Direct analysis of ionograms at magnetic dip angles of 26°–30°

At dip angles of 26°–30°, the group retardation of the extraordinary ray is a constant (depending on frequency) times the retardation of the ordinary ray reflected at the same true height. The true height can therefore be found directly from the virtual heights of the two rays, with no assumptions about the shape of other parts of the profile. Results for three days show that any valley between the E- and F-layers was less than about 15 km wide, from sunrise to sunset. At night the addition of a simple extraordinary ray correction reduces the errors in standard methods of analysis from about 50 km to about 5 km, at frequencies near 2 MHz.