ardupilot/ArduPlane/landing.cpp

#include "Plane.h"

/*
  landing logic
 */

/*
  update navigation for landing. Called when on landing approach or
  final flare
 */
bool Plane::verify_land()
{
    // we don't 'verify' landing in the sense that it never completes,
    // so we don't verify command completion. Instead we use this to
    // adjust final landing parameters

    // when aborting a landing, mimic the verify_takeoff with steering hold. Once
    // the altitude has been reached, restart the landing sequence
    if (flight_stage == AP_SpdHgtControl::FLIGHT_LAND_ABORT) {

        throttle_suppressed = false;
        auto_state.land_complete = false;
        auto_state.land_pre_flare = false;
        nav_controller->update_heading_hold(get_bearing_cd(prev_WP_loc, next_WP_loc));

        // see if we have reached abort altitude
        if (adjusted_relative_altitude_cm() > auto_state.takeoff_altitude_rel_cm) {
            next_WP_loc = current_loc;
            mission.stop();
            bool success = restart_landing_sequence();
            mission.resume();
            if (!success) {
                // on a restart failure lets RTL or else the plane may fly away with nowhere to go!
                set_mode(RTL, MODE_REASON_MISSION_END);
            }
            // make sure to return false so it leaves the mission index alone
        }
        return false;
    }

    float height = height_above_target();

    // use rangefinder to correct if possible
    height -= rangefinder_correction();

    /* Set land_complete (which starts the flare) under 3 conditions:
       1) we are within LAND_FLARE_ALT meters of the landing altitude
       2) we are within LAND_FLARE_SEC of the landing point vertically
          by the calculated sink rate (if LAND_FLARE_SEC != 0)
       3) we have gone past the landing point and don't have
          rangefinder data (to prevent us keeping throttle on 
          after landing if we've had positive baro drift)
    */
#if RANGEFINDER_ENABLED == ENABLED
    bool rangefinder_in_range = rangefinder_state.in_range;
#else
    bool rangefinder_in_range = false;
#endif

    // flare check:
    // 1) below flare alt/sec requires approach stage check because if sec/alt are set too
    //    large, and we're on a hard turn to line up for approach, we'll prematurely flare by
    //    skipping approach phase and the extreme roll limits will make it hard to line up with runway
    // 2) passed land point and don't have an accurate AGL
    // 3) probably crashed (ensures motor gets turned off)

    bool on_approach_stage = (flight_stage == AP_SpdHgtControl::FLIGHT_LAND_APPROACH ||
                              flight_stage == AP_SpdHgtControl::FLIGHT_LAND_PREFLARE);
    bool below_flare_alt = (height <= g.land_flare_alt);
    bool below_flare_sec = (aparm.land_flare_sec > 0 && height <= auto_state.sink_rate * aparm.land_flare_sec);
    bool probably_crashed = (g.crash_detection_enable && fabsf(auto_state.sink_rate) < 0.2f && !is_flying());

    if ((on_approach_stage && below_flare_alt) ||
        (on_approach_stage && below_flare_sec && (auto_state.wp_proportion > 0.5)) ||
        (!rangefinder_in_range && auto_state.wp_proportion >= 1) ||
        probably_crashed) {

        if (!auto_state.land_complete) {
            auto_state.post_landing_stats = true;
            if (!is_flying() && (millis()-auto_state.last_flying_ms) > 3000) {
                gcs_send_text_fmt(MAV_SEVERITY_CRITICAL, "Flare crash detected: speed=%.1f", (double)gps.ground_speed());
            } else {
                gcs_send_text_fmt(MAV_SEVERITY_INFO, "Flare %.1fm sink=%.2f speed=%.1f dist=%.1f",
                                  (double)height, (double)auto_state.sink_rate,
                                  (double)gps.ground_speed(),
                                  (double)get_distance(current_loc, next_WP_loc));
            }
            auto_state.land_complete = true;
            update_flight_stage();
        }


        if (gps.ground_speed() < 3) {
            // reload any airspeed or groundspeed parameters that may have
            // been set for landing. We don't do this till ground
            // speed drops below 3.0 m/s as otherwise we will change
            // target speeds too early.
            g.airspeed_cruise_cm.load();
            g.min_gndspeed_cm.load();
            aparm.throttle_cruise.load();
        }
    } else if (!auto_state.land_complete && !auto_state.land_pre_flare && aparm.land_pre_flare_airspeed > 0) {
        bool reached_pre_flare_alt = g.land_pre_flare_alt > 0 && (height <= g.land_pre_flare_alt);
        bool reached_pre_flare_sec = g.land_pre_flare_sec > 0 && (height <= auto_state.sink_rate * g.land_pre_flare_sec);
        if (reached_pre_flare_alt || reached_pre_flare_sec) {
            auto_state.land_pre_flare = true;
            update_flight_stage();
        }
    }

    /*
      when landing we keep the L1 navigation waypoint 200m ahead. This
      prevents sudden turns if we overshoot the landing point
     */
    struct Location land_WP_loc = next_WP_loc;
	int32_t land_bearing_cd = get_bearing_cd(prev_WP_loc, next_WP_loc);
    location_update(land_WP_loc,
                    land_bearing_cd*0.01f, 
                    get_distance(prev_WP_loc, current_loc) + 200);
    nav_controller->update_waypoint(prev_WP_loc, land_WP_loc);

    // once landed and stationary, post some statistics
    // this is done before disarm_if_autoland_complete() so that it happens on the next loop after the disarm
    if (auto_state.post_landing_stats && !arming.is_armed()) {
        auto_state.post_landing_stats = false;
        gcs_send_text_fmt(MAV_SEVERITY_INFO, "Distance from LAND point=%.2fm", (double)get_distance(current_loc, next_WP_loc));
    }

    // check if we should auto-disarm after a confirmed landing
    disarm_if_autoland_complete();

    /*
      we return false as a landing mission item never completes

      we stay on this waypoint unless the GCS commands us to change
      mission item, reset the mission, command a go-around or finish
      a land_abort procedure.
     */
    return false;
}

/*
    If land_DisarmDelay is enabled (non-zero), check for a landing then auto-disarm after time expires
 */
void Plane::disarm_if_autoland_complete()
{
    if (g.land_disarm_delay > 0 && 
        auto_state.land_complete && 
        !is_flying() && 
        arming.arming_required() != AP_Arming::NO &&
        arming.is_armed()) {
        /* we have auto disarm enabled. See if enough time has passed */
        if (millis() - auto_state.last_flying_ms >= g.land_disarm_delay*1000UL) {
            if (disarm_motors()) {
                gcs_send_text(MAV_SEVERITY_INFO,"Auto disarmed");
            }
        }
    }
}

void Plane::adjust_landing_slope_for_rangefinder_bump(void)
{
#if RANGEFINDER_ENABLED == ENABLED
    // check the rangefinder correction for a large change. When found, recalculate the glide slope. This is done by
    // determining the slope from your current location to the land point then following that back up to the approach
    // altitude and moving the prev_wp to that location. From there
    float correction_delta = fabsf(rangefinder_state.last_stable_correction) - fabsf(rangefinder_state.correction);

    if (g.land_slope_recalc_shallow_threshold <= 0 ||
            fabsf(correction_delta) < g.land_slope_recalc_shallow_threshold) {
        return;
    }

    rangefinder_state.last_stable_correction = rangefinder_state.correction;

    float corrected_alt_m = (adjusted_altitude_cm() - next_WP_loc.alt)*0.01f - rangefinder_state.correction;
    float total_distance_m = get_distance(prev_WP_loc, next_WP_loc);
    float top_of_glide_slope_alt_m = total_distance_m * corrected_alt_m / auto_state.wp_distance;
    prev_WP_loc.alt = top_of_glide_slope_alt_m*100 + next_WP_loc.alt;

    // re-calculate auto_state.land_slope with updated prev_WP_loc
    setup_landing_glide_slope();
    
    if (rangefinder_state.correction >= 0) { // we're too low or object is below us
        // correction positive means we're too low so we should continue on with
        // the newly computed shallower slope instead of pitching/throttling up

    } else if (g.land_slope_recalc_steep_threshold_to_abort > 0) {
        // correction negative means we're too high and need to point down (and speed up) to re-align
        // to land on target. A large negative correction means we would have to dive down a lot and will
        // generating way too much speed that we can not bleed off in time. It is better to remember
        // the large baro altitude offset and abort the landing to come around again with the correct altitude
        // offset and "perfect" slope.

        // calculate projected slope with projected alt
        float new_slope_deg = degrees(atan(auto_state.land_slope));
        float initial_slope_deg = degrees(atan(auto_state.initial_land_slope));

        // is projected slope too steep?
        if (new_slope_deg - initial_slope_deg > g.land_slope_recalc_steep_threshold_to_abort) {
            GCS_MAVLINK::send_statustext_all(MAV_SEVERITY_INFO, "Steep landing slope (%.0fm %.1fdeg)",
                                             (double)rangefinder_state.correction, (double)(new_slope_deg - initial_slope_deg));
            GCS_MAVLINK::send_statustext_all(MAV_SEVERITY_INFO, "aborting landing!");
            auto_state.land_alt_offset = rangefinder_state.correction;
            auto_state.commanded_go_around = 1;
            g.land_slope_recalc_steep_threshold_to_abort = 0; // disable this feature so we only perform it once
        }
    }
#endif
}

/*
  a special glide slope calculation for the landing approach

  During the land approach use a linear glide slope to a point
  projected through the landing point. We don't use the landing point
  itself as that leads to discontinuities close to the landing point,
  which can lead to erratic pitch control
 */
void Plane::setup_landing_glide_slope(void)
{
        float total_distance = get_distance(prev_WP_loc, next_WP_loc);

        // If someone mistakenly puts all 0's in their LAND command then total_distance
        // will be calculated as 0 and cause a divide by 0 error below.  Lets avoid that.
        if (total_distance < 1) {
            total_distance = 1;
        }

        // height we need to sink for this WP
        float sink_height = (prev_WP_loc.alt - next_WP_loc.alt)*0.01f;

        // current ground speed
        float groundspeed = ahrs.groundspeed();
        if (groundspeed < 0.5f) {
            groundspeed = 0.5f;
        }

        // calculate time to lose the needed altitude
        float sink_time = total_distance / groundspeed;
        if (sink_time < 0.5f) {
            sink_time = 0.5f;
        }

        // find the sink rate needed for the target location
        float sink_rate = sink_height / sink_time;

        // the height we aim for is the one to give us the right flare point
        float aim_height = aparm.land_flare_sec * sink_rate;
        if (aim_height <= 0) {
            aim_height = g.land_flare_alt;
        } 
            
        // don't allow the aim height to be too far above LAND_FLARE_ALT
        if (g.land_flare_alt > 0 && aim_height > g.land_flare_alt*2) {
            aim_height = g.land_flare_alt*2;
        }

        // calculate slope to landing point
        bool is_first_calc = is_zero(auto_state.land_slope);
        auto_state.land_slope = (sink_height - aim_height) / total_distance;
        if (is_first_calc) {
            gcs_send_text_fmt(MAV_SEVERITY_INFO, "Landing glide slope %.1f degrees", (double)degrees(atanf(auto_state.land_slope)));
        }


        // time before landing that we will flare
        float flare_time = aim_height / SpdHgt_Controller->get_land_sinkrate();

        // distance to flare is based on ground speed, adjusted as we
        // get closer. This takes into account the wind
        float flare_distance = groundspeed * flare_time;
        
        // don't allow the flare before half way along the final leg
        if (flare_distance > total_distance/2) {
            flare_distance = total_distance/2;
        }

        // project a point 500 meters past the landing point, passing
        // through the landing point
        const float land_projection = 500;
        int32_t land_bearing_cd = get_bearing_cd(prev_WP_loc, next_WP_loc);

        // now calculate our aim point, which is before the landing
        // point and above it
        Location loc = next_WP_loc;
        location_update(loc, land_bearing_cd*0.01f, -flare_distance);
        loc.alt += aim_height*100;

        // calculate point along that slope 500m ahead
        location_update(loc, land_bearing_cd*0.01f, land_projection);
        loc.alt -= auto_state.land_slope * land_projection * 100;

        // setup the offset_cm for set_target_altitude_proportion()
        target_altitude.offset_cm = loc.alt - prev_WP_loc.alt;

        // calculate the proportion we are to the target
        float land_proportion = location_path_proportion(current_loc, prev_WP_loc, loc);

        // now setup the glide slope for landing
        set_target_altitude_proportion(loc, 1.0f - land_proportion);

        // stay within the range of the start and end locations in altitude
        constrain_target_altitude_location(loc, prev_WP_loc);
}

/*
     Restart a landing by first checking for a DO_LAND_START and
     jump there. Otherwise decrement waypoint so we would re-start
     from the top with same glide slope. Return true if successful.
 */
bool Plane::restart_landing_sequence()
{
    if (mission.get_current_nav_cmd().id != MAV_CMD_NAV_LAND) {
        return false;
    }

    uint16_t do_land_start_index = mission.get_landing_sequence_start();
    uint16_t prev_cmd_with_wp_index = mission.get_prev_nav_cmd_with_wp_index();
    bool success = false;
    uint16_t current_index = mission.get_current_nav_index();
    AP_Mission::Mission_Command cmd;

    if (mission.read_cmd_from_storage(current_index+1,cmd) &&
            cmd.id == MAV_CMD_NAV_CONTINUE_AND_CHANGE_ALT &&
            (cmd.p1 == 0 || cmd.p1 == 1) &&
            mission.set_current_cmd(current_index+1))
    {
        // if the next immediate command is MAV_CMD_NAV_CONTINUE_AND_CHANGE_ALT to climb, do it
        gcs_send_text_fmt(MAV_SEVERITY_NOTICE, "Restarted landing sequence. Climbing to %dm", cmd.content.location.alt/100);
        success =  true;
    }
    else if (do_land_start_index != 0 &&
            mission.set_current_cmd(do_land_start_index))
    {
        // look for a DO_LAND_START and use that index
        gcs_send_text_fmt(MAV_SEVERITY_NOTICE, "Restarted landing via DO_LAND_START: %d",do_land_start_index);
        success =  true;
    }
    else if (prev_cmd_with_wp_index != AP_MISSION_CMD_INDEX_NONE &&
               mission.set_current_cmd(prev_cmd_with_wp_index))
    {
        // if a suitable navigation waypoint was just executed, one that contains lat/lng/alt, then
        // repeat that cmd to restart the landing from the top of approach to repeat intended glide slope
        gcs_send_text_fmt(MAV_SEVERITY_NOTICE, "Restarted landing sequence at waypoint %d", prev_cmd_with_wp_index);
        success =  true;
    } else {
        gcs_send_text_fmt(MAV_SEVERITY_WARNING, "Unable to restart landing sequence");
        success =  false;
    }

    if (success) {
        // exit landing stages if we're no longer executing NAV_LAND
        update_flight_stage();
    }
    return success;
}

/* 
   find the nearest landing sequence starting point (DO_LAND_START) and
   switch to that mission item.  Returns false if no DO_LAND_START
   available.
 */
bool Plane::jump_to_landing_sequence(void) 
{
    uint16_t land_idx = mission.get_landing_sequence_start();
    if (land_idx != 0) {
        if (mission.set_current_cmd(land_idx)) {

            // in case we're in RTL
            set_mode(AUTO, MODE_REASON_UNKNOWN);

            //if the mission has ended it has to be restarted
            if (mission.state() == AP_Mission::MISSION_STOPPED) {
                mission.resume();
            }

            gcs_send_text(MAV_SEVERITY_INFO, "Landing sequence start");
            return true;
        }            
    }

    gcs_send_text(MAV_SEVERITY_WARNING, "Unable to start landing sequence");
    return false;
}

/*
  the height above field elevation that we pass to TECS
 */
float Plane::tecs_hgt_afe(void)
{
    /*
      pass the height above field elevation as the height above
      the ground when in landing, which means that TECS gets the
      rangefinder information and thus can know when the flare is
      coming.
    */
    float hgt_afe;
    if (flight_stage == AP_SpdHgtControl::FLIGHT_LAND_FINAL ||
        flight_stage == AP_SpdHgtControl::FLIGHT_LAND_PREFLARE ||
        flight_stage == AP_SpdHgtControl::FLIGHT_LAND_APPROACH) {
        hgt_afe = height_above_target();
        hgt_afe -= rangefinder_correction();
    } else {
        // when in normal flight we pass the hgt_afe as relative
        // altitude to home
        hgt_afe = relative_altitude();
    }
    return hgt_afe;
}