Abstract: A digital data preprocessor is disclosed wherein consecutive samples of ny raw digital data telemetered from a dropsonde and decommutated are stored in a first storage stack of registers. Adjacent samples are subtracted, the adjacent first-differences therefrom are subtracted, and the resulting second-differences are stored in a second storage stack of registers. By a comparison of raw data with the second-differences, raw data distorted by noise are measured and subsequently restored to a new value calculated from valid values of raw data samples adjacent to the distorted value. The second-differences are fed through a series of adders to provide a sequence of second-sliding-sums which are stored in a third stack of storage registers. A significant value selector tests the sliding-sums using three different criteria. The results of the tests are used as a basis for selecting raw data samples from the stack for pre-output smoothing of the raw data.