A role for high power millimeter wave sources in atmospheric remote sensing

High power radiation sources (gyrotrons) at 94 GHz and above have been developed at NRL and other places. Recently, applications of these sources to atmospheric sensing have been studied. Sources in the atmospheric window at 94 GHz are particularly useful for the remote sensing of clouds due to the fact that Rayleigh scattering from the droplets in the cloud have cross section which is proportional to λ-4. With a variant of coherent on receive data processing, one can extract both the range resolution and the Doppler information from oscillator signals. Earlier 94 GHz data on clouds was very limited because the transmitter average power was only 4 Watts. Gyrotrons have the capability of bringing to bear at least two orders of magnitude increase in average power, and can thereby enhance the capability of previous studies. The quasioptical gyrotron, developed at NRL, brings an additional capability of tunability in addition to high power. Tuning from 85--130 GHz has been demonstrated, and harmonic operation could open the possibility of tuning up to 260 GHz. Ground based radiometry in this frequency range has been extremely useful in detecting upper atmosphere trace elements. One possible application to remote sensing is the measurement of transmission of this radiation through the upper atmosphere. This would require a two way transmission path obtained by reflecting the radiation from a small satellite. Alternatively one could put the source or detector in a geosynchronous orbit to measure upper atmosphere trace impurities. The latter could open the possibility of simultaneous worldwide measurement of the entire absorption line