When calculating true-height profiles from ionosonde data, it is important to account for all underlying ionisation. The true-height profile program POLAN estimates the start height in one of two ways.

In the simplest method, the start height is estimated from the virtual height of the lowest frequency echoes on an ionogram. This method uses the assumption that there is no ionisation below this height and therefore the virtual height represents the true-height of these echoes.

A more complex approach makes use of a synoptic model which calculates the
most likely start height due to parameters such as latitude, date, local time
and solar zenith angle (the position of the sun with respect to the ionosonde).
This procedure is outlined in * Starting models for the real height analysis
of ionograms by J. E. Titheridge, J.A.T.P, 48, pp 435-446 1986 *.

Ionosondes | WDC

4-DEC-1997 Chris Davis