The simple answer to the question “why polarization diversity” is that at 94 GHz we need to have two pulses that are very close together because the motion of the satellite combined with the finite beamwidth of the radar results in a rapid decorrelation of the phases of the return signals. If the pulses are close together, they would both be in the cloud at the same time: to distinguish them, WIVERN transmits pairs of pulses with alternating polarization—H (red) and V (blue)—spaced by a short time separation, T_hv, and receives the return signals in both polarizations (see Panel A). Because of the orthogonality of the polarizations, the horizontally and vertically polarized pulses are transmitted and backscattered, and then propagate through the atmosphere independently so that the returns from the two closely spaced pulses can be separated. Cross-polar interferences are typically weak and WIVERN signal processing includes ways for removing their effects. In Panel C, the red H pulse is at height h, and at a short time later (e.g.,T_hv = 20 μs), the blue V pulse has moved (e.g., 3 km) along the slant path and is at height h (Panel D). The estimation of the Doppler velocity is based on the phase change between the back-scattered signals of these two pulses. Panel B (green-shaded region) shows that for a T_hv of only 20 μs the targets have not had time to reshuffle, so the phases remain correlated and can be measured accurately. The maximum unambiguous phase shift value of ±180° is reached when the cloud particle targets move one quarter of a wavelength (800 μm at W band), equivalent, with T_hv = 20 μs, to a folding velocity of ±40 m s−1 (top x axis in Panel B). Contrast this with the situation encountered in a conventional pulse pair (Panel E) when the phase change is derived between two pulses with the same polarization, with the second red H pulse trailing 130 μs (≈20 km) behind the first so that only one pulse at a time is in the troposphere. Because of the fast movement of the satellite, the targets would appear to have been almost completely reshuffled (red- shaded region in Panel B), the phase difference would be very noisy, and the maximum “folding” velocity only about 6 m s−1, far below that required for retrieving line-of-sight winds.