ardupilot/ArduPlane/landing.pde

/// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-

/*
  landing logic
 */

/*
  update navigation for landing. Called when on landing approach or
  final flare
 */
static bool 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

    float height = height_above_target();

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

    // calculate the sink rate.
    float sink_rate;
    Vector3f vel;
    if (ahrs.get_velocity_NED(vel)) {
        sink_rate = vel.z;
    } else if (gps.status() >= AP_GPS::GPS_OK_FIX_3D && gps.have_vertical_velocity()) {
        sink_rate = gps.velocity().z;
    } else {
        sink_rate = -barometer.get_climb_rate();        
    }

    // low pass the sink rate to take some of the noise out
    auto_state.land_sink_rate = 0.8f * auto_state.land_sink_rate + 0.2f*sink_rate;
    
    /* 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
       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 (height <= g.land_flare_alt ||
        height <= -auto_state.land_sink_rate * g.land_flare_sec ||
        (!rangefinder_state.in_range && location_passed_point(current_loc, prev_WP_loc, next_WP_loc))) {

        if (!auto_state.land_complete) {
            gcs_send_text_fmt(PSTR("Flare %.1fm sink=%.2f speed=%.1f"), 
                              height, auto_state.land_sink_rate, gps.ground_speed());
        }
        auto_state.land_complete = true;

        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();
        }
    }

    /*
      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);

    /*
      we always return false as a landing mission item never
      completes - we stay on this waypoint unless the GCS commands us
      to change mission item or reset the mission
     */
    return false;
}


/*
  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
 */
static void setup_landing_glide_slope(void)
{
        Location loc = next_WP_loc;

        // project a poiunt 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);
        float land_slope = ((next_WP_loc.alt - prev_WP_loc.alt)*0.01f) / (float)wp_totalDistance;
        location_update(loc, land_bearing_cd*0.01f, land_projection);
        loc.alt += 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_distance = get_distance(current_loc, loc);
        float land_total_distance = get_distance(prev_WP_loc, loc);

        // now setup the glide slope for landing
        set_target_altitude_proportion(loc, land_distance / land_total_distance);

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