Developed by ESA and the Chinese National Space Science Center, this prototype millimetre-wave instrument is designed to sound the depth of typhoons and rain clouds from geostationary orbit. This 3-m diameter prototype, undergoing ground testing in Beijiing, makes use of interferometry to deliver high-resolution images from 36 000 km up. Separate signals from multiple antennas are precisely correlated together to produce a picture of otherwise impossible sharpness. These antennas are mounted on a circular frame, then set rotating at a rate of once per minute, allowing them to fill in further details. China’s 54 GHz 21-antenna instrument making up the outer circle is called the Geostationary Interferometic Microwave Sounder II, while ESA’s making up the inner circle is the 183 GHz 24-antenna Geosounder II, designed for ESA by Omnisys Instruments in Sweden.