From 0a60b7addddc302c66556c0f25ba1e71d14cf53d Mon Sep 17 00:00:00 2001 From: Keith Packard Date: Tue, 16 Jul 2019 09:56:39 -0700 Subject: [PATCH] ao-tools/lib: Add atmosphere model Converts between pressure and altitude Signed-off-by: Keith Packard --- ao-tools/lib/Makefile.am | 2 + ao-tools/lib/ao-atmosphere.c | 175 +++++++++++++++++++++++++++++++++++ ao-tools/lib/ao-atmosphere.h | 24 +++++ 3 files changed, 201 insertions(+) create mode 100644 ao-tools/lib/ao-atmosphere.c create mode 100644 ao-tools/lib/ao-atmosphere.h diff --git a/ao-tools/lib/Makefile.am b/ao-tools/lib/Makefile.am index de6c1946..47f9d30c 100644 --- a/ao-tools/lib/Makefile.am +++ b/ao-tools/lib/Makefile.am @@ -40,6 +40,8 @@ libao_tools_a_SOURCES = \ chbevl.c \ mconf.h \ cephes.h \ + ao-atmosphere.c \ + ao-atmosphere.h \ ao-hex.c \ ao-hex.h \ ao-editaltos.c \ diff --git a/ao-tools/lib/ao-atmosphere.c b/ao-tools/lib/ao-atmosphere.c new file mode 100644 index 00000000..f11ca7f6 --- /dev/null +++ b/ao-tools/lib/ao-atmosphere.c @@ -0,0 +1,175 @@ +/* + * Copyright © 2019 Keith Packard + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation, either version 2 of the License, or + * (at your option) any later version. + * + * This program is distributed in the hope that it will be useful, but + * WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + * General Public License for more details. + */ + +#include +#include "ao-atmosphere.h" + +#define GRAVITY 9.80665 + +/* + * Pressure Sensor Model, version 1.1 + * + * written by Holly Grimes + * + * Uses the International Standard Atmosphere as described in + * "A Quick Derivation relating altitude to air pressure" (version 1.03) + * from the Portland State Aerospace Society, except that the atmosphere + * is divided into layers with each layer having a different lapse rate. + * + * Lapse rate data for each layer was obtained from Wikipedia on Sept. 1, 2007 + * at site MAXIMUM_ALTITUDE) /* FIX ME: use sensor data to improve model */ + return 0; + + /* calculate the base temperature and pressure for the atmospheric layer + associated with the inputted altitude */ + for(layer_number = 0; layer_number < NUMBER_OF_LAYERS - 1 && altitude > base_altitude[layer_number + 1]; layer_number++) { + delta_z = base_altitude[layer_number + 1] - base_altitude[layer_number]; + if (lapse_rate[layer_number] == 0.0) { + exponent = GRAVITATIONAL_ACCELERATION * delta_z + / AIR_GAS_CONSTANT / base_temperature; + base_pressure *= exp(exponent); + } + else { + base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0; + exponent = GRAVITATIONAL_ACCELERATION / + (AIR_GAS_CONSTANT * lapse_rate[layer_number]); + base_pressure *= pow(base, exponent); + } + base_temperature += delta_z * lapse_rate[layer_number]; + } + + /* calculate the pressure at the inputted altitude */ + delta_z = altitude - base_altitude[layer_number]; + if (lapse_rate[layer_number] == 0.0) { + exponent = GRAVITATIONAL_ACCELERATION * delta_z + / AIR_GAS_CONSTANT / base_temperature; + pressure = base_pressure * exp(exponent); + } + else { + base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0; + exponent = GRAVITATIONAL_ACCELERATION / + (AIR_GAS_CONSTANT * lapse_rate[layer_number]); + pressure = base_pressure * pow(base, exponent); + } + + return pressure; +} + + +/* outputs the altitude associated with the given pressure. the altitude + returned is measured with respect to the mean sea level */ +double +ao_pressure_to_altitude(double pressure) +{ + + double next_base_temperature = LAYER0_BASE_TEMPERATURE; + double next_base_pressure = LAYER0_BASE_PRESSURE; + + double altitude; + double base_pressure; + double base_temperature; + double base; /* base for function to determine base pressure of next layer */ + double exponent; /* exponent for function to determine base pressure + of next layer */ + double coefficient; + int layer_number; /* identifies layer in the atmosphere */ + int delta_z; /* difference between two altitudes */ + + if (pressure < 0) /* illegal pressure */ + return -1; + if (pressure < MINIMUM_PRESSURE) /* FIX ME: use sensor data to improve model */ + return MAXIMUM_ALTITUDE; + + /* calculate the base temperature and pressure for the atmospheric layer + associated with the inputted pressure. */ + layer_number = -1; + do { + layer_number++; + base_pressure = next_base_pressure; + base_temperature = next_base_temperature; + delta_z = base_altitude[layer_number + 1] - base_altitude[layer_number]; + if (lapse_rate[layer_number] == 0.0) { + exponent = GRAVITATIONAL_ACCELERATION * delta_z + / AIR_GAS_CONSTANT / base_temperature; + next_base_pressure *= exp(exponent); + } + else { + base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0; + exponent = GRAVITATIONAL_ACCELERATION / + (AIR_GAS_CONSTANT * lapse_rate[layer_number]); + next_base_pressure *= pow(base, exponent); + } + next_base_temperature += delta_z * lapse_rate[layer_number]; + } + while(layer_number < NUMBER_OF_LAYERS - 1 && pressure < next_base_pressure); + + /* calculate the altitude associated with the inputted pressure */ + if (lapse_rate[layer_number] == 0.0) { + coefficient = (AIR_GAS_CONSTANT / GRAVITATIONAL_ACCELERATION) + * base_temperature; + altitude = base_altitude[layer_number] + + coefficient * log(pressure / base_pressure); + } + else { + base = pressure / base_pressure; + exponent = AIR_GAS_CONSTANT * lapse_rate[layer_number] + / GRAVITATIONAL_ACCELERATION; + coefficient = base_temperature / lapse_rate[layer_number]; + altitude = base_altitude[layer_number] + + coefficient * (pow(base, exponent) - 1); + } + + return altitude; +} diff --git a/ao-tools/lib/ao-atmosphere.h b/ao-tools/lib/ao-atmosphere.h new file mode 100644 index 00000000..0a24cb02 --- /dev/null +++ b/ao-tools/lib/ao-atmosphere.h @@ -0,0 +1,24 @@ +/* + * Copyright © 2019 Keith Packard + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation, either version 2 of the License, or + * (at your option) any later version. + * + * This program is distributed in the hope that it will be useful, but + * WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + * General Public License for more details. + */ + +#ifndef _AO_ATMOSPHERE_H_ +#define _AO_ATMOSPHERE_H_ + +double +ao_altitude_to_pressure(double altitude); + +double +ao_pressure_to_altitude(double pressure); + +#endif /* _AO_ATMOSPHERE_H_ */ -- 2.30.2