The MicroWave Imager (MWI) Instrument is part of the payload complement of the MetOp-SG Satellites type B. MWI is a conical scanning radiometer (scan speed of 45 rpm), with multiple frequency channels covering the frequency range from 18.7 GHz to 183.3 GHz. The MicroWave Imager will provide precipitation monitoring as well as sea ice extent information.
The On Ground Calibration Targets (OGCT) are temperature controlled microwave blackbodies allowing the MWI instrument RF stimulation with a known reference for the on-ground radiometric performance and calibration tests either in thermal-vacuum conditions or in ambient conditions. The instrument will be calibrated on ground with temperature accuracy better than 0.3 K, before flight. For the thermal-vacuum tests two OGCTs are required: a Thermal-Vacuum Earth Target (TVET) that is a variable temperature target simulating the variable brightness temperature of the measured scene (e.g. from 80K to 335K) to be placed in front of the instrument main reflector and a Thermal-Vacuum Cold Sky Target (TVCT) that is a fixed temperature target to be placed in front of the Cold Sky reflector simulating the cold sky temperature (<80 K). An Ambient Cold Sky Target (ACT) is required to perform tests in laboratory at ambient conditions, with the challenge to maintain a liquid nitrogen temperature in a standard pressure environment.
The thermal design of the OGCTs is described and the results of the thermal analysis are presented. The attention will be focused on the challenges of developing the thermal model of the different targets, since a very high accuracy is requested in terms of temperature gradients, at first, and brightness temperature knowledge as the final aim. The objectives of the design and analysis activities have been to construct robust, easily usable thermal models of high detail to address these challenges. In this framework, the thermal model is extremely important. In fact, during the tests it is not possible to put any kind of sensors on the targets surface because it is very fragile and, anyway, they would affect the measurement. In addition, other measuring systems have been deemed not suitable. Hence, the reliability on the thermal model has to be quite high.
The challenges concerning the present thermal modelling are mainly due to computational limits: in fact, the targets are composed of a very huge number of elements whose temperature difference and gradients knowledge has to be precise as much as possible. In other words, this can be seen as dealing with a very large number of nodes and shapes. The geometries have been built using the Systema Thermica Python API and a 32-core workstation in order to deal with such heavy thermal models; this is the best solution to generate these targets for a total number of more than six thousands absorber pyramids for the TVCT and more than one thousand for the TVET, taking into account that a reasonably fine mesh is quite important. The two vacuum targets will be simulated with the instrument in test configuration and rotating at nominal speed.
Concerning the ambient target, the thermal development design concept will be described, supported by a conservative thermal analysis performed on a small sample, estimating the convection contribution.
Finally, concerning some MWI frequencies, the results in terms of brightness temperature are presented and matched to the requirements.