Israel Duran

MIRAS Calibration and Performance: Results From the SMOS In-Orbit Commissioning Phase

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.

Impact of Correlator Efficiency Errors on SMOS Land–Sea Contamination

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.

Spatial biases analysis and mitigation methods in SMOS images

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.

SMOS image reconstruction quality assessment

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.

Enhanced SMOS amplitude calibration using external target

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.

First results on MIRAS calibration and overall SMOS performance

After the successful launching of the SMOS satellite, the first continuous streams of data are being processed and carefully analyzed in the frame of the SMOS In-Orbit Commissioning phase. Results regarding instrument calibration parameters retrieval, both internal and external, and brightness temperature imaging are presented. Images of ocean, ice and land are given as examples.

The MIRAS “all-licef” calibration mode

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.

Mitigation of land-sea contamination in SMOS

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.

Residual calibration error impact on SMOS SLL performance

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.

Some results on SMOS-MIRAS calibration and Imaging

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.