The N(h)-Working Party, a Group in Commission III of URS! (URS! Information Bulletin No. 112, p. 12) suggested these calculations of ordinary and extraordinary refractive indices n , vertical group refractive indices c/U , and virtual o, x o, x 1 heights h (f), for an Epstein, cosine and parabolic layer model. c ist the free 0 X 1 space velocity of light . U and U denote vertical ordinary and extraordinary 0 X group velocities. The data are intended to facilitate real height (h) computations from 1 1 observed, h (f)-traces. Especially the h (f)-data are intended also to allow o, x o, x for tests of existing reduction methods. For ionization minimum investigations an additional set of tables is presented. These tables represent the virtual paths 1 6 h (f) of sounding pulses which penetrate an ionospheric layer of parabolic o, x 1 shape; they can be used together with the abovementioned standard h (f)-curves o, x 1 to give ordinary and extraordinary h (f)-traces for any combination of a lower O, X parabolic layer and an upper Epstein, cosine or parabolic electron density distribution. Ordinary group refractive index tables have already been published by D. H. SHINN [ 1] 1 and by Vv. BECKER [ 2] * Their value s of cp , the angle of inclination of the earth s magnetic field, are slightly different from those used here. These tables may be used 1 as additional sets for interpolation purposes. D. H.