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+#!/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;
+}
+
+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 MP3H6115A pressure sensor
+ *
+ * From the data sheet:
+ *
+ * Pressure range: 15-115 kPa
+ * Voltage at 115kPa: 2.82
+ * Output scale: 27mV/kPa
+ *
+ *
+ * 27 mV/kPa * 2047 / 3300 counts/mV = 16.75 counts/kPa
+ * 2.82V * 2047 / 3.3 counts/V = 1749 counts/115 kPa
+ */
+
+real counts_per_kPa = 27 * 2047 / 3300;
+real counts_at_101_3kPa = 1674;
+
+real count_to_kPa(real count)
+{
+	return (count / 2047 + 0.095) / 0.009;
+}
+
+for (real count = 0; count <= 2047; count++) {
+	real	kPa = count_to_kPa(count);
+	real	meters = pressure_to_altitude(kPa * 1000);
+	printf ("	%d,	/* %6.2g kPa %d count */\n",
+		floor (meters + 0.5), kPa, count);
+}