#!/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 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; }