AP_Math: moved constants from AP_Baro

libraries/AP_Math/definitions.h

@@ -67,6 +67,15 @@ static const double WGS84_B = (WGS84_A * (1 - WGS84_F));
 // Eccentricity of the Earth
 static const double WGS84_E = (sqrt(2 * WGS84_F - WGS84_F * WGS84_F));
 
+// air density at 15C at sea level in kg/m^3
+#define AIR_DENSITY_SEA_LEVEL    1.225f
+
+#define C_TO_KELVIN 273.15f
+
+// Gas Constant is from Aerodynamics for Engineering Students, Third Edition, E.L.Houghton and N.B.Carruthers
+#define ISA_GAS_CONSTANT 287.26f
+#define ISA_LAPSE_RATE 0.0065f
+
 /*
   use AP_ prefix to prevent conflict with OS headers, such as NuttX
   clock.h