Radio Frequency Interference (RFI) is a major concern for both real and synthetic aperture radiometers. After the lessons learnt from SMOS, ESA is preparing the next generation of L-band interferometric radiometers with RFI mitigation integrated into the cross-correlators. This work presents a preliminary design and results of a pre-correlation RFI mitigation algorithm tailored for interferometric radiometers. The results show that the correlation error introduced by the RFI is reduced on average to the half, with peaks of 20 dB of mitigation.

In February 2019 a Project Authorization Request was approved by the Institute of Electrical and Electronics Engineers (IEEE) Standards Association with the title “Standard for Global Navigation Satellite System Reflectometry (GNSS-R) Data and Metadata Content”. A Working Group has been assembled to draft this standard with the purpose of unifying and documenting GNSS-R measurements, calibration procedures, and product level definitions. The Working Group (http://www.grss-ieee.org/community/technical-committees/standards-or-earth-observations/) includes members, collaborators, and contributors from academia, international space agencies, and private industry. In a recent face-to-face meeting held during the ARSI+KEO 2019 Conference, the need was recognized to develop a standard with a wide range of operations, providing procedure guidelines independently of constraints imposed by current limitations on geophysical parameters retrieval algorithms. As such, this effort aims to establish the fundamentals of a potential virtual network of satellites providing inter-comparable data to the scientific community.

PRETTY is a 3U Cubesat mission by a consortium of Technical University Graz, Seibersdorf Laboratories and RUAG Space GmbH with a planned launch in 2022. The satellite is based on the OPS-SAT platform [1] and will host two payloads, a radiation monitor and a passive reflectometer. Within the present publication we will discuss the current status of the reflectometer payload including the most relevant risks and also their mitigation by prototype developments and measurements. A first operational version of the instrument using commercial off the shelf (COTS) demonstration boards, hosting flight representative components for the RF front end as well as for the digital signal processing has been assembled and integrated with the corresponding hard- and software. While measurement results based on using the output of a global navigation satellite system (GNSS) simulator as input to the PRETTY instrument have been presented earlier, we focus within this publication on the result of field tests on representative hardware in order to evaluate the expected L1 band environment for the mission.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission was launched 2 Nov 2009 and, to date, is still in good health, providing valuable L-band observations of the Earth surface [1]. A number of products are obtained from these, including thin sea ice [2], frost/thaw soils [3], high winds [4], ocean surface wind [5] and Sun brightness temperature [6], besides the main mission measurements of soil moisture and sea surface salinity [7] [8]. This paper deals with the description and early results of some technology activities conducted by ESA applying the lessons learnt by SMOS and in preparation of an advanced L-band radiometer mission.

In this work, a methodology to correct the Faraday Rotation Angle (FRA) is presented. It consists of calculating a systematic error pattern introduced by MIRAS, which is calculated in zones where the FRA tends to zero. Once calculated, this error is subtracted in the rest of the measurements. In both cases, the FRA is calculated following a process of minimization of the equation that relates the SMOS full polarization radiometric measurements to that parameter.

A method for internally calibrating microwave total power radiometers by using only one level of noise injection is presented. It is based on having a previous accurate characterization of the receiver noise temperature, which used de facto as a second calibration standard. The method proves to be at least equivalent to the classical two level, as demonstrated through their intercomparison using the data provided by the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) on board the European Space Agency Soil Moisture and Ocean Salinity (SMOS) Satellite. The long-term stability in terms of retrieved brightness temperature using both methods has similar trends with a small advantage for the one-point approach proposed here.

Temporal stability of the radiometer MIRAS, on board the SMOS satellite, is analyzed using long series of data spanning for about 6 years. Brightness temperature retrieved from one orbit per day over the Pacific Ocean is compared against a forward model and the difference between both is analyzed. Plots of the average in central latitudes as a function of time show small seasonal ripples (~±0.5 K) that depend on the calibration strategy chosen. An extremely low drift of only several mK/year is found and in some cases disappears. Hovmoller plots for the same data, consisting of brightness temperature biases as a function of latitude and time, confirm the small ripples and show some latitude dependent structures, probably coming from geophysical signatures.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission [1] has been in orbit for over 9 years, and its Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) in two dimensions is working well. The data products are generated using version v620 of the Level-1 operational processor, a version which entered into operation in Spring 2015. During last year a comprehensive data set was processed using a new processor version v720 and the assessment of the results is expected to be completed by mid 2019. In parallel to this evaluation of v720, the following version v730 of the Level-1 processor of SMOS has been already produced. This latter version is intended for investigating the capability to reduce Radio Frequency Interferences (RFI) by applying image processing techniques. This paper describes the major features and status of the two mentioned versions of the SMOS Level-1 processor, and importantly, aims at updating the remote sensing community on those aspects of the SMOS mission.

In this work, a refined methodology to correct the Faraday Rotation angle (FRA) for the SMOS mission is proposed. The method is based on calculating the FRA using the SMOS full-pol radiometric data to obtain VTEC maps applying spatiotemporal filtering techniques, to then making possible the compensation of the FRA effect. By this way, the FRA would be corrected from the data that is measuring the satellite and not using an external database.

This work is to demonstrate spaceborne altimetry over sea ice using coherent component of reflected Global Navigation Satellite System (GNSS) signal. The raw samples of sea ice reflected GNSS signal collected by the UK TechDemoSat-l (TDS-l) mission are processed. Strong coherent components are found in sea ice reflected GNSS signal received in space at different elevation angles, which would make it possible to perform precise surface altimetry measurements using the carrier phase information of the reflected signals. In addition, the coherent reflected signals can be integrated coherently for a longer period to increase the signal to noise ratio, which would make it possible to demonstrate the interferometric GNSS-R (iGNSS-R) altimetry technique in space without very high gain antenna. In this presentation, the altimetric results of coherent GNSS reflections with both carrier phase and code phase will be discussed.

Radio Frequency Interferences (RFI) are still an important source of contamination in SMOS data. The application of nodal sampling (NS) to brightness temperature images helps to mitigate the degradation that RFIs produce in geophysical retrievals. Nodal sampling has been extensive and successfully tested over open ocean and in the proximity to coastal regions. In this work, we assess the performances of NS over strongly RFI-contaminated ocean regions, particularly over semi-enclosed seas. These regions are especially challenging because of the strong contamination caused by the nearby RFI sources over land and the residual land-sea contamination.

The European Space Agency has recently studied, assessed or approved GNSS Reflectometry mission concepts based partly or fully on carrier phase observables for altimetry applications. Examples of these are, respectively, the GEROS-ISS experiment on board the International Space Station, the G- TERN Earth Explorer proposal or the PRETTY cubesat mission, currently in Phase B. This paper touches upon a few topics which are specific to carrier phase GNSS-R altimetry, namely, the involved geometry, the observables, the link budget (related to the aperture of the receiving antenna) and the carrier phase ambiguity resolution.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission [1] has been in orbit for over 8 years, and its Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) in two dimensions is working well. The data for this whole period has been and is being processed with the operational version of the current Level-l processor (version v620). Also a representative part of the same data set has been processed with a working version of a new processor (v720) which is now in preparation so that homogenous records of brightness temperatures have been made available. These rich and long data records have allowed learning important lessons from the in-flight experience, and shall eventually lead into the consolidation of the new Level-l processor version (v720) with its corresponding auxiliary calibration and configuration files. Once the improvements are confirmed the new processor version shall be recommended for the operational chain.

In this work, a new methodology to estimate the Faraday Rotation Angle (FRA) from SMOS full-pol brightness temperature is presented. This approach is focused on retrieving the Vertical Total Electron Content (VTEC) and then, the FRA. for most of the SMOS overpass, with the final aim of improving geophysical retrievals.

This paper analyzes the ocean altimetry precision of global navigation satellite systems' reflectometry (GNSS-R) by characterizing the noise statistics of the measured waveform. For this purpose, we first investigate the dependence of the altimetry precision on the statistics of the observed waveform and then derive an analytical model of the waveform statistics after incoherent averaging. The main data set used for altimetry analysis and model validation is the GPS L5 signals obtained from an airborne experiment. Two different delay estimators, based on the leading-edge derivative (DER) and the waveform fitting (FIT) over the leading edge, have been implemented. Later, the relationship between the statistics of the observed waveform and the altimetry precision is derived and validated for both the estimators with the airborne data. Then, the analytical model of the statistics of the incoherently averaged waveform is built from the correlation properties of the complex waveform, and also validated with both airborne and the TechDemoSat-1 spaceborne data. After that, the proposed waveform statistics and altimetry precision models are applied to spaceborne cases with different orbit altitudes. The altimetry performance of different onboard processing methods, i.e., conventional GNSS-R and interferometric GNSS-R, and delay estimators, i.e., DER and FIT, are preliminarily assessed. Finally, the dependence of altimetry precision on different parameters is analyzed and a simplified model of altimetry precision is proposed for the spaceborne case.

It has recently been found that the visibility denormalization process introduces a spatial error distribution due to small sporadic offset jumps in the PMS detectors. The radiometric impact of this error at system level is very low. However, due to the good performance of the SMOS instrument, a study has recently been conducted to evaluate the amplitude of such visibility errors and develop a mitigation technique. The main results of this study are summarized in this presentation.

This work is to analyse the ocean altimetry precision of Global Navigation Satellite Systems Reflectometry (GNSS-R) based on the statistics of observed waveform. The GPS L5 signals obtained from an airborne experiment are used for this study. The altimetry precision is analysed for two specific algorithms, based on the leading edge derivative (DER) and the leading edge fitting (FIT), in which the relationship between the altimetry precision and the waveform statistics is established and validated. Then, we introduce an analytical model of the statistics of the incoherently averaged waveform, which has been validated with both airborne and spaceborne data. Finally, as an example, the altimetry precision is predicted for the GEROS-ISS mission, and preliminary results are presented.

Since the nadir 1-second altimetry precision was first estimated to be of 56 cm when the PARIS concept was developed back in 1993 (it was assumed the interferometric processing of GPS P-code signals and a 4×4 m×m receiving antenna at 700 km orbital altitude) [1], many detailed analyses and experiments have been conducted leading to far more optimistic values, as low as some 15 cm (using the interferometric processing and a 1 m2antenna from 400 km altitude). This paper presents the instrument and concept advances that have allowed such improved expectations.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission has been in orbit for over 7 years, with its Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) functioning well. This 7 year period has provided a wealth of information which has enabled us to understand and consolidate the performance of the payload in great detail. More importantly, we know now the things that work well, those that need improvement, and how the instrument could be enhanced if we were to build it again. This paper presents the lessons learnt from SMOS after 7 years in orbit.

The SMOS mission is a joint program led by ESA with participation of CNES in France and CDTI in Spain. SMOS satellite carries a single payload on board, MIRAS, a Microwave Imaging Radiometer with Aperture Synthesis. The satellite has been in-orbit for over 7 years, providing for the first time and for this entire time, the longest record of space-sensed soil moisture and ocean salinity measurement [1], as well as new applications such as measurements of thin ice thickness [2] and surface ocean winds in storms [3].

It 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 approaches.

The GNSS rEflectometry, Radio Occultation and Scatterometry onboard the International Space Station (GEROS-ISS) is an innovative experiment for climate research, proposed in 2011 within a call of the European Space Agency (ESA) for installation at the ISS. This international proposal was the only one selected for further studies by ESA out of ~25 submitted ones. In this work, an updated assessment of the instrument performance for the near-nadir altimetry (GNSS-R) mode and scatterometry is assessed, including the effect of the inter-modulation signals, and the expected degradation due to presence of radio-frequency interference coming from DME/TACAN systems. These results correspond to the study lead by Airbus Defence and Space-Spain, one of the two parallel Phase A studies conducted by ESA.

Since each of the individual elements of the MIRAS array is a total power radiometer, the zero-spacing visibility can be obtained by the average of all the corresponding antenna temperatures. The main advantage of this option with respect to using the NIR measurements is that amplitude calibration is more consistent between zero-spacing visibility and the rest. On the other hand, total power radiometers are not usually as stable as noise injection radiometers, so a small loose of stability could be expected. Preliminary results show, however, similar performance.

SMOS is the acronym for the Soil Moisture and Ocean Salinity mission by the European Space Agency (ESA) [1]. Its single payload, the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), was successfully launched in November 2009. Along more than six years of operation, SMOS calibration and imaging algorithms have undergone a continuous evolution to further improve the accuracy of the retrieved geophysical parameters for a variety of scientific applications related with soil moisture and ocean salinity [2]. This paper analyzes a simplified iterative image reconstruction method that exclusively takes into account the dominant terms in the full-polarimetric equation [3]. This simplified method, that reduces computation time up to 50% at the cost of small radiometric performance degradation, is useful to the L1 teams to fine tune the calibration of the instrument when processing a large amount of data is required.

We present in this paper the work done to test the synoptic capabilities of the GNSS-R interferometric technique as a means towards sea surface altimetry estimation, which is an integrated part of the assessment of the GEROS-ISS mission. For this purpose, a new software receiver has been developed and tested: SPIR. With the lessons learned after a first flight campaign, some system modifications were performed and a second experimental campaign has been recently carried on. Initial results show interferometric waveforms that present the quality required for altimetric analysis.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission has been in orbit for over 6 years, and its Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) in two dimensions keeps working well. The data for almost this whole period has been reprocessed with the new fully polarimetric version (v620) of the Level-1 processor which also includes refined calibration schema for the antenna losses. This reprocessing has allowed the assessment of an improved performance benchmark, a better understanding of the observations, and the preparation of a new version (v700) of the Level-1 processor with further potential.

The Software PARIS Interferometric Receiver (SPIR) is a multichannel GNSS-R recording receiver. It is capable to record the GNSS signals at L1, L2 or L5 bands from eight up-looking and eight down-looking antenna elements simultaneously at a rate of 80 MHz. A software post-processor processes the recorded data off-line to obtain altimetric estimates. Its development has been carried out in order to validate experimentally the synoptic capabilities of GNSS-R using the PARIS Interferometric Technique and to allow the comparison of different GNSS-R processing techniques on exactly the same set of recorded data. The paper presents the instrument's main characteristics and preliminary results.

The Soil Moisture and Ocean Salinity (SMOS) mission by the European Space Agency (ESA) was launched in 2009 [1]. Since then, it has produced a continuous stream of valuable geophysical data over land, ice and ocean [2] However, a problem which does persist is the so-called land-sea contamination (LSC) effect, an artificial increase of brightness temperature around large land masses that enters into the ocean several hundreds of kilometers. This artifact has been found to be caused by an inconsistency between the multiplicative error that affects the zero baseline (antenna temperature) and the error affecting the rest of baselines (correlations). This inconsistency is related to about 2% measurement overestimation of the correlation efficiency (fringe washing function at the origin) that produces a scene dependent bias in the retrieved brightness temperature [3]. It must be pointed out that correlation efficiency is periodically measured on-board by means of an internal noise injection scheme [4]. Since antenna frequency response is not included in the noise injection loop, this paper is devoted to assess its potential impact in SMOS end-to-end increased correlation loss.

GEROS-ISS (GEROS hereafter) stands for GNSS REflectometry, Radio Occultation and Scatterometry onboard the International Space Station. It is a scientific experiment, proposed to the European Space Agency (ESA) in 2011 for installation aboard the ISS. The main focus of GEROS is the dedicated use of signals from the currently available Global Navigation Satellite Systems (GNSS) for remote sensing of the System Earth with focus to Climate Change characterisation. The GEROS mission idea and the current status are briefly reviewed.

In this paper, the SOMOSTA (Soil Moisture Monitoring Station) experiment on soil moisture monitoring by Global Navigation Satellite System Reflected signals(GNSS-R) at the Valencia Anchor Station is introduced.

GNSS Reflectometry, Scatterometry and Radio Occultation aboard ISS is the mission concept under study within the European Space Agency. Its core payload consists of an interferometric GNSS-Reflectometry ocean altimeter/scatterometer which does not need to generate any clean replicas of the GNSS codes for its operation. This paper describes a new interferometric technique by which such payload could also perform radio occultation as an add-on, without requiring any additional hardware resources, like the generation of clean code replicas or a storage of them. Two possibilities are studied. The first one consists of performing the complex autocorrelation function of the received signal transmitted by a rising or setting GNSS satellite. The autocorrelation function is evaluated around time epochs that are multiples of the period of suitable codes found in the modulation of the navigation signals. Satellite discrimination has to be performed spatially, through the antenna pattern. The second possibility consists in acquiring the reference signal separately from the occultation event which, in turn, has two options depending upon the geometry at which the reference is recorded: zenithal and top of the atmosphere. The signal-to-noise ratio, the satellite discrimination, and the impact of clock errors are assessed.

GNSS Reflectometry, Radio Occultation and Scatterometry on-board the ISS ("GEROS") is a proposed experiment to install an instrument on the Space Station that would perform reflectometry and radio occultation using GNSS signals of opportunity. In this paper the analysis of visible GNSS reflections and radio occultation events to support the design of the payload instrument performed in the context of Phase A studies (feasibility phase) is here presented.

Since its launch in November 2009, the Soil Moisture and Ocean Salinity (SMOS) mission by the European Space Agency (ESA) has undergone a continuous calibration and imaging procedures improvement to produce valuable geophysical data over land, ice and ocean. However, a problem which does persist is the so-called land-sea contamination (LSC) effect. This paper unveils the nature of this artifact, which is caused by a multiplicative (scene dependent) bias in the retrieved brightness temperature images. The origin of LSC is traced down to SMOS calibration parameters to yield a simple correction scheme which is validated against several geophysical scenarios. This paper shows how autoconsistency rules in interferometric synthesis together with redundant and complementary calibration procedures provide a robust SMOS calibration scheme.

The GNSS-R concept is a well-known spaceborne remote sensing technique that, by exploiting GNSS signals reflected from the earth, allows performing simultaneous observations of the Earth on several points over a very wide swath, very promising for applications such as mesoscale altimetry. However, for a single observation on earth, due to the intrinsic low power transmitted by the GNSS satellites, this technique provides reduced performance when compared to conventional spaceborne radars. In order to overcome this limitation, an innovative processing and retracking concept for GNSS-R altimeters based on the acquisition of the full delay-Doppler Map has been recently proposed by the authors. This differs with respect to the conventional GNSS-R processing technique, which exploits only the zero-Doppler power waveform of the DDM. The proposed processing adapts the SAR delay-Doppler concept of spaceborne radar altimeters for use in a GNSS-R system. This processing yields additional multi-look with respect to conventional GNSS-R concepts, and translates into an improvement of the altimetry performance estimated to be up to 35% for practical cases with low SNR. The present paper, after a brief introduction of the delay-Doppler altimetry processing, focuses on the performance analysis of this technique when applied to the for the reference case of the proposed GNSS-R GEROS-ISS experiment to be embarked on the International Space Station.

SMOS is the acronym for the Soil Moisture and Ocean Salinity mission by the European Space Agency (ESA) [1]. Its single payload, the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), was successfully launched in November 2009 to provide a continuous flow of fully polarimetric brightness temperature images. After more than five years of operation, SMOS has proven to be highly useful for a variety of scientific applications related to soil moisture over land, ocean salinity and winds over ocean, as well as specific studies over the ice covered surfaces [2][3]. Along these years of operation, SMOS calibration and imaging algorithms have undergone a continuous evolution to further improve the accuracy of the retrieved geophysical parameters. These improvements are particularly useful in Ocean Salinity applications where accuracy and stability requirements are very demanding and spatial and temporal averaging is required to achieve SMOS mission objectives. This paper analyzes the impact on ocean images of the so-called “floor error”, an image reconstruction artifact that, currently, is significant on the 3rdand 4thStokes parameters. Error mitigation techniques are evaluated by computing spatial bias over the ocean and also by providing a 4thStokes global error map over the ocean that takes into account SMOS full spatial and temporal averaging capabilities.

For a given geometry, the coherence time of a GNSS-R waveform can be initially related to the significant wave height. However, when the coherence time is analyzed, several issues shall be considered, such as the geometry (i.e altitude, elevation angle). This work presents a review of the coherence time and its dependence on the geometry. In addition, the impact that the SWH has on both the coherence time and elevation angles is analyzed. Real data from different scenarios (spaceborne, airborne, and ground based) are used in this study.

The effectiveness of incoherent averaging depends on the level of correlation between consecutive waveforms, which can vary with the geometry and with the sea state conditions. In addition it is also dependent on the lag position, since the level of correlation is not the same for all the lags. An initial estimation of the lag correlation can be done by means the ratio (R) between the actual number of incoherent averages and the effective number of incoherent averages. This work is structured in two parts: In the first part the concept of using of the ratio R as a new observable to estimate the significant wave height is detailed. In the second part, data from the TIGRIS experiment is used in order to relate the R with the significant wave height, comparing its performance with the performance obtained with the coherence time.

Land-sea contamination observed in Soil Moisture and Ocean Salinity (SMOS) brightness temperature images is found to have two main contributions: the floor error inherent of image reconstruction and a multiplicative error either in the antenna temperature or in the visibility samples measured by the correlator. The origin of this last one is traced down to SMOS calibration parameters to yield a simple correction scheme, which is validated against several geophysical scenarios. Autoconsistency rules in interferometric synthesis together with redundant and complementary calibration procedures provide a robust SMOS calibration scheme.

Faraday rotation is dynamically computed from L-band radiometric data acquired by the Soil Moisture and Ocean Salinity satellite. Improved full polarimetric image reconstruction and smart spatiotemporal filtering are used to obtain good estimations of Faraday rotation, which compare very well with the values theoretically predicted from available ionosphere and geomagnetic field data.

In Global Navigation Satellite System Reflectometry (GNSS-R), power waveforms or Delay-Doppler Maps (DDM) are incoherently averaged to reduce the standard deviation of the fluctuations caused by the speckle and thermal noise. This letter proposes a simple and innovative processing concept based on the computation of the variance of the complex waveforms. By this approach, the coherent part of the signal is subtracted from the incoherent averaged waveform. It can be useful, for example, in scenarios where reflected signal is contaminated by the direct signal, as ground-based scenarios or in future experiments such as GEROS-ISS due to the nearby multipath of direct signals.

The Synthetic Aperture Interferometric Radiometer (SAIR) Performance Simulator (SAIRPS) is a very versatile software tool developed by Deimos Engenharia (Portugal) and the Universitat Politècnica de Catalunya-Barcelona Tech (Spain) for the European Space Agency to simulate and compute the figures of merit of the performance of arbitrary SAIRs, using the OpenSF framework. The description and developments of this simulator were previously reported in the open literature and previous IGARSS. This paper summarizes the developments in this third year of the project, including the end-to-end validation tests, and the radiative transfer module that has been added to improve the versatility of the tool.

The aim of this paper is to investigate on these image reconstruction limitations in order to eventually design an improved methodology, able to further reduce the present artifacts of the SMOS images.

Typhoon motoring using Global Navigation Satellite System (GNSS) signals is a new application of the GNSS-R technique, with increasing interest in the last years. Examples of that can be the CYGNSS (Cyclone Global Navigation Satellite System) spaceborne mission proposed by NASA, or the TIGRIS (Typhoon Investigation using GNSS-R Interferometric Signals) experiment, conducted in the framework of ESA-China cooperation. This paper focuses on the TIGRIS experiment, presenting the preliminary results obtained using the interferometric GNSS-R (iGNSS-R) technique.

SMOS Side Lobe Level (SLL), which is caused by the limited coverage of the measured visibility samples in the frequency domain, is responsible for non-negligible radiometric errors at pixels placed close to large brightness temperature transitions (e.g. coastline, ice-sea border, RFI, sun reflection, etc.). In order to mitigate this effect the visibility samples are tapered by means of a Blackman window before the image inversion procedure is undertaken. However, the theoretical SLL performance of such a specific taper is degraded by residual visibility calibration errors. Simulations performed in this work show that SLL performance is very sensitive to calibration errors and that the SLL performance of the rectangular window cannot be improved to a large extend, even in the case that calibration errors are constrained by very stringent requirements. Therefore, smarter inversion techniques or data handling procedures must be devised in the future to effectively deal with this issue.

The bistatically reflected global navigation satellite systems (GNSS) signals have become an attractive tool for spaceborne ocean altimetry. The interferometric processing that can exploit the full bandwidth of the available GNSS signals without the knowledge of the actual ranging codes was proposed for the PARIS IoD mission to improve the ranging precision. This letter presents a novel on-board processing method which utilizes the interferometry of partial GNSS signal components to explore a further improvement of the altimetry precision. The scheme of extracting partial signal components and interferometry procedure, for GPS L1 band as an example, are illustrated. The assessment and comparison of achievable altimetric performance of the proposed method including the altimetric sensitivity, the resolution per pulse, the signal-to-noise ratio, and the overall altimetric precision are also introduced. The extension of the proposed method to other existing and planned GNSS systems and signals would guarantee an improvement in the overall performance of the PARIS concept.

This paper revises the precision of altimetric measurements made with signals of the Global Navigation Satellite Systems (GNSS) reflected (GNSS-R) off the sea surface. In particular, we investigate the performance of two different GNSS-R techniques, referred to here as the clean-replica and interferometric approaches. The former has been used in GNSS-R campaigns since the late 1990s, while the latter has only been tested once, in 2010, from an 18-m-high bridge in static conditions and estuary waters. In 2011, we conducted an airborne experiment over the Baltic Sea at 3-km altitude to test the interferometric concept in dynamic and rougher conditions. The campaign also flew a clean-replica GNSS-R instrument with the purpose of comparing both approaches. We have analyzed with detail the data sets to extract and validate models of the noise present in both techniques. After predicting the noise models and verifying these with aircraft data, we used them to obtain the precision of altimetric measurements and to extrapolate the performance analysis to spaceborne scenarios. The main conclusions are that the suggested noise model agrees with measured data and that the GNSS-R interferometric technique is at least two times better in precision than a technique based on using a clean replica of the publicly available GPS code. This represents a factor of at least four times finer along-track resolution. A precision of 22 cm in 65-km along-track averaging should be achievable using near-nadir interferometric GNSS-R observations from a low earth orbiter.

Earth-reflected Global Navigation Satellite System (GNSS) have become an attractive tool to be employed in low-Earth-orbit spaceborne missions, for applications such as ocean mesoscale altimetry and scatterometry. For such techniques, several on-board processing strategies have been proposed, either based on the correlation with on-board generated signal or based on ‘blind’ interferometric processing, which involves the correlation between received direct and reflected signals. This paper will provide a comparison of these two proposed GNSS-R processing techniques, highlighting the possible achieved performance for a typical spaceborne scenario. The performance comparison will be carried out by analyzing the widely used Cramer-Rao Bound, which takes into account the full statistical properties of the reflected signals and provides accurate comparison.

Reflectometry using Global Navigation Satellite System's signals of opportunity (GNSS-R) was originally conceived for mesoscale altimetry [1], although its applicability to sea state determination, soil moisture, vegetation, snow monitoring... has already been demonstrated. In December 2012 the Phase A studies of ESA's PAssive Reflectometry and Interferometry System In-orbit Demonstration (PARIS IoD) mission ended. In conventional GNSS-R the GNSS signals scattered over the Earth's surface are cross-correlated with a locally generated replica of the transmitted signal shifted in frequency (Δfd) and in delay (Δτ). PARIS is called an interferometric GNSS-R (iGNSS-R) system because the direct and the scattered signals are cross-correlated in order to use the whole signal's bandwidth, and improve the altimetric precision, despite the large bandwidth signals are not publicly available. This work presents a methodology to evaluate the performance of iGNSS-R altimeters. It is then applied to a PARIS IoD-like case, in which the receivers' bandwidths have been optimized in terms of altimetric resolution.

SMOS brightness temperature images show some residual artifacts as a function of spatial directions. Due to having different antenna patterns in each element, even if knowing them perfectly, a minimum reconstruction error inherent to the inversion algorithm exists. On top of that, antenna patterns measurement errors and other effects such as cross-polarization terms, increase the spatial biases. Accurate characterization of these sources of error allows improving the quality of the SMOS images. Good results in this direction are reported.

This paper analyzes the consistency of SMOS fully polarimetric measurement mode by comparing measured data over the ocean with the expected results. These last are based on an accurate ocean model provided by LOCEAN and simulated taking into account both the co- and cross-polar field antenna patterns. The successful image reconstruction by means of the so-called model full-pol G matrix (M-FPG) method validates SMOS main full-pol features: the full-pol switching scheme, the accuracy of the full-pol antenna pattern measurements and the full-polarimetric visibility equations.

Tracking, in radar altimetry, is the positioning of the waveforms in the correlation window. This letter presents a tracking strategy in spaceborne altimetry using global navigation satellite system reflectometry. First, the tracking procedure is illustrated, and the tracking parameters are discussed one by one: the determination of the correlation window, the accuracy of specular delay guess, and the tracking-refresh period. Based on the results, the proposed European Space Agency Passive Reflectometry and Interferometry System In-Orbit Demonstrator tracking case study is examined.

This work has been conducted in the framework of several projects devoted to assess the performance of the Soil Moisture and Ocean Salinity (SMOS) mission full-pol measurement mode. Since its launch in November 2009, SMOS is producing dual-polarization brightness temperature synthesized images that are yielding a high scientific return. However, these images are affected by a non-negligible spatial amplitude error, the so-called spatial bias (SB), that degrades geophysical parameter retrieval. This effect is particularly detrimental in SMOS polarimetric images where spatial bias is masking the polarimetric physical signature to a large extend. This paper presents a method to mitigate SMOS spatial bias by taking into account the co- and cross-polar antenna patterns in the image reconstruction algorithm through the, so called, full-pol G-matrix (FPG). The method is validated by producing spatial bias maps out of the comparison between SMOS full-pol images and an accurate polarimetric brightness temperature model of the ocean. This model has been provided to SMOS ESLs (Expert Support Laboratories) by LOCEAN (Laboratoire d'Océanographie et du Climat, France) as a test bench to validate and improve SMOS Level 1 (L1) data. Finally, a radiometric performance summary table comparing spatial bias and radiometric sensitivity between this new FPG approach and SMOS current co-polar G-matrix approach (CPG) is provided. This paper presents the best quality SMOS polarimetric images, which may lead a breakthrough in the science returns of the mission.

This paper summarizes the rationale for the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission routine calibration plan, including the analysis of the calibration parameter annual variability, and the performances and stability of SMOS images after one year of data. SMOS spends 1.68% of the total observation time in calibration. The instrument performs well within expectations with regard to accuracy and radiometric sensitivity, although spatial ripples are present in SMOS images. Several mechanisms are currently used or under investigation to mitigate this problem. Also, a loss antenna model has recently been introduced to correct for physical temperature-induced effects. This antenna model successfully corrects observed orbital variations, but has difficulties in correcting brightness temperature long-term drifting, as assessed using relatively well-known targets other than the external calibration region-cold space.

This paper focuses on the theoretical basis and general procedures for the characterization of antenna loss using the cold sky calibration sequences. The main objective is to find an improved procedure to compensate for the long- and short-term drifts observed in SMOS data. Future versions of the SMOS level 1 processor will include the procedures detailed here.

The objective of the Super MIRAS activity is to define optimum array geometrical configurations and corresponding system architectures for future high spatial resolution L-band synthetic aperture interferometric radiometers. The definition of the array geometrical configurations is performed with the objective of maximizing both spatial resolution and radiometric sensitivity. A trade-off between different instrument configurations has concluded that the optimal array geometry is a 16m diameter hexagon with 49 antennas per arm. The selected instrument configuration includes two parallel chains for X and Y polarizations with the capability to deliver at each snapshot the four stokes parameter. Additional details including instrument budgets, calibration requirements/strategy, mechanical analysis and instrument performance are being studied.

The PARIS (Passive Reflectometry and Interferometry System) in orbit demonstrator is a novel altimeter concept recently proposed by the European Space Agency. PARIS has the primary objective of observing ocean mesoscale features using Global Navigation Satellite Systems (GNSS) reflected signals, with an expected sea surface height estimation accuracy of about 20 cm, and with a very high simultaneous spatial-temporal sampling of the ocean. To date, the altimetry performance has been preliminary predicted by means of simple analytical methods, which rely on a number of assumptions on the estimators, not predicting the optimum performance. This paper will present a prediction of the height precision based on the statistical analysis of the well-known Cramer-Rao estimation bound.

This paper presents the progress made in the calibration and image reconstruction of the brightness temperature data provided by the SMOS mission of the European Space Agency. This progress has been made thanks to the learning through the accumulation of results since the launch of the satellite. Current performance of the payload on-board SMOS, the MIRAS interferometer, our main findings on its behavior and open lines of investigation are included in this contribution.

This paper presents the progress made in the preparation of the PARIS In-Orbit Demonstration mission.

Earth-reflected Global Navigation Satellite System (GNSS) signals have become an attractive tool for remote sensing. Recently it has been proposed as an alternative to the conventional altimetry to estimate the surface height However GNSS-R altimetry offers lower bandwidths and signals power compared to radar altimeters. This implies a poorer altimetry precision, accuracy and resolution per pulse.Also the altimetry precision will be impacted by the impact of the thermal, and speckle noise mainly. This paper gives a critical review of the cross-correlation waveform model.

We report preliminary results of an aircraft experiment aimed to proof the PARIS Interferometric Technique. The experiment was performed in the Gulf of Finland during a two hours flight. We installed a PARIS Interferometric Receiver together with a GOLD-RTR instrument to collect reflected C/A, P(Y) and M-code GPS signals. The collected data has been analyzed to produce altimetric observables with both techniques.

This paper analyzes the degree of correlation of MIRAS/SMOS redundant baselines by analyzing the radiometric noise reduction when N simultaneous redundant visibilities over the Ocean are averaged. This analysis benefits from the high degree of redundancy that is available along each of the three arms in MIRAS “Y” shape array to verify the spatial decorrelation of SMOS visibility samples due to the Earth as an extended source of thermal radiation.

In a synthetic aperture radiometer, the sidelobe level is determined by the shape of the synthetic pattern, which, in turn, depends on the window used to weight the visibility samples. Reduced sidelobes are obtained when the weighted visibility is smooth with no abrupt changes in the entire spatial frequency domain. Since standard 2-D windows have circular symmetry, the sidelobes are reduced if visibility samples are chosen only within a circle. This technique has been successfully tested with Soil Moisture and Ocean Salinity data, demonstrating an important reduction of sidelobes with some degradation of spatial resolution.

Soil Moisture and Ocean Salinity (SMOS) brightness temperature synthesized images are obtained after a comprehensive error correction procedure that takes into account both on-ground and in-flight calibration measurements. However, the final images are still contaminated by small, although nonnegligible, spatial errors: the so-called pixel bias. Since spatial errors in the 2-D SMOS images are not zero mean along track, these errors produce clearly visible artifacts aligned to this direction. Fortunately, spatial errors have been found to be very stable and can be minimized once the image distortion pattern is properly measured by observing a target at a uniform brightness temperature distribution. This letter describes the procedure to compute a multiplicative mask that largely reduces spatial errors over the ocean. Preliminary results to assess the mask performance are also presented by computing the reduction of the rms spatial error for a number of targets selected to have significant temporal and geographical diversity.

In this paper, we present an analysis of the radiometric performance of the three 1.4-GHz noise injection radiometers of the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) satellite. The units measure the antenna temperature, which contributes to the average brightness temperature level of SMOS retrievals. We assess the radiometric resolution of the receivers, the similarity between their measurements, and their thermal stability. For these purposes, we use SMOS measurement data gathered during the first year of the orbital operations of the satellite, which was launched in November 2009. The main results from the analysis are that the units meet the design requirements with a margin. Also, we present a new thermal model for the radiometers to further enhance their stability.

The introductory overview and 27 papers in this special issue present information on instrument performance and first results of the SMOS mission.

The European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission was launched on the 2nd of November 2009. The first six months after launch, the so-called commissioning phase, were dedicated to test the functionalities of the spacecraft, the instrument, and the ground segment including the data processors. This phase was successfully completed in May 2010, and SMOS has since been in the routine operations phase and providing data products to the science community for over a year. The performance of the instrument has been within specifications. A parallel processing chain has been providing brightness temperatures in near-real time to operational centers, e.g., the European Centre for Medium-Range Weather Forecasts. Data quality has been within specifications; however, radio-frequency interference (RFI) has been detected over large parts of Europe, China, Southern Asia, and the Middle East. Detecting and flagging contaminated observations remains a challenge as well as contacting national authorities to localize and eliminate RFI sources emitting in the protected band. The generation of Level 2 soil moisture and ocean salinity data is an ongoing activity with continuously improved processors. This article will summarize the mission status after one year of operations and present selected first results.

The fundamental concept of interferometric aperture synthesis microwave radiometry for Earth observation is reviewed. The measurement principles and a discussion on the signal processing techniques for image reconstruction are briefly summarized. Examples of real measurements from the sensor onboard the ESA SMOS mission are used to illustrate the image reconstruction techniques.

The PARIS in orbit demonstrator is a novel altimeter concept recently proposed by the European Space Agency with has the objective of observing ocean mesoscale features with an expected sea surface height estimation accuracy of about 20 cm and unprecedented simultaneous spatial-temporal sampling of the ocean. This altimeter is based on the well known PARIS (also defined as GNSS-R) technique, which performs altimetry by exploiting the properties of GNSS signals scattered off the ocean surface. The PARIS IoD is novel because it is based on the so called "interferometric" concept, which, by cross-correlating direct and reflected navigations signals, allows exploiting at the maximum possible the spectral characteristics of GPS and Galileo signals and thus maximizing the height accuracy of such altimeter. This paper will present an analysis of the expected altimetry performance of such instrument by analyzing the major error contributors.

SMOS is the acronym for the Soil Moisture and Ocean Salinity mission by the European Space Agency (ESA) [1]. Its single payload, the Mcrowave Imaging Radiometer using Aperture Synthesis (MΓRAS), was successfully launched in November 2009. A six months Commissioning Phase was devoted to bring the satellite into a fully operational condition and to characterize the payload using specific orbits to check all instrument modes [2]. An on- going activity is devoted to analyze the contribution of each single calibration procedure to the overall radiometric accuracy in order to assess the dominant sources of spatial errors. In this framework, this paper is devoted to assess the performance of the phase calibration procedures by means of the so called Redundant Space Calibration (RSC)1.

Systematic spatial errors in SMOS brightness temperature images are successfully estimated by using a statistical analysis of measured data. A constant multiplicative mask is derived by averaging spatial errors of a large number of snapshots over the ocean. The mask has been obtained for the alias- free field of view region without the need of any geophysical model. It can be considered as part of the instrument characterization and is totally independent of the target to measure. When this mask is applied to regular SMOS brightness temperatures, the spatial artifacts are clearly reduced.

SMOS is the acronym for the Soil Moisture and Ocean Salinity mission by the European Space Agency (ESA) [1]. Its single payload, the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), was launched in November 2009. After a six months Commissioning Phase SMOS entered in operational mode in May 2010. Since then SMOS has been delivering a large amount of data to successfully produce the first relevant scientific results. In order to provide accurate measurements, MIRAS requires a complex multi-step calibration procedure that was successfully tested both during pre-flight ground tests and Commissioning Phase activities. Additionally, an assessment of SMOS system performance in terms of short and long term stability, radiometric sensitivity and radiometric accuracy was also produced. In this context, this work is devoted to provide a high level overview of MIRAS calibration scheme by focusing on the rationale behind it.

After the successful launching of the Soil Moisture and Ocean Salinity satellite in November 2009, continuous streams of data started to be regularly downloaded and made available to be processed. The first six months of operation were fully dedicated to the In-Orbit Commissioning Phase, with an intense activity aimed at bringing the satellite and instrument into a fully operational condition. Concerning the payload Microwave Imaging Radiometer with Aperture Synthesis, it was fully characterized using specific orbits dedicated to check all instrument modes. The procedures, already defined during the on-ground characterization, were repeated so as to obtain realistic temperature characterization and updated internal calibration parameters. External calibration maneuvers were tested for the first time and provided absolute instrument calibration, as well as corrections to internal calibration data. Overall, performance parameters, such as stability, radiometric sensitivity and radiometric accuracy were evaluated. The main results of this activity are presented in this paper, showing that the instrument delivers stable and well-calibrated data thanks to the combination of external and internal calibration and to an accurate thermal characterization. Finally, the quality of the visibility calibration is demonstrated by producing brightness temperature images in the alias-free field of view using standard inversion techniques. Images of ocean, ice, and land are given as examples.

L-band radiometry is widely considered the best technique for Earth observing satellites to measure sea surface salinity (SSS). Interferometric aperture synthesis is a new technology applicable in spaceborne remote sensing at low frequencies. The challenge of the technology comes with decreased radiometric resolution and complexity in calibration compared to conventional radiometer systems. Due to these issues and the overall newness of the concept, validation of the technology for salinity retrieval purposes is desired. In this paper, we describe an intense measurement campaign carried out with the complete interferometric aperture synthesis radiometer system HUT-2-D, designed and operated by the Helsinki University of Technology. The campaign aimed at the detection of a changing salinity level in the Baltic Sea, in the coastal areas of Finland. We describe the campaign comprising details of the ground truth collection, sea surface emission modeling, and radiometric data analysis. We have a special emphasis on the assessment of the impact of the sea state on the radiometric measurements, which is considered one of the major obstacles for SSS retrieval at the L-band. For this purpose, we present a new correlation between sea roughness information collected with the Global Navigation Satellite System reflectometer and radiometric data measured by an L-band radiometer system.

Mesoscale ocean altimetry remains a challenge in satellite remote sensing. Conventional nadir-looking radar altimeters can make observations only along the satellite ground track, and many of them are needed to sample the sea surface at the required spatial and temporal resolutions. The Passive Reflectometry and Interferometry System (PARIS) using Global Navigation Satellite Systems (GNSS) reflected signals was proposed as a means to perform ocean altimetry along several tracks simultaneously spread over a wide swath. The bandwidth limitation of the GNSS signals and the large ionospheric delay at L-band are however issues which deserve careful attention in the design and performance of a PARIS ocean altimeter. This paper describes such an instrument specially conceived to fully exploit the GNSS signals for best altimetric performance and to provide multifrequency observations to correct for the ionospheric delay. Furthermore, an in-orbit demonstration mission that would prove the expected altimetric accuracy suited for mesoscale ocean science is proposed.

After the six-month long In-Orbit Commissioning Phase (IOCP) the SMOS satellite started to work in its fully operational mode. During the IOCP, the payload MIRAS was completely characterized, both in short- and long-term, and the optimum calibration rate for in-flight operation was established. The results show that the amplitude of the visibility is very stable, thus allowing a very low calibration rate, and that the phase has a systematic and periodic variation, easily tracked with short but frequent internal calibration sequences. Absolute calibration for antenna temperature is carried out by external maneuvers to account for drift in the reference Noise Injection Radiometer. Brightness temperature images of good quality are obtained by inverting the calibrated visibility. The images show features compatible with ocean salinity over ocean and soil moisture over land.

The performance requirements of the SMOS payload are demanding in terms of spatial resolution, accuracy, stability and precision, all critical to fulfill its scientific objectives. For this reason a commissioning plan for MIRAS was carefully devised to verify, calibrate and characterize all instrument parameters which could have an impact on its performance. This presentation describes the most important results from the instrument commissioning phase.

The Corbella's revision of the fundamental equation of the interferometric aperture synthesis radiometry is verified using airborne and spaceborne data. This experimental verification uses the measurements of the airborne instrument HUT-2D from the Aalto University (Helsinki), and the unique spaceborne microwave imaging radiometer by aperture synthesis (MIRAS) from the ESA's Soil Moisture and Ocean Salinity (SMOS) mission. The data acquired with those sensors support the Corbella's revision of the visibility function. The revised function predicts that visibilities depend on the contrast between the target's brightness temperature and the backward noise of the receivers emitted through the antennas.

A proof-of-concept instrument for PARIS-IOD, based on a full-custom dedicated GNSS-Reflections receiver has been developed, and the novel interferometric concept tested in several experiments using signals generated by a SPIRENT equipment. The preliminary analysis has resulted in the detection of 1-cm delay-jumps, using 33 group-delay observables of 1 second, with an associated dispersion of the order of 2-cm. A signal-to-noise degradation of the order of ~3 dB with respect to the one expected for the Bridge Experiment would keep the 1-second sigma error below 3-cm.

Demonstrator of the Full Polarization Interferometric Radiometer (FPIR) is a light weight high resolution imaging polarimeter at X-band. It is based on techniques of one dimensional aperture synthesis. Differing from ESTAR-like precedents on polarimetric mode, conical antenna beam and 2-point calibration, FPIR employs two interlaced arrays. They are orthogonally polarized, therefore enable polarimetric application. Main subsystems, including the antenna, receiver and digital correlator, are introduced. First brightness image of dual polarizations with FPIR demonstrator are presented and compared with the optical image.

Soil Moisture and Ocean Salinity, European Space Agency, is the first satellite mission addressing the challenge of measuring sea surface salinity from space. It uses an L-band microwave interferometric radiometer with aperture synthesis (MIRAS) that generates brightness temperature images, from which both geophysical variables are computed. The retrieval of salinity requires very demanding performances of the instrument in terms of calibration and stability. This paper highlights the importance of ocean salinity for the Earth's water cycle and climate; provides a detailed description of the MIRAS instrument, its principles of operation, calibration, and image-reconstruction techniques; and presents the algorithmic approach implemented for the retrieval of salinity from MIRAS observations, as well as the expected accuracy of the obtained results.

It is now well understood that data on soil moisture and sea surface salinity (SSS) are required to improve meteorological and climate predictions. These two quantities are not yet available globally or with adequate temporal or spatial sampling. It is recognized that a spaceborne L-band radiometer with a suitable antenna is the most promising way of fulfilling this gap. With these scientific objectives and technical solution at the heart of a proposed mission concept the European Space Agency (ESA) selected the Soil Moisture and Ocean Salinity (SMOS) mission as its second Earth Explorer Opportunity Mission. The development of the SMOS mission was led by ESA in collaboration with the Centre National d'Etudes Spatiales (CNES) in France and the Centro para el Desarrollo Tecnologico Industrial (CDTI) in Spain. SMOS carries a single payload, an L-Band 2-D interferometric radiometer operating in the 1400-1427-MHz protected band . The instrument receives the radiation emitted from Earth's surface, which can then be related to the moisture content in the first few centimeters of soil over land, and to salinity in the surface waters of the oceans. SMOS will achieve an unprecedented maximum spatial resolution of 50 km at L-band over land (43 km on average over the field of view), providing multiangular dual polarized (or fully polarized) brightness temperatures over the globe. SMOS has a revisit time of less than 3 days so as to retrieve soil moisture and ocean salinity data, meeting the mission's science objectives. The caveat in relation to its sampling requirements is that SMOS will have a somewhat reduced sensitivity when compared to conventional radiometers. The SMOS satellite was launched successfully on November 2, 2009.

The purpose of the Soil Moisture and Ocean Salinity (SMOS) mission is to measure soil moisture and sea surface salinity (SSS). The measurement of SSS using microwave radiometry requires a very sensitive instrument. In SMOS, the image is formed using the interferometric technique complemented by the average brightness temperature, or zero baseline, to set the absolute level of the image. Therefore, the measurement of the zero baseline is very critical for the success of the mission. In this paper, the radiometric resolution and stability of the radiometers dedicated to the measurement of zero baseline on SMOS are estimated. The results of a measurement campaign carried out in an anechoic electromagnetic compatibility chamber are used. The results show that the zero-baseline radiometers have the potential for relative accuracy better than 20 mK, depending on the integration scenario, satisfying the mission requirement for SSS retrieval.

This paper presents the results of the processing of the data acquired with the Airborne Microwave Imaging Radiometer with Aperture Synthesis, the European Space Agency's airborne 2-D aperture synthesis radiometer, in a circular flight over the sea in the vicinity of Helsinki (Finland). The goal of the analysis of these data is the detection of the galactic glint on the sea surface. These results are the first measurements with an L-band 2-D aperture synthesis radiometer that show the signature of the galaxy signal reflected on the sea surface.

We describe the development and implementation of a method for extracting altimetric information using the Passive Reflectometry and Interferometry System (PARIS), i.e., from GPS signals after their reflection off the sea surface. We have formalized one idea laid out in the description of a bistatic system for ocean altimetry using the GPS signal, by Hajj and Zuffada (Jet Propulsion Laboratory), and have extended it to real situations encountered in PARIS aircraft experiments. Second, we have developed the corresponding algorithms to produce real-time altimetric observables to be used in dedicated digital signal processors. Finally, we have applied this method to estimate sea-surface height from one flight experiment in the North Sea off the coast of Norway.

The sensitivity of a radiometer to the brightness temperature variations of the target is one of the critical parameters of any radiometric measurement. The sensitivity is limited by the noise in the measurement and the stability of the instrument. In this study measurements in an anechoic chamber are used to evaluate the sensitivity limit of radiometers, which are the noise injection radiometers of European Space Agency's SMOS (Soil Moisture Ocean Salinity) satellite. The sensitivity is especially important for the measurement of sea surface salinity (SSS), one of the retrieval objectives of SMOS. The methods used include comparisons of the measurements to the physical temperature estimation of the chamber, both in relative and absolute sense, and comparison of different radiometer channels with each other. The analysis demonstrates the reliability of the methods in consistency of both brightness temperature values and physical temperature measurements. Finally, the results show that the noise injection radiometers satisfy the measurement requirement for SSS retrieval.

Measuring ocean mesoscale variability is one of the main objectives of next generation satellite altimeters. Among the alternative solutions proposed to overcome the limitations of conventional altimeters, the PARIS concept has been proposed, which exploits GNSS signals reflected from the ocean surface and allows performing altimetry along several points simultaneously over a wide swath with sub-decimeter accuracy in spatial scales of 50-100 Km. The present paper proposes to adopt for a PARIS altimeter a multilook unfocused Doppler processing in order to improve height precision and accuracy. The proposed processing concept is a simplified extension to PARIS of the delay/Doppler processing adopted in conventional radar altimetry. The analyses reported in the paper show that, for open access GPS L5 navigation signal, an improvement of height precision up to 30% could be achieved without modifications to the RF front-end.

The performance of the power detectors used to denormalize the digital correlations in interferometric radiometers may be degraded due to the non-linear behavior of the diode response. This work presents the comparison of two methods used to characterize and correct the non-linear response of the detectors. The results are illustrated by presenting the characterization of the power detectors in the MIRAS (Microwave Imaging Radiometer with Aperture Synthesis) instrument, the single payload of the ESA-SMOS mission.

Interferometnc aperture synthesis is a new technology proposed for space-borne remote sensing of sea surface salinity. In the lack of availability of a complete instrument of this kind, validation of the technology for salinity retrieval purposes is desired. In this paper we describe an intense measurement campaign carried out with a complete interferometnc aperture synthesis radiometer system HUT-2D. The campaign aimed for the detection of a changing salinity level in the Baltic Sea. We have a special emphasis on the assessment of the impact of sea state on the radiometnc measurements, which is considered one of the major obstacles for sea surface salinity retrieval at L-band. For this purpose we study sea roughness information collected with Global Navigation Satellite System Reflectometer.

Mesoscale ocean altimetry remains being a challenging area for satellite observations. Conventional nadir looking radar altimeters can make observations only along the satellite ground track and many of them are needed to sample the sea surface at the required spatial and temporal resolutions. The Passive Reflectometry and Interferometry System (PARIS) using GNSS reflected signals was proposed as a means to perform ocean altimetry along several tracks simultaneously spread over a wide swath. The present paper describes such an instrument specially conceived to fully exploit the GNSS signals for best altimetric performance, and to provide multi-frequency observations to correct for the ionospheric delay. Instrument calibration strategy is also discussed. Furthermore an in-orbit demonstration mission is proposed that would prove the expected altimetric accuracy suited for mesoscale ocean science.

The on-ground characterization of the synthetic aperture radiometer onboard the Soil Moisture and Ocean Salinity mission is described. Characterization includes basic functionality, internal calibration, thermal cycling, response to point and flat sources, self-radio-frequency interference, and others. The description of the different tests performed as well as the detailed results are provided. The results show that the instrument is very stable and has all gains and offsets consistent with the ones obtained at subsystem level. On the other hand, the phase of the visibility has a larger variation with temperature than expected, a small signal leakage from the local oscillators is present, and a small interference from the X-band transmitter during short periods of time has been detected. The implementation of internal-calibration procedures, along with the accurate thermal characterization performed, have been used to produce highly accurate brightness-temperature values well within specifications.

This paper presents a complete analysis of the height precision of a GNSS-R space-based altimeter exploiting the forthcoming signals of the European Galileo satellite navigation system. The major differences between Galileo and GPS transmitted signals are shown and their impact on altimetric performance is carefully pinpointed, analysed and justified. Simulated cross-correlation altimetric waveforms are evaluated and the performance analysed by means of an analytical model for the evaluation of height measurement precision. The analysis shows that, thanks to the unique features in terms of bandwidth, TX power and modulation schemes, a GNSS-R altimeter based on Galileo signals-in-space could easily meet the requirements on precision for mesoscale ocean altimetry.

This paper summarizes the procedure to estimate the Fringe Washing Function shape. Several tests were conducted at the Maxwell anechoic chamber at ESA-ESTEC facilities on spring 2007. A wrap-up of theoretical and experimental results after data analysis is presented.

The Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) is the single payload of the SMOS (Soil Moisture and Ocean Salinity) mission by the European Space Agency (ESA) to be launched by spring 2009. This work presents the success criteria tool that has been designed to easily assess the impact of any perturbation on the payload system performance. The tool is illustrated by showing some of the main results obtained during the MIRAS system performance tests by the prime contractor, EADS-CASA Espacio, Spain, at ESA premises in ESTEC, The Netherlands, and after payload integration with the Proteus platform at Thales Alenia Space in Cannes, France.

This paper presents a method to check the consistency of the amplitude calibration in interferometric radiometers devoted to Earth observation This tool is based in the intrinsic properties of the internal calibration scheme and can be used to systematically assess in-orbit the quality of the amplitude calibration coefficients. This technique is illustrated taking into account the results of the validation and system performance tests recently undertaken by the MIRAS instrument, the single payload of the Soil Moisture and Ocean Salinity mission of the European Space Agency.

The performance of the power detectors used to denormalize the digital correlations in interferometric radiometers is degraded due to the non-linear behavior of the diode response. This work presents a comprehensive analysis of the impact of detector non-linearity and related correction techniques in the performance of the MIRAS (Microwave Imaging Radiometer with Aperture Synthesis) instrument, which is the single payload of the European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) mission.

This paper discusses the sensitivities achieved by either real or synthetic aperture radiometers. In the discussions of both kinds of radiometers, sensitivity is normalized to effective integration time, which is defined to relate the sensitivities of different kind of instruments. Derived relations explicitly demonstrate that the sensitivity by synthetic aperture radiometer is comparable to that of real aperture radiometer.

The Soil Moisture and Ocean Salinity (SMOS) mission is a joint ESA-CNES (F)-CDTI (E) mission within the ESA Living Planet Program, and it was the second ESA Earth Explorer Opportunity Mission to be selected. The mission objectives of SMOS are to provide soil moisture and ocean salinity observations for weather forecasting, climate monitoring, and the global freshwater cycle. This paper will describe the scientific campaigns performed to date, as well as the plans for the on-orbit calibration and validation activities.

This paper describes AMIRAS, an airborne demonstrator of the Microwave Imaging Radiometer with Aperture Synthesis, which is the instrument onboard ESA's Soil Moisture and Ocean Salinity (SMOS) mission. The main electrical, mechanical, thermal, and control elements of the demonstrator are shown, together with its capabilities and performances as demonstrator of the spaceborne instrument. AMIRAS main tests inside an anechoic chamber, field ground experiments, and its first two maiden flights are reported, and some results of these tests are highlighted. AMIRAS will further be used in some calibration and validation campaigns of the SMOS mission.

This paper describes a practical method to calibrate the microwave imaging radiometer with aperture synthesis (MIRAS) in orbit using external targets. Together with the in-orbit validation of the antenna patterns, the method ensures the generation of valid and calibrated visibilities from raw measurements with minimum impact from instrument errors, particularly antenna errors. It has been possible to devise this strategy only after the derivation of the Corbella equation, which correctly describes how MIRAS works. The proposed in-orbit calibration and validation approach is fully based on this equation. In a markedly different way to the point target response method used to calibrate other imaging systems, the technique put forward is essentially based on measuring a ldquoflatrdquo target such as the cold sky near the Galactic poles. This is why it has been called the ldquoflat target transformationrdquo.

The European Space Agency's Soil Moisture and Ocean Salinity satellite comprises a single payload instrument known as the Microwave Interferometric Radiometer with Aperture Synthesis (MIRAS) coupled to a PROTEUS platform. MIRAS synthesizes a large aperture from a reasonably sized 2-D array of passive microwave radiometers. By using interferometric techniques, the required coverage and spatial resolution can be achieved without the need for a large antenna. This paper describes the MIRAS instrument, its observation modes, the imaging geometry, and data products.

In this paper, some results of the MIRAS-SMOS demonstrator campaigns are presented. The follow two campaigns were held: an image validation campaign at IRTA side and a flight demonstrator campaign at Finland. The ultimate objective of the airborne demonstrator is to demonstrate its imaging capabilities by applying improved calibration and inversion algorithms to natural low-contrast targets.

This paper presents a new design of an X-band imaging interferometric radiometer needed for wind vector, precipitation and vegetation measurement with acceptable complexity, light weight and low power consumption. It introduces polarimetric capability to conventional 1D imaging interferometer by employing a polarimetric patch antenna. Conical beam is generated by beam squint. Novel 2-point calibration is realized for interferometer by dual beam or additional cold sky pointing horn and matched load.

Since the late 80’s when interferometric radiometry started to be applied to Earth observation, much literature has addressed the question of radiometric sensitivity of the instrument, but all references are based on a relatively simple system model, where only incident target noise and receiver noise are taken into account. After Corbella presented his more complex equation of the visibilities to include antenna pattern characterization and antenna coupling, a review of the general expression of sensitivity is also necessary to include the impacts of antenna coupling and other system imperfections. This paper concerns about the sensitivity of a microwave imaging interferometric radiometer. By defining a general signal model of the total output of the receiver, the sensitivity is discussed in both redundancy and non-redundancy cases.

The fundamental equation of aperture synthesis microwave radiometry has been recently revised into a new so-called Corbella equation to include antenna coupling effects and the interferometry formulation into a single equation. This equation has been experimentally demonstrated for the first time with a full prototype of the Microwave Imaging Radiometer with Aperture Synthesis. Several tests have been carried out to demonstrate the new visibility equation with different targets. The test results show better agreement with simulations based on the Corbella equation than the old fundamental equation. An initial analysis of the influence of the antenna pattern characterization errors shows the importance of these errors in the final results according to the new formulation

SMOS, acronym for Soil Moisture and Ocean Salinity, is the second selected opportunity mission on "The Living Planet Program: Earth Explorer Opportunity Missions" of the European Space Agency (ESA) [1]. The MIRAS (Microwave Imaging Radiometer with Aperture Synthesis) is the single SMOS pay- load, which is compounded of 69 L-band receivers to form a Y shape interferometer with full on-board calibration capability. The Spanish company EADS-CASA is currently leading the development of a Small scale Airborne prototype of the MIRAS (SAM), in the frame of the MDPP-3 project (MIRAS Demonstrator Pilot Project, stage 3) sponsored by the ESA. Several European institutions, including the Polytechnic University of Catalonia, are participating in the development of the MDPP-3 instrument and test campaigns. The demonstrator consists of 12 receivers called LICEF (4 per arm), 1 reference radiometer (NIR) and includes the same internal calibration system scheme foreseen for MIRAS. The instrument has been designed to be flown in the Skyvan of the LST/HUT (Laboratory of Space Technology, Helsinki University of Technology), where several airborne campaigns over land and sea will be undertaken in order to asses the capability of SMOS calibration and retrieval algorithms to provide soil moisture and ocean salinity.

In this letter, we explore a method to obtain accurate ocean heights using measurements of the global positioning system (GPS) carrier phase after reflection on the sea surface. A carrier tracking algorithm is employed in measuring travel path differences between GPS direct and reflected signals collected from antennas suspended over a marine estuary, when roughness guarantees partially coherent reflections at the GPS L1 frequency. This technique proves to be sensitive to surface roughness and able to follow tide variations with a precision better than 5 cm (1-sigma) for sea states with significant wave heights below 10 cm. It is expected that this technique could be further extended to rougher sea states using GPS frequency combinations with longer synthetic wavelengths.

This paper reviews the relative calibration of an interferometric radiometer taking into account the experimental results of the first batch of receivers developed in the frame of the European Space Agency's Soil Moisture and Ocean Salinity mission. Measurements show state-of-the-art baseline performance as long as the system is capable of correcting the effect of orbital temperature swing. A method to validate internal calibration during in-orbit deep-sky views and to correct linearity errors is also presented.

This paper reviews the relative calibration of an interferometric radiometer taking into account the experimental results of the first batch of receivers developed in the frame of the European Space Agency's Soil Moisture and Ocean Salinity mission. Measurements show state-of-the-art baseline performance as long as the system is capable of correcting the effect of orbital temperature swing. A method to validate internal calibration during in-orbit deep-sky views and to correct linearity errors is also presented.

The effect of the frequency dependence of the antenna voltage patterns within the bandwidth of an aperture synthesis interferometric radiometer is studied and quantified using actual antenna voltage patterns measured at different frequencies for the first nine receivers of the Microwave Imaging Radiometer with Aperture Synthesis instrument aboard the European Space Agency's Soil Moisture and Ocean Salinity mission. A technique is proposed to mitigate this effect.

End-to-end calibration of the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS) radiometer refers to processing the measured raw data up to dual-polarization brightness temperature maps over the earth's surface, which is the level 1 product of the Soil Moisture and Ocean Salinity (SMOS) mission. The process starts with a self-correction of comparators offset and quadrature error and is followed by the calibration procedure itself. This one is based on periodically injecting correlated and uncorrelated noise to all receivers in order to measure their relevant parameters, which are then used to correct the raw data. This can deal with most of the errors associated with the receivers but does not correct for antenna errors, which must be included in the image reconstruction algorithm. Relative S-parameters of the noise injection network and of the input switch are needed as additional data, whereas the whole process is independent of the exact value of the noise source power and of the distribution network physical temperature. On the other hand, the approach relies on having at least one very well-calibrated reference receiver, which is implemented as a noise injection radiometer. The result is the calibrated visibility function, which is inverted by the image reconstruction algorithm to get the brightness temperature as a function of the director cosines at the antenna reference plane. The final step is a coordinate rotation to obtain the horizontal and vertical brightness temperature maps over the earth. The procedures presented are validated using a complete SMOS simulator previously developed by the authors.

A complete and systematic noise analysis of radiometer front-ends, including both total power and correlation measurements, is presented. The procedure uses the concepts of noise waves and S-parameters, widely used in microwave systems design and takes into account full noise characterization of receivers including mismatch effects. The general formulation is compatible with known total power radiometer analysis and is specially appropriate in correlation radiometers for which the effect of nonideal components, such as input isolators, is analyzed. Along with numerical simulations, simple formulas are given to compute the measured visibility in nonideal conditions. The analysis is validated using experimental results consisting of correlation measurements of four receivers placed inside an anechoic chamber. Good agreement between theoretical predictions and experimental data is observed.

This article is concerned with the apodization windows to be applied to brightness temperature maps reconstructed from complex visibilities provided by the MIRAS (Microwave Imaging Radiometer with Aperture Synthesis) instrument on board the SMOS (Soil Moisture and Ocean Salinity space mission) spacecraft in order to achieve a close to uniform pixel at the Earth's surface level

The SMOS data product steps from raw data to brightness temperature maps are described. Originally ordered in the sequence of snapshots, the data are rearranged for the level 1c Brightness Temperature maps according to their location in a Discrete Global Grid (DGG). A number of candidate DGGs are introduced and their suitability is evaluated.

Soil Moisture and Ocean Salinity (SMOS) is an Earth Explorer Opportunity Mission from the European Space Agency with a launch date in 2007. Its goal is to produce global maps of soil moisture and ocean salinity variables for climatic studies using a new dual-polarization L-band (1400-1427 MHz) radiometer Microwave Imaging Radiometer by Aperture Synthesis (MIRAS). SMOS will have multiangular observation capability and can be optionally operated in full-polarimetric mode. At this frequency the sensitivity of the brightness temperature (T/sub B/) to the sea surface salinity (SSS) is low: 0.5 K/psu for a sea surface temperature (SST) of 20/spl deg/C, decreasing to 0.25 K/psu for a SST of 0/spl deg/C. Since other variables than SSS influence the T/sub B/ signal (sea surface temperature, surface roughness and foam), the accuracy of the SSS measurement will degrade unless these effects are properly accounted for. The main objective of the ESA-sponsored Wind and Salinity Experiment (WISE) field experiments has been the improvement of our understanding of the sea state effects on T/sub B/ at different incidence angles and polarizations. This understanding will help to develop and improve sea surface emissivity models to be used in the SMOS SSS retrieval algorithms. This paper summarizes the main results of the WISE field experiments on sea surface emissivity at L-band and its application to a performance study of multiangular sea surface salinity retrieval algorithms. The processing of the data reveals a sensitivity of T/sub B/ to wind speed extrapolated at nadir of /spl sim/0.23-0.25 K/(m/s), increasing at horizontal (H) polarization up to /spl sim/0.5 K/(m/s), and decreasing at vertical (V) polarization down to /spl sim/-0.2 K/(m/s) at 65/spl deg/ incidence angle. The sensitivity of T/sub B/ to significant wave height extrapolated to nadir is /spl sim/1 K/m, increasing at H-polarization up to /spl sim/1.5 K/m, and decreasing at V-polarization down to -0.5 K/m at 65/spl deg/. A modulation of the instantaneous brightness temperature T/sub B/(t) is found to be correlated with the measured sea surface slope spectra. Peaks in T/sub B/(t) are due to foam, which has allowed estimates of the foam brightness temperature and, taking into account the fractional foam coverage, the foam impact on the sea surface brightness temperature. It is suspected that a small azimuthal modulation /spl sim/0.2-0.3 K exists for low to moderate wind speeds. However, much larger values (4-5 K peak-to-peak) were registered during a strong storm, which could be due to increased foam. These sensitivities are satisfactorily compared to numerical models, and multiangular T/sub B/ data have been successfully used to retrieve sea surface salinity.

This paper describes a new method to compensate the Faraday rotation in polarimetric radiometric measurements under the assumption that the polarimetric brightness temperature matrix of the target is diagonal. Simulations results of how this method can be used within the Soil Moisture and Ocean Salinity mission and the Microwave Imaging Radiometer with Aperture Synthesis imaging radiometer are presented.

An interferometric radiometer prototype at 1.4 GHz was successfully demonstrated. The system was developed for the image validation of MIRAS (Microwave Imaging Radiometer with Aperture Synthesis). The instrument, image processing procedure, and measurement results are presented. The future development of the system is discussed.

The WISE 2000 and WISE 2001 field campaigns were sponsored by ESA to gather experimental data to improve the knowledge of the L-band brightness temperature dependence with wind speed at different incidence angles and azimuth angles. The goal is to help the development of sea surface salinity retrieval algorithms for SMOS Earth Explorer Mission of the European Space Agency. The L-band AUtomatic RAdiometer (LAURA) plus other sensors to characterize the sea surface state were installed at the Casablanca oil rig, 40 km south east off the coast of Tarragona in Spain. During WISE 2000 wind conditions were low-to-moderate, but during WISE 2001 two strong storms beat the Catalonian coast with sustained winds higher than 100 km/h at the platform meteorological station (69 m height). The first results of the radiometric measurements (azimuth and elevation scans) acquired with LAURA during the WInd and Salinity Experiment (WISE-2001) are presented.

Sea surface salinity can be measured by passive microwave remote sensing at L-band. In May 1999, the European Space Agency (ESA) selected the Soil Moisture and Ocean Salinity (SMOS) Earth Explorer Opportunity Mission to provide global coverage of soil moisture and ocean salinity. To determine the effect of wind on the sea surface emissivity, ESA sponsored the Wind and Salinity Experiment (WISE 2000). This paper describes the field campaign, the measurements acquired with emphasis in the radiometric measurements at L-band, their comparison with numerical models, and the implications for the remote sensing of sea salinity.

The L-band Microwave Imaging Radiometer with Aperture Synthesis (MIRAS), scheduled to be flown as single payload on board the European Soil Moisture and Ocean Salinity (SMOS) mission, has a very wide field of view and synthesizes narrow beams by means of two-dimensional (2-D) interferometry, the same concept used in radio astronomy. Wide field of view is indeed a key feature of this radiometer, which leads naturally to the measurement of the full vector of brightness temperatures of the image. This paper analyzes the theory of polarimetry in the 2-D wide-field-of-view microwave interferometry and describes the way MIRAS will measure the polarimetric brightness temperatures.

This paper presents the passive reflectometry and interferometry system (PARIS) concept and how it originated in the European Space Agency (ESA), Noordwijk, The Netherlands, in 1993 as a novel method to perform mesoscale ocean altimetry. The PARIS concept uses signals of opportunity such as the signals from the global navigation satellite systems (GNSS), which are reflected off the ocean surface to perform mesoscale ocean altimetry. Essentially, the relative delay between the direct and the reflected signals received from a Low Earth Orbit satellite provides information about sea surface height. The paper describes an original experiment on sea surface altimetry using GPS-reflected signals. The objective of the experiment was to demonstrate the potential of the PARIS concept. This experiment is the first one ever published on performing sea surface height estimations using reflected navigation signals in a controlled environment. The key result of the experiment is the demonstration of a root mean squared (RMS) height accuracy within 5 s of 1% of the used code chip (3 m for C/A code). Direct extrapolation of this experimental result to the 10-times higher chip rate P-code signal allows us to predict a height error of 30 cm in 5 s, provided adequate models are used to take into account systematic effects. The paper ends presenting the potential of the PARIS concept for long term ocean altimetric observations in view of the current trends of the GNSS systems.

A constellation of satellite radar altimeters for mesoscale ocean applications has been recommended since the beginning of this decade. Studies show that eight monostatic satellites are needed to achieve an ideal sample spacing of seven days revisit time and 50-km spatial sampling. However, for this number of satellites the required cost of such a constellation is very high. With the appearance of Global Navigation Satellite Systems (GNSS), such as GPS and GLONASS, there is a great opportunity for constellations of small satellites to operate in a cooperative bistatic way, i.e., with their sensors synchronized both in time and space. The benefits of this concept are the subject of this paper. In this article, the authors study a constellation of bistatic altimeters with fewer satellites than the monostatic constellation but with the same sampling performance. This paper first introduces the mesoscale ocean requirements. The configuration of the bistatic constellation follows together with an analysis of its coverage and required pulse-limited footprints. Suitable gate lengths, pulse repetition frequency, synchronization, pointing requirements, and link budget are discussed. The tracker and estimation process are also described as well as calibration issues. The goal of the whole design of the altimeter instrument itself has been to achieve simultaneous monostatic and bistatic operation with the simplest architecture.