[fw/altos] / src / ao_flight.c

/*
 * Copyright © 2009 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.
 */

#ifndef AO_FLIGHT_TEST
#include "ao.h"
#endif

#ifndef HAS_ACCEL
#error Please define HAS_ACCEL
#endif

#ifndef HAS_GPS
#error Please define HAS_GPS
#endif

#ifndef HAS_USB
#error Please define HAS_USB
#endif

/* Main flight thread. */

__pdata enum ao_flight_state    ao_flight_state;        /* current flight state */
__pdata uint16_t                ao_flight_tick;         /* time of last data */
__pdata uint16_t                ao_flight_prev_tick;    /* time of previous data */
__xdata int16_t                 ao_ground_pres;         /* startup pressure */
__pdata uint16_t                ao_launch_tick;         /* time of launch detect */
#if HAS_ACCEL
__pdata int16_t                 ao_ground_accel;        /* startup acceleration */
#endif

/*
 * track min/max data over a long interval to detect
 * resting
 */
__pdata uint16_t                ao_interval_end;
__pdata int16_t                 ao_interval_min_height;
__pdata int16_t                 ao_interval_max_height;

__data uint8_t ao_flight_adc;
__pdata int16_t ao_raw_pres;
__xdata uint8_t ao_flight_force_idle;

#if HAS_ACCEL
__pdata int16_t ao_raw_accel, ao_raw_accel_prev;
__pdata int16_t ao_accel_2g;

/* Accelerometer calibration
 *
 * We're sampling the accelerometer through a resistor divider which
 * consists of 5k and 10k resistors. This multiplies the values by 2/3.
 * That goes into the cc1111 A/D converter, which is running at 11 bits
 * of precision with the bits in the MSB of the 16 bit value. Only positive
 * values are used, so values should range from 0-32752 for 0-3.3V. The
 * specs say we should see 40mV/g (uncalibrated), multiply by 2/3 for what
 * the A/D converter sees (26.67 mV/g). We should see 32752/3300 counts/mV,
 * for a final computation of:
 *
 * 26.67 mV/g * 32767/3300 counts/mV = 264.8 counts/g
 *
 * Zero g was measured at 16000 (we would expect 16384).
 * Note that this value is only require to tell if the
 * rocket is standing upright. Once that is determined,
 * the value of the accelerometer is averaged for 100 samples
 * to find the resting accelerometer value, which is used
 * for all further flight computations
 */

#define GRAVITY 9.80665

#define ACCEL_NOSE_UP   (ao_accel_2g >> 2)

#endif

/*
 * Barometer calibration
 *
 * We directly sample the barometer. The specs say:
 *
 * Pressure range: 15-115 kPa
 * Voltage at 115kPa: 2.82
 * Output scale: 27mV/kPa
 *
 * If we want to detect launch with the barometer, we need
 * a large enough bump to not be fooled by noise. At typical
 * launch elevations (0-2000m), a 200Pa pressure change cooresponds
 * to about a 20m elevation change. This is 5.4mV, or about 3LSB.
 * As all of our calculations are done in 16 bits, we'll actually see a change
 * of 16 times this though
 *
 * 27 mV/kPa * 32767 / 3300 counts/mV = 268.1 counts/kPa
 */

/* We also have a clock, which can be used to sanity check things in
 * case of other failures
 */

#define BOOST_TICKS_MAX AO_SEC_TO_TICKS(15)

#define to_fix16(x) ((int16_t) ((x) * 65536.0 + 0.5))
#define to_fix32(x) ((int32_t) ((x) * 65536.0 + 0.5))
#define from_fix(x)     ((x) >> 16)

#include "ao_kalman.h"

__pdata int16_t                 ao_ground_height;
__pdata int16_t                 ao_height;
__pdata int16_t                 ao_speed;
__pdata int16_t                 ao_accel;
__pdata int16_t                 ao_max_height;

static __pdata int32_t          ao_k_height;
static __pdata int32_t          ao_k_speed;
static __pdata int32_t          ao_k_accel;

#define AO_K_STEP_100           to_fix16(0.01)
#define AO_K_STEP_2_2_100       to_fix16(0.00005)

#define AO_K_STEP_10            to_fix16(0.1)
#define AO_K_STEP_2_2_10        to_fix16(0.005)

/*
 * Above this height, the baro sensor doesn't work
 */
#define AO_MAX_BARO_HEIGHT      12000

/*
 * Above this speed, baro measurements are unreliable
 */
#define AO_MAX_BARO_SPEED       200

static void
ao_kalman_predict(void)
{
#ifdef AO_FLIGHT_TEST
        if (ao_flight_tick - ao_flight_prev_tick > 5) {
                ao_k_height += ((int32_t) ao_speed * AO_K_STEP_10 +
                                (int32_t) ao_accel * AO_K_STEP_2_2_10) >> 4;
                ao_k_speed += (int32_t) ao_accel * AO_K_STEP_10;

                return;
        }
        if (ao_flight_debug) {
                printf ("predict speed %g + (%g * %g) = %g\n",
                        ao_k_speed / (65536.0 * 16.0), ao_accel / 16.0, AO_K_STEP_100 / 65536.0,
                        (ao_k_speed + (int32_t) ao_accel * AO_K_STEP_100) / (65536.0 * 16.0));
        }
#endif
        ao_k_height += ((int32_t) ao_speed * AO_K_STEP_100 +
                        (int32_t) ao_accel * AO_K_STEP_2_2_100) >> 4;
        ao_k_speed += (int32_t) ao_accel * AO_K_STEP_100;
}

static __pdata int16_t ao_error_h;
static __pdata int16_t ao_raw_alt;
static __pdata int16_t ao_raw_height;
static __pdata int16_t ao_error_h_sq_avg;

static void
ao_kalman_err_height(void)
{
        int16_t e;
        int16_t height_distrust;
#if HAS_ACCEL
        int16_t speed_distrust;
#endif

        ao_error_h = ao_raw_height - (int16_t) (ao_k_height >> 16);

        e = ao_error_h;
        if (e < 0)
                e = -e;
        if (e > 127)
                e = 127;
#if HAS_ACCEL
        ao_error_h_sq_avg -= ao_error_h_sq_avg >> 2;
        ao_error_h_sq_avg += (e * e) >> 2;
#else
        ao_error_h_sq_avg -= ao_error_h_sq_avg >> 4;
        ao_error_h_sq_avg += (e * e) >> 4;
#endif

        height_distrust = ao_raw_height - AO_MAX_BARO_HEIGHT;
#if HAS_ACCEL
        /* speed is stored * 16, but we need to ramp between 200 and 328, so
         * we want to multiply by 2. The result is a shift by 3.
         */
        speed_distrust = (ao_speed - AO_MS_TO_SPEED(AO_MAX_BARO_SPEED)) >> (4 - 1);
        if (speed_distrust <= 0)
                speed_distrust = 0;
        else if (speed_distrust > height_distrust)
                height_distrust = speed_distrust;
#endif
        if (height_distrust <= 0)
                height_distrust = 0;

        if (height_distrust) {
#ifdef AO_FLIGHT_TEST
                int     old_ao_error_h = ao_error_h;
#endif
                if (height_distrust > 0x100)
                        height_distrust = 0x100;
                ao_error_h = (int16_t) (((int32_t) ao_error_h * (0x100 - height_distrust)) >> 8);
#ifdef AO_FLIGHT_TEST
                if (ao_flight_debug) {
                        printf("over height %g over speed %g distrust: %g height: error %d -> %d\n",
                               (double) (ao_raw_height - AO_MAX_BARO_HEIGHT),
                               (ao_speed - AO_MS_TO_SPEED(AO_MAX_BARO_SPEED)) / 16.0,
                               height_distrust / 256.0,
                               old_ao_error_h, ao_error_h);
                }
#endif
        }
}

static void
ao_kalman_correct_baro(void)
{
        ao_kalman_err_height();
#ifdef AO_FLIGHT_TEST
        if (ao_flight_tick - ao_flight_prev_tick > 5) {
                ao_k_height += (int32_t) AO_BARO_K0_10 * ao_error_h;
                ao_k_speed  += (int32_t) AO_BARO_K1_10 * ao_error_h;
                ao_k_accel  += (int32_t) AO_BARO_K2_10 * ao_error_h;
                return;
        }
#endif
        ao_k_height += (int32_t) AO_BARO_K0_100 * ao_error_h;
        ao_k_speed  += (int32_t) AO_BARO_K1_100 * ao_error_h;
        ao_k_accel  += (int32_t) AO_BARO_K2_100 * ao_error_h;
}

#if HAS_ACCEL
static __pdata int16_t ao_error_a;
static __pdata int32_t ao_accel_scale;

static void
ao_kalman_err_accel(void)
{
        int32_t accel;

        accel = (ao_ground_accel - ao_raw_accel) * ao_accel_scale;

        /* Can't use ao_accel here as it is the pre-prediction value still */
        ao_error_a = (accel - ao_k_accel) >> 16;
}

static void
ao_kalman_correct_both(void)
{
        ao_kalman_err_height();
        ao_kalman_err_accel();

#ifdef AO_FLIGHT_TEST
        if (ao_flight_tick - ao_flight_prev_tick > 5) {
                if (ao_flight_debug) {
                        printf ("correct speed %g + (%g * %g) + (%g * %g) = %g\n",
                                ao_k_speed / (65536.0 * 16.0),
                                (double) ao_error_h, AO_BOTH_K10_10 / 65536.0,
                                (double) ao_error_a, AO_BOTH_K11_10 / 65536.0,
                                (ao_k_speed +
                                 (int32_t) AO_BOTH_K10_10 * ao_error_h +
                                 (int32_t) AO_BOTH_K11_10 * ao_error_a) / (65536.0 * 16.0));
                }
                ao_k_height +=
                        (int32_t) AO_BOTH_K00_10 * ao_error_h +
                        (int32_t) AO_BOTH_K01_10 * ao_error_a;
                ao_k_speed +=
                        (int32_t) AO_BOTH_K10_10 * ao_error_h +
                        (int32_t) AO_BOTH_K11_10 * ao_error_a;
                ao_k_accel +=
                        (int32_t) AO_BOTH_K20_10 * ao_error_h +
                        (int32_t) AO_BOTH_K21_10 * ao_error_a;
                return;
        }
        if (ao_flight_debug) {
                printf ("correct speed %g + (%g * %g) + (%g * %g) = %g\n",
                        ao_k_speed / (65536.0 * 16.0),
                        (double) ao_error_h, AO_BOTH_K10_100 / 65536.0,
                        (double) ao_error_a, AO_BOTH_K11_100 / 65536.0,
                        (ao_k_speed +
                         (int32_t) AO_BOTH_K10_100 * ao_error_h +
                         (int32_t) AO_BOTH_K11_100 * ao_error_a) / (65536.0 * 16.0));
        }
#endif
        ao_k_height +=
                (int32_t) AO_BOTH_K00_100 * ao_error_h +
                (int32_t) AO_BOTH_K01_100 * ao_error_a;
        ao_k_speed +=
                (int32_t) AO_BOTH_K10_100 * ao_error_h +
                (int32_t) AO_BOTH_K11_100 * ao_error_a;
        ao_k_accel +=
                (int32_t) AO_BOTH_K20_100 * ao_error_h +
                (int32_t) AO_BOTH_K21_100 * ao_error_a;
}

#ifdef FORCE_ACCEL
static void
ao_kalman_correct_accel(void)
{
        ao_kalman_err_accel();

        if (ao_flight_tick - ao_flight_prev_tick > 5) {
                ao_k_height +=(int32_t) AO_ACCEL_K0_10 * ao_error_a;
                ao_k_speed  += (int32_t) AO_ACCEL_K1_10 * ao_error_a;
                ao_k_accel  += (int32_t) AO_ACCEL_K2_10 * ao_error_a;
                return;
        }
        ao_k_height += (int32_t) AO_ACCEL_K0_100 * ao_error_a;
        ao_k_speed  += (int32_t) AO_ACCEL_K1_100 * ao_error_a;
        ao_k_accel  += (int32_t) AO_ACCEL_K2_100 * ao_error_a;
}
#endif
#endif /* HAS_ACCEL */

__xdata int32_t ao_raw_pres_sum;

#ifdef HAS_ACCEL
__xdata int32_t ao_raw_accel_sum;
#endif

/* Landing is detected by getting constant readings from both pressure and accelerometer
 * for a fairly long time (AO_INTERVAL_TICKS)
 */
#define AO_INTERVAL_TICKS       AO_SEC_TO_TICKS(5)

#define abs(a)  ((a) < 0 ? -(a) : (a))

void
ao_flight(void)
{
        __pdata static uint16_t nsamples = 0;

        ao_flight_adc = ao_adc_head;
        ao_raw_pres = 0;
#if HAS_ACCEL
        ao_raw_accel_prev = 0;
        ao_raw_accel = 0;
#endif
        ao_flight_tick = 0;
        for (;;) {
                ao_wakeup(DATA_TO_XDATA(&ao_flight_adc));
                ao_sleep(DATA_TO_XDATA(&ao_adc_head));
                while (ao_flight_adc != ao_adc_head) {
                        __xdata struct ao_adc *ao_adc;
                        ao_flight_prev_tick = ao_flight_tick;

                        /* Capture a sample */
                        ao_adc = &ao_adc_ring[ao_flight_adc];
                        ao_flight_tick = ao_adc->tick;
                        ao_raw_pres = ao_adc->pres;
                        ao_raw_alt = ao_pres_to_altitude(ao_raw_pres);
                        ao_raw_height = ao_raw_alt - ao_ground_height;
#if HAS_ACCEL
                        ao_raw_accel = ao_adc->accel;
#if HAS_ACCEL_REF
                        /*
                         * Ok, the math here is a bit tricky.
                         *
                         * ao_raw_accel:  ADC output for acceleration
                         * ao_accel_ref:  ADC output for the 5V reference.
                         * ao_cook_accel: Corrected acceleration value
                         * Vcc:           3.3V supply to the CC1111
                         * Vac:           5V supply to the accelerometer
                         * accel:         input voltage to accelerometer ADC pin
                         * ref:           input voltage to 5V reference ADC pin
                         *
                         *
                         * Measured acceleration is ratiometric to Vcc:
                         *
                         *     ao_raw_accel   accel
                         *     ------------ = -----
                         *        32767        Vcc
                         *
                         * Measured 5v reference is also ratiometric to Vcc:
                         *
                         *     ao_accel_ref    ref
                         *     ------------ = -----
                         *        32767        Vcc
                         *
                         *
                         *      ao_accel_ref = 32767 * (ref / Vcc)
                         *
                         * Acceleration is measured ratiometric to the 5V supply,
                         * so what we want is:
                         *
                         *      ao_cook_accel    accel
                         *      ------------- =  -----
                         *          32767         ref
                         *
                         *
                         *                      accel    Vcc
                         *                    = ----- *  ---
                         *                       Vcc     ref
                         *
                         *                      ao_raw_accel       32767
                         *                    = ------------ *  ------------
                         *                         32737        ao_accel_ref
                         *
                         * Multiply through by 32767:
                         *
                         *                      ao_raw_accel * 32767
                         *      ao_cook_accel = --------------------
                         *                          ao_accel_ref
                         *
                         * Now, the tricky part. Getting this to compile efficiently
                         * and keeping all of the values in-range.
                         *
                         * First off, we need to use a shift of 16 instead of * 32767 as SDCC
                         * does the obvious optimizations for byte-granularity shifts:
                         *
                         *      ao_cook_accel = (ao_raw_accel << 16) / ao_accel_ref
                         *
                         * Next, lets check our input ranges:
                         *
                         *      0 <= ao_raw_accel <= 0x7fff             (singled ended ADC conversion)
                         *      0x7000 <= ao_accel_ref <= 0x7fff        (the 5V ref value is close to 0x7fff)
                         *
                         * Plugging in our input ranges, we get an output range of 0 - 0x12490,
                         * which is 17 bits. That won't work. If we take the accel ref and shift
                         * by a bit, we'll change its range:
                         *
                         *      0xe000 <= ao_accel_ref<<1 <= 0xfffe
                         *
                         *      ao_cook_accel = (ao_raw_accel << 16) / (ao_accel_ref << 1)
                         *
                         * Now the output range is 0 - 0x9248, which nicely fits in 16 bits. It
                         * is, however, one bit too large for our signed computations. So, we
                         * take the result and shift that by a bit:
                         *
                         *      ao_cook_accel = ((ao_raw_accel << 16) / (ao_accel_ref << 1)) >> 1
                         *
                         * This finally creates an output range of 0 - 0x4924. As the ADC only
                         * provides 11 bits of data, we haven't actually lost any precision,
                         * just dropped a bit of noise off the low end.
                         */
                        ao_raw_accel = (uint16_t) ((((uint32_t) ao_raw_accel << 16) / (ao_accel_ref[ao_flight_adc] << 1))) >> 1;
                        ao_adc->accel = ao_raw_accel;
#endif
#endif

                        if (ao_flight_state > ao_flight_idle) {
                                ao_kalman_predict();
#if HAS_ACCEL
                                if (ao_flight_state <= ao_flight_coast) {
#ifdef FORCE_ACCEL
                                        ao_kalman_correct_accel();
#else
                                        ao_kalman_correct_both();
#endif
                                } else
#endif
                                        ao_kalman_correct_baro();
                                ao_height = from_fix(ao_k_height);
                                ao_speed = from_fix(ao_k_speed);
                                ao_accel = from_fix(ao_k_accel);
                                if (ao_height > ao_max_height)
                                        ao_max_height = ao_height;
                        }
                        ao_flight_adc = ao_adc_ring_next(ao_flight_adc);
                }

                switch (ao_flight_state) {
                case ao_flight_startup:

                        /* startup state:
                         *
                         * Collect 512 samples of acceleration and pressure
                         * data and average them to find the resting values
                         */
                        if (nsamples < 512) {
#if HAS_ACCEL
                                ao_raw_accel_sum += ao_raw_accel;
#endif
                                ao_raw_pres_sum += ao_raw_pres;
                                ++nsamples;
                                continue;
                        }
                        ao_config_get();
#if HAS_ACCEL
                        ao_ground_accel = ao_raw_accel_sum >> 9;
                        ao_accel_2g = ao_config.accel_minus_g - ao_config.accel_plus_g;
                        ao_accel_scale = to_fix32(GRAVITY * 2 * 16) / ao_accel_2g;
#endif
                        ao_ground_pres = ao_raw_pres_sum >> 9;
                        ao_ground_height = ao_pres_to_altitude(ao_ground_pres);

                        /* Check to see what mode we should go to.
                         *  - Invalid mode if accel cal appears to be out
                         *  - pad mode if we're upright,
                         *  - idle mode otherwise
                         */
#if HAS_ACCEL
                        if (ao_config.accel_plus_g == 0 ||
                            ao_config.accel_minus_g == 0 ||
                            ao_ground_accel < ao_config.accel_plus_g - ACCEL_NOSE_UP ||
                            ao_ground_accel > ao_config.accel_minus_g + ACCEL_NOSE_UP)
                        {
                                /* Detected an accel value outside -1.5g to 1.5g
                                 * (or uncalibrated values), so we go into invalid mode
                                 */
                                ao_flight_state = ao_flight_invalid;

                        } else
#endif
                                if (!ao_flight_force_idle
#if HAS_ACCEL
                                    && ao_ground_accel < ao_config.accel_plus_g + ACCEL_NOSE_UP
#endif
                                        )
                        {
                                /* Set pad mode - we can fly! */
                                ao_flight_state = ao_flight_pad;
#if HAS_USB
                                /* Disable the USB controller in flight mode
                                 * to save power
                                 */
                                ao_usb_disable();
#endif

                                /* Disable packet mode in pad state */
                                ao_packet_slave_stop();

                                /* Turn on telemetry system */
                                ao_rdf_set(1);
                                ao_telemetry_set_interval(AO_TELEMETRY_INTERVAL_PAD);

                                /* signal successful initialization by turning off the LED */
                                ao_led_off(AO_LED_RED);
                        } else {
                                /* Set idle mode */
                                ao_flight_state = ao_flight_idle;
 
                                /* signal successful initialization by turning off the LED */
                                ao_led_off(AO_LED_RED);
                        }
                        /* wakeup threads due to state change */
                        ao_wakeup(DATA_TO_XDATA(&ao_flight_state));

                        break;
                case ao_flight_pad:

                        /* pad to boost:
                         *
                         * barometer: > 20m vertical motion
                         *             OR
                         * accelerometer: > 2g AND velocity > 5m/s
                         *
                         * The accelerometer should always detect motion before
                         * the barometer, but we use both to make sure this
                         * transition is detected. If the device
                         * doesn't have an accelerometer, then ignore the
                         * speed and acceleration as they are quite noisy
                         * on the pad.
                         */
                        if (ao_height > AO_M_TO_HEIGHT(20)
#if HAS_ACCEL
                            || (ao_accel > AO_MSS_TO_ACCEL(20) &&
                                ao_speed > AO_MS_TO_SPEED(5))
#endif
                                )
                        {
                                ao_flight_state = ao_flight_boost;
                                ao_launch_tick = ao_flight_tick;

                                /* start logging data */
                                ao_log_start();

                                /* Increase telemetry rate */
                                ao_telemetry_set_interval(AO_TELEMETRY_INTERVAL_FLIGHT);

                                /* disable RDF beacon */
                                ao_rdf_set(0);

#if HAS_GPS
                                /* Record current GPS position by waking up GPS log tasks */
                                ao_wakeup(&ao_gps_data);
                                ao_wakeup(&ao_gps_tracking_data);
#endif

                                ao_wakeup(DATA_TO_XDATA(&ao_flight_state));
                                break;
                        }
                        break;
                case ao_flight_boost:

                        /* boost to fast:
                         *
                         * accelerometer: start to fall at > 1/4 G
                         *              OR
                         * time: boost for more than 15 seconds
                         *
                         * Detects motor burn out by the switch from acceleration to
                         * deceleration, or by waiting until the maximum burn duration
                         * (15 seconds) has past.
                         */
                        if ((ao_accel < AO_MSS_TO_ACCEL(-2.5) && ao_height > AO_M_TO_HEIGHT(100)) ||
                            (int16_t) (ao_flight_tick - ao_launch_tick) > BOOST_TICKS_MAX)
                        {
#if HAS_ACCEL
                                ao_flight_state = ao_flight_fast;
#else
                                ao_flight_state = ao_flight_coast;
#endif
                                ao_wakeup(DATA_TO_XDATA(&ao_flight_state));
                                break;
                        }
                        break;
#if HAS_ACCEL
                case ao_flight_fast:
                        /*
                         * This is essentially the same as coast,
                         * but the barometer is being ignored as
                         * it may be unreliable.
                         */
                        if (ao_speed < AO_MS_TO_SPEED(AO_MAX_BARO_SPEED))
                        {
                                ao_flight_state = ao_flight_coast;
                                ao_wakeup(DATA_TO_XDATA(&ao_flight_state));
                                break;
                        }
                        break;
#endif
                case ao_flight_coast:

                        /* apogee detect: coast to drogue deploy:
                         *
                         * speed: < 0
                         *
                         * Also make sure the model altitude is tracking
                         * the measured altitude reasonably closely; otherwise
                         * we're probably transsonic.
                         */
                        if (ao_speed < 0
#if !HAS_ACCEL
                            && (ao_raw_alt >= AO_MAX_BARO_HEIGHT || ao_error_h_sq_avg < 100)
#endif
                                )
                        {
                                /* ignite the drogue charge */
                                ao_ignite(ao_igniter_drogue);

                                /* slow down the telemetry system */
                                ao_telemetry_set_interval(AO_TELEMETRY_INTERVAL_RECOVER);

                                /*
                                 * Start recording min/max height
                                 * to figure out when the rocket has landed
                                 */

                                /* initialize interval values */
                                ao_interval_end = ao_flight_tick + AO_INTERVAL_TICKS;

                                ao_interval_min_height = ao_interval_max_height = ao_height;

                                /* and enter drogue state */
                                ao_flight_state = ao_flight_drogue;
                                ao_wakeup(DATA_TO_XDATA(&ao_flight_state));
                        }

                        break;
                case ao_flight_drogue:

                        /* drogue to main deploy:
                         *
                         * barometer: reach main deploy altitude
                         *
                         * Would like to use the accelerometer for this test, but
                         * the orientation of the flight computer is unknown after
                         * drogue deploy, so we ignore it. Could also detect
                         * high descent rate using the pressure sensor to
                         * recognize drogue deploy failure and eject the main
                         * at that point. Perhaps also use the drogue sense lines
                         * to notice continutity?
                         */
                        if (ao_height <= ao_config.main_deploy)
                        {
                                ao_ignite(ao_igniter_main);
                                ao_flight_state = ao_flight_main;
                                ao_wakeup(DATA_TO_XDATA(&ao_flight_state));
                        }

                        /* fall through... */
                case ao_flight_main:

                        /* drogue/main to land:
                         *
                         * barometer: altitude stable and within 1000m of the launch altitude
                         */

                        if (ao_height < ao_interval_min_height)
                                ao_interval_min_height = ao_height;
                        if (ao_height > ao_interval_max_height)
                                ao_interval_max_height = ao_height;

                        if ((int16_t) (ao_flight_tick - ao_interval_end) >= 0) {
                                if (ao_height < AO_M_TO_HEIGHT(1000) &&
                                    ao_interval_max_height - ao_interval_min_height < AO_M_TO_HEIGHT(5))
                                {
                                        ao_flight_state = ao_flight_landed;

                                        /* turn off the ADC capture */
                                        ao_timer_set_adc_interval(0);
                                        /* Enable RDF beacon */
                                        ao_rdf_set(1);

                                        ao_wakeup(DATA_TO_XDATA(&ao_flight_state));
                                }
                                ao_interval_min_height = ao_interval_max_height = ao_height;
                                ao_interval_end = ao_flight_tick + AO_INTERVAL_TICKS;
                        }
                        break;
                case ao_flight_landed:
                        break;
                }
        }
}

static __xdata struct ao_task   flight_task;

void
ao_flight_init(void)
{
        ao_flight_state = ao_flight_startup;
        ao_add_task(&flight_task, ao_flight, "flight");
}