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Diffstat (limited to 'src/util/atmosphere.5c')
| -rw-r--r-- | src/util/atmosphere.5c | 153 |
1 files changed, 153 insertions, 0 deletions
diff --git a/src/util/atmosphere.5c b/src/util/atmosphere.5c new file mode 100644 index 00000000..9b5107f0 --- /dev/null +++ b/src/util/atmosphere.5c @@ -0,0 +1,153 @@ +#!/usr/bin/nickle -f +/* + * 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). + */ + +const real GRAVITATIONAL_ACCELERATION = -9.80665; +const real AIR_GAS_CONSTANT = 287.053; +const int NUMBER_OF_LAYERS = 7; +const real MAXIMUM_ALTITUDE = 84852; +const real MINIMUM_PRESSURE = 0.3734; +const real LAYER0_BASE_TEMPERATURE = 288.15; +const real LAYER0_BASE_PRESSURE = 101325; + +/* lapse rate and base altitude for each layer in the atmosphere */ +const real[NUMBER_OF_LAYERS] lapse_rate = { + -0.0065, 0.0, 0.001, 0.0028, 0.0, -0.0028, -0.002 +}; +const int[NUMBER_OF_LAYERS] 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 */ +real altitude_to_pressure(real altitude) { + + real base_temperature = LAYER0_BASE_TEMPERATURE; + real base_pressure = LAYER0_BASE_PRESSURE; + + real pressure; + real base; /* base for function to determine pressure */ + real exponent; /* exponent for function to determine pressure */ + int layer_number; /* identifies layer in the atmosphere */ + int 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 */ +real pressure_to_altitude(real pressure) { + + real next_base_temperature = LAYER0_BASE_TEMPERATURE; + real next_base_pressure = LAYER0_BASE_PRESSURE; + + real altitude; + real base_pressure; + real base_temperature; + real base; /* base for function to determine base pressure of next layer */ + real exponent; /* exponent for function to determine base pressure + of next layer */ + real 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; +} |