Starting models for the real height analysis of ionograms

Procedures are described for use in the real height analysis of ionogram data using the ordinary ray only to allow for the presence of underlying (low density) ionization. A controlled extrapolation of the virtual heights, with upper and lower limits, gives a useful correction under most conditions. For more accurate and consistent results, a synoptic model is used to give a mean starting height at a fixed frequency of 0.5 MHz. Constraints are placed on the profile shape between 0.5 MHz and the lowest observed frequency, f1, to minimize the variations with different methods of analysis and different values of f1. Suitable model starting heights are described for day and night conditions, and presented in tabular and graphical form. Equations are also given from which the model starting height can be calculated directly as a function of the local time, the month, and the station latitude.