FY-3 meteorological satellite is the 2nd generation of China’s polar-orbit meteorological satellite

and the Microwave Humidity Sounder（MWHS） is one of its main payloads.The basic principle of atmospheric humidity sounding by microwave radiometer and the technical specifications of MWHS onboard FY-3 satellite are briefly introduced in this paper.The block diagram and operating modes of MWHS are presented.MWHS is a dual-frequency

five-channel millimeter wave radiometer which is a total power type microwave radiometer based on a heterodyne receiver

and performs cross-track scanning.The frequencies of MWHS include both the atmospheric transparent windows of 150GHz and water vapour absorption lines around 183.31GHz.The 150GHz radiometer has two polarizations of V and H

and the 183.31GHz radiometer has three channels of 183.31±1GHz

183.31±3GHz and 183.31±7GHz

respectively.The swath width of about 2700km is implemented by cross-track scanning of the antenna beams.Final testing verifies that the performance of MWHS satisfies technical requirements of specifications.MWHS consists of three units: antenna and receiver unit

electronic unit and power supply unit.Antenna and receiver unit collcets emission from the atmosphere. The received signal is focused to the horn-feed and the first element of the high frequency front end

and then down-converted by a double side band mixer to Intermediate Frequency;IF signal is been down-converted to Low Frequency（LF） by a detector

and then integrated.The electronic unit digitizes the LF signal

controls the scanning mechanism and measures physical temperature of the on-board hot target for calibration

and communicates with satellite through a 1553B data bus.The power supply unit performs DC/DC conversion

distributies the DC lines to the various sub assemblies and switches between the nominal and the redundant unit.One motor drives two separated reflectors for the 150GHz and the 183.31GHz channels

the scanning period is 2.667s.Main beams of the antenna scan over the observing swath（±53.35° from nadir） in the cross-track direction at a constant periodicity of 1.71s.During each period

two-point calibration are performed to calibrate the receiver gain and noise.The calibration is implemented by observing an onboard hot target and the background emission of the cold sky.