This library contains functions for computing indicated airspeed, equivalent airspeed, true airspeed, pressure altitude, density altitude, and estimating outside air temperature (OAT) and air density. This library is compatible with Arduino and with CMake build systems.

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• Contributing guide

Use the Arduino Library Manager to install this library or clone to your Arduino/libraries folder. Additionally, the Bolder Flight Systems Units library must be installed. This library is added as:

#include "airdata.h"

An example Arduino executable is located at examples/arduino/airdata_example/airdata_example.ino. Teensy 3.x, 4.x, and LC devices are used for testing under Arduino and this library should be compatible with other devices.

CMake is used to build this library, which is exported as a library target called airdata. The header is added as:

#include "airdata.h"

The library can be also be compiled stand-alone using the CMake idiom of creating a build directory and then, from within that directory issuing:

cmake ..
make

This will build the library, an example executable called airdata_example, and an executable for testing using the Google Test framework, called airdata_test. The example executable source file is located at examples/cmake/airdata_example.cc.

This library is within the namespace bfs.

Standard day sea level speed of sound (m/s), pressure (Pa), and temperature(C, K) are defined as constants SEA_LEVEL_SPEED_OF_SOUND_MPS, SEA_LEVEL_PRESSURE_PA, SEA_LEVEL_TEMPERATURE_C, and SEA_LEVEL_TEMPERATURE_K.

float a_mps = bfs::SEA_LEVEL_SPEED_OF_SOUND_MPS;
float p0_pa = bfs::SEA_LEVEL_PRESSURE_PA;
float t0_c = bfs::SEA_LEVEL_TEMPERATURE_C;
float t0_k = bfs::SEA_LEVEL_TEMPERATURE_K;

Environmental lapse rate (K/m or C/m) is defined as a constant LAPSE_RATE_KPM.

float l = bfs::LAPSE_RATE_KPM;

The gas constant (J/(kg mol)) and molecular mas (kg/mol) of air are defined as constants GAS_CONSTANT_JPKGMOL and MOLECULAR_MASS_AIR_KGPMOL.

float r = bfs::GAS_CONSTANT_JPKGMOL;
float m = bfs::MOLECULAR_MASS_AIR_KGPMOL;

float Ias_mps(const float p) Returns indicated airspeed (m/s) given differential pressure (Pa)

float ias = bfs::Ias_mps(500.0f);
std::cout << "IAS (m/s): " << ias << std::endl;  // 28.5457

float Eas_mps(const float dp, const float sp) Returns equivalent airspeed (m/s) given differential pressure (Pa) and static pressure (Pa)

float eas = bfs::Eas_mps(500.0f, 90000.0f);
std::cout << "EAS (m/s): " << eas << std::endl;  // 28.5429

float Tas_mps(const float eas, const float t) Returns true airspeed (m/s) given equivalent airspeed (m/s) and temperature (C)

float tas = bfs::Tas_mps(eas, 17);
std::cout << "TAS (m/s): " << tas << std::endl;  // 28.6418

float PressureAltitude_m(const float p) Returns pressure altitue (m) given static pressure (Pa)

float pa = bfs::PressureAltitude_m(90000);
std::cout << "Pressure Altitude (m): " << pa << std::endl;  // 988.1

float DensityAltitude_m(const float p, const float t) Returns density altitude (m) given static pressure (Pa) and temperature (C)

float da = bfs::DensityAltitude_m(90000, 10);
std::cout << "Density Altitude (m): " << da << std::endl;  // 1039.88

float Oat_c(const float t, const float agl) Returns estimated outside air temperature (C) as a function of surface temperature (C) and altitude above ground (m). Uses a standard lapse rate of 6.5 C per km.

float oat = bfs::Oat_c(15, 400);
std::cout << "OAT (C): " << oat << std::endl;  // 12.4

float AirDensity_kgpm3(const float p, const float t) Returns air density (kg/m^3) given pressure (Pa) and temperature (C)

float density = bfs::AirDensity_kgpm3(101325, 15);
std::cout << "Air Density (kg/m^3): " << density << std::endl;  // 1.225