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AP_Baro: expose get_altitude_difference()

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this is useful for pressure altitude calculations
This commit is contained in:
Andrew Tridgell 2014-04-11 16:34:02 +10:00
parent b3d84cec81
commit 41dd280739
2 changed files with 28 additions and 18 deletions
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40
libraries/AP_Baro/AP_Baro.cpp
View File

@ -129,13 +129,33 @@ void AP_Baro::update_calibration()
_ground_temperature.set(get_temperature());
}
// 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)
{
float ret;
#if HAL_CPU_CLASS <= HAL_CPU_CLASS_16
// on slower CPUs use a less exact, but faster, calculation
float scaling = base_pressure / pressure;
float temp = _ground_temperature + 273.15f;
ret = logf(scaling) * temp * 29.271267f;
#else
// on faster CPUs use a more exact calculation
float scaling = pressure / base_pressure;
float temp = _ground_temperature + 273.15f;
// 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;
}
// return current altitude estimate relative to time that calibrate()
// was called. Returns altitude in meters
// note that this relies on read() being called regularly to get new data
float AP_Baro::get_altitude(void)
{
float scaling, temp;
if (_ground_pressure == 0) {
// called before initialisation
return 0;
@ -146,21 +166,7 @@ float AP_Baro::get_altitude(void)
return _altitude + _alt_offset;
}
#if HAL_CPU_CLASS <= HAL_CPU_CLASS_16
// on slower CPUs use a less exact, but faster, calculation
scaling = (float)_ground_pressure / (float)get_pressure();
temp = ((float)_ground_temperature) + 273.15f;
_altitude = logf(scaling) * temp * 29.271267f;
#else
// on faster CPUs use a more exact calculation
scaling = (float)get_pressure() / (float)_ground_pressure;
temp = ((float)_ground_temperature) + 273.15f;
// This is an exact calculation that is within +-2.5m of the standard atmosphere tables
// in the troposphere (up to 11,000 m amsl).
_altitude = 153.8462f * temp * (1.0f - expf(0.190259f * logf(scaling)));
#endif
_altitude = get_altitude_difference(_ground_pressure, get_pressure());
_last_altitude_t = _last_update;

6
libraries/AP_Baro/AP_Baro.h
View File

@ -41,6 +41,10 @@ public:
// of the last calibrate() call
float get_altitude(void);
// get altitude difference in meters relative given a base
// pressure in Pascal
float get_altitude_difference(float base_pressure, float pressure);
// get scale factor required to convert equivalent to true airspeed
float get_EAS2TAS(void);
@ -67,7 +71,7 @@ public:
}
// get last time sample was taken (in ms)
uint32_t get_last_update() { return _last_update; };
uint32_t get_last_update() const { return _last_update; };
static const struct AP_Param::GroupInfo var_info[];