ardupilot/libraries/AP_Mission/AP_Mission.cpp

/// @file    AP_Mission.cpp
/// @brief   Handles the MAVLINK command mission stack.  Reads and writes mission to storage.

#include "AP_Mission.h"
#include <AP_Terrain/AP_Terrain.h>
#include <GCS_MAVLink/GCS.h>
#include <AP_AHRS/AP_AHRS.h>

const AP_Param::GroupInfo AP_Mission::var_info[] = {

    // @Param: TOTAL
    // @DisplayName: Total mission commands
    // @Description: The number of mission mission items that has been loaded by the ground station. Do not change this manually.
    // @Range: 0 32766
    // @Increment: 1
    // @User: Advanced
    // @ReadOnly: True
    AP_GROUPINFO_FLAGS("TOTAL",  0, AP_Mission, _cmd_total, 0, AP_PARAM_FLAG_INTERNAL_USE_ONLY),

    // @Param: RESTART
    // @DisplayName: Mission Restart when entering Auto mode
    // @Description: Controls mission starting point when entering Auto mode (either restart from beginning of mission or resume from last command run)
    // @Values: 0:Resume Mission, 1:Restart Mission
    // @User: Advanced
    AP_GROUPINFO("RESTART",  1, AP_Mission, _restart, AP_MISSION_RESTART_DEFAULT),

    // @Param: OPTIONS
    // @DisplayName: Mission options bitmask
    // @Description: Bitmask of what options to use in missions.
    // @Bitmask: 0:Clear Mission on reboot, 1:Use distance to land calc on battery failsafe,2:ContinueAfterLand
    // @Bitmask{Copter}: 0:Clear Mission on reboot, 2:ContinueAfterLand
    // @Bitmask{Rover, Sub}: 0:Clear Mission on reboot
    // @User: Advanced
    AP_GROUPINFO("OPTIONS",  2, AP_Mission, _options, AP_MISSION_OPTIONS_DEFAULT),

    AP_GROUPEND
};

extern const AP_HAL::HAL& hal;

// storage object
StorageAccess AP_Mission::_storage(StorageManager::StorageMission);

HAL_Semaphore AP_Mission::_rsem;

///
/// public mission methods
///

/// init - initialises this library including checks the version in eeprom matches this library
void AP_Mission::init()
{
    // check_eeprom_version - checks version of missions stored in eeprom matches this library
    // command list will be cleared if they do not match
    check_eeprom_version();

    // If Mission Clear bit is set then it should clear the mission, otherwise retain the mission.
    if (AP_MISSION_MASK_MISSION_CLEAR & _options) {
        gcs().send_text(MAV_SEVERITY_INFO, "Clearing Mission");
        clear();
    }

    _last_change_time_ms = AP_HAL::millis();
}

/// start - resets current commands to point to the beginning of the mission
///     To-Do: should we validate the mission first and return true/false?
void AP_Mission::start()
{
    _flags.state = MISSION_RUNNING;

    reset(); // reset mission to the first command, resets jump tracking

    // advance to the first command
    if (!advance_current_nav_cmd()) {
        // on failure set mission complete
        complete();
    }
}

/// stop - stops mission execution.  subsequent calls to update() will have no effect until the mission is started or resumed
void AP_Mission::stop()
{
    _flags.state = MISSION_STOPPED;
}

/// resume - continues the mission execution from where we last left off
///     previous running commands will be re-initialized
void AP_Mission::resume()
{
    // if mission had completed then start it from the first command
    if (_flags.state == MISSION_COMPLETE) {
        start();
        return;
    }

    // if mission had stopped then restart it
    if (_flags.state == MISSION_STOPPED) {
        _flags.state = MISSION_RUNNING;

        // if no valid nav command index restart from beginning
        if (_nav_cmd.index == AP_MISSION_CMD_INDEX_NONE) {
            start();
            return;
        }
    }

    // ensure cache coherence
    if (!read_cmd_from_storage(_nav_cmd.index, _nav_cmd)) {
        // if we failed to read the command from storage, then the command may have
        // been from a previously loaded mission it is illogical to ever resume
        // flying to a command that has been excluded from the current mission
        start();
        return;
    }

    // rewind the mission wp if the repeat distance has been set via MAV_CMD_DO_SET_RESUME_REPEAT_DIST
    if (_repeat_dist > 0 && _wp_index_history[LAST_WP_PASSED] != AP_MISSION_CMD_INDEX_NONE) {
        // if not already in a resume state calculate the position to rewind to
        Mission_Command tmp_cmd;
        if (!_flags.resuming_mission && calc_rewind_pos(tmp_cmd)) {
            _resume_cmd = tmp_cmd;
        }

        // resume mission to rewound position
        if (_resume_cmd.index != AP_MISSION_CMD_INDEX_NONE && start_command(_resume_cmd)) {
            _nav_cmd = _resume_cmd;
            _flags.nav_cmd_loaded = true;
            // set flag to prevent history being re-written
            _flags.resuming_mission = true;
            return;
        }
    }

    // restart active navigation command. We run these on resume()
    // regardless of whether the mission was stopped, as we may be
    // re-entering AUTO mode and the nav_cmd callback needs to be run
    // to setup the current target waypoint

    if (_flags.do_cmd_loaded && _do_cmd.index != AP_MISSION_CMD_INDEX_NONE) {
        // restart the active do command, which will also load the nav command for us
        set_current_cmd(_do_cmd.index);
    } else if (_flags.nav_cmd_loaded) {
        // restart the active nav command
        set_current_cmd(_nav_cmd.index);
    }

    // Note: if there is no active command then the mission must have been stopped just after the previous nav command completed
    //      update will take care of finding and starting the nav command
}

/// check if the next nav command is a takeoff, skipping delays
bool AP_Mission::is_takeoff_next(uint16_t cmd_index)
{
    Mission_Command cmd = {};
    // check a maximum of 16 items, remembering that missions can have
    // loops in them
    for (uint8_t i=0; i<16; i++, cmd_index++) {
        if (!get_next_nav_cmd(cmd_index, cmd)) {
            return false;
        }
        switch (cmd.id) {
        // any of these are considered a takeoff command:
        case MAV_CMD_NAV_VTOL_TAKEOFF:
        case MAV_CMD_NAV_TAKEOFF:
        case MAV_CMD_NAV_TAKEOFF_LOCAL:
            return true;
        // any of these are considered "skippable" when determining if
        // we "start with a takeoff command"
        case MAV_CMD_NAV_DELAY:
            continue;
        default:
            return false;
        }
    }
    return false;
}

/// check mission starts with a takeoff command
bool AP_Mission::starts_with_takeoff_cmd()
{
    uint16_t cmd_index = _restart ? AP_MISSION_CMD_INDEX_NONE : _nav_cmd.index;
    if (cmd_index == AP_MISSION_CMD_INDEX_NONE) {
        cmd_index = AP_MISSION_FIRST_REAL_COMMAND;
    }
    return is_takeoff_next(cmd_index);
}

/*
    return true if MIS_OPTIONS is set to allow continue of mission
    logic after a land and the next waypoint is a takeoff. If this
    is false then after a landing is complete the vehicle should 
    disarm and mission logic should stop
*/
bool AP_Mission::continue_after_land_check_for_takeoff()
{
    if (!continue_after_land()) {
        return false;
    }
    if (_nav_cmd.index == AP_MISSION_CMD_INDEX_NONE) {
        return false;
    }
    return is_takeoff_next(_nav_cmd.index+1);
}

/// start_or_resume - if MIS_AUTORESTART=0 this will call resume(), otherwise it will call start()
void AP_Mission::start_or_resume()
{
    if (_restart == 1 && !_force_resume) {
        start();
    } else {
        resume();
        _force_resume = false;
    }
}

/// reset - reset mission to the first command
void AP_Mission::reset()
{
    _flags.nav_cmd_loaded  = false;
    _flags.do_cmd_loaded   = false;
    _flags.do_cmd_all_done = false;
    _flags.in_landing_sequence = false;
    _nav_cmd.index         = AP_MISSION_CMD_INDEX_NONE;
    _do_cmd.index          = AP_MISSION_CMD_INDEX_NONE;
    _prev_nav_cmd_index    = AP_MISSION_CMD_INDEX_NONE;
    _prev_nav_cmd_wp_index = AP_MISSION_CMD_INDEX_NONE;
    _prev_nav_cmd_id       = AP_MISSION_CMD_ID_NONE;
    init_jump_tracking();
    reset_wp_history();
}

/// clear - clears out mission
///     returns true if mission was running so it could not be cleared
bool AP_Mission::clear()
{
    // do not allow clearing the mission while it is running
    if (_flags.state == MISSION_RUNNING) {
        return false;
    }

    // remove all commands
    _cmd_total.set_and_save(0);

    // clear index to commands
    _nav_cmd.index = AP_MISSION_CMD_INDEX_NONE;
    _do_cmd.index = AP_MISSION_CMD_INDEX_NONE;
    _flags.nav_cmd_loaded = false;
    _flags.do_cmd_loaded = false;

    // return success
    return true;
}


/// trucate - truncate any mission items beyond index
void AP_Mission::truncate(uint16_t index)
{
    if ((unsigned)_cmd_total > index) {
        _cmd_total.set_and_save(index);
    }
}

/// update - ensures the command queues are loaded with the next command and calls main programs command_init and command_verify functions to progress the mission
///     should be called at 10hz or higher
void AP_Mission::update()
{
    // exit immediately if not running or no mission commands
    if (_flags.state != MISSION_RUNNING || _cmd_total == 0) {
        return;
    }

    update_exit_position();

    // save persistent waypoint_num for watchdog restore
    hal.util->persistent_data.waypoint_num = _nav_cmd.index;

    // check if we have an active nav command
    if (!_flags.nav_cmd_loaded || _nav_cmd.index == AP_MISSION_CMD_INDEX_NONE) {
        // advance in mission if no active nav command
        if (!advance_current_nav_cmd()) {
            // failure to advance nav command means mission has completed
            complete();
            return;
        }
    } else {
        // run the active nav command
        if (verify_command(_nav_cmd)) {
            // market _nav_cmd as complete (it will be started on the next iteration)
            _flags.nav_cmd_loaded = false;
            // immediately advance to the next mission command
            if (!advance_current_nav_cmd()) {
                // failure to advance nav command means mission has completed
                complete();
                return;
            }
        }
    }

    // check if we have an active do command
    if (!_flags.do_cmd_loaded) {
        advance_current_do_cmd();
    } else {
        // check the active do command
        if (verify_command(_do_cmd)) {
            // mark _do_cmd as complete
            _flags.do_cmd_loaded = false;
        }
    }
}

bool AP_Mission::verify_command(const Mission_Command& cmd)
{
    switch (cmd.id) {
    // do-commands always return true for verify:
    case MAV_CMD_DO_GRIPPER:
    case MAV_CMD_DO_SET_SERVO:
    case MAV_CMD_DO_SET_RELAY:
    case MAV_CMD_DO_REPEAT_SERVO:
    case MAV_CMD_DO_REPEAT_RELAY:
    case MAV_CMD_DO_DIGICAM_CONFIGURE:
    case MAV_CMD_DO_DIGICAM_CONTROL:
    case MAV_CMD_DO_SET_CAM_TRIGG_DIST:
    case MAV_CMD_DO_PARACHUTE:
    case MAV_CMD_DO_SEND_SCRIPT_MESSAGE:
    case MAV_CMD_DO_SPRAYER:
    case MAV_CMD_DO_AUX_FUNCTION:
    case MAV_CMD_DO_SET_RESUME_REPEAT_DIST:
    case MAV_CMD_DO_GIMBAL_MANAGER_PITCHYAW:
        return true;
    default:
        return _cmd_verify_fn(cmd);
    }
}

bool AP_Mission::start_command(const Mission_Command& cmd)
{
    // check for landing related commands and set in_landing_sequence flag
    if (is_landing_type_cmd(cmd.id) || cmd.id == MAV_CMD_DO_LAND_START) {
        set_in_landing_sequence_flag(true);
    }

    gcs().send_text(MAV_SEVERITY_INFO, "Mission: %u %s", cmd.index, cmd.type());
    switch (cmd.id) {
    case MAV_CMD_DO_AUX_FUNCTION:
        return start_command_do_aux_function(cmd);
    case MAV_CMD_DO_GRIPPER:
        return start_command_do_gripper(cmd);
    case MAV_CMD_DO_SET_SERVO:
    case MAV_CMD_DO_SET_RELAY:
    case MAV_CMD_DO_REPEAT_SERVO:
    case MAV_CMD_DO_REPEAT_RELAY:
        return start_command_do_servorelayevents(cmd);
    case MAV_CMD_DO_CONTROL_VIDEO:
    case MAV_CMD_DO_DIGICAM_CONFIGURE:
    case MAV_CMD_DO_DIGICAM_CONTROL:
    case MAV_CMD_DO_SET_CAM_TRIGG_DIST:
        return start_command_camera(cmd);
    case MAV_CMD_DO_PARACHUTE:
        return start_command_parachute(cmd);
    case MAV_CMD_DO_SEND_SCRIPT_MESSAGE:
        return start_command_do_scripting(cmd);
    case MAV_CMD_DO_SPRAYER:
        return start_command_do_sprayer(cmd);
    case MAV_CMD_DO_SET_RESUME_REPEAT_DIST:
        return command_do_set_repeat_dist(cmd);
    case MAV_CMD_DO_GIMBAL_MANAGER_PITCHYAW:
        return start_command_do_gimbal_manager_pitchyaw(cmd);
    default:
        return _cmd_start_fn(cmd);
    }
}

///
/// public command methods
///

/// add_cmd - adds a command to the end of the command list and writes to storage
///     returns true if successfully added, false on failure
///     cmd.index is updated with it's new position in the mission
bool AP_Mission::add_cmd(Mission_Command& cmd)
{
    // attempt to write the command to storage
    bool ret = write_cmd_to_storage(_cmd_total, cmd);

    if (ret) {
        // update command's index
        cmd.index = _cmd_total;
        // increment total number of commands
        _cmd_total.set_and_save(_cmd_total + 1);
    }

    return ret;
}

/// replace_cmd - replaces the command at position 'index' in the command list with the provided cmd
///     replacing the current active command will have no effect until the command is restarted
///     returns true if successfully replaced, false on failure
bool AP_Mission::replace_cmd(uint16_t index, const Mission_Command& cmd)
{
    // sanity check index
    if (index >= (unsigned)_cmd_total) {
        return false;
    }

    // attempt to write the command to storage
    return write_cmd_to_storage(index, cmd);
}

/// is_nav_cmd - returns true if the command's id is a "navigation" command, false if "do" or "conditional" command
bool AP_Mission::is_nav_cmd(const Mission_Command& cmd)
{
    // NAV commands all have ids below MAV_CMD_NAV_LAST, plus some exceptions
    return (cmd.id <= MAV_CMD_NAV_LAST ||
            cmd.id == MAV_CMD_NAV_SET_YAW_SPEED ||
            cmd.id == MAV_CMD_NAV_SCRIPT_TIME ||
            cmd.id == MAV_CMD_NAV_ATTITUDE_TIME);
}

/// get_next_nav_cmd - gets next "navigation" command found at or after start_index
///     returns true if found, false if not found (i.e. reached end of mission command list)
///     accounts for do_jump commands but never increments the jump's num_times_run (advance_current_nav_cmd is responsible for this)
bool AP_Mission::get_next_nav_cmd(uint16_t start_index, Mission_Command& cmd)
{
    // search until the end of the mission command list
    for (uint16_t cmd_index = start_index; cmd_index < (unsigned)_cmd_total; cmd_index++) {
        // get next command
        if (!get_next_cmd(cmd_index, cmd, false)) {
            // no more commands so return failure
            return false;
        }
        // if found a "navigation" command then return it
        if (is_nav_cmd(cmd)) {
            return true;
        }
    }

    // if we got this far we did not find a navigation command
    return false;
}

/// get the ground course of the next navigation leg in centidegrees
/// from 0 36000. Return default_angle if next navigation
/// leg cannot be determined
int32_t AP_Mission::get_next_ground_course_cd(int32_t default_angle)
{
    Mission_Command cmd;
    if (!get_next_nav_cmd(_nav_cmd.index+1, cmd)) {
        return default_angle;
    }
    // special handling for nav commands with no target location
    if (cmd.id == MAV_CMD_NAV_GUIDED_ENABLE ||
        cmd.id == MAV_CMD_NAV_DELAY) {
        return default_angle;
    }
    if (cmd.id == MAV_CMD_NAV_SET_YAW_SPEED) {
        return (_nav_cmd.content.set_yaw_speed.angle_deg * 100);
    }
    return _nav_cmd.content.location.get_bearing_to(cmd.content.location);
}

// set_current_cmd - jumps to command specified by index
bool AP_Mission::set_current_cmd(uint16_t index, bool rewind)
{
    // read command to check for DO_LAND_START
    Mission_Command cmd;
    if (!read_cmd_from_storage(index, cmd) || (cmd.id != MAV_CMD_DO_LAND_START)) {
        _flags.in_landing_sequence = false;
    }

    // mission command has been set and not as rewind command, don't track history.
    if (!rewind) {
        reset_wp_history();
    }

    // sanity check index and that we have a mission
    if (index >= (unsigned)_cmd_total || _cmd_total == 1) {
        return false;
    }

    // stop the current running do command
    _do_cmd.index = AP_MISSION_CMD_INDEX_NONE;
    _flags.do_cmd_loaded = false;
    _flags.do_cmd_all_done = false;

    // stop current nav cmd
    _flags.nav_cmd_loaded = false;

    // if index is zero then the user wants to completely restart the mission
    if (index == 0 || _flags.state == MISSION_COMPLETE) {
        _prev_nav_cmd_id    = AP_MISSION_CMD_ID_NONE;
        _prev_nav_cmd_index = AP_MISSION_CMD_INDEX_NONE;
        _prev_nav_cmd_wp_index = AP_MISSION_CMD_INDEX_NONE;
        // reset the jump tracking to zero
        init_jump_tracking();
        if (index == 0) {
            index = 1;
        }
    }

    // if the mission is stopped or completed move the nav_cmd index to the specified point and set the state to stopped
    // so that if the user resumes the mission it will begin at the specified index
    if (_flags.state != MISSION_RUNNING) {
        // search until we find next nav command or reach end of command list
        while (!_flags.nav_cmd_loaded) {
            // get next command
            if (!get_next_cmd(index, cmd, true)) {
                _nav_cmd.index = AP_MISSION_CMD_INDEX_NONE;
                return false;
            }

            // check if navigation or "do" command
            if (is_nav_cmd(cmd)) {
                // set current navigation command
                _nav_cmd = cmd;
                _flags.nav_cmd_loaded = true;
            } else {
                // set current do command
                if (!_flags.do_cmd_loaded) {
                    _do_cmd = cmd;
                    _flags.do_cmd_loaded = true;
                }
            }
            // move onto next command
            index = cmd.index+1;
        }

        // if we have not found a do command then set flag to show there are no do-commands to be run before nav command completes
        if (!_flags.do_cmd_loaded) {
            _flags.do_cmd_all_done = true;
        }

        // if we got this far then the mission can safely be "resumed" from the specified index so we set the state to "stopped"
        _flags.state = MISSION_STOPPED;
        return true;
    }

    // the state must be MISSION_RUNNING, allow advance_current_nav_cmd() to manage starting the item
    if (!advance_current_nav_cmd(index)) {
        // on failure set mission complete
        complete();
        return false;
    }

    // if we got this far we must have successfully advanced the nav command
    return true;
}

// restart current navigation command.  Used to handle external changes to mission
// returns true on success, false if mission is not running or current nav command is invalid
bool AP_Mission::restart_current_nav_cmd()
{
    // return immediately if mission is not running
    if (_flags.state != MISSION_RUNNING) {
        return false;
    }

    // return immediately if nav command index is invalid
    const uint16_t nav_cmd_index = get_current_nav_index();
    if ((nav_cmd_index == 0) || (nav_cmd_index >= num_commands())) {
        return false;
    }

    return set_current_cmd(_nav_cmd.index);
}

// returns false on any issue at all.
bool AP_Mission::set_item(uint16_t index, mavlink_mission_item_int_t& src_packet)
{
    // this is the on-storage format
    AP_Mission::Mission_Command cmd;

    // can't handle request for anything bigger than the mission size+1...
    if (index > num_commands()) {
        return false;
    }

    // convert from mavlink-ish format to storage format, if we can.
    if (mavlink_int_to_mission_cmd(src_packet, cmd) != MAV_MISSION_ACCEPTED) {
        return false;
    }

    // A request to set the 'next' item after the end is how we add an extra
    //  item to the list, thus allowing us to write entire missions if needed.
    if (index == num_commands()) {
        return add_cmd(cmd);
    }

    // replacing an existing mission item...
    return AP_Mission::replace_cmd(index, cmd);
}

bool AP_Mission::get_item(uint16_t index, mavlink_mission_item_int_t& ret_packet) const
{
    // setting ret_packet.command = -1  and/or returning false
    //  means it contains invalid data after it leaves here.

    // this is the on-storage format
    AP_Mission::Mission_Command cmd;

    // can't handle request for anything bigger than the mission size...
    if (index >= num_commands()) {
        ret_packet.command = -1;
        return false;
    }

    // minimal placeholder values during read-from-storage
    ret_packet.target_system = 1;     // unused sysid
    ret_packet.target_component =  1; // unused compid

    // 0=home, higher number/s = mission item number.
    ret_packet.seq = index;

    // retrieve mission from eeprom
    if (!read_cmd_from_storage(ret_packet.seq, cmd)) {
        ret_packet.command = -1;
        return false;
    }
    // convert into mavlink-ish format for lua and friends.
    if (!mission_cmd_to_mavlink_int(cmd, ret_packet)) {
        ret_packet.command = -1;
        return false;
    }

    // set packet's current field to 1 if this is the command being executed
    if (cmd.id == (uint16_t)get_current_nav_cmd().index) {
        ret_packet.current = 1;
    } else {
        ret_packet.current = 0;
    }

    // set auto continue to 1, becasue that's what's done elsewhere.
    ret_packet.autocontinue = 1;     // 1 (true), 0 (false)
    ret_packet.command = cmd.id;

    return true;
}


struct PACKED Packed_Location_Option_Flags {
    uint8_t relative_alt : 1;           // 1 if altitude is relative to home
    uint8_t unused1      : 1;           // unused flag (defined so that loiter_ccw uses the correct bit)
    uint8_t loiter_ccw   : 1;           // 0 if clockwise, 1 if counter clockwise
    uint8_t terrain_alt  : 1;           // this altitude is above terrain
    uint8_t origin_alt   : 1;           // this altitude is above ekf origin
    uint8_t loiter_xtrack : 1;          // 0 to crosstrack from center of waypoint, 1 to crosstrack from tangent exit location
    uint8_t type_specific_bit_0 : 1;    // each mission item type can use this for storing 1 bit of extra data
};

struct PACKED PackedLocation {
    union {
        Packed_Location_Option_Flags flags;                    ///< options bitmask (1<<0 = relative altitude)
        uint8_t options;                                /// allows writing all flags to eeprom as one byte
    };
    // by making alt 24 bit we can make p1 in a command 16 bit,
    // allowing an accurate angle in centi-degrees. This keeps the
    // storage cost per mission item at 15 bytes, and allows mission
    // altitudes of up to +/- 83km
    int32_t alt:24;                                     ///< param 2 - Altitude in centimeters (meters * 100) see LOCATION_ALT_MAX_M
    int32_t lat;                                        ///< param 3 - Latitude * 10**7
    int32_t lng;                                        ///< param 4 - Longitude * 10**7
};

union PackedContent {
    // location
    PackedLocation location;      // Waypoint location

    // raw bytes, for reading/writing to eeprom. Note that only 10
    // bytes are available if a 16 bit command ID is used
    uint8_t bytes[12];

};

assert_storage_size<PackedContent, 12> assert_storage_size_PackedContent;

/// load_cmd_from_storage - load command from storage
///     true is return if successful
bool AP_Mission::read_cmd_from_storage(uint16_t index, Mission_Command& cmd) const
{
    WITH_SEMAPHORE(_rsem);

    // special handling for command #0 which is home
    if (index == 0) {
        cmd = {};
        cmd.id = MAV_CMD_NAV_WAYPOINT;
        cmd.content.location = AP::ahrs().get_home();
        return true;
    }

    if (index >= (unsigned)_cmd_total) {
        return false;
    }

    // ensure all bytes of cmd are zeroed
    cmd = {};

    // Find out proper location in memory by using the start_byte position + the index
    // we can load a command, we don't process it yet
    // read WP position
    const uint16_t pos_in_storage = 4 + (index * AP_MISSION_EEPROM_COMMAND_SIZE);

    PackedContent packed_content {};

    const uint8_t b1 = _storage.read_byte(pos_in_storage);
    if (b1 == 0) {
        cmd.id = _storage.read_uint16(pos_in_storage+1);
        cmd.p1 = _storage.read_uint16(pos_in_storage+3);
        _storage.read_block(packed_content.bytes, pos_in_storage+5, 10);
    } else {
        cmd.id = b1;
        cmd.p1 = _storage.read_uint16(pos_in_storage+1);
        _storage.read_block(packed_content.bytes, pos_in_storage+3, 12);
    }

    if (stored_in_location(cmd.id)) {
#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
        // NOTE!  no 16-bit command may be stored_in_location as only
        // 10 bytes are available for storage and lat/lon/alt required
        // 4*sizeof(float) == 12 bytes of storage.
        if (b1 == 0) {
            AP_HAL::panic("May not store location for 16-bit commands");
        }
#endif
        // Location is not PACKED; field-wise copy it:
        cmd.content.location.relative_alt = packed_content.location.flags.relative_alt;
        cmd.content.location.loiter_ccw = packed_content.location.flags.loiter_ccw;
        cmd.content.location.terrain_alt = packed_content.location.flags.terrain_alt;
        cmd.content.location.origin_alt = packed_content.location.flags.origin_alt;
        cmd.content.location.loiter_xtrack = packed_content.location.flags.loiter_xtrack;
        cmd.content.location.alt = packed_content.location.alt;
        cmd.content.location.lat = packed_content.location.lat;
        cmd.content.location.lng = packed_content.location.lng;

        if (packed_content.location.flags.type_specific_bit_0) {
            cmd.type_specific_bits = 1U << 0;
        }
    } else {
        // all other options in Content are assumed to be packed:
        static_assert(sizeof(cmd.content) >= 12,
                      "content is big enough to take bytes");
        // (void *) cast to specify gcc that we know that we are copy byte into a non trivial type and leaving 4 bytes untouched
        memcpy((void *)&cmd.content, packed_content.bytes, 12);
    }

    // set command's index to it's position in eeprom
    cmd.index = index;

    // return success
    return true;
}

bool AP_Mission::stored_in_location(uint16_t id)
{
    switch (id) {
    case MAV_CMD_NAV_WAYPOINT:
    case MAV_CMD_NAV_LOITER_UNLIM:
    case MAV_CMD_NAV_LOITER_TURNS:
    case MAV_CMD_NAV_LOITER_TIME:
    case MAV_CMD_NAV_LAND:
    case MAV_CMD_NAV_TAKEOFF:
    case MAV_CMD_NAV_CONTINUE_AND_CHANGE_ALT:
    case MAV_CMD_NAV_LOITER_TO_ALT:
    case MAV_CMD_NAV_SPLINE_WAYPOINT:
    case MAV_CMD_NAV_GUIDED_ENABLE:
    case MAV_CMD_DO_SET_HOME:
    case MAV_CMD_DO_LAND_START:
    case MAV_CMD_DO_GO_AROUND:
    case MAV_CMD_DO_SET_ROI:
    case MAV_CMD_NAV_VTOL_TAKEOFF:
    case MAV_CMD_NAV_VTOL_LAND:
    case MAV_CMD_NAV_PAYLOAD_PLACE:
        return true;
    default:
        return false;
    }
}

/// write_cmd_to_storage - write a command to storage
///     index is used to calculate the storage location
///     true is returned if successful
bool AP_Mission::write_cmd_to_storage(uint16_t index, const Mission_Command& cmd)
{
    WITH_SEMAPHORE(_rsem);

    // range check cmd's index
    if (index >= num_commands_max()) {
        return false;
    }

    PackedContent packed {};
    if (stored_in_location(cmd.id)) {
        // Location is not PACKED; field-wise copy it:
        packed.location.flags.relative_alt = cmd.content.location.relative_alt;
        packed.location.flags.loiter_ccw = cmd.content.location.loiter_ccw;
        packed.location.flags.terrain_alt = cmd.content.location.terrain_alt;
        packed.location.flags.origin_alt = cmd.content.location.origin_alt;
        packed.location.flags.loiter_xtrack = cmd.content.location.loiter_xtrack;
        packed.location.flags.type_specific_bit_0 = cmd.type_specific_bits & (1U<<0);
        packed.location.alt = cmd.content.location.alt;
        packed.location.lat = cmd.content.location.lat;
        packed.location.lng = cmd.content.location.lng;
    } else {
        // all other options in Content are assumed to be packed:
        static_assert(sizeof(packed.bytes) >= 12,
                      "packed.bytes is big enough to take content");
        memcpy(packed.bytes, &cmd.content, 12);
    }

    // calculate where in storage the command should be placed
    uint16_t pos_in_storage = 4 + (index * AP_MISSION_EEPROM_COMMAND_SIZE);

    if (cmd.id < 256) {
        _storage.write_byte(pos_in_storage, cmd.id);
        _storage.write_uint16(pos_in_storage+1, cmd.p1);
        _storage.write_block(pos_in_storage+3, packed.bytes, 12);
    } else {
        // if the command ID is above 256 we store a 0 followed by the 16 bit command ID
        _storage.write_byte(pos_in_storage, 0);
        _storage.write_uint16(pos_in_storage+1, cmd.id);
        _storage.write_uint16(pos_in_storage+3, cmd.p1);
        _storage.write_block(pos_in_storage+5, packed.bytes, 10);
    }

    // remember when the mission last changed
    _last_change_time_ms = AP_HAL::millis();

    // return success
    return true;
}

/// write_home_to_storage - writes the special purpose cmd 0 (home) to storage
///     home is taken directly from ahrs
void AP_Mission::write_home_to_storage()
{
    Mission_Command home_cmd = {};
    home_cmd.id = MAV_CMD_NAV_WAYPOINT;
    home_cmd.content.location = AP::ahrs().get_home();
    write_cmd_to_storage(0,home_cmd);
}

MAV_MISSION_RESULT AP_Mission::sanity_check_params(const mavlink_mission_item_int_t& packet)
{
    uint8_t nan_mask;
    switch (packet.command) {
    case MAV_CMD_NAV_WAYPOINT:
        nan_mask = ~(1 << 3); // param 4 can be nan
        break;
    case MAV_CMD_NAV_LAND:
        nan_mask = ~(1 << 3); // param 4 can be nan
        break;
    case MAV_CMD_NAV_TAKEOFF:
        nan_mask = ~(1 << 3); // param 4 can be nan
        break;
    case MAV_CMD_NAV_VTOL_TAKEOFF:
        nan_mask = ~(1 << 3); // param 4 can be nan
        break;
    case MAV_CMD_NAV_VTOL_LAND:
        nan_mask = ~((1 << 2) | (1 << 3)); // param 3 and 4 can be nan
        break;
    default:
        nan_mask = 0xff;
        break;
    }

    if (((nan_mask & (1 << 0)) && isnan(packet.param1)) ||
        isinf(packet.param1)) {
        return MAV_MISSION_INVALID_PARAM1;
    }
    if (((nan_mask & (1 << 1)) && isnan(packet.param2)) ||
        isinf(packet.param2)) {
        return MAV_MISSION_INVALID_PARAM2;
    }
    if (((nan_mask & (1 << 2)) && isnan(packet.param3)) ||
        isinf(packet.param3)) {
        return MAV_MISSION_INVALID_PARAM3;
    }
    if (((nan_mask & (1 << 3)) && isnan(packet.param4)) ||
        isinf(packet.param4)) {
        return MAV_MISSION_INVALID_PARAM4;
    }
    return MAV_MISSION_ACCEPTED;
}

// mavlink_int_to_mission_cmd - converts mavlink message to an AP_Mission::Mission_Command object which can be stored to eeprom
//  return MAV_MISSION_ACCEPTED on success, MAV_MISSION_RESULT error on failure
MAV_MISSION_RESULT AP_Mission::mavlink_int_to_mission_cmd(const mavlink_mission_item_int_t& packet, AP_Mission::Mission_Command& cmd)
{
    // command's position in mission list and mavlink id
    cmd.index = packet.seq;
    cmd.id = packet.command;
    cmd.content.location = {};

    MAV_MISSION_RESULT param_check = sanity_check_params(packet);
    if (param_check != MAV_MISSION_ACCEPTED) {
        return param_check;
    }

    // command specific conversions from mavlink packet to mission command
    switch (cmd.id) {

    case 0:
        // this is reserved for storing 16 bit command IDs
        return MAV_MISSION_INVALID;

    case MAV_CMD_NAV_WAYPOINT: {                        // MAV ID: 16
        /*
          the 15 byte limit means we can't fit both delay and radius
          in the cmd structure. When we expand the mission structure
          we can do this properly
         */
#if APM_BUILD_TYPE(APM_BUILD_ArduPlane)
        // acceptance radius in meters and pass by distance in meters
        uint16_t acp = packet.param2;           // param 2 is acceptance radius in meters is held in low p1
        uint16_t passby = packet.param3;        // param 3 is pass by distance in meters is held in high p1

        // limit to 255 so it does not wrap during the shift or mask operation
        passby = MIN(0xFF,passby);
        acp = MIN(0xFF,acp);

        cmd.p1 = (passby << 8) | (acp & 0x00FF);
#else
        // delay at waypoint in seconds (this is for copters???)
        cmd.p1 = packet.param1;
#endif
    }
    break;

    case MAV_CMD_NAV_LOITER_UNLIM:                      // MAV ID: 17
        cmd.p1 = fabsf(packet.param3);                  // store radius as 16bit since no other params are competing for space
        cmd.content.location.loiter_ccw = (packet.param3 < 0);    // -1 = counter clockwise, +1 = clockwise
        break;

    case MAV_CMD_NAV_LOITER_TURNS: {                    // MAV ID: 18
        // number of turns is stored in the lowest bits.  radii below
        // 255m are stored in the top 8 bits as an 8-bit integer.
        // Radii above 255m are stored divided by 10 and a bit set in
        // storage so that on retrieval they are multiplied by 10
        cmd.p1 = MIN(255, packet.param1); // store number of times to circle in low p1
        uint8_t radius_m;
        const float abs_radius = fabsf(packet.param3);
        if (abs_radius <= 255) {
            radius_m = abs_radius;
        } else {
            radius_m = MIN(255, abs_radius * 0.1);
            cmd.type_specific_bits = 1U << 0;
        }
        cmd.p1 |= (radius_m<<8);   // store radius in high byte of p1
        cmd.content.location.loiter_ccw = (packet.param3 < 0);
        cmd.content.location.loiter_xtrack = (packet.param4 > 0); // 0 to xtrack from center of waypoint, 1 to xtrack from tangent exit location
    }
    break;

    case MAV_CMD_NAV_LOITER_TIME:                       // MAV ID: 19
        cmd.p1 = packet.param1;                         // loiter time in seconds uses all 16 bits, 8bit seconds is too small. No room for radius.
        cmd.content.location.loiter_ccw = (packet.param3 < 0);
        cmd.content.location.loiter_xtrack = (packet.param4 > 0); // 0 to xtrack from center of waypoint, 1 to xtrack from tangent exit location
        break;

    case MAV_CMD_NAV_RETURN_TO_LAUNCH:                  // MAV ID: 20
        break;

    case MAV_CMD_NAV_LAND:                              // MAV ID: 21
        cmd.p1 = packet.param1;                         // abort target altitude(m)  (plane only)
        if (!isnan(packet.param4)) {
            cmd.content.location.loiter_ccw = is_negative(packet.param4); // yaw direction, (plane deepstall only)
        }
        break;

    case MAV_CMD_NAV_TAKEOFF:                           // MAV ID: 22
        cmd.p1 = packet.param1;                         // minimum pitch (plane only)
        break;

    case MAV_CMD_NAV_CONTINUE_AND_CHANGE_ALT:           // MAV ID: 30
        cmd.p1 = packet.param1;                         // Climb/Descend
        // 0 = Neutral, cmd complete at +/- 5 of indicated alt.
        // 1 = Climb, cmd complete at or above indicated alt.
        // 2 = Descend, cmd complete at or below indicated alt.
        break;

    case MAV_CMD_NAV_LOITER_TO_ALT:                     // MAV ID: 31
        cmd.p1 = fabsf(packet.param2);                  // param2 is radius in meters
        cmd.content.location.loiter_ccw = (packet.param2 < 0);
        cmd.content.location.loiter_xtrack = (packet.param4 > 0); // 0 to xtrack from center of waypoint, 1 to xtrack from tangent exit location
        break;

    case MAV_CMD_NAV_SPLINE_WAYPOINT:                   // MAV ID: 82
#if APM_BUILD_TYPE(APM_BUILD_ArduPlane)
        return MAV_MISSION_UNSUPPORTED;
#else
        cmd.p1 = packet.param1;                         // delay at waypoint in seconds
        break;
#endif

    case MAV_CMD_NAV_GUIDED_ENABLE:                     // MAV ID: 92
        cmd.p1 = packet.param1;                         // on/off. >0.5 means "on", hand-over control to external controller
        break;

    case MAV_CMD_NAV_DELAY:                            // MAV ID: 93
        cmd.content.nav_delay.seconds = packet.param1; // delay in seconds
        cmd.content.nav_delay.hour_utc = packet.param2;// absolute time's hour (utc)
        cmd.content.nav_delay.min_utc = packet.param3;// absolute time's min (utc)
        cmd.content.nav_delay.sec_utc = packet.param4; // absolute time's second (utc)
        break;

    case MAV_CMD_CONDITION_DELAY:                       // MAV ID: 112
        cmd.content.delay.seconds = packet.param1;      // delay in seconds
        break;

    case MAV_CMD_CONDITION_DISTANCE:                    // MAV ID: 114
        cmd.content.distance.meters = packet.param1;    // distance in meters from next waypoint
        break;

    case MAV_CMD_CONDITION_YAW:                         // MAV ID: 115
        cmd.content.yaw.angle_deg = packet.param1;      // target angle in degrees
        cmd.content.yaw.turn_rate_dps = packet.param2;  // 0 = use default turn rate otherwise specific turn rate in deg/sec
        cmd.content.yaw.direction = packet.param3;      // -1 = ccw, +1 = cw
        cmd.content.yaw.relative_angle = packet.param4; // lng=0: absolute angle provided, lng=1: relative angle provided
        break;

    case MAV_CMD_DO_JUMP:                               // MAV ID: 177
        cmd.content.jump.target = packet.param1;        // jump-to command number
        cmd.content.jump.num_times = packet.param2;     // repeat count
        break;

    case MAV_CMD_DO_CHANGE_SPEED:                       // MAV ID: 178
        cmd.content.speed.speed_type = packet.param1;   // 0 = airspeed, 1 = ground speed
        cmd.content.speed.target_ms = packet.param2;    // target speed in m/s
        cmd.content.speed.throttle_pct = packet.param3; // throttle as a percentage from 1 ~ 100%
        break;

    case MAV_CMD_DO_SET_HOME:
        cmd.p1 = packet.param1;                         // p1=0 means use current location, p=1 means use provided location
        break;

    case MAV_CMD_DO_SET_RELAY:                          // MAV ID: 181
        cmd.content.relay.num = packet.param1;          // relay number
        cmd.content.relay.state = packet.param2;        // 0:off, 1:on
        break;

    case MAV_CMD_DO_REPEAT_RELAY:                       // MAV ID: 182
        cmd.content.repeat_relay.num = packet.param1;           // relay number
        cmd.content.repeat_relay.repeat_count = packet.param2;  // count
        cmd.content.repeat_relay.cycle_time = packet.param3;    // time converted from seconds to milliseconds
        break;

    case MAV_CMD_DO_SET_SERVO:                          // MAV ID: 183
        cmd.content.servo.channel = packet.param1;      // channel
        cmd.content.servo.pwm = packet.param2;          // PWM
        break;

    case MAV_CMD_DO_REPEAT_SERVO:                       // MAV ID: 184
        cmd.content.repeat_servo.channel = packet.param1;      // channel
        cmd.content.repeat_servo.pwm = packet.param2;          // PWM
        cmd.content.repeat_servo.repeat_count = packet.param3; // count
        cmd.content.repeat_servo.cycle_time = packet.param4;   // time in seconds
        break;

    case MAV_CMD_DO_LAND_START:                         // MAV ID: 189
        break;

    case MAV_CMD_DO_GO_AROUND:                          // MAV ID: 191
        break;

    case MAV_CMD_DO_SET_ROI:                            // MAV ID: 201
        cmd.p1 = packet.param1;                         // 0 = no roi, 1 = next waypoint, 2 = waypoint number, 3 = fixed location, 4 = given target (not supported)
        break;

    case MAV_CMD_DO_DIGICAM_CONFIGURE:                  // MAV ID: 202
        cmd.content.digicam_configure.shooting_mode = packet.param1;
        cmd.content.digicam_configure.shutter_speed = packet.param2;
        cmd.content.digicam_configure.aperture = packet.param3;
        cmd.content.digicam_configure.ISO = packet.param4;
        cmd.content.digicam_configure.exposure_type = packet.x;
        cmd.content.digicam_configure.cmd_id = packet.y;
        cmd.content.digicam_configure.engine_cutoff_time = packet.z;
        break;

    case MAV_CMD_DO_DIGICAM_CONTROL:                    // MAV ID: 203
        cmd.content.digicam_control.session = packet.param1;
        cmd.content.digicam_control.zoom_pos = packet.param2;
        cmd.content.digicam_control.zoom_step = packet.param3;
        cmd.content.digicam_control.focus_lock = packet.param4;
        cmd.content.digicam_control.shooting_cmd = packet.x;
        cmd.content.digicam_control.cmd_id = packet.y;
        break;

    case MAV_CMD_DO_MOUNT_CONTROL:                      // MAV ID: 205
        cmd.content.mount_control.pitch = packet.param1;
        cmd.content.mount_control.roll = packet.param2;
        cmd.content.mount_control.yaw = packet.param3;
        break;

    case MAV_CMD_DO_SET_CAM_TRIGG_DIST:                 // MAV ID: 206
        cmd.content.cam_trigg_dist.meters = packet.param1;  // distance between camera shots in meters
        cmd.content.cam_trigg_dist.trigger = packet.param3; // when enabled, camera triggers once immediately
        break;

    case MAV_CMD_DO_FENCE_ENABLE:                       // MAV ID: 207
        cmd.p1 = packet.param1;                         // action 0=disable, 1=enable
        break;

    case MAV_CMD_DO_AUX_FUNCTION:
        cmd.content.auxfunction.function = packet.param1;
        cmd.content.auxfunction.switchpos = packet.param2;
        break;

    case MAV_CMD_DO_PARACHUTE:                         // MAV ID: 208
        cmd.p1 = packet.param1;                        // action 0=disable, 1=enable, 2=release.  See PARACHUTE_ACTION enum
        break;

    case MAV_CMD_DO_INVERTED_FLIGHT:                    // MAV ID: 210
        cmd.p1 = packet.param1;                         // normal=0 inverted=1
        break;

    case MAV_CMD_DO_GRIPPER:                            // MAV ID: 211
        cmd.content.gripper.num = packet.param1;        // gripper number
        cmd.content.gripper.action = packet.param2;     // action 0=release, 1=grab.  See GRIPPER_ACTION enum
        break;

    case MAV_CMD_DO_GUIDED_LIMITS:                      // MAV ID: 222
        cmd.p1 = packet.param1;                         // max time in seconds the external controller will be allowed to control the vehicle
        cmd.content.guided_limits.alt_min = packet.param2;  // min alt below which the command will be aborted.  0 for no lower alt limit
        cmd.content.guided_limits.alt_max = packet.param3;  // max alt above which the command will be aborted.  0 for no upper alt limit
        cmd.content.guided_limits.horiz_max = packet.param4;// max horizontal distance the vehicle can move before the command will be aborted.  0 for no horizontal limit
        break;

    case MAV_CMD_DO_AUTOTUNE_ENABLE:                    // MAV ID: 211
        cmd.p1 = packet.param1;                         // disable=0 enable=1
        break;

    case MAV_CMD_NAV_ALTITUDE_WAIT:                     // MAV ID: 83
        cmd.content.altitude_wait.altitude = packet.param1;
        cmd.content.altitude_wait.descent_rate = packet.param2;
        cmd.content.altitude_wait.wiggle_time = packet.param3;
        break;

    case MAV_CMD_NAV_VTOL_TAKEOFF:
        break;

    case MAV_CMD_NAV_VTOL_LAND:
        break;

    case MAV_CMD_DO_VTOL_TRANSITION:
        cmd.content.do_vtol_transition.target_state = packet.param1;
        break;

    case MAV_CMD_DO_SET_REVERSE:
        cmd.p1 = packet.param1; // 0 = forward, 1 = reverse
        break;

    case MAV_CMD_DO_ENGINE_CONTROL:
        cmd.content.do_engine_control.start_control = (packet.param1>0);
        cmd.content.do_engine_control.cold_start = (packet.param2>0);
        cmd.content.do_engine_control.height_delay_cm = packet.param3*100;
        break;

    case MAV_CMD_NAV_PAYLOAD_PLACE:
        cmd.p1 = packet.param1*100; // copy max-descend parameter (m->cm)
        break;

    case MAV_CMD_NAV_SET_YAW_SPEED:
        cmd.content.set_yaw_speed.angle_deg = packet.param1;        // target angle in degrees
        cmd.content.set_yaw_speed.speed = packet.param2;            // speed in meters/second
        cmd.content.set_yaw_speed.relative_angle = packet.param3;   // 0 = absolute angle, 1 = relative angle
        break;

    case MAV_CMD_DO_WINCH:                              // MAV ID: 42600
        cmd.content.winch.num = packet.param1;          // winch number
        cmd.content.winch.action = packet.param2;       // action (0 = relax, 1 = length control, 2 = rate control).  See WINCH_ACTION enum
        cmd.content.winch.release_length = packet.param3;   // cable distance to unwind in meters, negative numbers to wind in cable
        cmd.content.winch.release_rate = packet.param4; // release rate in meters/second
        break;

    case MAV_CMD_DO_SET_RESUME_REPEAT_DIST:
        cmd.p1 = packet.param1; // Resume repeat distance (m)
        break;

    case MAV_CMD_DO_SPRAYER:
        cmd.p1 = packet.param1;                        // action 0=disable, 1=enable
        break;

    case MAV_CMD_DO_SEND_SCRIPT_MESSAGE:
        cmd.p1 = packet.param1;
        cmd.content.scripting.p1 = packet.param2;
        cmd.content.scripting.p2 = packet.param3;
        cmd.content.scripting.p3 = packet.param4;
        break;

    case MAV_CMD_NAV_SCRIPT_TIME:
        cmd.content.nav_script_time.command = packet.param1;
        cmd.content.nav_script_time.timeout_s = packet.param2;
        cmd.content.nav_script_time.arg1 = packet.param3;
        cmd.content.nav_script_time.arg2 = packet.param4;
        break;

    case MAV_CMD_NAV_ATTITUDE_TIME:
        cmd.content.nav_attitude_time.time_sec = constrain_float(packet.param1, 0, UINT16_MAX);
        cmd.content.nav_attitude_time.roll_deg = (fabsf(packet.param2) <= 180) ? packet.param2 : 0;
        cmd.content.nav_attitude_time.pitch_deg = (fabsf(packet.param3) <= 90) ? packet.param3 : 0;
        cmd.content.nav_attitude_time.yaw_deg = ((packet.param4 >= -180) && (packet.param4 <= 360)) ? packet.param4 : 0;
        cmd.content.nav_attitude_time.climb_rate = packet.x;
        break;

    case MAV_CMD_DO_PAUSE_CONTINUE:
        cmd.p1 = packet.param1;
        break;

    case MAV_CMD_DO_GIMBAL_MANAGER_PITCHYAW:
        cmd.content.gimbal_manager_pitchyaw.pitch_angle_deg = packet.param1;
        cmd.content.gimbal_manager_pitchyaw.yaw_angle_deg = packet.param2;
        cmd.content.gimbal_manager_pitchyaw.pitch_rate_degs = packet.param3;
        cmd.content.gimbal_manager_pitchyaw.yaw_rate_degs = packet.param4;
        cmd.content.gimbal_manager_pitchyaw.flags = packet.x;
        cmd.content.gimbal_manager_pitchyaw.gimbal_id = packet.z;
        break;

    default:
        // unrecognised command
        return MAV_MISSION_UNSUPPORTED;
    }

    // copy location from mavlink to command
    if (stored_in_location(cmd.id)) {

        // sanity check location
        if (!check_lat(packet.x)) {
            return MAV_MISSION_INVALID_PARAM5_X;
        }
        if (!check_lng(packet.y)) {
            return MAV_MISSION_INVALID_PARAM6_Y;
        }
        if (isnan(packet.z) || fabsf(packet.z) >= LOCATION_ALT_MAX_M) {
            return MAV_MISSION_INVALID_PARAM7;
        }

        cmd.content.location.lat = packet.x;
        cmd.content.location.lng = packet.y;

        cmd.content.location.alt = packet.z * 100.0f;       // convert packet's alt (m) to cmd alt (cm)

        switch (packet.frame) {

        case MAV_FRAME_MISSION:
        case MAV_FRAME_GLOBAL:
        case MAV_FRAME_GLOBAL_INT:
            cmd.content.location.relative_alt = 0;
            break;

        case MAV_FRAME_GLOBAL_RELATIVE_ALT:
        case MAV_FRAME_GLOBAL_RELATIVE_ALT_INT:
            cmd.content.location.relative_alt = 1;
            break;

#if AP_TERRAIN_AVAILABLE
        case MAV_FRAME_GLOBAL_TERRAIN_ALT:
        case MAV_FRAME_GLOBAL_TERRAIN_ALT_INT:
            // we mark it as a relative altitude, as it doesn't have
            // home alt added
            cmd.content.location.relative_alt = 1;
            // mark altitude as above terrain, not above home
            cmd.content.location.terrain_alt = 1;
            break;
#endif

        default:
            return MAV_MISSION_UNSUPPORTED_FRAME;
        }
    }

    // if we got this far then it must have been successful
    return MAV_MISSION_ACCEPTED;
}

MAV_MISSION_RESULT AP_Mission::convert_MISSION_ITEM_to_MISSION_ITEM_INT(const mavlink_mission_item_t &packet,
        mavlink_mission_item_int_t &mav_cmd)
{
    // TODO: rename mav_cmd to mission_item_int
    // TODO: rename packet to mission_item
    mav_cmd.param1 = packet.param1;
    mav_cmd.param2 = packet.param2;
    mav_cmd.param3 = packet.param3;
    mav_cmd.param4 = packet.param4;
    mav_cmd.z = packet.z;
    mav_cmd.seq = packet.seq;
    mav_cmd.command = packet.command;
    mav_cmd.target_system = packet.target_system;
    mav_cmd.target_component = packet.target_component;
    mav_cmd.frame = packet.frame;
    mav_cmd.current = packet.current;
    mav_cmd.autocontinue = packet.autocontinue;
    mav_cmd.mission_type = packet.mission_type;

    /*
      the strategy for handling both MISSION_ITEM and MISSION_ITEM_INT
      is to pass the lat/lng in MISSION_ITEM_INT straight through, and
      for MISSION_ITEM multiply by 1e7 here. We need an exception for
      any commands which use the x and y fields not as
      latitude/longitude.
     */
    switch (packet.command) {
    case MAV_CMD_DO_DIGICAM_CONTROL:
    case MAV_CMD_DO_DIGICAM_CONFIGURE:
    case MAV_CMD_NAV_ATTITUDE_TIME:
    case MAV_CMD_DO_GIMBAL_MANAGER_PITCHYAW:
        mav_cmd.x = packet.x;
        mav_cmd.y = packet.y;
        break;

    default:
        // all other commands use x and y as lat/lon. We need to
        // multiply by 1e7 to convert to int32_t
        if (!check_lat(packet.x)) {
            return MAV_MISSION_INVALID_PARAM5_X;
        }
        if (!check_lng(packet.y)) {
            return MAV_MISSION_INVALID_PARAM6_Y;
        }
        mav_cmd.x = packet.x * 1.0e7f;
        mav_cmd.y = packet.y * 1.0e7f;
        break;
    }

    return MAV_MISSION_ACCEPTED;
}

MAV_MISSION_RESULT AP_Mission::convert_MISSION_ITEM_INT_to_MISSION_ITEM(const mavlink_mission_item_int_t &item_int,
        mavlink_mission_item_t &item)
{
    item.param1 = item_int.param1;
    item.param2 = item_int.param2;
    item.param3 = item_int.param3;
    item.param4 = item_int.param4;
    item.z = item_int.z;
    item.seq = item_int.seq;
    item.command = item_int.command;
    item.target_system = item_int.target_system;
    item.target_component = item_int.target_component;
    item.frame = item_int.frame;
    item.current = item_int.current;
    item.autocontinue = item_int.autocontinue;
    item.mission_type = item_int.mission_type;

    switch (item_int.command) {
    case MAV_CMD_DO_DIGICAM_CONTROL:
    case MAV_CMD_DO_DIGICAM_CONFIGURE:
    case MAV_CMD_NAV_ATTITUDE_TIME:
    case MAV_CMD_DO_GIMBAL_MANAGER_PITCHYAW:
        item.x = item_int.x;
        item.y = item_int.y;
        break;

    default:
        // all other commands use x and y as lat/lon. We need to
        // multiply by 1e-7 to convert to float
        item.x = item_int.x * 1.0e-7f;
        item.y = item_int.y * 1.0e-7f;
        if (!check_lat(item.x)) {
            return MAV_MISSION_INVALID_PARAM5_X;
        }
        if (!check_lng(item.y)) {
            return MAV_MISSION_INVALID_PARAM6_Y;
        }
        break;
    }

    return MAV_MISSION_ACCEPTED;
}

// mavlink_cmd_long_to_mission_cmd - converts a mavlink cmd long to an AP_Mission::Mission_Command object which can be stored to eeprom
// return MAV_MISSION_ACCEPTED on success, MAV_MISSION_RESULT error on failure
MAV_MISSION_RESULT AP_Mission::mavlink_cmd_long_to_mission_cmd(const mavlink_command_long_t& packet, AP_Mission::Mission_Command& cmd)
{
    mavlink_mission_item_int_t miss_item = {0};

    miss_item.param1 = packet.param1;
    miss_item.param2 = packet.param2;
    miss_item.param3 = packet.param3;
    miss_item.param4 = packet.param4;

    miss_item.command = packet.command;
    miss_item.target_system = packet.target_system;
    miss_item.target_component = packet.target_component;

    return mavlink_int_to_mission_cmd(miss_item, cmd);
}

// mission_cmd_to_mavlink_int - converts an AP_Mission::Mission_Command object to a mavlink message which can be sent to the GCS
//  return true on success, false on failure
bool AP_Mission::mission_cmd_to_mavlink_int(const AP_Mission::Mission_Command& cmd, mavlink_mission_item_int_t& packet)
{
    // command's position in mission list and mavlink id
    packet.seq = cmd.index;
    packet.command = cmd.id;

    // set defaults
    packet.current = 0;     // 1 if we are passing back the mission command that is currently being executed
    packet.param1 = 0;
    packet.param2 = 0;
    packet.param3 = 0;
    packet.param4 = 0;
    packet.autocontinue = 1;

    // command specific conversions from mission command to mavlink packet
    switch (cmd.id) {
    case 0:
        // this is reserved for 16 bit command IDs
        return false;

    case MAV_CMD_NAV_WAYPOINT:                          // MAV ID: 16
#if APM_BUILD_TYPE(APM_BUILD_ArduPlane)
        // acceptance radius in meters

        packet.param2 = LOWBYTE(cmd.p1);        // param 2 is acceptance radius in meters is held in low p1
        packet.param3 = HIGHBYTE(cmd.p1);       // param 3 is pass by distance in meters is held in high p1
#else
        // delay at waypoint in seconds
        packet.param1 = cmd.p1;
#endif
        break;

    case MAV_CMD_NAV_LOITER_UNLIM:                      // MAV ID: 17
        packet.param3 = (float)cmd.p1;
        if (cmd.content.location.loiter_ccw) {
            packet.param3 *= -1;
        }
        break;

    case MAV_CMD_NAV_LOITER_TURNS:                      // MAV ID: 18
        packet.param1 = LOWBYTE(cmd.p1);                // number of times to circle is held in low byte of p1
        packet.param3 = HIGHBYTE(cmd.p1);               // radius is held in high byte of p1
        if (cmd.content.location.loiter_ccw) {
            packet.param3 = -packet.param3;
        }
        if (cmd.type_specific_bits & (1U<<0)) {
            packet.param3 *= 10;
        }
        packet.param4 = cmd.content.location.loiter_xtrack; // 0 to xtrack from center of waypoint, 1 to xtrack from tangent exit location
        break;

    case MAV_CMD_NAV_LOITER_TIME:                       // MAV ID: 19
        packet.param1 = cmd.p1;                         // loiter time in seconds
        if (cmd.content.location.loiter_ccw) {
            packet.param3 = -1;
        } else {
            packet.param3 = 1;
        }
        packet.param4 = cmd.content.location.loiter_xtrack; // 0 to xtrack from center of waypoint, 1 to xtrack from tangent exit location
        break;

    case MAV_CMD_NAV_RETURN_TO_LAUNCH:                  // MAV ID: 20
        break;

    case MAV_CMD_NAV_LAND:                              // MAV ID: 21
        packet.param1 = cmd.p1;                        // abort target altitude(m)  (plane only)
        packet.param4 = cmd.content.location.loiter_ccw ? -1 : 1; // yaw direction, (plane deepstall only)
        break;

    case MAV_CMD_NAV_TAKEOFF:                           // MAV ID: 22
        packet.param1 = cmd.p1;                         // minimum pitch (plane only)
        break;

    case MAV_CMD_NAV_CONTINUE_AND_CHANGE_ALT:           // MAV ID: 30
        packet.param1 = cmd.p1;                         // Climb/Descend
        // 0 = Neutral, cmd complete at +/- 5 of indicated alt.
        // 1 = Climb, cmd complete at or above indicated alt.
        // 2 = Descend, cmd complete at or below indicated alt.
        break;

    case MAV_CMD_NAV_LOITER_TO_ALT:                     // MAV ID: 31
        packet.param2 = cmd.p1;                        // loiter radius(m)
        if (cmd.content.location.loiter_ccw) {
            packet.param2 = -packet.param2;
        }
        packet.param4 = cmd.content.location.loiter_xtrack; // 0 to xtrack from center of waypoint, 1 to xtrack from tangent exit location
        break;

    case MAV_CMD_NAV_SPLINE_WAYPOINT:                   // MAV ID: 82
        packet.param1 = cmd.p1;                         // delay at waypoint in seconds
        break;

    case MAV_CMD_NAV_GUIDED_ENABLE:                     // MAV ID: 92
        packet.param1 = cmd.p1;                         // on/off. >0.5 means "on", hand-over control to external controller
        break;

    case MAV_CMD_NAV_DELAY:                            // MAV ID: 93
        packet.param1 = cmd.content.nav_delay.seconds; // delay in seconds
        packet.param2 = cmd.content.nav_delay.hour_utc; // absolute time's day of week (utc)
        packet.param3 = cmd.content.nav_delay.min_utc; // absolute time's hour (utc)
        packet.param4 = cmd.content.nav_delay.sec_utc; // absolute time's min (utc)
        break;

    case MAV_CMD_CONDITION_DELAY:                       // MAV ID: 112
        packet.param1 = cmd.content.delay.seconds;      // delay in seconds
        break;

    case MAV_CMD_CONDITION_DISTANCE:                    // MAV ID: 114
        packet.param1 = cmd.content.distance.meters;    // distance in meters from next waypoint
        break;

    case MAV_CMD_CONDITION_YAW:                         // MAV ID: 115
        packet.param1 = cmd.content.yaw.angle_deg;      // target angle in degrees
        packet.param2 = cmd.content.yaw.turn_rate_dps;  // 0 = use default turn rate otherwise specific turn rate in deg/sec
        packet.param3 = cmd.content.yaw.direction;      // -1 = ccw, +1 = cw
        packet.param4 = cmd.content.yaw.relative_angle; // 0 = absolute angle provided, 1 = relative angle provided
        break;

    case MAV_CMD_DO_JUMP:                               // MAV ID: 177
        packet.param1 = cmd.content.jump.target;        // jump-to command number
        packet.param2 = cmd.content.jump.num_times;     // repeat count
        break;

    case MAV_CMD_DO_CHANGE_SPEED:                       // MAV ID: 178
        packet.param1 = cmd.content.speed.speed_type;   // 0 = airspeed, 1 = ground speed
        packet.param2 = cmd.content.speed.target_ms;    // speed in m/s
        packet.param3 = cmd.content.speed.throttle_pct; // throttle as a percentage from 1 ~ 100%
        break;

    case MAV_CMD_DO_SET_HOME:                           // MAV ID: 179
        packet.param1 = cmd.p1;                         // p1=0 means use current location, p=1 means use provided location
        break;

    case MAV_CMD_DO_SET_RELAY:                          // MAV ID: 181
        packet.param1 = cmd.content.relay.num;          // relay number
        packet.param2 = cmd.content.relay.state;        // 0:off, 1:on
        break;

    case MAV_CMD_DO_REPEAT_RELAY:                       // MAV ID: 182
        packet.param1 = cmd.content.repeat_relay.num;           // relay number
        packet.param2 = cmd.content.repeat_relay.repeat_count;  // count
        packet.param3 = cmd.content.repeat_relay.cycle_time;    // time in seconds
        break;

    case MAV_CMD_DO_SET_SERVO:                          // MAV ID: 183
        packet.param1 = cmd.content.servo.channel;      // channel
        packet.param2 = cmd.content.servo.pwm;          // PWM
        break;

    case MAV_CMD_DO_REPEAT_SERVO:                       // MAV ID: 184
        packet.param1 = cmd.content.repeat_servo.channel;       // channel
        packet.param2 = cmd.content.repeat_servo.pwm;           // PWM
        packet.param3 = cmd.content.repeat_servo.repeat_count;  // count
        packet.param4 = cmd.content.repeat_servo.cycle_time;    // time in milliseconds converted to seconds
        break;

    case MAV_CMD_DO_LAND_START:                         // MAV ID: 189
        break;

    case MAV_CMD_DO_GO_AROUND:                          // MAV ID: 191
        break;

    case MAV_CMD_DO_SET_ROI:                            // MAV ID: 201
        packet.param1 = cmd.p1;                         // 0 = no roi, 1 = next waypoint, 2 = waypoint number, 3 = fixed location, 4 = given target (not supported)
        break;

    case MAV_CMD_DO_DIGICAM_CONFIGURE:                  // MAV ID: 202
        packet.param1 = cmd.content.digicam_configure.shooting_mode;
        packet.param2 = cmd.content.digicam_configure.shutter_speed;
        packet.param3 = cmd.content.digicam_configure.aperture;
        packet.param4 = cmd.content.digicam_configure.ISO;
        packet.x = cmd.content.digicam_configure.exposure_type;
        packet.y = cmd.content.digicam_configure.cmd_id;
        packet.z = cmd.content.digicam_configure.engine_cutoff_time;
        break;

    case MAV_CMD_DO_DIGICAM_CONTROL:                    // MAV ID: 203
        packet.param1 = cmd.content.digicam_control.session;
        packet.param2 = cmd.content.digicam_control.zoom_pos;
        packet.param3 = cmd.content.digicam_control.zoom_step;
        packet.param4 = cmd.content.digicam_control.focus_lock;
        packet.x = cmd.content.digicam_control.shooting_cmd;
        packet.y = cmd.content.digicam_control.cmd_id;
        break;

    case MAV_CMD_DO_MOUNT_CONTROL:                      // MAV ID: 205
        packet.param1 = cmd.content.mount_control.pitch;
        packet.param2 = cmd.content.mount_control.roll;
        packet.param3 = cmd.content.mount_control.yaw;
        break;

    case MAV_CMD_DO_SET_CAM_TRIGG_DIST:                 // MAV ID: 206
        packet.param1 = cmd.content.cam_trigg_dist.meters;  // distance between camera shots in meters
        packet.param3 = cmd.content.cam_trigg_dist.trigger; // when enabled, camera triggers once immediately
        break;

    case MAV_CMD_DO_FENCE_ENABLE:                       // MAV ID: 207
        packet.param1 = cmd.p1;                         // action 0=disable, 1=enable
        break;

    case MAV_CMD_DO_PARACHUTE:                          // MAV ID: 208
        packet.param1 = cmd.p1;                         // action 0=disable, 1=enable, 2=release.  See PARACHUTE_ACTION enum
        break;

    case MAV_CMD_DO_SPRAYER:
        packet.param1 = cmd.p1;                         // action 0=disable, 1=enable
        break;

    case MAV_CMD_DO_AUX_FUNCTION:
        packet.param1 = cmd.content.auxfunction.function;
        packet.param2 = cmd.content.auxfunction.switchpos;
        break;

    case MAV_CMD_DO_INVERTED_FLIGHT:                    // MAV ID: 210
        packet.param1 = cmd.p1;                         // normal=0 inverted=1
        break;

    case MAV_CMD_DO_GRIPPER:                            // MAV ID: 211
        packet.param1 = cmd.content.gripper.num;        // gripper number
        packet.param2 = cmd.content.gripper.action;     // action 0=release, 1=grab.  See GRIPPER_ACTION enum
        break;

    case MAV_CMD_DO_GUIDED_LIMITS:                      // MAV ID: 222
        packet.param1 = cmd.p1;                         // max time in seconds the external controller will be allowed to control the vehicle
        packet.param2 = cmd.content.guided_limits.alt_min;  // min alt below which the command will be aborted.  0 for no lower alt limit
        packet.param3 = cmd.content.guided_limits.alt_max;  // max alt above which the command will be aborted.  0 for no upper alt limit
        packet.param4 = cmd.content.guided_limits.horiz_max;// max horizontal distance the vehicle can move before the command will be aborted.  0 for no horizontal limit
        break;

    case MAV_CMD_DO_AUTOTUNE_ENABLE:
        packet.param1 = cmd.p1;                         // disable=0 enable=1
        break;

    case MAV_CMD_DO_SET_REVERSE:
        packet.param1 = cmd.p1;   // 0 = forward, 1 = reverse
        break;

    case MAV_CMD_NAV_ALTITUDE_WAIT:                     // MAV ID: 83
        packet.param1 = cmd.content.altitude_wait.altitude;
        packet.param2 = cmd.content.altitude_wait.descent_rate;
        packet.param3 = cmd.content.altitude_wait.wiggle_time;
        break;

    case MAV_CMD_NAV_VTOL_TAKEOFF:
        break;

    case MAV_CMD_NAV_VTOL_LAND:
        break;

    case MAV_CMD_DO_VTOL_TRANSITION:
        packet.param1 = cmd.content.do_vtol_transition.target_state;
        break;

    case MAV_CMD_DO_ENGINE_CONTROL:
        packet.param1 = cmd.content.do_engine_control.start_control?1:0;
        packet.param2 = cmd.content.do_engine_control.cold_start?1:0;
        packet.param3 = cmd.content.do_engine_control.height_delay_cm*0.01f;
        break;

    case MAV_CMD_NAV_PAYLOAD_PLACE:
        packet.param1 = cmd.p1/100.0f; // copy max-descend parameter (cm->m)
        break;

    case MAV_CMD_NAV_SET_YAW_SPEED:
        packet.param1 = cmd.content.set_yaw_speed.angle_deg;        // target angle in degrees
        packet.param2 = cmd.content.set_yaw_speed.speed;            // speed in meters/second
        packet.param3 = cmd.content.set_yaw_speed.relative_angle;   // 0 = absolute angle, 1 = relative angle
        break;

    case MAV_CMD_DO_WINCH:
        packet.param1 = cmd.content.winch.num;              // winch number
        packet.param2 = cmd.content.winch.action;           // action (0 = relax, 1 = length control, 2 = rate control).  See WINCH_ACTION enum
        packet.param3 = cmd.content.winch.release_length;   // cable distance to unwind in meters, negative numbers to wind in cable
        packet.param4 = cmd.content.winch.release_rate;     // release rate in meters/second
        break;

    case MAV_CMD_DO_SET_RESUME_REPEAT_DIST:
        packet.param1 = cmd.p1; // Resume repeat distance (m)
        break;

    case MAV_CMD_DO_SEND_SCRIPT_MESSAGE:
        packet.param1 = cmd.p1;
        packet.param2 = cmd.content.scripting.p1;
        packet.param3 = cmd.content.scripting.p2;
        packet.param4 = cmd.content.scripting.p3;
        break;

    case MAV_CMD_NAV_SCRIPT_TIME:
        packet.param1 = cmd.content.nav_script_time.command;
        packet.param2 = cmd.content.nav_script_time.timeout_s;
        packet.param3 = cmd.content.nav_script_time.arg1;
        packet.param4 = cmd.content.nav_script_time.arg2;
        break;

    case MAV_CMD_NAV_ATTITUDE_TIME:
        packet.param1 = cmd.content.nav_attitude_time.time_sec;
        packet.param2 = cmd.content.nav_attitude_time.roll_deg;
        packet.param3 = cmd.content.nav_attitude_time.pitch_deg;
        packet.param4 = cmd.content.nav_attitude_time.yaw_deg;
        packet.x = cmd.content.nav_attitude_time.climb_rate;
        break;

    case MAV_CMD_DO_PAUSE_CONTINUE:
        packet.param1 = cmd.p1;
        break;

    case MAV_CMD_DO_GIMBAL_MANAGER_PITCHYAW:
        packet.param1 = cmd.content.gimbal_manager_pitchyaw.pitch_angle_deg;
        packet.param2 = cmd.content.gimbal_manager_pitchyaw.yaw_angle_deg;
        packet.param3 = cmd.content.gimbal_manager_pitchyaw.pitch_rate_degs;
        packet.param4 = cmd.content.gimbal_manager_pitchyaw.yaw_rate_degs;
        packet.x = cmd.content.gimbal_manager_pitchyaw.flags;
        packet.z = cmd.content.gimbal_manager_pitchyaw.gimbal_id;
        break;

    default:
        // unrecognised command
        return false;
    }

    // copy location from mavlink to command
    if (stored_in_location(cmd.id)) {
        packet.x = cmd.content.location.lat;
        packet.y = cmd.content.location.lng;

        packet.z = cmd.content.location.alt * 0.01f;   // cmd alt in cm to m
        if (cmd.content.location.relative_alt) {
            packet.frame = MAV_FRAME_GLOBAL_RELATIVE_ALT;
        } else {
            packet.frame = MAV_FRAME_GLOBAL;
        }
#if AP_TERRAIN_AVAILABLE
        if (cmd.content.location.terrain_alt) {
            // this is a above-terrain altitude
            if (!cmd.content.location.relative_alt) {
                // refuse to return non-relative terrain mission
                // items. Internally we do have these, and they
                // have home.alt added, but we should never be
                // returning them to the GCS, as the GCS doesn't know
                // our home.alt, so it would have no way to properly
                // interpret it
                return false;
            }
            packet.z = cmd.content.location.alt * 0.01f;
            packet.frame = MAV_FRAME_GLOBAL_TERRAIN_ALT;
        }
#else
        // don't ever return terrain mission items if no terrain support
        if (cmd.content.location.terrain_alt) {
            return false;
        }
#endif
    }

    // if we got this far then it must have been successful
    return true;
}

///
/// private methods
///

/// complete - mission is marked complete and clean-up performed including calling the mission_complete_fn
void AP_Mission::complete()
{
    // flag mission as complete
    _flags.state = MISSION_COMPLETE;
    _flags.in_landing_sequence = false;

    // callback to main program's mission complete function
    _mission_complete_fn();
}

/// advance_current_nav_cmd - moves current nav command forward
///     do command will also be loaded
///     accounts for do-jump commands
//      returns true if command is advanced, false if failed (i.e. mission completed)
bool AP_Mission::advance_current_nav_cmd(uint16_t starting_index)
{
    // exit immediately if we're not running
    if (_flags.state != MISSION_RUNNING) {
        return false;
    }

    // exit immediately if current nav command has not completed
    if (_flags.nav_cmd_loaded) {
        return false;
    }

    // stop the current running do command
    _do_cmd.index = AP_MISSION_CMD_INDEX_NONE;
    _flags.do_cmd_loaded = false;
    _flags.do_cmd_all_done = false;

    // get starting point for search
    uint16_t cmd_index = starting_index > 0 ? starting_index - 1 : _nav_cmd.index;
    if (cmd_index == AP_MISSION_CMD_INDEX_NONE) {
        // start from beginning of the mission command list
        cmd_index = AP_MISSION_FIRST_REAL_COMMAND;
    } else {
        // start from one position past the current nav command
        cmd_index++;
    }

    // avoid endless loops
    uint8_t max_loops = 255;

    // search until we find next nav command or reach end of command list
    while (!_flags.nav_cmd_loaded && max_loops-- > 0) {
        // get next command
        Mission_Command cmd;
        if (!get_next_cmd(cmd_index, cmd, true)) {
            return false;
        }

        // check if navigation or "do" command
        if (is_nav_cmd(cmd)) {
            // save previous nav command index
            _prev_nav_cmd_id = _nav_cmd.id;
            _prev_nav_cmd_index = _nav_cmd.index;
            // save separate previous nav command index if it contains lat,long,alt
            if (!(cmd.content.location.lat == 0 && cmd.content.location.lng == 0)) {
                _prev_nav_cmd_wp_index = _nav_cmd.index;
            }
            // set current navigation command and start it
            _nav_cmd = cmd;
            if (start_command(_nav_cmd)) {
                _flags.nav_cmd_loaded = true;
            }
            // save a loaded wp index in history array for when _repeat_dist is set via MAV_CMD_DO_SET_RESUME_REPEAT_DIST
            // and prevent history being re-written until vehicle returns to interrupted position
            if (_repeat_dist > 0 && !_flags.resuming_mission && _nav_cmd.index != AP_MISSION_CMD_INDEX_NONE && !(_nav_cmd.content.location.lat == 0 && _nav_cmd.content.location.lng == 0)) {
                // update mission history. last index position is always the most recent wp loaded.
                for (uint8_t i=0; i<AP_MISSION_MAX_WP_HISTORY-1; i++) {
                    _wp_index_history[i] = _wp_index_history[i+1];
                }
                _wp_index_history[AP_MISSION_MAX_WP_HISTORY-1] = _nav_cmd.index;
            }
            // check if the vehicle is resuming and has returned to where it was interrupted
            if (_flags.resuming_mission && _nav_cmd.index == _wp_index_history[AP_MISSION_MAX_WP_HISTORY-1]) {
                // vehicle has resumed previous position
                gcs().send_text(MAV_SEVERITY_INFO, "Mission: Returned to interrupted WP");
                _flags.resuming_mission = false;
            }

        } else {
            // set current do command and start it (if not already set)
            if (!_flags.do_cmd_loaded) {
                _do_cmd = cmd;
                _flags.do_cmd_loaded = true;
                start_command(_do_cmd);
            }
        }
        // move onto next command
        cmd_index = cmd.index+1;
    }

    if (max_loops == 0) {
        // infinite loop.  This can happen if there's a loop involving
        // only nav commands (no DO commands) which won't start()
        return false;
    }

    // if we have not found a do command then set flag to show there are no do-commands to be run before nav command completes
    if (!_flags.do_cmd_loaded) {
        _flags.do_cmd_all_done = true;
    }

    // if we got this far we must have successfully advanced the nav command
    return true;
}

/// advance_current_do_cmd - moves current do command forward
///     accounts for do-jump commands
void AP_Mission::advance_current_do_cmd()
{
    // exit immediately if we're not running or we've completed all possible "do" commands
    if (_flags.state != MISSION_RUNNING || _flags.do_cmd_all_done) {
        return;
    }

    // get starting point for search
    uint16_t cmd_index = _do_cmd.index;
    if (cmd_index == AP_MISSION_CMD_INDEX_NONE) {
        cmd_index = AP_MISSION_FIRST_REAL_COMMAND;
    } else {
        // start from one position past the current do command
        cmd_index = _do_cmd.index + 1;
    }

    // find next do command
    Mission_Command cmd;
    if (!get_next_do_cmd(cmd_index, cmd)) {
        // set flag to stop unnecessarily searching for do commands
        _flags.do_cmd_all_done = true;
        return;
    }

    // set current do command and start it
    _do_cmd = cmd;
    _flags.do_cmd_loaded = true;
    start_command(_do_cmd);
}

/// get_next_cmd - gets next command found at or after start_index
///     returns true if found, false if not found (i.e. mission complete)
///     accounts for do_jump commands
///     increment_jump_num_times_if_found should be set to true if advancing the active navigation command
bool AP_Mission::get_next_cmd(uint16_t start_index, Mission_Command& cmd, bool increment_jump_num_times_if_found, bool send_gcs_msg)
{
    uint16_t cmd_index = start_index;
    Mission_Command temp_cmd;
    uint16_t jump_index = AP_MISSION_CMD_INDEX_NONE;

    // search until the end of the mission command list
    uint8_t max_loops = 64;
    while (cmd_index < (unsigned)_cmd_total) {
        // load the next command
        if (!read_cmd_from_storage(cmd_index, temp_cmd)) {
            // this should never happen because of check above but just in case
            return false;
        }

        // check for do-jump command
        if (temp_cmd.id == MAV_CMD_DO_JUMP) {

            if (max_loops-- == 0) {
                return false;
            }

            // check for invalid target
            if ((temp_cmd.content.jump.target >= (unsigned)_cmd_total) || (temp_cmd.content.jump.target == 0)) {
                // To-Do: log an error?
                return false;
            }

            // check for endless loops
            if (!increment_jump_num_times_if_found && jump_index == cmd_index) {
                // we have somehow reached this jump command twice and there is no chance it will complete
                // To-Do: log an error?
                return false;
            }

            // record this command so we can check for endless loops
            if (jump_index == AP_MISSION_CMD_INDEX_NONE) {
                jump_index = cmd_index;
            }

            // check if jump command is 'repeat forever'
            if (temp_cmd.content.jump.num_times == AP_MISSION_JUMP_REPEAT_FOREVER) {
                // continue searching from jump target
                cmd_index = temp_cmd.content.jump.target;
            } else {
                // get number of times jump command has already been run
                int16_t jump_times_run = get_jump_times_run(temp_cmd);
                if (jump_times_run < temp_cmd.content.jump.num_times) {
                    // update the record of the number of times run
                    if (increment_jump_num_times_if_found && !_flags.resuming_mission) {
                        increment_jump_times_run(temp_cmd, send_gcs_msg);
                    }
                    // continue searching from jump target
                    cmd_index = temp_cmd.content.jump.target;
                } else {
                    // jump has been run specified number of times so move search to next command in mission
                    cmd_index++;
                }
            }
        } else {
            // this is a non-jump command so return it
            cmd = temp_cmd;
            return true;
        }
    }

    // if we got this far we did not find a navigation command
    return false;
}

/// get_next_do_cmd - gets next "do" or "conditional" command after start_index
///     returns true if found, false if not found
///     stops and returns false if it hits another navigation command before it finds the first do or conditional command
///     accounts for do_jump commands but never increments the jump's num_times_run (advance_current_nav_cmd is responsible for this)
bool AP_Mission::get_next_do_cmd(uint16_t start_index, Mission_Command& cmd)
{
    Mission_Command temp_cmd;

    // check we have not passed the end of the mission list
    if (start_index >= (unsigned)_cmd_total) {
        return false;
    }

    // get next command
    if (!get_next_cmd(start_index, temp_cmd, false)) {
        // no more commands so return failure
        return false;
    } else if (is_nav_cmd(temp_cmd)) {
        // if it's a "navigation" command then return false because we do not progress past nav commands
        return false;
    } else {
        // this must be a "do" or "conditional" and is not a do-jump command so return it
        cmd = temp_cmd;
        return true;
    }
}

///
/// jump handling methods
///

// init_jump_tracking - initialise jump_tracking variables
void AP_Mission::init_jump_tracking()
{
    for (uint8_t i=0; i<AP_MISSION_MAX_NUM_DO_JUMP_COMMANDS; i++) {
        _jump_tracking[i].index = AP_MISSION_CMD_INDEX_NONE;
        _jump_tracking[i].num_times_run = 0;
    }
}

/// get_jump_times_run - returns number of times the jump command has been run
int16_t AP_Mission::get_jump_times_run(const Mission_Command& cmd)
{
    // exit immediately if cmd is not a do-jump command or target is invalid
    if ((cmd.id != MAV_CMD_DO_JUMP) || (cmd.content.jump.target >= (unsigned)_cmd_total) || (cmd.content.jump.target == 0)) {
        // To-Do: log an error?
        return AP_MISSION_JUMP_TIMES_MAX;
    }

    // search through jump_tracking array for this cmd
    for (uint8_t i=0; i<AP_MISSION_MAX_NUM_DO_JUMP_COMMANDS; i++) {
        if (_jump_tracking[i].index == cmd.index) {
            return _jump_tracking[i].num_times_run;
        } else if (_jump_tracking[i].index == AP_MISSION_CMD_INDEX_NONE) {
            // we've searched through all known jump commands and haven't found it so allocate new space in _jump_tracking array
            _jump_tracking[i].index = cmd.index;
            _jump_tracking[i].num_times_run = 0;
            return 0;
        }
    }

    // if we've gotten this far then the _jump_tracking array must be full
    // To-Do: log an error?
    return AP_MISSION_JUMP_TIMES_MAX;
}

/// increment_jump_times_run - increments the recorded number of times the jump command has been run
void AP_Mission::increment_jump_times_run(Mission_Command& cmd, bool send_gcs_msg)
{
    // exit immediately if cmd is not a do-jump command
    if (cmd.id != MAV_CMD_DO_JUMP) {
        // To-Do: log an error?
        return;
    }

    // search through jump_tracking array for this cmd
    for (uint8_t i=0; i<AP_MISSION_MAX_NUM_DO_JUMP_COMMANDS; i++) {
        if (_jump_tracking[i].index == cmd.index) {
            _jump_tracking[i].num_times_run++;
            if (send_gcs_msg) {
                gcs().send_text(MAV_SEVERITY_INFO, "Mission: %u Jump %i/%i", _jump_tracking[i].index, _jump_tracking[i].num_times_run, cmd.content.jump.num_times);
            }
            return;
        } else if (_jump_tracking[i].index == AP_MISSION_CMD_INDEX_NONE) {
            // we've searched through all known jump commands and haven't found it so allocate new space in _jump_tracking array
            _jump_tracking[i].index = cmd.index;
            _jump_tracking[i].num_times_run = 1;
            return;
        }
    }

    // if we've gotten this far then the _jump_tracking array must be full
    // To-Do: log an error
    return;
}

// check_eeprom_version - checks version of missions stored in eeprom matches this library
// command list will be cleared if they do not match
void AP_Mission::check_eeprom_version()
{
    uint32_t eeprom_version = _storage.read_uint32(0);

    // if eeprom version does not match, clear the command list and update the eeprom version
    if (eeprom_version != AP_MISSION_EEPROM_VERSION) {
        if (clear()) {
            _storage.write_uint32(0, AP_MISSION_EEPROM_VERSION);
        }
    }
}

/*
  return total number of commands that can fit in storage space
 */
uint16_t AP_Mission::num_commands_max(void) const
{
    // -4 to remove space for eeprom version number
    return (_storage.size() - 4) / AP_MISSION_EEPROM_COMMAND_SIZE;
}

// find the nearest landing sequence starting point (DO_LAND_START) and
// return its index.  Returns 0 if no appropriate DO_LAND_START point can
// be found.
uint16_t AP_Mission::get_landing_sequence_start()
{
    struct Location current_loc;

    if (!AP::ahrs().get_location(current_loc)) {
        return 0;
    }

    uint16_t landing_start_index = 0;
    float min_distance = -1;

    // Go through mission looking for nearest landing start command
    for (uint16_t i = 1; i < num_commands(); i++) {
        Mission_Command tmp;
        if (!read_cmd_from_storage(i, tmp)) {
            continue;
        }
        if (tmp.id == MAV_CMD_DO_LAND_START) {
            if (!tmp.content.location.initialised() && !get_next_nav_cmd(i, tmp)) {
                // command does not have a valid location and cannot get next valid
                continue;
            }
            float tmp_distance = tmp.content.location.get_distance(current_loc);
            if (min_distance < 0 || tmp_distance < min_distance) {
                min_distance = tmp_distance;
                landing_start_index = i;
            }
        }
    }

    return landing_start_index;
}

/*
   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 AP_Mission::jump_to_landing_sequence(void)
{
    uint16_t land_idx = get_landing_sequence_start();
    if (land_idx != 0 && set_current_cmd(land_idx)) {

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

        gcs().send_text(MAV_SEVERITY_INFO, "Landing sequence start");
        _flags.in_landing_sequence = true;
        return true;
    }

    gcs().send_text(MAV_SEVERITY_WARNING, "Unable to start landing sequence");
    return false;
}

// jumps the mission to the closest landing abort that is planned, returns false if unable to find a valid abort
bool AP_Mission::jump_to_abort_landing_sequence(void)
{
    struct Location current_loc;

    uint16_t abort_index = 0;
    if (AP::ahrs().get_location(current_loc)) {
        float min_distance = FLT_MAX;

        for (uint16_t i = 1; i < num_commands(); i++) {
            Mission_Command tmp;
            if (!read_cmd_from_storage(i, tmp)) {
                continue;
            }
            if (tmp.id == MAV_CMD_DO_GO_AROUND) {
                float tmp_distance = tmp.content.location.get_distance(current_loc);
                if (tmp_distance < min_distance) {
                    min_distance = tmp_distance;
                    abort_index = i;
                }
            }
        }
    }

    if (abort_index != 0 && set_current_cmd(abort_index)) {

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

        _flags.in_landing_sequence = false;

        gcs().send_text(MAV_SEVERITY_INFO, "Landing abort sequence start");
        return true;
    }

    gcs().send_text(MAV_SEVERITY_WARNING, "Unable to start find a landing abort sequence");
    return false;
}

// check which is the shortest route to landing an RTL via a DO_LAND_START or continuing on the current mission plan
bool AP_Mission::is_best_land_sequence(void)
{
    // check if there is even a running mission to interupt
    if (_flags.state != MISSION_RUNNING) {
        return false;
    }

    // check if aircraft has already jumped to a landing sequence
    if (_flags.in_landing_sequence) {
        return true;
    }

    // check if MIS_OPTIONS bit set to allow distance calculation to be done
    if (!(_options & AP_MISSION_MASK_DIST_TO_LAND_CALC)) {
        return false;
    }

    // The decision to allow a failsafe to interupt a potential landing approach
    // is a distance travelled minimization problem.  Look forward in
    // mission to evaluate the shortest remaining distance to land.

    // go through the mission for the nearest DO_LAND_START first as this is the most probable route
    // to a landing with the minimum number of WP.
    uint16_t do_land_start_index = get_landing_sequence_start();
    if (do_land_start_index == 0) {
        // then no DO_LAND_START commands are in mission and normal failsafe behaviour should be maintained
        return false;
    }

    // get our current location
    Location current_loc;
    if (!AP::ahrs().get_location(current_loc)) {
        // we don't know where we are!!
        return false;
    }

    // get distance to landing if travelled to nearest DO_LAND_START via RTL
    float dist_via_do_land;
    if (!distance_to_landing(do_land_start_index, dist_via_do_land, current_loc)) {
        // cant get a valid distance to landing
        return false;
    }

    // get distance to landing if continue along current mission path
    float dist_continue_to_land;
    if (!distance_to_landing(_nav_cmd.index, dist_continue_to_land, current_loc)) {
        // cant get a valid distance to landing
        return false;
    }

    // compare distances
    if (dist_via_do_land >= dist_continue_to_land) {
        // then the mission should carry on uninterrupted as that is the shorter distance
        gcs().send_text(MAV_SEVERITY_NOTICE, "Rejecting RTL: closer land if mis continued");
        return true;
    } else {
        // allow failsafes to interrupt the current mission
        return false;
    }
}

// Approximate the distance travelled to get to a landing.  DO_JUMP commands are observed in look forward.
bool AP_Mission::distance_to_landing(uint16_t index, float &tot_distance, Location prev_loc)
{
    Mission_Command temp_cmd;
    tot_distance = 0.0f;
    bool ret;

    // back up jump tracking to reset after distance calculation
    jump_tracking_struct _jump_tracking_backup[AP_MISSION_MAX_NUM_DO_JUMP_COMMANDS];
    for (uint8_t i=0; i<AP_MISSION_MAX_NUM_DO_JUMP_COMMANDS; i++) {
        _jump_tracking_backup[i] = _jump_tracking[i];
    }

    // run through remainder of mission to approximate a distance to landing
    for (uint8_t i=0; i<255; i++) {
        // search until the end of the mission command list
        for (uint16_t cmd_index = index; cmd_index < (unsigned)_cmd_total; cmd_index++) {
            // get next command
            if (!get_next_cmd(cmd_index, temp_cmd, true, false)) {
                // we got to the end of the mission
                ret = false;
                goto reset_do_jump_tracking;
            }
            if (temp_cmd.id == MAV_CMD_NAV_WAYPOINT || temp_cmd.id == MAV_CMD_NAV_SPLINE_WAYPOINT || is_landing_type_cmd(temp_cmd.id)) {
                break;
            } else if (is_nav_cmd(temp_cmd) || temp_cmd.id == MAV_CMD_CONDITION_DELAY) {
                // if we receive a nav command that we dont handle then give up as cant measure the distance e.g. MAV_CMD_NAV_LOITER_UNLIM
                ret = false;
                goto reset_do_jump_tracking;
            }
        }
        index = temp_cmd.index+1;

        if (!(temp_cmd.content.location.lat == 0 && temp_cmd.content.location.lng == 0)) {
            // add distance to running total
            float disttemp = prev_loc.get_distance(temp_cmd.content.location);
            tot_distance = tot_distance + disttemp;

            // store wp location as previous
            prev_loc = temp_cmd.content.location;
        }

        if (is_landing_type_cmd(temp_cmd.id)) {
            // reached a landing!
            ret = true;
            goto reset_do_jump_tracking;
        }
    }
    // reached end of loop without getting to a landing
    ret = false;

reset_do_jump_tracking:
    for (uint8_t i=0; i<AP_MISSION_MAX_NUM_DO_JUMP_COMMANDS; i++) {
        _jump_tracking[i] = _jump_tracking_backup[i];
    }

    return ret;
}

// check if command is a landing type command.
bool AP_Mission::is_landing_type_cmd(uint16_t id) const
{
    switch (id) {
    case MAV_CMD_NAV_LAND:
    case MAV_CMD_NAV_VTOL_LAND:
    case MAV_CMD_DO_PARACHUTE:
        return true;
    default:
        return false;
    }
}

const char *AP_Mission::Mission_Command::type() const
{
    switch (id) {
    case MAV_CMD_NAV_WAYPOINT:
        return "WP";
    case MAV_CMD_NAV_SPLINE_WAYPOINT:
        return "SplineWP";
    case MAV_CMD_NAV_RETURN_TO_LAUNCH:
        return "RTL";
    case MAV_CMD_NAV_LOITER_UNLIM:
        return "LoitUnlim";
    case MAV_CMD_NAV_LOITER_TIME:
        return "LoitTime";
    case MAV_CMD_NAV_GUIDED_ENABLE:
        return "GuidedEnable";
    case MAV_CMD_NAV_LOITER_TURNS:
        return "LoitTurns";
    case MAV_CMD_NAV_LOITER_TO_ALT:
        return "LoitAltitude";
    case MAV_CMD_NAV_SET_YAW_SPEED:
        return "SetYawSpd";
    case MAV_CMD_CONDITION_DELAY:
        return "CondDelay";
    case MAV_CMD_CONDITION_DISTANCE:
        return "CondDist";
    case MAV_CMD_DO_CHANGE_SPEED:
        return "ChangeSpeed";
    case MAV_CMD_DO_SET_HOME:
        return "SetHome";
    case MAV_CMD_DO_SET_SERVO:
        return "SetServo";
    case MAV_CMD_DO_SET_RELAY:
        return "SetRelay";
    case MAV_CMD_DO_REPEAT_SERVO:
        return "RepeatServo";
    case MAV_CMD_DO_REPEAT_RELAY:
        return "RepeatRelay";
    case MAV_CMD_DO_CONTROL_VIDEO:
        return "CtrlVideo";
    case MAV_CMD_DO_DIGICAM_CONFIGURE:
        return "DigiCamCfg";
    case MAV_CMD_DO_DIGICAM_CONTROL:
        return "DigiCamCtrl";
    case MAV_CMD_DO_SET_CAM_TRIGG_DIST:
        return "SetCamTrigDst";
    case MAV_CMD_DO_SET_ROI:
        return "SetROI";
    case MAV_CMD_DO_SET_REVERSE:
        return "SetReverse";
    case MAV_CMD_DO_SET_RESUME_REPEAT_DIST:
        return "SetRepeatDist";
    case MAV_CMD_DO_GUIDED_LIMITS:
        return "GuidedLimits";
    case MAV_CMD_NAV_TAKEOFF:
        return "Takeoff";
    case MAV_CMD_NAV_LAND:
        return "Land";
    case MAV_CMD_NAV_CONTINUE_AND_CHANGE_ALT:
        return "ContinueAndChangeAlt";
    case MAV_CMD_NAV_ALTITUDE_WAIT:
        return "AltitudeWait";
    case MAV_CMD_NAV_VTOL_TAKEOFF:
        return "VTOLTakeoff";
    case MAV_CMD_NAV_VTOL_LAND:
        return "VTOLLand";
    case MAV_CMD_DO_INVERTED_FLIGHT:
        return "InvertedFlight";
    case MAV_CMD_DO_FENCE_ENABLE:
        return "FenceEnable";
    case MAV_CMD_DO_AUTOTUNE_ENABLE:
        return "AutoTuneEnable";
    case MAV_CMD_DO_VTOL_TRANSITION:
        return "VTOLTransition";
    case MAV_CMD_DO_ENGINE_CONTROL:
        return "EngineControl";
    case MAV_CMD_CONDITION_YAW:
        return "CondYaw";
    case MAV_CMD_DO_LAND_START:
        return "LandStart";
    case MAV_CMD_NAV_DELAY:
        return "Delay";
    case MAV_CMD_DO_GRIPPER:
        return "Gripper";
    case MAV_CMD_NAV_PAYLOAD_PLACE:
        return "PayloadPlace";
    case MAV_CMD_DO_PARACHUTE:
        return "Parachute";
    case MAV_CMD_DO_SPRAYER:
        return "Sprayer";
    case MAV_CMD_DO_AUX_FUNCTION:
        return "AuxFunction";
    case MAV_CMD_DO_MOUNT_CONTROL:
        return "MountControl";
    case MAV_CMD_DO_WINCH:
        return "Winch";
    case MAV_CMD_DO_SEND_SCRIPT_MESSAGE:
        return "Scripting";
    case MAV_CMD_DO_JUMP:
        return "Jump";
    case MAV_CMD_DO_GO_AROUND:
        return "Go Around";
    case MAV_CMD_NAV_SCRIPT_TIME:
        return "NavScriptTime";
    case MAV_CMD_NAV_ATTITUDE_TIME:
        return "NavAttitudeTime";
    case MAV_CMD_DO_PAUSE_CONTINUE:
        return "PauseContinue";
    case MAV_CMD_DO_GIMBAL_MANAGER_PITCHYAW:
        return "GimbalPitchYaw";
    default:
#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
        AP_HAL::panic("Mission command with ID %u has no string", id);
#endif
        return "?";
    }
}

bool AP_Mission::contains_item(MAV_CMD command) const
{
    for (int i = 1; i < num_commands(); i++) {
        Mission_Command tmp;
        if (!read_cmd_from_storage(i, tmp)) {
            continue;
        }
        if (tmp.id == command) {
            return true;
        }
    }
    return false;
}

// reset the mission history to prevent recalling previous mission histories after a mission restart.
void AP_Mission::reset_wp_history(void)
{
    for (uint8_t i = 0; i<AP_MISSION_MAX_WP_HISTORY; i++) {
        _wp_index_history[i] = AP_MISSION_CMD_INDEX_NONE;
    }
    _resume_cmd.index = AP_MISSION_CMD_INDEX_NONE;
    _flags.resuming_mission = false;
    _repeat_dist = 0;
}

// store the latest reported position incase of mission exit and rewind resume
void AP_Mission::update_exit_position(void)
{
    if (!AP::ahrs().get_location(_exit_position)) {
        _exit_position.lat = 0;
        _exit_position.lng = 0;
    }
}

// calculate the location of the _resume_cmd wp and set as current
bool AP_Mission::calc_rewind_pos(Mission_Command& rewind_cmd)
{
    // check for a recent history
    if (_wp_index_history[LAST_WP_PASSED] == AP_MISSION_CMD_INDEX_NONE) {
        // no saved history so can't rewind
        return false;
    }

    // check that we have a valid exit position
    if (_exit_position.lat == 0 && _exit_position.lng == 0) {
        return false;
    }

    Mission_Command temp_cmd;
    float rewind_distance = _repeat_dist; //(m)
    uint16_t resume_index;
    Location prev_loc = _exit_position;

    for (int8_t i = (LAST_WP_PASSED); i>=0; i--) {

        // to get this far there has to be at least one 'passed wp' stored in history.  This is to check incase
        // of history array no being completely filled with valid waypoints upon resume.
        if (_wp_index_history[i] == AP_MISSION_CMD_INDEX_NONE) {
            // no more stored history
            resume_index = i+1;
            break;
        }

        if (!read_cmd_from_storage(_wp_index_history[i], temp_cmd)) {
            // if read from storage failed then don't use rewind
            return false;
        }

        // calculate distance
        float disttemp = prev_loc.get_distance(temp_cmd.content.location); //(m)
        rewind_distance -= disttemp;
        resume_index = i;

        if (rewind_distance <= 0.0f) {
            // history rewound enough distance to meet _repeat_dist requirement
            rewind_cmd = temp_cmd;
            break;
        }

        // store wp location as previous
        prev_loc = temp_cmd.content.location;
    }

    if (rewind_distance > 0.0f) {
        // then the history array was rewound all of the way without finding a wp distance > _repeat_dist distance.
        // the last read temp_cmd will be the furthest cmd back in the history array so resume to that.
        rewind_cmd = temp_cmd;
        return true;
    }

    // if we have got this far the desired rewind distance lies between two waypoints stored in history array.
    // calculate the location for the mission to resume

    // the last wp read from storage is the wp that is before the resume wp in the mission order
    Location passed_wp_loc = temp_cmd.content.location;

    // fetch next destination wp
    if (!read_cmd_from_storage(_wp_index_history[resume_index+1], temp_cmd)) {
        // if read from storage failed then don't use rewind
        return false;
    }

    // determine the length of the mission leg that the resume wp lies in
    float leg_length = passed_wp_loc.get_distance(temp_cmd.content.location); //(m)

    // calculate the percentage along the leg that resume wp will be positioned
    float leg_percent = fabsf(rewind_distance)/leg_length;

    // calculate difference vector of mission leg
    Vector3f dist_vec = passed_wp_loc.get_distance_NED(temp_cmd.content.location);

    // calculate the resume wp position
    rewind_cmd.content.location.offset(dist_vec.x * leg_percent, dist_vec.y * leg_percent);
    rewind_cmd.content.location.alt -= dist_vec.z * leg_percent * 100; //(cm)

    // The rewind_cmd.index has the index of the 'last passed wp' from the history array.  This ensures that the mission order
    // continues as planned without further intervention.  The resume wp is not written to memory so will not perminantely change the mission.

    // if we got this far then mission rewind position was successfully calculated
    return true;
}

// singleton instance
AP_Mission *AP_Mission::_singleton;

namespace AP
{

AP_Mission *mission()
{
    return AP_Mission::get_singleton();
}

}