Abstract
The assessment of the Metop-SG microwave imager radiometric performance is based on a complete set of thermal vacuum tests. Such an extensive experimental campaign requires the development of large calibration targets (black bodies) with controlled systematics and low residual uncertainties. The significant size of such targets, which are based on pyramidal emitting surfaces, does not allow for a complete minimization of the thermal gradients below the required accuracy level. This article presents the modeling approach that has been developed to deal with both such thermal gradients and the proximity of the baffle. Results on the frequency dependence of the cold target brightness temperature (ECCOSORB CR-117) are reported between 18.7 and 183 GHz. The complete uncertainty budget provides 1-sigma errors levels lower than 0.22 K. The systematic effect of thermal coupling with the earth target has been found as relevant as the coupling to the chamber. This interdependence requires a look-up-table approach to maintain the low uncertainty levels mentioned above.
Highlights
THE MicroWave Imager (MWI) is a conical scanning radiometer operating from 18.7 GHz to 183 GHz [1] that will be placed onboard the Metop Second Generation (SG) satellites
The thermal vacuum (TV)-CT will be used for complete TV cycles where the chamber and TV-ET temperatures will be varied from -150° C to 20°C and from 100 to 335 K, respectively
A multiphysics approach has been adopted to compute the brightness temperature in presence of longitudinal gradients, transverse gradients, frequency dispersion of RF absorber parameters and baffle reflection phenomena. The latter have been found to be negligible. This confirms the correct sizing of the TV Cold Target (TV-CT)
Summary
THE MicroWave Imager (MWI) is a conical scanning radiometer operating from 18.7 GHz to 183 GHz [1] that will be placed onboard the Metop Second Generation (SG) satellites. The MWI radiometric performances i.e., sensitivity, linearity and accuracy will be verified in thermal vacuum (TV) environment over the full operating temperature range of the instrument [4]. Such an activity requires the development of two TV calibration targets: the TV Earth Target (TV-ET, to be placed in front of the main reflector, see Fig. 1) to mimic a variable scene temperature from about 100 K to 335 K and the TV Cold Target (TV-CT) operating at about 80 K (to be placed in front of the cold-sky reflector). To the best of the authors knowledge, the brightness temperature evaluation with the effect of baffles, specular and diffuse target reflectivity, the corresponding uncertainty budget and the evaluation of the interdependence between multiple calibration targets are not yet available in the literature
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