AP_Baro: make C_TO_KELVIN a function macro; create KELVIN_TO_C

These are in celsius

libraries/AP_Baro/AP_Baro.cpp

@@ -375,7 +375,7 @@ void AP_Baro::update_calibration()
 float AP_Baro::get_altitude_difference(float base_pressure, float pressure) const
 {
     float ret;
-    float temp    = get_ground_temperature() + C_TO_KELVIN;
+    float temp    = C_TO_KELVIN(get_ground_temperature());
     float scaling = pressure / base_pressure;
 
     // This is an exact calculation that is within +-2.5m of the standard
@@ -405,7 +405,7 @@ float AP_Baro::get_EAS2TAS(void)
     // only estimate lapse rate for the difference from the ground location
     // provides a more consistent reading then trying to estimate a complete
     // ISA model atmosphere
-    float tempK = get_ground_temperature() + C_TO_KELVIN - ISA_LAPSE_RATE * altitude;
+    float tempK = C_TO_KELVIN(get_ground_temperature()) - ISA_LAPSE_RATE * altitude;
     const float eas2tas_squared = SSL_AIR_DENSITY / (pressure / (ISA_GAS_CONSTANT * tempK));
     if (!is_positive(eas2tas_squared)) {
         return 1.0f;

libraries/AP_Baro/AP_Baro_HIL.cpp

@@ -66,7 +66,7 @@ void AP_Baro::SimpleUnderWaterAtmosphere(
     // \f$T(D)\f$ Temperature underwater at given temperature
     // \f$S\f$ Surface temperature at the surface
     // \f$T(D)\approx\frac{S}{1.8 \cdot 10^{-4} \cdot S \cdot T + 1}\f$
-    const float seaTempSurface = SSL_AIR_TEMPERATURE - C_TO_KELVIN; // Celsius
+    const float seaTempSurface = KELVIN_TO_C(SSL_AIR_TEMPERATURE); // Celsius
     const float S = seaTempSurface * 0.338f;
     theta = 1.0f / ((1.8e-4f) * S * (alt * 1e3f) + 1.0f);
 }

libraries/AP_Baro/AP_Baro_SITL.cpp

@@ -118,14 +118,14 @@ void AP_Baro_SITL::_timer()
 
     AP_Baro::SimpleAtmosphere(sim_alt * 0.001f, sigma, delta, theta);
     float p = SSL_AIR_PRESSURE * delta;
-    float T = SSL_AIR_TEMPERATURE * theta - C_TO_KELVIN;
+    float T = KELVIN_TO_C(SSL_AIR_TEMPERATURE * theta);
 
     temperature_adjustment(p, T);
 #else
     float rho, delta, theta;
     AP_Baro::SimpleUnderWaterAtmosphere(-sim_alt * 0.001f, rho, delta, theta);
     float p = SSL_AIR_PRESSURE * delta;
-    float T = SSL_AIR_TEMPERATURE * theta - C_TO_KELVIN;
+    float T = KELVIN_TO_C(SSL_AIR_TEMPERATURE * theta);
 #endif
 
     // add in correction for wind effects

libraries/AP_Baro/AP_Baro_UAVCAN.cpp

@@ -168,7 +168,7 @@ void AP_Baro_UAVCAN::handle_temperature(AP_UAVCAN* ap_uavcan, uint8_t node_id, c
     }
     {
         WITH_SEMAPHORE(driver->_sem_baro);
-        driver->_temperature = cb.msg->static_temperature - C_TO_KELVIN;
+        driver->_temperature = KELVIN_TO_C(cb.msg->static_temperature);
     }
 }