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/****************************************************************************
*
* Copyright (C) 2012-2013 PX4 Development Team. All rights reserved.
* Author: Lorenz Meier <lm@inf.ethz.ch>
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
* 3. Neither the name PX4 nor the names of its contributors may be
* used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
* ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*
****************************************************************************/
/**
* @file airspeed.c
* Airspeed estimation
*
* @author Lorenz Meier <lm@inf.ethz.ch>
*
*/
#include <stdio.h>
#include <math.h>
#include <geo/geo.h>
#include "airspeed.h"
/**
* Calculate indicated airspeed.
*
* Note that the indicated airspeed is not the true airspeed because it
* lacks the air density compensation. Use the calc_true_airspeed functions to get
* the true airspeed.
*
* @param differential_pressure total_ pressure - static pressure
* @return indicated airspeed in m/s
*/
float calc_indicated_airspeed(float differential_pressure)
{
if (differential_pressure > 0.0f) {
return sqrtf((2.0f*differential_pressure) / CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C);
} else {
return -sqrtf((2.0f*fabsf(differential_pressure)) / CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C);
}
}
/**
* Calculate true airspeed from indicated airspeed.
*
* Note that the true airspeed is NOT the groundspeed, because of the effects of wind
*
* @param speed_indicated current indicated airspeed
* @param pressure_ambient pressure at the side of the tube/airplane
* @param temperature_celsius air temperature in degrees celcius
* @return true airspeed in m/s
*/
float calc_true_airspeed_from_indicated(float speed_indicated, float pressure_ambient, float temperature_celsius)
{
return speed_indicated * sqrtf(CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C / get_air_density(pressure_ambient, temperature_celsius));
}
/**
* Directly calculate true airspeed
*
* Note that the true airspeed is NOT the groundspeed, because of the effects of wind
*
* @param total_pressure pressure inside the pitot/prandtl tube
* @param static_pressure pressure at the side of the tube/airplane
* @param temperature_celsius air temperature in degrees celcius
* @return true airspeed in m/s
*/
float calc_true_airspeed(float total_pressure, float static_pressure, float temperature_celsius)
{
float density = get_air_density(static_pressure, temperature_celsius);
if (density < 0.0001f || !isfinite(density)) {
density = CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C;
}
float pressure_difference = total_pressure - static_pressure;
if (pressure_difference > 0) {
return sqrtf((2.0f*(pressure_difference)) / density);
} else {
return -sqrtf((2.0f*fabsf(pressure_difference)) / density);
}
}
float get_air_density(float static_pressure, float temperature_celsius)
{
return static_pressure / (CONSTANTS_AIR_GAS_CONST * (temperature_celsius - CONSTANTS_ABSOLUTE_NULL_CELSIUS));
}
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