 The MIRS microwave radiometer–spectrometer is described. The instrument is being developed using a conceptual design that was the first stage of ground preparations for the Convergence Space Experiment (SE). At the next stage of SE implementation, it is planned to use the proposed technical solutions as design documentation for manufacturing test and flight prototypes. MIRS is a multi-frequency scanner and sounder intended to record the characteristic radiothermal radiation emitted by the system of the Earth’s surface and atmosphere. The set of frequencies of the received radiation is in the range of 10 to 200 GHz. The device is to be installed on the Russian Orbital Segment (ROS) of the International Space Station (ISS). Conical scanning of the Earth’s surface is done by rotating an antenna unit mounted on a fixed base. With a 1.3 s period of rotation and a constant Earth incidence angle of 53.1 deg, continuous coverage of the Earth’s surface is ensured in a swath 800 km wide with spatial resolution of 10 km for the highest-frequency channels. Nine radiometer–polarimeters laid out as a full-power radiometer are used as radiation receivers. The MIRS antenna system is an offset parabolic antenna with separated irradiators. The optimum design solutions allowing the main problems of the SE to be solved and the MIRS parameters to be maintained under operating conditions on board the ISS are chosen for the conceptual design.