The Jason Microwave Radiometer (JMR) measures atmospheric water vapor content to determine how water vapor impacts radar signal propagation. Its measurements also can be used directly for studying other atmospheric phenomena, particularly rain. The JMR is a passive receiver that collects radiation reflected by the ocean at frequencies of 18.7, 23.8, and 34 GHz. To determine atmospheric water vapor content accurately, sea surface and cloud contributions need to be eliminated from the signal received by the radiometer. To accomplish this, JMR uses different frequencies, each of which is more sensitive than the others to one of these contributions. The main 23.8 GHz frequency is used to measure water vapor; the 34 GHz channel provides the correction for non-rain bearing clouds, and the 18.7 GHz channel is highly sensitive to wind-driven variations in the sea surface. The water vapor signal is determined by combining the measurements from all frequencies.