SMOS floor error impact and migation on ocean imaging

Three flight models (FMs) of the reference radiometer of the Soil Moisture and Ocean Salinity (SMOS) mission have been developed and tested. SMOS is a joint mission of the European Space Agency, Centre National d'Etudes Spatiales of France, and Centre for the Development of Industrial Technology of Spain. The reference radiometer is a noise injection radiometer (NIR); the NIR subsystem FM has been developed by Elektrobit Microwave, Ltd., in collaboration with the Laboratory of Space Technology of Helsinki University of Technology, which has acted as a subcontractor. The NIR subsystem will be integrated into the microwave imaging radiometer using aperture synthesis (MIRAS) payload in 2006. MIRAS will be the sole instrument onboard the SMOS satellite. MIRAS has 66 total power receiver units (light and cost-effective front end) and three NIR units. The purpose of the NIR subsystem is 1) to provide precise measurement of the average brightness temperature scene for absolute calibration of the MIRAS image map, 2) to measure the noise temperature level of the internal active calibration sources of MIRAS [referred to as the calibration subsystem (CAS)], and 3) to form interferometer baselines, so-called mixed baselines, with the regular receiver units. The performance of the NIR is a decisive factor of overall MIRAS performance. In this paper, we present the design solutions for the NIR FMs, which enable the achievement of the mission goals set for the NIR subsystem. The results of the NIR test campaign, proving that the performance and environmental requirements are fulfilled, are also presented, and the outcome of the ground calibration campaign is analyzed. Furthermore, the orbital calibration scheme is depicted; the calibration scheme enables the NIR to measure its targets with precision.

Three Flight Models (FM) of the reference ra- diometer of European Space Agency's SMOS mission have been developed and tested. SMOS stands for Soil Moisture and Ocean Salinity. The reference radiometer is a Noise Injection Radiometer and hence named as NIR; the NIR FM's have been developed by Elektrobit Microwave Ltd. in collaboration with Laboratory of Space Technology of Helsinki University of Technology, which has acted as a subcontractor. The NIRs will be integrated into the MIRAS (Microwave Imaging Radiometer Using Aperture Synthesis) payload in 2006. The MIRAS is the sole instrument onboard SMOS satellite. MIRAS has 66 regular receiver units (called LICEF) and three NIR units. The purpose of NIR subsystem is (1) to provide precise measurement of the average brightness temperature scene for absolute calibration of the MIRAS image map, (2) to measure the noise temperature level of internal active calibration sources for individual receiver calibration (named Calibration Subsystem, CAS), and (3) to form interferometer baselines, so-called mixed baselines, with the regular receiver units. The performance of the NIR is a decisive factor of the overall MIRAS performance. In this paper we present the design of the NIR flight models, the test campaign results, and both ground and orbital calibration techniques including the ground calibration results.

The Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) [1] is the single payload of the SMOS (Soil Moisture and Ocean Salinity) mission of the European Space Agency (ESA), to be launched on spring 2009 [2]. MIRAS performance was successfully tested during spring 2007 by the prime contractor, EADS-CASA Espacio Spain, at ESA premises in ESTEC and after payload integration with the Proteus platform at Thales Alenia Space in Cannes, France. This work presents the results of specific tests designed to assess the impact of a number of possible operating conditions and/or perturbations on MIRAS system performance. The major challenge to easily assess the impact of any perturbation comes from the large number of measurements that have to be dealed with.

The recently developed Spherical Wave Expansion-to-Plane Wave Expansion (SWE-to-PWE) antenna diagnostics technique is employed in an investigation of the antenna system in the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) for ESA's Soil Moisture and Ocean Salinity (SMOS) mission. The SWE-to-PWE antenna diagnostics technique successfully identifies the sources of the anomalies detected in 2 of the 138 MIRAS antenna far- field patterns that were measured during the on-ground calibration at the DTU-ESA Spherical Near-Field Antenna Test Facility in 2006. In addition to its obvious value for the SMOS mission, this investigation also provides an experimental validation of the SWE-to-PWE antenna diagnostics technique. (6 pages)

Session B1 : Earth Observation Missions and International Cooperation The main objective of the SMOS mission is to deliver crucial variables of the land surfaces: soil moisture, and of ocean surfaces: sea surface salinity fields. The mission should also deliver information in the field of the cryosphere. The baseline SMOS payload is an L band (1.4 GHz) 2D interferometric radiometer, Y shaped, with three arms 4 m long for a mass (including electronics and structure) of 350 kg. The radiometer is accommodated on a generic PROTEUS platform for a total mass of 650 kg. The folded satellite is compatible with most launchers. SMOS aims at providing, over the open ocean, global salinity maps with an accuracy better than 0.1 PSU every few days, with a 200 km spatial resolution; over the land surfaces, global maps of soil moisture, with an accuracy better than 0.035 m/m every 3 days, with a space resolution better than 50 km, as well as vegetation water content with an accuracy of 0.2 kgm. SMOS is demonstrator whose mission has broad and ambitious scientific objectives based on an innovative approach and proved concepts. Its goal is to demonstrate the use of L-band 2-D interferometry data for possible operational uses, and to pave the way for future, upgraded, technical implementation. MIRAS (Microwave Imaging Radiometer using Aperture Synthesis) capitalises on the ESTAR design, and it embodies major improvements. The two-dimensional MIRAS interferometer allows to measure TB at large incidences, for two polarisations. Moreover, the instrument records instantaneously a whole scene; as the satellite moves, a given point within the 2D field of view is observed from different view angles. One then obtains a series of independent measurements, which allows to retrieve surface parameters with much improved accuracy. Most of the system elements have heritage. The instrument will benefit from the technological developments made by ESA in the framework of MIRAS. The mission design is based on the work accomplished by the RAMSES team through contract funded activities and CNES in-house engineering. The platform is a standard PROTEUS bus, which is a generic Earth observing platform designed by CNES and ALCATEL. The SMOS Ground Segment will be made up of two main components; the Spacecraft Operations Ground Segment (SOGS) and the Payload Mission Ground Segment (PMGS). The SOGS, located at CNES, will be responsible for spacecraft S-Band telemetry, spacecraft data and payload health and status archival, spacecraft commanding and validation and relay (uplink) of payload commands. The PMGS will be responsible for performing science data processing, archival and distribution. It will be also in charge of payload programming, and generate payload commands and transmit them to the SOGS. Finally, it will provide software tools to verify the payload calibration and to deliver calibration parameters for data processing. It will be presented in this paper : an overview of the SMOS thematic aspect, the principle of instrument technical concept, the system architecture, the satellite configuration.

Sea surface salinity retrievals from HUT-2D L-band radiometric measurements

The SMOS(Soil Moisture and Ocean Salinity) Mission was selected in May 1999 by the European Space Agency to provide global and frequent soil moisture and sea surface salinity maps. SMOS' single payload is MIRAS(Microwave Imaging Radiometer by Aperture Synthesis), an L-band 2D aperture synthesis interferometric radiometer with multiangular observation capabilities. To achieve the ultimate goal of 0.1 psu error in ocean salinity retrievals, most studies assume the independence of measurements both in time and space so that the standard deviation of the retrieval errors decreases with the inverse of square root of the number of measurements being averaged. However, biases in the brightness temperature images originated from errors in the noise injection radiometers. Sun contributions to the antenna temperature, and imaging under aliasing conditions need to be compensated before attempting to retrieve the sea surface salinity. This work presents an external calibration suitable to perform salinity retrievals with an acceptable error. This study has been performed using the SMOS End-to-end Performance Simulator(SEPS) including thermal noise, all instrumental error sources, current error-correction and image reconstruction algorithms, and correction of atmospheric and sky noises. Simulation results show a retrieved salinity error of /spl sim/1 psu in one overpass over open ocean, which can be reduced afterwards by spatio-temporal averaging, much better than the -5 psu error that appears when image biases are not corrected.

The SMOS Payload is a single instrument, the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), which is a passive microwave 2D-interferometric radiometer working in the L-Band. The MIRAS antenna aperture is synthesized from 69 antenna elements arranged in a Y-shaped configuration. In this paper, the current status of the SMOS payload after more than 6 years in orbit is overviewed from four different perspectives: (1) Operational Performance, indicating how some anomalies have been dealt with in orbit and the availability figures accounting for more than 99%, (2) Thermal Performance, where we review the thermal requirements of the instrument and we analyze the temperature evolution of the system in some examples, (3) Calibration Strategy, being quite demanding for MIRAS, have required only few changes since launch, and (4) RFI Management, where we provide an overview of the RFI situation since launch and the progress achieved after 6 years of due actions by ESA to switchoff or mitigate the impact of interference sources.

The SMOS (Soil Moisture and Ocean Salinity) mission is an Earth Explorer Opportunity Mission from the European Space Agency that will be launched in Spring 2009. SMOSpsila single payload is a new type of radiometer called MIRAS (Microwave Imaging Radiometer by Aperture Synthesis) operating at L-band in which brightness temperature images are formed by a Fourier synthesis technique. An updated overview of the current status of the image reconstruction algorithms in 2D Aperture Synthesis Radiometers for Earth Observation, with special emphasis in the SMOS mission is presented.

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 ESA Earth Explorer SMOS (Soil Moisture and Ocean Salinity) mission will carry the first polar-orbiting 2-D interferometric radiometer (MIRAS- Microwave Imaging Radiometer using Aperture Synthesis) acquiring data of emitted microwave radiation at the frequency of 1.4 GHz (L-band). The SMOS mission products will provide different data types, each with its own characteristics. SMOS products are classified according to the different levels of processing and each product will contain specific data. The aim of this paper is to highlight the innovative satellite instrument and to present to the scientific user community the content of each product type.

Soil moisture is an important hydrologic variable due to its influence on plant growth and the surface energy budget. Most current soil moisture networks measure soil moisture at single points; these point measurements of soil moisture may not relate to the soil moisture of the larger, surrounding areas. Crop models and weather and climate models, which are sensitive to soil moisture, could be improved by having accurate soil moisture data that is relevant at scales larger than a point. Microwave C–band and X–band sensing satellites have attempted to retrieve soil moisture at relevant, large spatial scales needed for models, but these satellites are limited in their sensing depth, sensitivity to vegetation, and sensitivity to scattering by precipitation. Two recent satellite missions, the European Space Agency’s Soil Moisture and Ocean Salinity (SMOS) mission and NASA’s Soil Moisture Active Passive (SMAP) mission, retrieve soil moisture from L–band measurements of radiation, which improve upon the limitations of previous satellites. The retrieval of soil moisture from microwave satellite measurements requires knowledge of the vegetation optical thickness (τ), which is related to the vegetation water content (VWC). The multi-incidence angular design of SMOS provides its retrieval algorithm the advantage of retrieving soil moisture and τ simultaneously. SMAP will measure at a single incidence angle and will require an ancillary source of τ data. The current plan is for SMAP to use a climatology of normalized difference vegetation index (NDVI) data to generate a τ climatology. This dissertation analyzed this SMAP τ climatology by first validating SMOS τ and then by comparing SMOS τ to SMAP τ . The validation of SMOS τ was done by comparing SMOS τ during the growing season to crop yield estimates for each Iowa county. The result was that larger values of SMOS τ were correlated with larger crop yields, which suggested that SMOS τ is related to the growth of vegetation. SMOS τ was also found to be noisy and to behave strangely outside of the growing

Soil moisture plays a significant role in disciplines such as hydrology, meteorology and agriculture, and passive microwave remote sensing has become a widely used technique for global soil moisture estimation over the past three decades. Several satellite missions carrying radiometers have been launched over the past years. Among them are Japan Aerospace Exploration Agency's (JAXA's) Advanced Microwave Scanning Radiometer-EOS (AMSR-E) launched on NASA's Aqua satellite, European Space Agency's (ESA's) Soil Moisture and Ocean Salinity (SMOS) mission, JAXA's Advanced Microwave Scanning Radiometer 2 (AMSR2) onboard the GCOM-W satellite, and NASA's Soil Moisture Active Passive (SMAP) mission. Based on the availability of these four missions, there is an opportunity to develop a consistent inter-calibrated longterm soil moisture data record. This study focuses on the parametrization of the tau-omega model for soil moisture retrieval at L-, C- and X-band using brightness temperature observations from the four missions and in-situ soil moisture and soil temperature data from the SMAP core validation sites across various land cover types. The ancillary data sets used in the SMAP baseline algorithm are used for the retrievals at different frequencies. Simultaneous calibrations of the vegetation parameter $b$ and roughness parameter $h$ at both horizontal and vertical polarizations are performed. A set of model parameters to successfully retrieve soil moisture at different validation sites at L-, C- and X -band are presented. A preliminary comparison of SMAP and AMSR2 soil moisture retrievals against in situ observations at the Yanco (Australia) and TxSON (U.S.) sites showed the best accuracy and correlation at L-band (RMSD=0.03-0.05 m3/m3; R=0.92-0.94). The C-/X-band performance was not as satisfactory as L-band (RMSD=0.08-0.17 m3/m3; R=0.24-0.79). This also indicates that retrieval at higher frequencies can be very challenging when dense vegetation is present.

Capability for sea surface salinity observation was an important gap in ocean remote sensing in the last few decades of the 20th century. New technological developments during the 1990s at the European Space Agency led to the proposal of SMOS (Soil Moisture and Ocean Salinity), an Earth explorer opportunity mission based on the use of a microwave interferometric radiometer, MIRAS (Microwave Imaging Radiometer with Aperture Synthesis). SMOS, the first satellite ever addressing the observation of ocean salinity from space, was successfully launched in November 2009. The determination of salinity from the MIRAS radiometric measurements at 1.4 GHz is a complex procedure that requires high performance from the instrument and accurate modelling of several physical processes that impact on the microwave emission of the ocean’s surface. This paper introduces SMOS in the ocean remote sensing context, and summarizes the MIRAS principles of operation and the SMOS salinity retrieval approach. It describes the Spanish SMOS high-level data processing centre (CP34) and the SMOS Barcelona Expert Centre on Radiometric Calibration and Ocean Salinity (SMOS-BEC), and presents a preliminary validation of global sea surface salinity maps operationally produced by CP34.

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