This first paper in a series of three describing the automatic processing of digital ionograms discusses the scaling of topside ionograms in a ground-based minicomputer. The objective is to automatically obtain the vertical electron density profiles. The topside ionogram scaling algorithm finds the resonance and cutoff frequencies and the vertical O and X echo traces. Application of the scaling algorithm to digitized ISIS 1 and 2 ionograms illustrates its performance and demonstrates the feasibility to automatically scale topside ionograms. The ISIS studies show that polarization tagging of the O and X signals should be done in a digital satellite sounder to assure successful, real time scaling of complex ionograms.