/* Get an ISA pressure altitude. 
Given atmospheric pressure "P" in pascals, this routine returns the associated barometric altitude ("pressure altitude") using International Standard Atmosphere (ISA) pressure model coefficients and equations. */
scalar pressure_altitude(scalar P)
/* Get an ISA pressure altitude. 
Given atmospheric pressure "P" in pascals, this routine returns the associated barometric altitude ("pressure altitude") using International Standard Atmosphere (ISA) pressure model coefficients and equations. */
scalar pressure_altitude(scalar P)
{
	const scalar del ((P - P_bias) / Pb);
	if (Lb) {
		return (Hb + ((Tb / Lb) * (pow(del, RL_g0) - 1)));
	} else {
		return (Hb + (RT_g0 * log(del)));
	}
}