The measurement converts time delay to effective altitude of the ionospheric layer.
2.
The ionogram display shows the effective altitude of the ionospheric layer as a function of frequency.
3.
The critical frequency is the limiting frequency at or below which a radio wave is reflected by an ionospheric layer at vertical incidence.
4.
After studies of mathematics and physics in Freiburg and Munich ( with Arnold Sommerfeld ), under Jonathan Zenneck he wrote a thesis on partial reflection of radio waves in an ionospheric layer.