The Microwave Imager (MWI) is a conically scanning radiometer, capable of measuring thermal radiance emitted by the Earth, at high spatial resolution in the microwave region of the electromagnetic spectrum.
MWI has heritage from microwave imaging missions such as SSM/I (Special Sensor Microwave Imager) and SSMI/S (Special Sensor Microwave Imager Sounder), which have been flown as part of the US Defence Meteorological Satellite Program, and the NASA/JAXA Global Precipitation Measurement’s Microwave Imager (GMI), TRMM-TMI Tropical Rainfall Measuring Mission’s (TRMM) Microwave Imager (TMI) and The Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E).
The primary objective of the MWI is to support Numerical Weather Prediction at regional and global scales by providing cloud and precipitation products and all weather surface imagery.
It will provide all-weather surface imagery, including sea ice coverage and type, snow coverage and water equivalent. The primary objective of MWI is also to obtain sea surface winds and total column water vapour above oceans.
Other mission objectives include water vapour and temperature gross profiles, plus providing continuity of other key microwave imager channels (e.g. SSM/I, TRMM TMI, SSMIS, AMSR-E) in support of long-term climate records.
For Nowcasting and Very short Range Forecasting at regional scales, MWI mission is foreseen to fulfil key requirements on cloud liquid water and ice estimates, and precipitation estimates.
MWI frequency coverage is from 18 GHz up to 183 GHz. The table below shows the MWI channel specifications, NEΔT and footprint size. All MWI channels up to 89 GHz are measured with both *V- and **H polarisations. Channels above 89 GHz are measured at V polarisation only.
|Channel||Frequency||Bandwidth||Polarisation||Radiometric Sensitivity (***NEΔT)||Footprint Size at 3dB|
|MWI-1||18.7 GHz||200 MHz||V, H||0.8||50 km|
|MWI-2||23.8 GHz||400 MHz||V, H||0.7||50 km|
|MWI-3||31.4 GHz||200 MHz||V, H||0.9||30 km|
|MWI-4||50.3 GHz||400 MHz||V, H||1.1||30 km|
|MWI-5||52.61 GHz||400 MHz||V, H||1.1||30 km|
|MWI-6||53.24 GHz||400 MHz||V, H||1.1||30 km|
|MWI-7||53.75 GHz||400 MHz||V, H||1.1||30 km|
|MWI-8||89.0 GHz||4000 MHz||V, H||1.1||10 km|
|MWI-9||118.7503±3.2 GHz||2x500 MHz||V||1.3||10 km|
|MWI-10||118.7503±2.1 GHz||2x400 MHz||V||1.3||10 km|
|MWI-11||118.7503±1.4 GHz||2x400 MHz||V||1.3||10 km|
|MWI-12||118.7503±1.2 GHz||2x400 MHz||V||1.3||10 km|
|MWI-13||165.5±0.75 GHz||2x1350 MHz||V||1.2||10 km|
|MWI-14||183.31±7.0 GHz||2x2000 MHz||V||1.3||10 km|
|MWI-15||183.31±6.1 GHz||2x1500 MHz||V||1.2||10 km|
|MWI-16||183.31±4.9 GHz||2x1500 MHz||V||1.2||10 km|
|MWI-17||183.31±3.4 GHz||2x1500 MHz||V||1.2||10 km|
|MWI-18||183.31±2.0 GHz||2x1500 MHz||V||1.3||10 km|
*V — Vertical Polarisation
**H — Horizontal Polarisation
***NEΔT — Noise Equivalent Temperature change
The conically scanning MWI collects radiation coming from the Earth by means of a rotating offset parabolic reflector antenna and feed-horn cluster rotating together. The rotation of the slanted antennas allows conical scans with constant incidence angles of about 53°, depending on the channel frequency.
The observations are acquired within an angle of ± 65° in azimuth for the fore view, equivalent to a swath of about 1700 km from the altitude of the nominal orbit. MWI has a moderate antenna size providing on ground footprints ranging from 50 km down to 10 km, depending on frequency. The observation geometry can be seen in Figure 1.