Phase difference power spectra in atmospheric propagation through rain at 10.6 µm

The power spectrum of the phase difference fluctuations between two IR beams propagating side by side is strongly influenced by rain along the path. According to measurements over a 1.37-km path in convective rainfall averaging 18 mm/h, the power spectrum of phase difference fluctuations in rain extends over the 300-Hz to 2-kHz band, and it is as much as 18 dB above the no-rain spectrum at 600 Hz during periods of heavier rain (greater than 30 mm/h). The shape of the spectrum is different at different stages of the storm probably because of changing drop size distributions. A simple dimensionless parameter derived from the power spectrum of phase difference fluctuations is 0.186 plus or minus 20 percent for a rain rate of 41 (mm/h)km plus or minus 25 percent, but this value may depend on the chopped transmitter waveform. The simple optical design and data analysis procedure allow power spectra to be obtained from a single channel. It is concluded that measurement of phase difference fluctuations in 10.6-micron propagation should allow the path-averaged rainfall rate to be inferred over path lengths of at least 1.5 km if a properly calibrated system is used.