| The physics characteristics of rainfall are introduced in detail. On the basis of the Mie theory and dot-matching methods, the scattering characteristics of a single raindrop in millimeter wave band are calculated. According to the raindrop-size distribution, the rain scattering in millimeter wave band is calculated. Millimeter wave radar equation is given. The influences of rainfall on the performance of millimeter wave systems are investigated. Signal-to-noise ratio (SNR), signal-to-clutter ratio and clutter-to-noise ratio are calculated respectively. Then, the approach to the solution of the Fourier integral in closed form is adopted for millimeter wave pulse propagation through rain medium. Pulse distortion, in terms of percentage changing in pulse width, is studied. The changes of pulse frequency spectrum in Doppler Effect are analyzed when millimeter wave pulse propagates through rain medium. When the missile approaches to the target, the influences of rainfall effects on the performance of millimeter wave Doppler fuze systems are studied. Target and rainfall Doppler frequency spectrum are calculated respectively. The results show that when the millimeter wave radar has the short-distance detection, signal-to-clutter ratio is more than signal-to-noise ratio. So rainfall is an important impact factor. When the millimeter wave band pulse propagates through rain medium, it changes the spread and compress, and it also delays. The spread and compress of millimeter wave pulse frequency spectrum will happen in Doppler Effect during rain. When the missile approaches to the target, the Doppler frequency of rainfall in range gate which the target locates decreases and the intensity of Doppler frequency spectrum of rainfall in range gate which the target locates increases gradually with the decrease of the distance between missile and target. These results have an important practical meaning in detection of the millimeter wave radar in short distance and Doppler fuze during rain.
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