MetOp SG MWI "On Ground Calibration Targets": design and development

In this paper we present the driving concepts that led to the design of the On Ground Calibration Targets (OGCT) for the Micro-Wave Imager (MWI) to be flown on board of the Meteorological Operational Satellite - Second Generation (MetOp -SG) developed by ESA and EUMETSAT. The MicroWave Imager (MWI) Instrument is part of the payload on board the MetOp-SG Satellites type B. MWI is a conical scanning radiometer, with 26 frequency channels covering the frequency range from 18.7 GHz to 183.3 GHz. All MWI channels up to 89 GHz are measured with both V- and H polarizations. Channels above 89 GHz are measured at V polarization only. The spatial resolution, corresponding to the IFOV, ranges from 50 km at 18.7GHz to about10 km for the highest frequency channels. The main goal of the MWI is to serve operational meteorology and oceanography by providing precipitation monitoring as well as sea ice extent information also in support of long-term climate records. A crucial and critical part of the project is the instrument calibration to be provided by the On Ground Calibration Targets. OGCT, almost perfect, blackbodies allowing the MWI instrument RF stimulation from a known source. OGCT, planar arrays of pyramidal microwave-absorbing elements, shall provide on-ground radiometric performances and calibration tests either in thermal-vacuum conditions or in ambient conditions. For thermal-vacuum tests two targets will be used: Thermal-Vacuum Earth Target (TV-ET), a variable temperature target simulating the variable brightness temperature of the measured scene (nominally from 100K to 293K) to be placed in front of the instrument main reflector and Thermal-Vacuum Cold Sky Target (TV-CT), a fixed temperature target to be placed in front of the cold sky reflector simulating the cold sky temperature (<80 K). A third target, the Ambient Cold Target (A-CT), will be used to perform laboratory and calibration tests at ambient conditions (normal temperature and pressure). Concerning RF performances, the A-CT design is very similar to the TV-CT. Two A-CT targets will be provided, one to be used at instrument level, the second on the instrument integrated on the satellite. The main features of the TV targets (TV-CT and TV-ET) shall satisfy the following parameters: Return loss better than 35 dB (for incident angles within +/-20°); Coupling loss better than -35 dB; Brightness Temperature accuracy better than 0.3K (at 1 sigma). The main elements and performances of the OGCT will be shown in the current paper.