In the coming years, the European Space Agency (ESA) will be launching a series of new weather satellites to measure important meteorological data (such as precipitation, water vapor or temperature) better than ever before. The heart of the measuring devices consists of extremely sensitive indium gallium arsenide (InGaAs)-based microwave amplifiers developed at the Fraunhofer Institute for Applied Solid State Physics (IAF) of Freiburg, Germany that can perceive even very weak signals from the environment (key for more accurate weather predictions).
For whether predictions, meteorologists rely on computer simulations fed with thousands of pieces of measurement data. For decades, such data have mainly been supplied by satellites that use sensitive sensors to measure the temperature or the precipitation on the earth. The better these sensors are, the more accurate the measured values and hence the weather predictions. In the next two years, ESA will therefore be launching the second generation of its MetOp Meteorological Operational Satellites (six in total), equipped with state-of-the-art measurement technology. A total of €1.4bn has been earmarked for their construction (not including launch and operation).
Measurements in the upper atmosphere
With the satellites, small, highly precise amplifiers from Fraunhofer IAF that absorb microwave radiation will also be launched. The radiation is emitted from every surface of every single object. The amplifiers are calibrated to microwave frequencies because these provide key meteorological information: they capture microwaves emitted by water vapor, rain, fog or ice crystals – particularly also from ice crystals in the cirrus clouds high up in the atmosphere, which are believed to have an important effect on the climate and weather. Due to the microwave radiation, it is also possible to reach very precise conclusions about the temperature on the ground.
However, the signals received by the satellite’s microwave antennae are extremely weak – just a few nanowatts. To measure these microwave signals with any degree of reliability, highly sensitive amplifiers are needed. However, transistors made of conventional silicon would be far too insensitive.
“A characteristic of InGaAs is that it is very easily traversed by electrons, even if the electric field that drives the electrons is very small,” notes Dr Michael Schlechtweg, who heads Fraunhofer IAF’s High Frequency Electronic business unit. Accordingly, the electrons in the transistor are already set in motion by very weak microwave signals, which makes the transistor extremely sensitive. “Thanks to the microwave circuits developed by the Fraunhofer IAF, the MetOp satellite will be able to determine temperature, water vapor and type of precipitation even more precisely in the future. This increases the reliability of the weather forecast,” comments ESA project manager Ville Kangas.
Small electrodes with high accuracy
In addition to using InGaAs, the amplifiers’ small size contributes to making them so sensitive. The electrodes of the transistors are just 50-35nm long, enabling the measurement of such small electron currents (signals). “This extreme sensitivity and small size are the result of 25 years of research,” Schlechtweg says. “During this time, we have developed a highly accurate manufacturing process in which the amplifier circuits are built in 150 production steps [including forming the electrodes with an electron beam],” he adds.”
On the MetOp satellite, the amplifiers are used in three different microwave instruments that measure different things: precipitation, water vapor, ice crystals or the temperature. Different sensors therefore had to be produced, each calibrated to the corresponding microwave frequency (five frequency bands between 54GHz and 243GHz, with the higher the frequencies requiring higher-power amplifiers).
Fraunhofer IAF says that, in addition to ESA, a US firm has also recently inquired about the microwave amplifiers, so it is possible that they may also be launched aboard US satellites.