X-Git-Url: https://git.gag.com/?p=fw%2Faltos;a=blobdiff_plain;f=src%2Futil%2Fmake-altitude-pa;fp=src%2Futil%2Fmake-altitude-pa;h=190b36fcf4a92bee0780255ca7747314c709c004;hp=0000000000000000000000000000000000000000;hb=5d8b9d524d6424ff98dcc4155fe8b8bd892b6d8f;hpb=1541fc0bde71f503b1ae5757497e9e1e6d023111 diff --git a/src/util/make-altitude-pa b/src/util/make-altitude-pa new file mode 100644 index 00000000..190b36fc --- /dev/null +++ b/src/util/make-altitude-pa @@ -0,0 +1,275 @@ +#!/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; +} + +real feet_to_meters(real feet) +{ + return feet * (12 * 2.54 / 100); +} + +real meters_to_feet(real meters) +{ + return meters / (12 * 2.54 / 100); +} + +/* + * Values for our MS5607 + * + * From the data sheet: + * + * Pressure range: 10-1200 mbar (1000 - 120000 Pa) + * + * Pressure data is reported in units of Pa + */ + +typedef struct { + real m, b; + int m_i, b_i; +} line_t; + +line_t best_fit(real[] values, int first, int last) { + real sum_x = 0, sum_x2 = 0, sum_y = 0, sum_xy = 0; + int n = last - first + 1; + real m, b; + int m_i, b_i; + + for (int i = first; i <= last; i++) { + sum_x += i; + sum_x2 += i**2; + sum_y += values[i]; + sum_xy += values[i] * i; + } + m = (n*sum_xy - sum_y*sum_x) / (n*sum_x2 - sum_x**2); + b = sum_y/n - m*(sum_x/n); + return (line_t) { m = m, b = b }; +} + +real min_Pa = 0; +real max_Pa = 120000; + +/* Target is an array of < 2000 entries */ +int pa_sample_shift = 3; +int pa_part_shift = 3; + +int num_part = ceil(max_Pa / (2 ** (pa_part_shift + pa_sample_shift))); + +int num_samples = num_part << pa_part_shift; + +real sample_to_Pa(int sample) = sample << pa_sample_shift; + +real sample_to_altitude(int sample) = pressure_to_altitude(sample_to_Pa(sample)); + +int part_to_sample(int part) = part << pa_part_shift; + +real[num_samples] alt = { [n] = sample_to_altitude(n) }; + +int seg_len = 1 << pa_part_shift; + +line_t [num_part] fit = { + [n] = best_fit(alt, n * seg_len, n * seg_len + seg_len - 1) +}; + +int[num_samples/seg_len + 1] alt_part; + +alt_part[0] = floor (fit[0].b + 0.5); +alt_part[dim(fit)] = floor(fit[dim(fit)-1].m * dim(fit) * seg_len + fit[dim(fit)-1].b + 0.5); + +for (int i = 0; i < dim(fit) - 1; i++) { + real here, there; + here = fit[i].m * (i+1) * seg_len + fit[i].b; + there = fit[i+1].m * (i+1) * seg_len + fit[i+1].b; + alt_part[i+1] = floor ((here + there) / 2 + 0.5); +} + +real sample_to_fit_altitude(int sample) { + int sub = sample // seg_len; + int off = sample % seg_len; + line_t l = fit[sub]; + real r_v; + real i_v; + + r_v = sample * l.m + l.b; + i_v = (alt_part[sub] * (seg_len - off) + alt_part[sub+1] * off) / seg_len; + return i_v; +} + +real max_error = 0; +int max_error_sample = 0; +real total_error = 0; + +for (int sample = 0; sample < num_samples; sample++) { + real Pa = sample_to_Pa(sample); + real meters = pressure_to_altitude(Pa); + + real meters_approx = sample_to_fit_altitude(sample); + real error = abs(meters - meters_approx); + + total_error += error; + if (error > max_error) { + max_error = error; + max_error_sample = sample; + } +# printf (" %7d, /* %6.2f kPa %5d sample approx %d */\n", +# floor (meters + 0.5), Pa / 1000, sample, floor(sample_to_fit_altitude(sample) + 0.5)); +} + +printf ("/*max error %f at %7.3f%%. Average error %f*/\n", max_error, max_error_sample / (num_samples - 1) * 100, total_error / num_samples); + +printf ("#define NALT %d\n", dim(alt_part)); +printf ("#define ALT_SHIFT %d\n", pa_part_shift + pa_sample_shift); + +for (int part = 0; part < dim(alt_part); part++) { + real kPa = sample_to_Pa(part_to_sample(part)) / 1000; + printf ("%9d, /* %6.2f kPa */\n", + alt_part[part], kPa); +}