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The European Centre for Medium-Range Weather Forecasts (ECMWF) assimilates microwave imager observations for their information on humidity, cloud, and precipitation. However, Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) first-guess (FG) departure biases exhibit a 46-day oscillation with a peak-to-peak amplitude of up to 3 K in brightness temperature, which is linked to the precession of the equator crossing time of the TRMM orbit. The TMI bias has a diurnal cycle, but neither the Special Sensor Microwave Imager nor the Advanced Microwave Scanning Radiometer for the Earth Observing System shows a similar variation, so the bias must be in the TMI instrument itself. Its cause is probably solar heating of the main reflector, which is not perfectly reflective. This means that the instrument measures a combination of Earth emission and the physical temperature of the reflector. This effect has been partly corrected by the data providers, but their correction assumes a constant reflector temperature. In contrast, the ECMWF FG departures suggest that the reflector temperature varies by up to 70 K through the orbit. A method is presented for correcting the bias in the ECMWF assimilation system. It is also noted that when building future conical-scanning microwave imagers, it is important to provide reflectors that are as nonemissive as possible and to also carefully record the reflector's actual (possibly frequency-dependent) emissivity. The in-space temperature of the reflector surface should also be recorded so that it can be used in bias correction.