Graphical presentation of the relationships between the frequency, ionosphere electron density, and propagation incident angle measured from vertical are given in the expression:
Nec = 1.2 x 10^4 v^2 cos^2 a ...(from Tascione, 1988)
For the given circuit parameters:
v = 15.000 MHz
a = 72 d ...(h = 230 KM)
Then the Critical ionosphere density from the formula is
Nec = 2.59 x 10^5 cm^-3.
For densities lower than that value the signal will pass out of the ionosphere into space. For densities above that value the signal will continue to propogate, but with increasing absorbtion on this circuit.