Passive microwave satellite sensors of the next decade for observing ocean vector winds, temperature, and salinity: WSF-MWI, COWVR, AMSR3, CIMR

A series of new passive microwave satellite sensors will offer strongly enhanced capability to measure ocean surface vector winds (OSVW), sea surface temperature (SST) and salinity (SSS) during the next decade and beyond. Our presentation gives an overview of the features of these instruments and how we plan to use them in future Earth observations.

The Weather System Follow on Microwave Imager WSF-M MWI, operated by the US DoD scheduled to be launched in January 2024, is a follow-up of the US Navy’s WindSat. The sensor calibration system will continue the four-point calibration method implemented with GMI employing a combination of internal and external calibration targets. Like GMI, WSF-MWI is expected to reach absolute calibration accuracy.  As WindSat did, it will provide fully polarimetric measurements at X, Ku and Ka-band and thus be able to continue WindSat’s OSVW data record.

COVWR, developed by NASA JPL and US DoD, was launched in December 2022. Its novel cost-effective design consists of a fixed feedhorn bench and has only the antenna dish spinning. It is fully polarimetric at 3 frequencies within Ku, K, and Ka bands and can observe most Earth locations simultaneously using fore and aft looks. The 2-look capability strongly aids the measurement of wind direction as it does not have to rely on any external input from Numerical Weather Prediction Models, as for example scatterometers do.

JAXA’s AMSR3, to be launched in late 2023, continues the series that started with AMSR-E in 2002 and followed with AMSR-2 in 2012. The presence of the C-band channels is vital for globally measuring SST with passive microwave sensors. The global availability of microwave SST is essential for the scientific community, as it provides observations in the presence of clouds and aerosols, where infrared sensors fail. The AMSR3 sensor will observe at two C-band and at two X-band frequencies, which will result in increased capabilities to measure ocean wind speeds through rain, including in strong tropical and extratropical storms where most other passive microwave sensors cannot provide usable retrievals. It is possible to find combinations between the C- and X-band channels that minimize the impact of rain but are still sensitive to wind speed, which enables disentangling passive wind and rain signals.       

The ultimate passive microwave sensor for ocean observations will be ESA’s CIMR, which is anticipated to launch in 2028. CIMR has an antenna with 8-meter diameter, measures at 5 frequencies between L- and Ka-band, is fully polarimetric at each frequency and has fore and aft looks. These combined features will not only provide the capability for measuring global SST, OSVW and wind speeds in rain as mentioned above. The large antenna will allow these observations to occur at a spatial resolution of 15 km. This constitutes a significant enhancement over currently operating sensors, which reach resolutions of about 50-km for SST and 30-km for OSVW. Finally, the presence of L-band enables CIMR to also measure SSS and thus provide continued SSS satellite data after SMOS, Aquarius and SMAP.