// return altitude difference in meters between current pressure and a
// given base_pressure in Pascal
float AP_Baro::get_altitude_difference(float base_pressure, float pressure) const
{
float ret;
float temp = get_ground_temperature() + 273.15f;
float scaling = pressure / base_pressure;
// This is an exact calculation that is within +-2.5m of the standard
// atmosphere tables in the troposphere (up to 11,000 m amsl).
ret = 153.8462f * temp * (1.0f - expf(0.190259f * logf(scaling)));
return ret;
}
// return current scale factor that converts from equivalent to true airspeed
// valid for altitudes up to 10km AMSL
// assumes standard atmosphere lapse rate
float AP_Baro::get_EAS2TAS(void)
{
float altitude = get_altitude();
if ((fabsf(altitude - _last_altitude_EAS2TAS) < 25.0f) && !is_zero(_EAS2TAS)) {
// not enough change to require re-calculating
return _EAS2TAS;
}
// 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;
_EAS2TAS = safe_sqrt(1.225f / ((float)get_pressure() / (ISA_GAS_CONSTANT * tempK)));
_last_altitude_EAS2TAS = altitude;
return _EAS2TAS;
}
// return altitude difference in meters between current pressure and a
// given base_pressure in Pascal
float AP_Baro::get_altitude_difference(float base_pressure, float pressure) const
{
float ret;
float temp = get_ground_temperature() + 273.15f;
#if HAL_CPU_CLASS <= HAL_CPU_CLASS_16
// on slower CPUs use a less exact, but faster, calculation
float scaling = base_pressure / pressure;
ret = logf(scaling) * temp * 29.271267f;
#else
// on faster CPUs use a more exact calculation
float scaling = pressure / base_pressure;
// This is an exact calculation that is within +-2.5m of the standard atmosphere tables
// in the troposphere (up to 11,000 m amsl).
ret = 153.8462f * temp * (1.0f - expf(0.190259f * logf(scaling)));
#endif
return ret;
}
