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.
