1 files changed, 38 insertions, 0 deletions
diff --git a/src/ballistics.c b/src/ballistics.c
new file mode 100644
index 0000000..10d97da
--- /dev/null
+++ b/src/ballistics.c
@@ -0,0 +1,38 @@
+#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);
+}