MWS is a total power radiometer which measures the brightness temperature, at various altitudes, and delivers calibrated and geo-located atmospheric temperature and water vapour sounding data.
The MWS (Microwave Sounder) has a direct heritage from the microwave instruments AMSU-A (Advanced Micro-wave Sounding Unit A) and MHS (Micro-wave Humidity Sounder) on board EPS and NOAA satellites. The MWS set of channels is also similar to the ones of the Advanced Technology Microwave Sounder (ATMS) on board the NOAA/NASA Suomi-National Polar-orbiting Partnership (S-NPP) satellite.
The primary objective of the Microwave Sounder mission is to support Numerical Weather Prediction at regional and global scales by providing information on atmospheric temperature and water vapour profiles in clear and cloudy conditions. It also provides information on cloud liquid water columns.
Microwave soundings greatly enhance the National Meteorological Services’ (NMS) ability to initialise global and regional NWP models with realistic information on temperature and moisture. The frequent availability of detailed temperature and moisture soundings will also help to fulfil other key requirements common to Nowcasting and VSRF at regional scales, such as cloud microphysical structure.
The availability of MWS radiances (and derived temperatures soundings) is of primary importance to climate monitoring applications, due to the heritage of the temperature records from MSU (Microwave Sounding Unit) onwards (since 1979).
The spectral characteristics of MWS are enhanced, in comparison to AMSU-A and MHS, by the addition of two temperature and three humidity sounding channels. The new channel of MWS at 229 GHz will provide information on cirrus clouds which improve humidity sounding information. The MWS channel specifications are listed in the table.
|Channel||Centre frequency*||Bandwidth**||Polarisation||Nadir 3dB footprint size (IFOV)***||NEΔT(K)|
|MWS-5||53.246±0.08||2 x 140||QH/QV||20||0.4|
|MWS-6||53.596±0.115||2 x 170||QH/QV||20||0.4|
|MWS-7||53.948±0.081||2 x 142||QH/QV||20||0.4|
|MWS-12||57.290344±0.217||2 x 78||QH/QV||20||0.55|
|MWS-13||57.290344±0.3222±0.048||4 x 36||QH/QV||20||0.6|
|MWS-14||57.290344±0.3222±0.022||4 x 16||QH/QV||20||0.9|
|MWS-15||57.290344±0.3222±0.010||4 x 8||QH/QV||20||1.2|
|MWS-16||57.290344±0.3222±0.0045||4 x 3||QH/QV||20||2.0|
|MWS-18||164–167||2 x 1350||QH||17||0.5|
|MWS-19||183.311±7.0||2 x 2000||QV||17||0.4|
|MWS-20||183.311±4.5||2 x 2000||QV||17||0.4|
|MWS-21||183.311±3.0||2 x 1000||QV||17||0.6|
|MWS-22||183.311±1.8||2 x 1000||QV||17||0.6|
|MWS-23||183.311±1.0||2 x 500||QV||17||0.75|
*unit of Centre Frequency — GHz
**unit of Bandwidth — MHz
***unit of footprint size — km
****QH — Quasi-horizontal; polarisation vector is perpendicular to the scan plane at nadir
*****QV — Quasi-vertical; polarisation vector is parallel to the scan plane at nadir
Channels 3–16 have primary and backup receivers with opposite polarisations
The MWS measurement principle is essentially similar to that of its predecessor instruments (e.g. MHS, AMSU), the main difference being the inclusion of the complete range of radiometric frequency channels within a single instrument with a single main antenna. One particular difference is that the two lowest frequency channels (MWS-1 and MWS-2) will be over-sampled with respect to the footprint in the scanning direction, in order to allow the possibility of filtering out RFI (Radio Frequency Interference).
The instrument collects the radiation coming from the Earth using a scanning flat mirror (rotating reflector) which reflects the energy to the feed-horn assembly through a static parabolic reflector. The rotation of the mirror around an axis which is nearly parallel to the flight direction of the orbiter, results in the cross-track sensing of the instrument. The Earth is viewed at different scanning angles, symmetric around the nadir direction. The scan speed is constant during the Earth view, providing equally spaced measurements in viewing angle. With a scan duration of 2.254 seconds, and an angular sampling amounting to 1.04919°, there are 95 earth view pixels per scan, resulting in a maximum scanning angle of 49.31° and a swatch width of about 2210 km in mid-latitude regions.