src/ballistics.c


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#include <math.h>

#include "ballistics.h"
#include "constants.h"
#include "context.h"

double atmospheric_correction(double ballistic_coefficient,
							  double temperature,
							  double pressure,
							  double relative_humidity,
							  AtmosphereModel atmosphere_model) {
	double air_density = calculate_air_density(temperature, pressure, relative_humidity);

	if (atmosphere_model == ICAO)
		return (STD_AIR_DENSITY_ICAO / air_density) * ballistic_coefficient;
	else
		return (STD_AIR_DENSITY_ASM / air_density) * ballistic_coefficient;
}

double calculate_air_density(double temperature,
							 double pressure,
							 double relative_humidity) {
	pressure = pressure * 100;

	if (relative_humidity > 0.0) {
        double p_sat = 610.78 * pow(10, (7.5 * temperature - 273.15) /
                                        (temperature - 273.15 + 237.3));
        double vapor_pressure = relative_humidity * p_sat;
        double partial_pressure = pressure - vapor_pressure;

        return fma(partial_pressure,
				   DRY_AIR_MOLAR_MASS,
                   vapor_pressure * WATOR_VAPOR_MOLAR_MASS)
               / (UNIVERSAL_GAS_CONSTANT * temperature);
	}
	else
		return pressure / (SPECIFIC_GAST_CONSTANT_DRY_AIR * temperature);
}