The Microwave Humidity Sounder (MHS) is a self-calibrating microwave radiometer, observing the Earth with a field of view of ±50 degrees across nadir, in five frequency channels of the millimeter-wave band (89-190 GHz). MHS, together with the complementary AMSU-A instruments, provides the operational microwave sounding capaility for the NOAA-N, NOAA-N', and EUMETSAT Metop-A, Metop-B and Metop-C meteorological satellites.
Channels at 157 GHz and around the 183 GHz water vapor absorption line provide a humidity profile sounding capability, while the 89 GHz channel provides information on surface temperature and emissivity (in conjunction with AMSU-A channels) and detects cloud and precipitation contaminated pixels. The MHS instrument represents an improvement to the AMSU-B radiometer on board previous NOAA satellites, while providing continuity to its data. Two MHS instruments are planned to fly on NOAA-N and NOAA-N' satellites, and three on the METOP satellite series of the European Polar System (EPS).
MHS is a cross-track, line-scanned instrument. Ninety contiguous scene resolution cells are sampled in a continuous scan, covering 49.44444 degrees on each side of the sub-satellite path, with an antenna beam width of 1.11111 degrees at half power point. These scan patterns and geometric resolution translate to a 17-km diameter cell at nadir from the 870 km nominal orbital altitude.
A parabolic mirror is rotated to sample the Earth scene at 90 equidistant angular positions, at a rate of three scans every eight seconds, and at the same time to provide reference measurements against two calibration sources, i.e. an on-board blackbody target and a view on free space. The radiation is then conveyed on four feeds at 89, 157, 183 and 190 GHz via a "quasi-optical" arrangement of lenses, dichroic plates and a polarizing beam splitter.
Source: NOAA KLM User's Guide ( www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c3/sec3-9.htm )