+/*
+ * Copyright © 2012 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; version 2 of the License.
+ *
+ * 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.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
+ */
+
+exception non_hexchar(int c);
+exception file_ended();
+exception invalid_crc();
+
+int
+get_nonwhite(file f)
+{
+ int c;
+
+ for (;;) {
+ if (File::end(f))
+ raise file_ended();
+ if (!Ctype::isspace((c = File::getc(f))))
+ return c;
+ }
+}
+
+int
+get_hexc(file f)
+{
+ int c = get_nonwhite(f);
+
+ if ('0' <= c && c <= '9')
+ return c - '0';
+ if ('a' <= c && c <= 'f')
+ return c - 'a' + 10;
+ if ('A' <= c && c <= 'F')
+ return c - 'A' + 10;
+ raise non_hexchar(c);
+}
+
+int POLY = 0x8408;
+
+int
+log_crc(int crc, int byte)
+{
+ int i;
+
+ for (i = 0; i < 8; i++) {
+ if (((crc & 0x0001) ^ (byte & 0x0001)) != 0)
+ crc = (crc >> 1) ^ POLY;
+ else
+ crc = crc >> 1;
+ byte >>= 1;
+ }
+ return crc & 0xffff;
+}
+
+int file_crc;
+
+
+int
+get_hex(file f)
+{
+ int a = get_hexc(f);
+ int b = get_hexc(f);
+
+ int h = (a << 4) + b;
+
+ file_crc = log_crc(file_crc, h);
+ return h;
+}
+
+bool
+find_header(file f)
+{
+ while (!File::end(f)) {
+ if (get_nonwhite(f) == 'M' && get_nonwhite(f) == 'P')
+ return true;
+ }
+ return false;
+}
+
+int
+get_32(file f)
+{
+ int v = 0;
+ for (int i = 0; i < 4; i++) {
+ v += get_hex(f) << (i * 8);
+ }
+ return v;
+}
+
+int
+get_16(file f)
+{
+ int v = 0;
+ for (int i = 0; i < 2; i++) {
+ v += get_hex(f) << (i * 8);
+ }
+ return v;
+}
+
+int
+swap16(int i) {
+ return ((i << 8) & 0xff00) | ((i >> 8) & 0xff);
+}
+typedef struct {
+ int ground_baro;
+ int min_baro;
+ int[*] samples;
+} log_t;
+
+log_t
+get_log(file f) {
+ log_t log;
+
+ if (!find_header(f))
+ raise file_ended();
+ file_crc = 0xffff;
+ log.ground_baro = get_32(f);
+ log.min_baro = get_32(f);
+ int nsamples = get_16(f);
+ log.samples = (int[nsamples]) { [i] = get_16(f) };
+
+ int current_crc = swap16(~file_crc & 0xffff);
+ int crc = get_16(f);
+
+ if (crc != current_crc)
+ raise invalid_crc();
+ return log;
+}
+
+/*
+ * 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;
+}
+
+real feet_to_meters(real feet)
+{
+ return feet * (12 * 2.54 / 100);
+}
+
+real meters_to_feet(real meters)
+{
+ return meters / (12 * 2.54 / 100);
+}
+
+
+real time = 0;
+int sample = 0;
+real interval = 0.192;
+real ground_alt = 0;
+
+void show(int pa)
+{
+ printf ("%9.2f %9.1f %d\n", time, pressure_to_altitude(pa) - ground_alt, pa);
+ sample++;
+ time += interval;
+}
+
+int mix_in (int high, int low)
+{
+ return high - (high & 0xffff) + low;
+}
+
+bool closer (int target, int a, int b)
+{
+ return abs (target - a) < abs(target - b);
+}
+
+void
+dump_log(log_t log) {
+ int cur = log.ground_baro;
+
+ ground_alt = pressure_to_altitude(cur);
+ show(cur);
+ for (int l = 0; l < dim(log.samples); l++) {
+ int k = log.samples[l];
+ int same = mix_in(cur, k);
+ int up = mix_in(cur + 0x10000, k);
+ int down = mix_in(cur - 0x10000, k);
+
+ if (closer (cur, same, up)) {
+ if (closer (cur, same, down))
+ cur = same;
+ else
+ cur = down;
+ } else {
+ if (closer (cur, up, down))
+ cur = up;
+ else
+ cur = down;
+ }
+ show(cur);
+ }
+}
+
+
+log_t log = get_log(stdin);
+dump_log(log);
projects / fw / altos / commitdiff