2 * Copyright © 2010 Keith Packard <keithp@keithp.com>
4 * This program is free software; you can redistribute it and/or modify
5 * it under the terms of the GNU General Public License as published by
6 * the Free Software Foundation; version 2 of the License.
8 * This program is distributed in the hope that it will be useful, but
9 * WITHOUT ANY WARRANTY; without even the implied warranty of
10 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
11 * General Public License for more details.
13 * You should have received a copy of the GNU General Public License along
14 * with this program; if not, write to the Free Software Foundation, Inc.,
15 * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
19 * Sensor data conversion functions
23 public class AltosConvert {
25 * Pressure Sensor Model, version 1.1
27 * written by Holly Grimes
29 * Uses the International Standard Atmosphere as described in
30 * "A Quick Derivation relating altitude to air pressure" (version 1.03)
31 * from the Portland State Aerospace Society, except that the atmosphere
32 * is divided into layers with each layer having a different lapse rate.
34 * Lapse rate data for each layer was obtained from Wikipedia on Sept. 1, 2007
35 * at site <http://en.wikipedia.org/wiki/International_Standard_Atmosphere
37 * Height measurements use the local tangent plane. The postive z-direction is up.
39 * All measurements are given in SI units (Kelvin, Pascal, meter, meters/second^2).
40 * The lapse rate is given in Kelvin/meter, the gas constant for air is given
41 * in Joules/(kilogram-Kelvin).
44 static final double GRAVITATIONAL_ACCELERATION = -9.80665;
45 static final double AIR_GAS_CONSTANT = 287.053;
46 static final double NUMBER_OF_LAYERS = 7;
47 static final double MAXIMUM_ALTITUDE = 84852.0;
48 static final double MINIMUM_PRESSURE = 0.3734;
49 static final double LAYER0_BASE_TEMPERATURE = 288.15;
50 static final double LAYER0_BASE_PRESSURE = 101325;
52 /* lapse rate and base altitude for each layer in the atmosphere */
53 static final double[] lapse_rate = {
54 -0.0065, 0.0, 0.001, 0.0028, 0.0, -0.0028, -0.002
57 static final int[] base_altitude = {
58 0, 11000, 20000, 32000, 47000, 51000, 71000
61 /* outputs atmospheric pressure associated with the given altitude.
62 * altitudes are measured with respect to the mean sea level
65 cc_altitude_to_pressure(double altitude)
67 double base_temperature = LAYER0_BASE_TEMPERATURE;
68 double base_pressure = LAYER0_BASE_PRESSURE;
71 double base; /* base for function to determine pressure */
72 double exponent; /* exponent for function to determine pressure */
73 int layer_number; /* identifies layer in the atmosphere */
74 double delta_z; /* difference between two altitudes */
76 if (altitude > MAXIMUM_ALTITUDE) /* FIX ME: use sensor data to improve model */
79 /* calculate the base temperature and pressure for the atmospheric layer
80 associated with the inputted altitude */
81 for(layer_number = 0; layer_number < NUMBER_OF_LAYERS - 1 && altitude > base_altitude[layer_number + 1]; layer_number++) {
82 delta_z = base_altitude[layer_number + 1] - base_altitude[layer_number];
83 if (lapse_rate[layer_number] == 0.0) {
84 exponent = GRAVITATIONAL_ACCELERATION * delta_z
85 / AIR_GAS_CONSTANT / base_temperature;
86 base_pressure *= Math.exp(exponent);
89 base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0;
90 exponent = GRAVITATIONAL_ACCELERATION /
91 (AIR_GAS_CONSTANT * lapse_rate[layer_number]);
92 base_pressure *= Math.pow(base, exponent);
94 base_temperature += delta_z * lapse_rate[layer_number];
97 /* calculate the pressure at the inputted altitude */
98 delta_z = altitude - base_altitude[layer_number];
99 if (lapse_rate[layer_number] == 0.0) {
100 exponent = GRAVITATIONAL_ACCELERATION * delta_z
101 / AIR_GAS_CONSTANT / base_temperature;
102 pressure = base_pressure * Math.exp(exponent);
105 base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0;
106 exponent = GRAVITATIONAL_ACCELERATION /
107 (AIR_GAS_CONSTANT * lapse_rate[layer_number]);
108 pressure = base_pressure * Math.pow(base, exponent);
115 /* outputs the altitude associated with the given pressure. the altitude
116 returned is measured with respect to the mean sea level */
118 cc_pressure_to_altitude(double pressure)
121 double next_base_temperature = LAYER0_BASE_TEMPERATURE;
122 double next_base_pressure = LAYER0_BASE_PRESSURE;
125 double base_pressure;
126 double base_temperature;
127 double base; /* base for function to determine base pressure of next layer */
128 double exponent; /* exponent for function to determine base pressure
131 int layer_number; /* identifies layer in the atmosphere */
132 int delta_z; /* difference between two altitudes */
134 if (pressure < 0) /* illegal pressure */
136 if (pressure < MINIMUM_PRESSURE) /* FIX ME: use sensor data to improve model */
137 return MAXIMUM_ALTITUDE;
139 /* calculate the base temperature and pressure for the atmospheric layer
140 associated with the inputted pressure. */
144 base_pressure = next_base_pressure;
145 base_temperature = next_base_temperature;
146 delta_z = base_altitude[layer_number + 1] - base_altitude[layer_number];
147 if (lapse_rate[layer_number] == 0.0) {
148 exponent = GRAVITATIONAL_ACCELERATION * delta_z
149 / AIR_GAS_CONSTANT / base_temperature;
150 next_base_pressure *= Math.exp(exponent);
153 base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0;
154 exponent = GRAVITATIONAL_ACCELERATION /
155 (AIR_GAS_CONSTANT * lapse_rate[layer_number]);
156 next_base_pressure *= Math.pow(base, exponent);
158 next_base_temperature += delta_z * lapse_rate[layer_number];
160 while(layer_number < NUMBER_OF_LAYERS - 1 && pressure < next_base_pressure);
162 /* calculate the altitude associated with the inputted pressure */
163 if (lapse_rate[layer_number] == 0.0) {
164 coefficient = (AIR_GAS_CONSTANT / GRAVITATIONAL_ACCELERATION)
166 altitude = base_altitude[layer_number]
167 + coefficient * Math.log(pressure / base_pressure);
170 base = pressure / base_pressure;
171 exponent = AIR_GAS_CONSTANT * lapse_rate[layer_number]
172 / GRAVITATIONAL_ACCELERATION;
173 coefficient = base_temperature / lapse_rate[layer_number];
174 altitude = base_altitude[layer_number]
175 + coefficient * (Math.pow(base, exponent) - 1);
182 * Values for our MP3H6115A pressure sensor
184 * From the data sheet:
186 * Pressure range: 15-115 kPa
187 * Voltage at 115kPa: 2.82
188 * Output scale: 27mV/kPa
191 * 27 mV/kPa * 2047 / 3300 counts/mV = 16.75 counts/kPa
192 * 2.82V * 2047 / 3.3 counts/V = 1749 counts/115 kPa
195 static final double counts_per_kPa = 27 * 2047 / 3300;
196 static final double counts_at_101_3kPa = 1674.0;
199 cc_barometer_to_pressure(double count)
201 return ((count / 16.0) / 2047.0 + 0.095) / 0.009 * 1000.0;
205 cc_barometer_to_altitude(double baro)
207 double Pa = cc_barometer_to_pressure(baro);
208 return cc_pressure_to_altitude(Pa);
211 static final double count_per_mss = 27.0;
214 cc_accelerometer_to_acceleration(double accel, double ground_accel)
216 return (ground_accel - accel) / count_per_mss;
219 /* Value for the CC1111 built-in temperature sensor
220 * Output voltage at 0°C = 0.755V
221 * Coefficient = 0.00247V/°C
222 * Reference voltage = 1.25V
224 * temp = ((value / 32767) * 1.25 - 0.755) / 0.00247
225 * = (value - 19791.268) / 32768 * 1.25 / 0.00247
229 cc_thermometer_to_temperature(double thermo)
231 return (thermo - 19791.268) / 32728.0 * 1.25 / 0.00247;
235 cc_battery_to_voltage(double battery)
237 return battery / 32767.0 * 5.0;
241 cc_ignitor_to_voltage(double ignite)
243 return ignite / 32767 * 15.0;