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
capability for the NOAA-N, -N' 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 -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.
MHS Channels and Passband Characteristics*
Channel (See Note 1)
Central Frequency (GHz)
No of Passbands
RF Bandwidth (MHz)(See Note 2)
T (K) (See Note 3)
Polarization (See Note 4)
H1
89.0
1
2800
0.22
V
H2
157.0
1
2800
0.34
V
H3
183.311 ± 1.0
2
2 x 500
0.51
H
H4
183.311 ± 3.0
2
2 x 1000
0.40
H
H5
190.311
1
2200
0.46
V
Notes:
The five MHS channels provide data continuity with AMSU-B channels 16 to 20, with some minor changes in
frequency allocation and polarization, and improved performance.
The quoted values for the maximum bandwidths are double-sideband values and represent the maximum permissible
bandwidths at the 3 dB points.
Ground measured values for the first flight model (NOAA-N).
The V and H polarizations correspond respectively to electrical fields normal or parallel to the ground track
at nadir, both rotating by an angle equal to the scan angle for off-nadir directions.