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Radio altimeters operating in the microwave region must distinguish between desired signals returned from the ground and undesired signals returned from precipitation. Calculation of the relative ground and precipitation returns for a 0.1 microsecond pulse-duration altimeter requiring 10 to 1 desired-undesired signal ratio indicates a minimum wavelength of about 2 cm may be used for reliable operation in heavy precipitation. Curves have been computed for minimum wavelength at a given altitude for fixed range to rain and for equal rain and ground ranges, for various beam widths. Minimum wavelengths as long as 20 cm are indicated for some conditions. Use of circular polarization may permit altimeter operation at wavelengths less than 2 cm, even with intense precipitation.