The global Digisonde network provides a vast volume of ionogram-derived electron density profile data from some 50 stations, The current true height inversion program NHPC differs significantly from the original algorithms developed for real time calculation on PC 286 machines, NHPC successfully inverts about 90% of the autoscaled ionograms, A stand-alone version of NHPC can invert h'(f) traces from any digital or analog ionogram, For each layer, the N(h) profile is expressed in terms of shifted Chebyshev polynomials with a logarithmic argument containing the starting plasma frequency and the critical frequency of the layer, The peak height and a set of coefficients specify the electron density distribution of each layer, A valley model derived from incoherent scatter observations at Arecibo describes the E-F transition, The frequent problem of missing h'(f) data points at the beginning of a trace is addressed by analytical expansion in the true height (profile) domain rather than by direct extrapolation of the h'(f) trace, The validity of the Digisonde NHPC profiles has been verified by comparison with incoherent scatter radar profiles.
