/*
   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, either version 3 of the License, or
   (at your option) any later version.

   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, see <http://www.gnu.org/licenses/>.
 */

/*
 *   AP_Landing_Deepstall.cpp - Landing logic handler for ArduPlane for deepstall landings
 */

#include "AP_Landing.h"
#include <GCS_MAVLink/GCS.h>
#include <AP_HAL/AP_HAL.h>
#include <SRV_Channel/SRV_Channel.h>

// table of user settable parameters for deepstall
const AP_Param::GroupInfo AP_Landing_Deepstall::var_info[] = {

    // @Param: V_FWD
    // @DisplayName: Deepstall forward velocity
    // @Description: The forward velocity of the aircraft while stalled
    // @Range: 0 20
    // @Units: m/s
    // @User: Advanced
    AP_GROUPINFO("V_FWD", 1, AP_Landing_Deepstall, forward_speed, 1),

    // @Param: SLOPE_A
    // @DisplayName: Deepstall slope a
    // @Description: The a component of distance = a*wind + b
    // @User: Advanced
    AP_GROUPINFO("SLOPE_A", 2, AP_Landing_Deepstall, slope_a, 1),

    // @Param: SLOPE_B
    // @DisplayName: Deepstall slope b
    // @Description: The a component of distance = a*wind + b
    // @User: Advanced
    AP_GROUPINFO("SLOPE_B", 3, AP_Landing_Deepstall, slope_b, 1),

    // @Param: APP_EXT
    // @DisplayName: Deepstall approach extension
    // @Description: The forward velocity of the aircraft while stalled
    // @Range: 10 200
    // @Units: m
    // @User: Advanced
    AP_GROUPINFO("APP_EXT", 4, AP_Landing_Deepstall, approach_extension, 50),

    // @Param: V_DWN
    // @DisplayName: Deepstall velocity down
    // @Description: The downward velocity of the aircraft while stalled
    // @Range: 0 20
    // @Units: m/s
    // @User: Advanced
    AP_GROUPINFO("V_DWN", 5, AP_Landing_Deepstall, down_speed, 2),

    // @Param: SLEW_SPD
    // @DisplayName: Deepstall slew speed
    // @Description: The speed at which the elevator slews to deepstall
    // @Range: 0 2
    // @Units: s
    // @User: Advanced
    AP_GROUPINFO("SLEW_SPD", 6, AP_Landing_Deepstall, slew_speed, 0.5),

    // @Param: ELEV_PWM
    // @DisplayName: Deepstall elevator PWM
    // @Description: The PWM value in microseconds for the elevator at full deflection in deepstall
    // @Range: 900 2100
    // @Units: PWM
    // @User: Advanced
    AP_GROUPINFO("ELEV_PWM", 7, AP_Landing_Deepstall, elevator_pwm, 1500),

    // @Param: ARSP_MAX
    // @DisplayName: Deepstall enabled airspeed
    // @Description: The maximum aispeed where the deepstall steering controller is allowed to have control
    // @Range: 5 20
    // @Units: m/s
    // @User: Advanced
    AP_GROUPINFO("ARSP_MAX", 8, AP_Landing_Deepstall, handoff_airspeed, 15.0),

    // @Param: ARSP_MIN
    // @DisplayName: Deepstall minimum derating airspeed
    // @Description: Deepstall lowest airspeed where the deepstall controller isn't allowed full control
    // @Range: 5 20
    // @Units: m/s
    // @User: Advanced
    AP_GROUPINFO("ARSP_MIN", 9, AP_Landing_Deepstall, handoff_lower_limit_airspeed, 10.0),

    // @Param: L1
    // @DisplayName: Deepstall L1 period
    // @Description: Deepstall L1 navigational controller period
    // @Range: 5 50
    // @Units: m
    // @User: Advanced
    AP_GROUPINFO("L1", 10, AP_Landing_Deepstall, L1_period, 30.0),

    // @Param: L1_I
    // @DisplayName: Deepstall L1 I gain
    // @Description: Deepstall L1 integratior gain
    // @Range: 0 1
    // @User: Advanced
    AP_GROUPINFO("L1_I", 11, AP_Landing_Deepstall, L1_i, 0),

    // @Param: YAW_LIM
    // @DisplayName: Deepstall yaw rate limit
    // @Description: The yaw rate limit while navigating in deepstall
    // @Range: 0 90
    // @Units degrees per second
    // @User: Advanced
    AP_GROUPINFO("YAW_LIM", 12, AP_Landing_Deepstall, yaw_rate_limit, 10),

    // @Param: L1_TCON
    // @DisplayName: Deepstall L1 time constant
    // @Description: Time constant for deepstall L1 control
    // @Range: 0 1
    // @Units seconds
    // @User: Advanced
    AP_GROUPINFO("L1_TCON", 13, AP_Landing_Deepstall, time_constant, 0.4),

    // @Group: DS_
    // @Path: ../PID/PID.cpp
    AP_SUBGROUPINFO(ds_PID, "", 14, AP_Landing_Deepstall, PID),

    // @Param: ABORTALT
    // @DisplayName: Deepstall minimum abort altitude
    // @Description: The minimum altitude which the aircraft must be above to abort a deepstall landing
    // @Range: 0 50
    // @Units meters
    // @User: Advanced
    AP_GROUPINFO("ABORTALT", 15, AP_Landing_Deepstall, min_abort_alt, 0.0f),

    // @Param: AIL_SCL
    // @DisplayName: Aileron landing gain scalaing
    // @Description: A scalar to reduce or increase the aileron control
    // @Range: 0 2.0
    // @User: Advanced
    AP_GROUPINFO("AIL_SCL", 16, AP_Landing_Deepstall, aileron_scalar, 1.0f),

    AP_GROUPEND
};


// if DEBUG_PRINTS is defined statustexts will be sent to the GCS for debug purposes
// #define DEBUG_PRINTS

void AP_Landing_Deepstall::do_land(const AP_Mission::Mission_Command& cmd, const float relative_altitude)
{
    stage = DEEPSTALL_STAGE_FLY_TO_LANDING;
    ds_PID.reset();
    L1_xtrack_i = 0.0f;
    hold_level = false; // come out of yaw lock

    // load the landing point in, the rest of path building is deferred for a better wind estimate
    memcpy(&landing_point, &cmd.content.location, sizeof(Location));

    if (!landing_point.relative_alt && !landing_point.terrain_alt) {
        approach_alt_offset = cmd.p1;
        landing_point.alt += approach_alt_offset * 100;
    } else {
        approach_alt_offset = 0.0f;
    }
}

// currently identical to the slope aborts
void AP_Landing_Deepstall::verify_abort_landing(const Location &prev_WP_loc, Location &next_WP_loc, bool &throttle_suppressed)
{
    // when aborting a landing, mimic the verify_takeoff with steering hold. Once
    // the altitude has been reached, restart the landing sequence
    throttle_suppressed = false;
    landing.nav_controller->update_heading_hold(get_bearing_cd(prev_WP_loc, next_WP_loc));
}


/*
  update navigation for landing
 */
bool AP_Landing_Deepstall::verify_land(const Location &prev_WP_loc, Location &next_WP_loc, const Location &current_loc,
        const float height, const float sink_rate, const float wp_proportion, const uint32_t last_flying_ms,
        const bool is_armed, const bool is_flying, const bool rangefinder_state_in_range)
{
    switch (stage) {
    case DEEPSTALL_STAGE_FLY_TO_LANDING:
        if (get_distance(current_loc, landing_point) > fabsf(2 * landing.aparm.loiter_radius)) {
            landing.nav_controller->update_waypoint(current_loc, landing_point);
            return false;
        }
        stage = DEEPSTALL_STAGE_ESTIMATE_WIND;
        loiter_sum_cd = 0; // reset the loiter counter
        FALLTHROUGH;
    case DEEPSTALL_STAGE_ESTIMATE_WIND:
        {
        landing.nav_controller->update_loiter(landing_point, landing.aparm.loiter_radius, landing_point.loiter_ccw ? -1 : 1);
        if (!landing.nav_controller->reached_loiter_target() || (fabsf(height - approach_alt_offset) > DEEPSTALL_LOITER_ALT_TOLERANCE)) {
            // wait until the altitude is correct before considering a breakout
            return false;
        }
        // only count loiter progress when within the target altitude
        int32_t target_bearing = landing.nav_controller->target_bearing_cd();
        int32_t delta = wrap_180_cd(target_bearing - last_target_bearing);
        delta *= (landing_point.loiter_ccw ? -1 : 1);
        if (delta > 0) { // only accumulate turns in the correct direction
            loiter_sum_cd += delta;
        }
        last_target_bearing = target_bearing;
        if (loiter_sum_cd < 36000) {
            // wait until we've done at least one complete loiter at the correct altitude
            return false;
        }
        stage = DEEPSTALL_STAGE_WAIT_FOR_BREAKOUT;
        loiter_sum_cd = 0; // reset the loiter counter
        FALLTHROUGH;
        }
    case DEEPSTALL_STAGE_WAIT_FOR_BREAKOUT:
        // rebuild the approach path if we have done less then a full circle to allow it to be
        // more into the wind as the estimator continues to refine itself, and allow better
        // compensation on windy days. This is limited to a single full circle though, as on
        // a no wind day you could be in this loop forever otherwise.
        if (loiter_sum_cd < 36000) {
            build_approach_path(false);
        }
        if (!verify_breakout(current_loc, arc_entry, height - approach_alt_offset)) {
            int32_t target_bearing = landing.nav_controller->target_bearing_cd();
            int32_t delta = wrap_180_cd(target_bearing - last_target_bearing);
            if (delta > 0) { // only accumulate turns in the correct direction
                loiter_sum_cd += delta;
            }
            last_target_bearing = target_bearing;
            landing.nav_controller->update_loiter(landing_point, landing.aparm.loiter_radius, landing_point.loiter_ccw ? -1 : 1);
            return false;
        }
        stage = DEEPSTALL_STAGE_FLY_TO_ARC;
        memcpy(&breakout_location, &current_loc, sizeof(Location));
        FALLTHROUGH;
    case DEEPSTALL_STAGE_FLY_TO_ARC:
        if (get_distance(current_loc, arc_entry) > 2 * landing.aparm.loiter_radius) {
            landing.nav_controller->update_waypoint(breakout_location, arc_entry);
            return false;
        }
        stage = DEEPSTALL_STAGE_ARC;
        FALLTHROUGH;
    case DEEPSTALL_STAGE_ARC:
        {
        Vector2f groundspeed = landing.ahrs.groundspeed_vector();
        if (!landing.nav_controller->reached_loiter_target() ||
            (fabsf(wrap_180(target_heading_deg -
                            degrees(atan2f(-groundspeed.y, -groundspeed.x) + M_PI))) >= 10.0f)) {
            landing.nav_controller->update_loiter(arc, landing.aparm.loiter_radius, landing_point.loiter_ccw ? -1 : 1);
            return false;
        }
        stage = DEEPSTALL_STAGE_APPROACH;
        }
        FALLTHROUGH;
    case DEEPSTALL_STAGE_APPROACH:
        {
        Location entry_point;
        landing.nav_controller->update_waypoint(arc_exit, extended_approach);

        float height_above_target;
        if (is_zero(approach_alt_offset)) {
            landing.ahrs.get_relative_position_D_home(height_above_target);
            height_above_target = -height_above_target;
        } else {
            Location position;
            if (landing.ahrs.get_position(position)) {
                height_above_target = (position.alt - landing_point.alt + approach_alt_offset * 100) * 1e-2f;
            } else {
                height_above_target = approach_alt_offset;
            }
        }

        const float travel_distance = predict_travel_distance(landing.ahrs.wind_estimate(), height_above_target, false);

        memcpy(&entry_point, &landing_point, sizeof(Location));
        location_update(entry_point, target_heading_deg + 180.0, travel_distance);

        if (!location_passed_point(current_loc, arc_exit, entry_point)) {
            if (location_passed_point(current_loc, arc_exit, extended_approach)) {
                // this should never happen, but prevent against an indefinite fly away
                stage = DEEPSTALL_STAGE_FLY_TO_LANDING;
            }
            return false;
        }
        predict_travel_distance(landing.ahrs.wind_estimate(), height_above_target, true);
        stage = DEEPSTALL_STAGE_LAND;
        stall_entry_time = AP_HAL::millis();

        const SRV_Channel* elevator = SRV_Channels::get_channel_for(SRV_Channel::k_elevator);
        if (elevator != nullptr) {
            // take the last used elevator angle as the starting deflection
            // don't worry about bailing here if the elevator channel can't be found
            // that will be handled within override_servos
            initial_elevator_pwm = elevator->get_output_pwm();
        }
        }
        FALLTHROUGH;
    case DEEPSTALL_STAGE_LAND:
        // while in deepstall the only thing verify needs to keep the extended approach point sufficently far away
        landing.nav_controller->update_waypoint(current_loc, extended_approach);
        landing.disarm_if_autoland_complete_fn();
        return false;
    default:
        return true;
    }
}

bool AP_Landing_Deepstall::override_servos(void)
{
    if (stage != DEEPSTALL_STAGE_LAND) {
        return false;
    }

    SRV_Channel* elevator = SRV_Channels::get_channel_for(SRV_Channel::k_elevator);

    if (elevator == nullptr) {
        // deepstalls are impossible without these channels, abort the process
        gcs().send_text(MAV_SEVERITY_CRITICAL, "Deepstall: Unable to find the elevator channels");
        request_go_around();
        return false;
    }

    // calculate the progress on slewing the elevator
    float slew_progress = 1.0f;
    if (slew_speed > 0) {
        slew_progress  = (AP_HAL::millis() - stall_entry_time) / (100.0f * slew_speed);
    }

    // mix the elevator to the correct value
    elevator->set_output_pwm(linear_interpolate(initial_elevator_pwm, elevator_pwm,
                             slew_progress, 0.0f, 1.0f));

    // use the current airspeed to dictate the travel limits
    float airspeed;
    landing.ahrs.airspeed_estimate(&airspeed);

    // only allow the deepstall steering controller to run below the handoff airspeed
    if (slew_progress >= 1.0f || airspeed <= handoff_airspeed) {
        // run the steering conntroller
        float pid = update_steering();


        float travel_limit = constrain_float((handoff_airspeed - airspeed) /
                                             (handoff_airspeed - handoff_lower_limit_airspeed) *
                                             0.5f + 0.5f,
                                             0.5f, 1.0f);

        float output = constrain_float(pid, -travel_limit, travel_limit);
        SRV_Channels::set_output_scaled(SRV_Channel::k_aileron, output*4500*aileron_scalar);
        SRV_Channels::set_output_scaled(SRV_Channel::k_rudder, output*4500);
        SRV_Channels::set_output_scaled(SRV_Channel::k_throttle, 0); // this will normally be managed as part of landing,
                                                                     // but termination needs to set throttle control here
    }

    // hand off rudder control to deepstall controlled
    return true;
}

bool AP_Landing_Deepstall::request_go_around(void)
{
    float current_altitude_d;
    landing.ahrs.get_relative_position_D_home(current_altitude_d);

    if (is_zero(min_abort_alt) || -current_altitude_d > min_abort_alt) {
        landing.flags.commanded_go_around = true;
        return true;
    } else {
        return false;
    }
}

bool AP_Landing_Deepstall::is_throttle_suppressed(void) const
{
    return stage == DEEPSTALL_STAGE_LAND;
}

bool AP_Landing_Deepstall::is_flying_forward(void) const
{
    return stage != DEEPSTALL_STAGE_LAND;
}

bool AP_Landing_Deepstall::is_on_approach(void) const
{
    return stage == DEEPSTALL_STAGE_LAND;
}

bool AP_Landing_Deepstall::get_target_altitude_location(Location &location)
{
    memcpy(&location, &landing_point, sizeof(Location));
    return true;
}

int32_t AP_Landing_Deepstall::get_target_airspeed_cm(void) const
{
    if (stage == DEEPSTALL_STAGE_APPROACH ||
        stage == DEEPSTALL_STAGE_LAND) {
        return landing.pre_flare_airspeed * 100;
    } else {
        return landing.aparm.airspeed_cruise_cm;
    }
}

bool AP_Landing_Deepstall::send_deepstall_message(mavlink_channel_t chan) const
{
    CHECK_PAYLOAD_SIZE2(DEEPSTALL);
    mavlink_msg_deepstall_send(
        chan,
        landing_point.lat,
        landing_point.lng,
        stage >= DEEPSTALL_STAGE_WAIT_FOR_BREAKOUT ? arc_exit.lat : 0.0f,
        stage >= DEEPSTALL_STAGE_WAIT_FOR_BREAKOUT ? arc_exit.lng : 0.0f,
        stage >= DEEPSTALL_STAGE_WAIT_FOR_BREAKOUT ? arc_entry.lat : 0.0f,
        stage >= DEEPSTALL_STAGE_WAIT_FOR_BREAKOUT ? arc_entry.lng : 0.0f,
        landing_point.alt * 0.01,
        stage >= DEEPSTALL_STAGE_WAIT_FOR_BREAKOUT ? predicted_travel_distance : 0.0f,
        stage == DEEPSTALL_STAGE_LAND ? crosstrack_error : 0.0f,
        stage);
    return true;
}

const AP_Logger::PID_Info& AP_Landing_Deepstall::get_pid_info(void) const
{
    return ds_PID.get_pid_info();
}

void AP_Landing_Deepstall::Log(void) const {
    const AP_Logger::PID_Info& pid_info = ds_PID.get_pid_info();
    struct log_DSTL pkt = {
        LOG_PACKET_HEADER_INIT(LOG_DSTL_MSG),
        time_us          : AP_HAL::micros64(),
        stage            : (uint8_t)stage,
        target_heading   : target_heading_deg,
        target_lat       : landing_point.lat,
        target_lng       : landing_point.lng,
        target_alt       : landing_point.alt,
        crosstrack_error : (int16_t)(stage >= DEEPSTALL_STAGE_LAND ?
                             constrain_float(crosstrack_error * 1e2f, (float)INT16_MIN, (float)INT16_MAX) : 0),
        travel_distance  : (int16_t)(stage >= DEEPSTALL_STAGE_LAND ?
                             constrain_float(predicted_travel_distance * 1e2f, (float)INT16_MIN, (float)INT16_MAX) : 0),
        l1_i             : stage >= DEEPSTALL_STAGE_LAND ? L1_xtrack_i : 0.0f,
        loiter_sum_cd    : stage >= DEEPSTALL_STAGE_ESTIMATE_WIND ? loiter_sum_cd : 0,
        desired          : pid_info.desired,
        P                : pid_info.P,
        I                : pid_info.I,
        D                : pid_info.D,
    };
    AP::logger().WriteBlock(&pkt, sizeof(pkt));
}

// termination handling, expected to set the servo outputs
bool AP_Landing_Deepstall::terminate(void) {
    // if we were not in a deepstall, mark us as being in one
    if(!landing.flags.in_progress || stage != DEEPSTALL_STAGE_LAND) {
        stall_entry_time = AP_HAL::millis();
        ds_PID.reset();
        L1_xtrack_i = 0.0f;
        landing.flags.in_progress = true;
        stage = DEEPSTALL_STAGE_LAND;

        if(landing.ahrs.get_position(landing_point)) {
            build_approach_path(true);
        } else {
            hold_level = true;
        }
    }

    // set the servo ouptuts, this can fail, so this is the important return value for the AFS
    return override_servos();
}

void AP_Landing_Deepstall::build_approach_path(bool use_current_heading)
{
    float loiter_radius = landing.nav_controller->loiter_radius(landing.aparm.loiter_radius);

    Vector3f wind = landing.ahrs.wind_estimate();
    // TODO: Support a user defined approach heading
    target_heading_deg = use_current_heading ? landing.ahrs.yaw_sensor * 1e-2 : (degrees(atan2f(-wind.y, -wind.x)));

    memcpy(&extended_approach, &landing_point, sizeof(Location));
    memcpy(&arc_exit, &landing_point, sizeof(Location));

    //extend the approach point to 1km away so that there is always a navigational target
    location_update(extended_approach, target_heading_deg, 1000.0);

    float expected_travel_distance = predict_travel_distance(wind, is_zero(approach_alt_offset) ?  landing_point.alt * 0.01f : approach_alt_offset,
                                                             false);
    float approach_extension_m = expected_travel_distance + approach_extension;
    float loiter_radius_m_abs = fabsf(loiter_radius);
    // an approach extensions must be at least half the loiter radius, or the aircraft has a
    // decent chance to be misaligned on final approach
    approach_extension_m = MAX(approach_extension_m, loiter_radius_m_abs * 0.5f);

    location_update(arc_exit, target_heading_deg + 180, approach_extension_m);
    memcpy(&arc, &arc_exit, sizeof(Location));
    memcpy(&arc_entry, &arc_exit, sizeof(Location));

    float arc_heading_deg = target_heading_deg + (landing_point.loiter_ccw ? -90.0f : 90.0f);
    location_update(arc, arc_heading_deg, loiter_radius_m_abs);
    location_update(arc_entry, arc_heading_deg, loiter_radius_m_abs * 2);

#ifdef DEBUG_PRINTS
    // TODO: Send this information via a MAVLink packet
    gcs().send_text(MAV_SEVERITY_INFO, "Arc: %3.8f %3.8f",
                                     (double)(arc.lat / 1e7),(double)( arc.lng / 1e7));
    gcs().send_text(MAV_SEVERITY_INFO, "Loiter en: %3.8f %3.8f",
                                     (double)(arc_entry.lat / 1e7), (double)(arc_entry.lng / 1e7));
    gcs().send_text(MAV_SEVERITY_INFO, "Loiter ex: %3.8f %3.8f",
                                     (double)(arc_exit.lat / 1e7), (double)(arc_exit.lng / 1e7));
    gcs().send_text(MAV_SEVERITY_INFO, "Extended: %3.8f %3.8f",
                                     (double)(extended_approach.lat / 1e7), (double)(extended_approach.lng / 1e7));
    gcs().send_text(MAV_SEVERITY_INFO, "Extended by: %f (%f)", (double)approach_extension_m,
                                     (double)expected_travel_distance);
    gcs().send_text(MAV_SEVERITY_INFO, "Target Heading: %3.1f", (double)target_heading_deg);
#endif // DEBUG_PRINTS

}

float AP_Landing_Deepstall::predict_travel_distance(const Vector3f wind, const float height, const bool print)
{
    bool reverse = false;

    float course = radians(target_heading_deg);

    // a forward speed of 0 will result in a divide by 0
    float forward_speed_ms = MAX(forward_speed, 0.1f);

    Vector2f wind_vec(wind.x, wind.y); // work with the 2D component of wind
    float wind_length = MAX(wind_vec.length(), 0.05f); // always assume a slight wind to avoid divide by 0
    Vector2f course_vec(cosf(course), sinf(course));

    float offset = course - atan2f(-wind.y, -wind.x);

    // estimator for how far the aircraft will travel while entering the stall
    float stall_distance = slope_a * wind_length * cosf(offset) + slope_b;

    float theta = acosf(constrain_float((wind_vec * course_vec) / wind_length, -1.0f, 1.0f));
    if ((course_vec % wind_vec) > 0) {
        reverse = true;
        theta *= -1;
    }

    float cross_component = sinf(theta) * wind_length;
    float estimated_crab_angle = asinf(constrain_float(cross_component / forward_speed_ms, -1.0f, 1.0f));
    if (reverse) {
        estimated_crab_angle *= -1;
    }

    float estimated_forward = cosf(estimated_crab_angle) * forward_speed_ms + cosf(theta) * wind_length;

    if (is_positive(down_speed)) {
        predicted_travel_distance = (estimated_forward * height / down_speed) + stall_distance;
    } else {
        // if we don't have a sane downward speed in a deepstall (IE not zero, and not
        // an ascent) then just provide the stall_distance as a reasonable approximation
        predicted_travel_distance = stall_distance;
    }

    if(print) {
        // allow printing the travel distances on the final entry as its used for tuning
        gcs().send_text(MAV_SEVERITY_INFO, "Deepstall: Entry: %0.1f (m) Travel: %0.1f (m)",
                                         (double)stall_distance, (double)predicted_travel_distance);
    }

    return predicted_travel_distance;
}

bool AP_Landing_Deepstall::verify_breakout(const Location &current_loc, const Location &target_loc,
                                               const float height_error) const
{
    Vector2f location_delta = location_diff(current_loc, target_loc);
    const float heading_error = degrees(landing.ahrs.groundspeed_vector().angle(location_delta));

    // Check to see if the the plane is heading toward the land waypoint. We use 20 degrees (+/-10 deg)
    // of margin so that the altitude to be within 5 meters of desired

    if (heading_error <= 10.0  && fabsf(height_error) < DEEPSTALL_LOITER_ALT_TOLERANCE) {
            // Want to head in a straight line from _here_ to the next waypoint instead of center of loiter wp
            return true;
    }   
    return false;
}

float AP_Landing_Deepstall::update_steering()
{
    Location current_loc;
    if ((!landing.ahrs.get_position(current_loc) || !landing.ahrs.healthy()) && !hold_level) {
        // panic if no position source is available
        // continue the stall but target just holding the wings held level as deepstall should be a minimal
        // energy configuration on the aircraft, and if a position isn't available aborting would be worse
        gcs().send_text(MAV_SEVERITY_CRITICAL, "Deepstall: Invalid data from AHRS. Holding level");
        hold_level = true;
    }

    float desired_change = 0.0f;

    if (!hold_level) {
        uint32_t time = AP_HAL::millis();
        float dt = constrain_float(time - last_time, (uint32_t)10UL, (uint32_t)200UL) * 1e-3;
        last_time = time;

        Vector2f ab = location_diff(arc_exit, extended_approach);
        ab.normalize();
        Vector2f a_air = location_diff(arc_exit, current_loc);

        crosstrack_error = a_air % ab;
        float sine_nu1 = constrain_float(crosstrack_error / MAX(L1_period, 0.1f), -0.7071f, 0.7107f);
        float nu1 = asinf(sine_nu1);

        if (L1_i > 0) {
            L1_xtrack_i += nu1 * L1_i / dt;
            L1_xtrack_i = constrain_float(L1_xtrack_i, -0.5f, 0.5f);
            nu1 += L1_xtrack_i;
        }
        desired_change = wrap_PI(radians(target_heading_deg) + nu1 - landing.ahrs.yaw) / time_constant;
    }

    float yaw_rate = landing.ahrs.get_gyro().z;
    float yaw_rate_limit_rps = radians(yaw_rate_limit);
    float error = wrap_PI(constrain_float(desired_change, -yaw_rate_limit_rps, yaw_rate_limit_rps) - yaw_rate);

#ifdef DEBUG_PRINTS
    gcs().send_text(MAV_SEVERITY_INFO, "x: %f e: %f r: %f d: %f",
                                    (double)crosstrack_error,
                                    (double)error,
                                    (double)degrees(yaw_rate),
                                    (double)location_diff(current_loc, landing_point).length());
#endif // DEBUG_PRINTS

    return ds_PID.get_pid(error);
}