办事指南

Weather eyes

点击量:   时间:2019-03-08 08:12:21

By Kurt Kleiner in Washington DC WEATHER satellites using an ultra-sensitive water vapour detector will in future give earlier warnings that severe weather is on the way. Based on a very high speed microchip amplifier, the technology could also be used by military satellites to filter out the radiation emitted by water vapour and sense what might be hidden beneath clouds. “This chip will really increase the capability of these new instruments,” says Richard Lai, an engineer at TRW in Redondo Beach, California, who designed and built the device with a team at NASA’s Jet Propulsion Laboratory in Pasadena. The microchip’s secret lies in its ability to run at very high frequencies up to 190 gigahertz. Since water vapour absorbs radiation at a frequency of 183 gigahertz, a sensor tuned to that range will be able to see water vapour in the atmosphere because the vapour will block out the radiation, like a cloud blocks the Sun. And by tuning slightly up and down from that range, the satellite will be able to detect wavelengths that are able to get through the water vapour. By comparing the different readings, meteorologists can work out the temperature and amount of water in the atmosphere at different altitudes. The new microchip can amplify very faint signals in the high-frequency microwave band—and with very little noise. Lai’s group is the first to develop a complete integrated circuit amplifier of a type known as a monolithic microwave integrated circuit (MMIC)—and then persuade it to work at such a high frequency. Ever since transistors were first developed, researchers have been striving to optimise operating frequency. A 1-gigahertz transistor turns on and off a billion times per second. At those speeds, the physical properties of the transistor are crucial to its performance: if electron mobility is too low, they don’t have time to make it through the transistor’s gate electrode—so the length of the gate makes a big difference. So TRW and JPL turned to a type of transistor called a high electron mobility transistor (HEMT) to improve water detection. Using a technique called molecular beam epitaxy, TRW and JPL created a semiconductor by growing alternating layers of indium gallium arsenide, indium aluminium arsenide and indium phosphide on a substrate. These materials let them make a transistor with a very short 80 nanometre-long gate electrode. This allows the transistor to operate at very high frequency and, importantly, with very little noise. “We’re planning for the next generation of microwave sensors. We’ve demonstrated that MMICs will be the next type,” says Jamie Hawkins, a manager at the US National Oceanic and Atmospheric Administration Office of Systems Development in Suitland, Maryland. Accurate measures of atmospheric water vapour would make weather forecasters’ jobs much easier—even on Mars. Because the chips eat up little power, they might find a place on a future Mars probe, says Todd Gaier,