+/*
+ * Copyright © 2019 Keith Packard <keithp@keithp.com>
+ *
+ * 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 <math.h>
+#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 <http://en.wikipedia.org/wiki/International_Standard_Atmosphere
+ *
+ * Height measurements use the local tangent plane. The postive z-direction is up.
+ *
+ * All measurements are given in SI units (Kelvin, Pascal, meter, meters/second^2).
+ * The lapse rate is given in Kelvin/meter, the gas constant for air is given
+ * in Joules/(kilogram-Kelvin).
+ */
+
+#define GRAVITATIONAL_ACCELERATION (-GRAVITY)
+#define AIR_GAS_CONSTANT 287.053
+#define NUMBER_OF_LAYERS 7
+#define MAXIMUM_ALTITUDE 84852.0
+#define MINIMUM_PRESSURE 0.3734
+#define LAYER0_BASE_TEMPERATURE 288.15
+#define LAYER0_BASE_PRESSURE 101325
+
+ /* lapse rate and base altitude for each layer in the atmosphere */
+static const double lapse_rate[] = {
+ -0.0065, 0.0, 0.001, 0.0028, 0.0, -0.0028, -0.002
+};
+
+static const double base_altitude[] = {
+ 0, 11000, 20000, 32000, 47000, 51000, 71000
+};
+
+/* outputs atmospheric pressure associated with the given altitude.
+ * altitudes are measured with respect to the mean sea level
+ */
+double
+ao_altitude_to_pressure(double altitude)
+{
+ double base_temperature = LAYER0_BASE_TEMPERATURE;
+ double base_pressure = LAYER0_BASE_PRESSURE;
+
+ double pressure;
+ double base; /* base for function to determine pressure */
+ double exponent; /* exponent for function to determine pressure */
+ int layer_number; /* identifies layer in the atmosphere */
+ double delta_z; /* difference between two altitudes */
+
+ if (altitude > 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;
+}
projects / fw / altos / commitdiff