/*
   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

//
//  u-blox GPS driver for ArduPilot
//	Origin code by Michael Smith, Jordi Munoz and Jose Julio, DIYDrones.com
//  Substantially rewritten for new GPS driver structure by Andrew Tridgell
//
#include "AP_GPS.h"
#include "AP_GPS_UBLOX.h"
#include <AP_HAL/Util.h>
#include <AP_Logger/AP_Logger.h>
#include <GCS_MAVLink/GCS.h>

#if CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_NAVIO || \
    CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_BH
    #define UBLOX_SPEED_CHANGE  1
#else
    #define UBLOX_SPEED_CHANGE 0
#endif


#define UBLOX_DEBUGGING 0
#define UBLOX_FAKE_3DLOCK 0
#define CONFIGURE_PPS_PIN 0

extern const AP_HAL::HAL& hal;

#ifdef HAL_NO_GCS
#define GCS_SEND_TEXT(severity, format, args...)
#else
#define GCS_SEND_TEXT(severity, format, args...) gcs().send_text(severity, format, ##args)
#endif

#if UBLOX_DEBUGGING
 # define Debug(fmt, args ...)  do {hal.console->printf("%s:%d: " fmt "\n", __FUNCTION__, __LINE__, ## args); hal.scheduler->delay(1); } while(0)
#else
 # define Debug(fmt, args ...)
#endif

AP_GPS_UBLOX::AP_GPS_UBLOX(AP_GPS &_gps, AP_GPS::GPS_State &_state, AP_HAL::UARTDriver *_port) :
    AP_GPS_Backend(_gps, _state, _port),
    _next_message(STEP_PVT),
    _ublox_port(255),
    _unconfigured_messages(CONFIG_ALL),
    _hardware_generation(UBLOX_UNKNOWN_HARDWARE_GENERATION),
    next_fix(AP_GPS::NO_FIX),
    noReceivedHdop(true)
{
    // stop any config strings that are pending
    gps.send_blob_start(state.instance, nullptr, 0);

    // start the process of updating the GPS rates
    _request_next_config();

#if CONFIGURE_PPS_PIN
    _unconfigured_messages |= CONFIG_TP5;
#endif
}

void
AP_GPS_UBLOX::_request_next_config(void)
{
    // don't request config if we shouldn't configure the GPS
    if (gps._auto_config == AP_GPS::GPS_AUTO_CONFIG_DISABLE) {
        return;
    }

    // Ensure there is enough space for the largest possible outgoing message
    if (port->txspace() < (int16_t)(sizeof(struct ubx_header)+sizeof(struct ubx_cfg_nav_rate)+2)) {
        // not enough space - do it next time
        return;
    }

    if (_unconfigured_messages == CONFIG_RATE_SOL && havePvtMsg) {
        /*
          we don't need SOL if we have PVT and TIMEGPS. This is needed
          as F9P doesn't support the SOL message
         */
        _unconfigured_messages &= ~CONFIG_RATE_SOL;
    }

    Debug("Unconfigured messages: 0x%x Current message: %u\n", (unsigned)_unconfigured_messages, (unsigned)_next_message);

    // check AP_GPS_UBLOX.h for the enum that controls the order.
    // This switch statement isn't maintained against the enum in order to reduce code churn
    switch (_next_message++) {
    case STEP_PVT:
        if(!_request_message_rate(CLASS_NAV, MSG_PVT)) {
            _next_message--;
        }
        break;
    case STEP_TIMEGPS:
        if(!_request_message_rate(CLASS_NAV, MSG_TIMEGPS)) {
            _next_message--;
        }
        break;
    case STEP_PORT:
        _request_port();
        break;
    case STEP_POLL_SVINFO:
        // not required once we know what generation we are on
        if(_hardware_generation == UBLOX_UNKNOWN_HARDWARE_GENERATION) {
            if (!_send_message(CLASS_NAV, MSG_NAV_SVINFO, 0, 0)) {
                _next_message--;
            }
        }
        break;
    case STEP_POLL_SBAS:
        if (gps._sbas_mode != 2) {
            _send_message(CLASS_CFG, MSG_CFG_SBAS, nullptr, 0);
        } else {
            _unconfigured_messages &= ~CONFIG_SBAS;
        }
        break;
    case STEP_POLL_NAV:
        if (!_send_message(CLASS_CFG, MSG_CFG_NAV_SETTINGS, nullptr, 0)) {
            _next_message--;
        }
        break;
    case STEP_POLL_GNSS:
        if (!_send_message(CLASS_CFG, MSG_CFG_GNSS, nullptr, 0)) {
            _next_message--;
        }
        break;
    case STEP_POLL_TP5:
#if CONFIGURE_PPS_PIN
        if (!_send_message(CLASS_CFG, MSG_CFG_TP5, nullptr, 0)) {
            _next_message--;
        }
#endif
        break;
    case STEP_NAV_RATE:
        if (!_send_message(CLASS_CFG, MSG_CFG_RATE, nullptr, 0)) {
            _next_message--;
        }
        break;
    case STEP_POSLLH:
        if(!_request_message_rate(CLASS_NAV, MSG_POSLLH)) {
            _next_message--;
        }
        break;
    case STEP_STATUS:
        if(!_request_message_rate(CLASS_NAV, MSG_STATUS)) {
            _next_message--;
        }
        break;
    case STEP_SOL:
        if(!_request_message_rate(CLASS_NAV, MSG_SOL)) {
            _next_message--;
        }
        break;
    case STEP_VELNED:
        if(!_request_message_rate(CLASS_NAV, MSG_VELNED)) {
            _next_message--;
        }
        break;
    case STEP_DOP:
       if(! _request_message_rate(CLASS_NAV, MSG_DOP)) {
            _next_message--;
        }
        break;
    case STEP_MON_HW:
        if(!_request_message_rate(CLASS_MON, MSG_MON_HW)) {
            _next_message--;
        }
        break;
    case STEP_MON_HW2:
        if(!_request_message_rate(CLASS_MON, MSG_MON_HW2)) {
            _next_message--;
        }
        break;
    case STEP_RAW:
#if UBLOX_RXM_RAW_LOGGING
        if(gps._raw_data == 0) {
            _unconfigured_messages &= ~CONFIG_RATE_RAW;
        } else if(!_request_message_rate(CLASS_RXM, MSG_RXM_RAW)) {
            _next_message--;
        }
#else
        _unconfigured_messages & = ~CONFIG_RATE_RAW;
#endif
        break;
    case STEP_RAWX:
#if UBLOX_RXM_RAW_LOGGING
        if(gps._raw_data == 0) {
            _unconfigured_messages &= ~CONFIG_RATE_RAW;
        } else if(!_request_message_rate(CLASS_RXM, MSG_RXM_RAWX)) {
            _next_message--;
        }
#else
        _unconfigured_messages & = ~CONFIG_RATE_RAW;
#endif
        break;
    case STEP_VERSION:
        if(!_have_version && !hal.util->get_soft_armed()) {
            _request_version();
        } else {
            _unconfigured_messages &= ~CONFIG_VERSION;
        }
        // no need to send the initial rates, move to checking only
        _next_message = STEP_PVT;
        break;
    default:
        // this case should never be reached, do a full reset if it is hit
        _next_message = STEP_PVT;
        break;
    }
}

void
AP_GPS_UBLOX::_verify_rate(uint8_t msg_class, uint8_t msg_id, uint8_t rate) {
    uint8_t desired_rate;

    switch(msg_class) {
    case CLASS_NAV:
        switch(msg_id) {
        case MSG_POSLLH:
            desired_rate = havePvtMsg ? 0 : RATE_POSLLH;
            if(rate == desired_rate) {
                _unconfigured_messages &= ~CONFIG_RATE_POSLLH;
            } else {
                _configure_message_rate(msg_class, msg_id, desired_rate);
                _unconfigured_messages |= CONFIG_RATE_POSLLH;
                _cfg_needs_save = true;
            }
            break;
        case MSG_STATUS:
            desired_rate = havePvtMsg ? 0 : RATE_STATUS;
            if(rate == desired_rate) {
                _unconfigured_messages &= ~CONFIG_RATE_STATUS;
            } else {
                _configure_message_rate(msg_class, msg_id, desired_rate);
                _unconfigured_messages |= CONFIG_RATE_STATUS;
                _cfg_needs_save = true;
            }
            break;
        case MSG_SOL:
            desired_rate = havePvtMsg ? 0 : RATE_SOL;
            if(rate == desired_rate) {
                _unconfigured_messages &= ~CONFIG_RATE_SOL;
            } else {
                _configure_message_rate(msg_class, msg_id, desired_rate);
                _unconfigured_messages |= CONFIG_RATE_SOL;
                _cfg_needs_save = true;
            }
            break;
        case MSG_PVT:
            if(rate == RATE_PVT) {
                _unconfigured_messages &= ~CONFIG_RATE_PVT;
            } else {
                _configure_message_rate(msg_class, msg_id, RATE_PVT);
                _unconfigured_messages |= CONFIG_RATE_PVT;
                _cfg_needs_save = true;
            }
            break;
        case MSG_TIMEGPS:
            if(rate == RATE_TIMEGPS) {
                _unconfigured_messages &= ~CONFIG_RATE_TIMEGPS;
            } else {
                _configure_message_rate(msg_class, msg_id, RATE_TIMEGPS);
                _unconfigured_messages |= CONFIG_RATE_TIMEGPS;
                _cfg_needs_save = true;
            }
            break;
        case MSG_VELNED:
            desired_rate = havePvtMsg ? 0 : RATE_VELNED;
            if(rate == desired_rate) {
                _unconfigured_messages &= ~CONFIG_RATE_VELNED;
            } else {
                _configure_message_rate(msg_class, msg_id, desired_rate);
                _unconfigured_messages |= CONFIG_RATE_VELNED;
                _cfg_needs_save = true;
            }
            break;
        case MSG_DOP:
            if(rate == RATE_DOP) {
                _unconfigured_messages &= ~CONFIG_RATE_DOP;
            } else {
                _configure_message_rate(msg_class, msg_id, RATE_DOP);
                _unconfigured_messages |= CONFIG_RATE_DOP;
                _cfg_needs_save = true;
            }
            break;
        }
        break;
    case CLASS_MON:
        switch(msg_id) {
        case MSG_MON_HW:
            if(rate == RATE_HW) {
                _unconfigured_messages &= ~CONFIG_RATE_MON_HW;
            } else {
                _configure_message_rate(msg_class, msg_id, RATE_HW);
                _unconfigured_messages |= CONFIG_RATE_MON_HW;
                _cfg_needs_save = true;
            }
            break;
        case MSG_MON_HW2:
            if(rate == RATE_HW2) {
                _unconfigured_messages &= ~CONFIG_RATE_MON_HW2;
            } else {
                _configure_message_rate(msg_class, msg_id, RATE_HW2);
                _unconfigured_messages |= CONFIG_RATE_MON_HW2;
                _cfg_needs_save = true;
            }
            break;
        }
        break;
#if UBLOX_RXM_RAW_LOGGING
    case CLASS_RXM:
        switch(msg_id) {
        case MSG_RXM_RAW:
            if(rate == gps._raw_data) {
                _unconfigured_messages &= ~CONFIG_RATE_RAW;
            } else {
                _configure_message_rate(msg_class, msg_id, gps._raw_data);
                _unconfigured_messages |= CONFIG_RATE_RAW;
                _cfg_needs_save = true;
            }
            break;
        case MSG_RXM_RAWX:
            if(rate == gps._raw_data) {
                _unconfigured_messages &= ~CONFIG_RATE_RAW;
            } else {
                _configure_message_rate(msg_class, msg_id, gps._raw_data);
                _unconfigured_messages |= CONFIG_RATE_RAW;
                _cfg_needs_save = true;
            }
            break;
        }
        break;
#endif // UBLOX_RXM_RAW_LOGGING
    }
}

// Requests the ublox driver to identify what port we are using to communicate
void
AP_GPS_UBLOX::_request_port(void)
{
    if (port->txspace() < (int16_t)(sizeof(struct ubx_header)+2)) {
        // not enough space - do it next time
        return;
    }
    _send_message(CLASS_CFG, MSG_CFG_PRT, nullptr, 0);
}

// Ensure there is enough space for the largest possible outgoing message
// Process bytes available from the stream
//
// The stream is assumed to contain only messages we recognise.  If it
// contains other messages, and those messages contain the preamble
// bytes, it is possible for this code to fail to synchronise to the
// stream immediately.  Without buffering the entire message and
// re-processing it from the top, this is unavoidable. The parser
// attempts to avoid this when possible.
//
bool
AP_GPS_UBLOX::read(void)
{
    uint8_t data;
    int16_t numc;
    bool parsed = false;
    uint32_t millis_now = AP_HAL::millis();

    // walk through the gps configuration at 1 message per second
    if (millis_now - _last_config_time >= _delay_time) {
        _request_next_config();
        _last_config_time = millis_now;
        if (_unconfigured_messages) { // send the updates faster until fully configured
            if (!havePvtMsg && (_unconfigured_messages & CONFIG_REQUIRED_INITIAL)) {
                _delay_time = 300;
            } else {
                _delay_time = 750;
            }
        } else {
            _delay_time = 2000;
        }
    }

    if(!_unconfigured_messages && gps._save_config && !_cfg_saved &&
       _num_cfg_save_tries < 5 && (millis_now - _last_cfg_sent_time) > 5000 &&
       !hal.util->get_soft_armed()) {
        //save the configuration sent until now
        if (gps._save_config == 1 ||
            (gps._save_config == 2 && _cfg_needs_save)) {
            _save_cfg();
        }
    }

    numc = port->available();
    for (int16_t i = 0; i < numc; i++) {        // Process bytes received

        // read the next byte
        data = port->read();

	reset:
        switch(_step) {

        // Message preamble detection
        //
        // If we fail to match any of the expected bytes, we reset
        // the state machine and re-consider the failed byte as
        // the first byte of the preamble.  This improves our
        // chances of recovering from a mismatch and makes it less
        // likely that we will be fooled by the preamble appearing
        // as data in some other message.
        //
        case 1:
            if (PREAMBLE2 == data) {
                _step++;
                break;
            }
            _step = 0;
            Debug("reset %u", __LINE__);
            FALLTHROUGH;
        case 0:
            if(PREAMBLE1 == data)
                _step++;
            break;

        // Message header processing
        //
        // We sniff the class and message ID to decide whether we
        // are going to gather the message bytes or just discard
        // them.
        //
        // We always collect the length so that we can avoid being
        // fooled by preamble bytes in messages.
        //
        case 2:
            _step++;
            _class = data;
            _ck_b = _ck_a = data;                       // reset the checksum accumulators
            break;
        case 3:
            _step++;
            _ck_b += (_ck_a += data);                   // checksum byte
            _msg_id = data;
            break;
        case 4:
            _step++;
            _ck_b += (_ck_a += data);                   // checksum byte
            _payload_length = data;                     // payload length low byte
            break;
        case 5:
            _step++;
            _ck_b += (_ck_a += data);                   // checksum byte

            _payload_length += (uint16_t)(data<<8);
            if (_payload_length > sizeof(_buffer)) {
                Debug("large payload %u", (unsigned)_payload_length);
                // assume any payload bigger then what we know about is noise
                _payload_length = 0;
                _step = 0;
				goto reset;
            }
            _payload_counter = 0;                       // prepare to receive payload
            if (_payload_length == 0) {
                // bypass payload and go straight to checksum
                _step++;
            }
            break;

        // Receive message data
        //
        case 6:
            _ck_b += (_ck_a += data);                   // checksum byte
            if (_payload_counter < sizeof(_buffer)) {
                _buffer[_payload_counter] = data;
            }
            if (++_payload_counter == _payload_length)
                _step++;
            break;

        // Checksum and message processing
        //
        case 7:
            _step++;
            if (_ck_a != data) {
                Debug("bad cka %x should be %x", data, _ck_a);
                _step = 0;
				goto reset;
            }
            break;
        case 8:
            _step = 0;
            if (_ck_b != data) {
                Debug("bad ckb %x should be %x", data, _ck_b);
                break;                                                  // bad checksum
            }

            if (_parse_gps()) {
                parsed = true;
            }
            break;
        }
    }
    return parsed;
}

// Private Methods /////////////////////////////////////////////////////////////
void AP_GPS_UBLOX::log_mon_hw(void)
{
#ifndef HAL_NO_LOGGING
    if (!should_log()) {
        return;
    }
    struct log_Ubx1 pkt = {
        LOG_PACKET_HEADER_INIT(_ubx_msg_log_index(LOG_GPS_UBX1_MSG)),
        time_us    : AP_HAL::micros64(),
        instance   : state.instance,
        noisePerMS : _buffer.mon_hw_60.noisePerMS,
        jamInd     : _buffer.mon_hw_60.jamInd,
        aPower     : _buffer.mon_hw_60.aPower,
        agcCnt     : _buffer.mon_hw_60.agcCnt,
        config     : _unconfigured_messages,
    };
    if (_payload_length == 68) {
        pkt.noisePerMS = _buffer.mon_hw_68.noisePerMS;
        pkt.jamInd     = _buffer.mon_hw_68.jamInd;
        pkt.aPower     = _buffer.mon_hw_68.aPower;
        pkt.agcCnt     = _buffer.mon_hw_68.agcCnt;
    }
    AP::logger().WriteBlock(&pkt, sizeof(pkt));
#endif
}

void AP_GPS_UBLOX::log_mon_hw2(void)
{
#ifndef HAL_NO_LOGGING
    if (!should_log()) {
        return;
    }

    struct log_Ubx2 pkt = {
        LOG_PACKET_HEADER_INIT(_ubx_msg_log_index(LOG_GPS_UBX2_MSG)),
        time_us   : AP_HAL::micros64(),
        instance  : state.instance,
        ofsI      : _buffer.mon_hw2.ofsI,
        magI      : _buffer.mon_hw2.magI,
        ofsQ      : _buffer.mon_hw2.ofsQ,
        magQ      : _buffer.mon_hw2.magQ,
    };
    AP::logger().WriteBlock(&pkt, sizeof(pkt));
#endif
}

#if UBLOX_RXM_RAW_LOGGING
void AP_GPS_UBLOX::log_rxm_raw(const struct ubx_rxm_raw &raw)
{
#ifndef HAL_NO_LOGGING
    if (!should_log()) {
        return;
    }

    uint64_t now = AP_HAL::micros64();
    for (uint8_t i=0; i<raw.numSV; i++) {
        struct log_GPS_RAW pkt = {
            LOG_PACKET_HEADER_INIT(LOG_GPS_RAW_MSG),
            time_us    : now,
            iTOW       : raw.iTOW,
            week       : raw.week,
            numSV      : raw.numSV,
            sv         : raw.svinfo[i].sv,
            cpMes      : raw.svinfo[i].cpMes,
            prMes      : raw.svinfo[i].prMes,
            doMes      : raw.svinfo[i].doMes,
            mesQI      : raw.svinfo[i].mesQI,
            cno        : raw.svinfo[i].cno,
            lli        : raw.svinfo[i].lli
        };
        AP::logger().WriteBlock(&pkt, sizeof(pkt));
    }
#endif
}

void AP_GPS_UBLOX::log_rxm_rawx(const struct ubx_rxm_rawx &raw)
{
#ifndef HAL_NO_LOGGING
    if (!should_log()) {
        return;
    }

    uint64_t now = AP_HAL::micros64();

    struct log_GPS_RAWH header = {
        LOG_PACKET_HEADER_INIT(LOG_GPS_RAWH_MSG),
        time_us    : now,
        rcvTow     : raw.rcvTow,
        week       : raw.week,
        leapS      : raw.leapS,
        numMeas    : raw.numMeas,
        recStat    : raw.recStat
    };
    AP::logger().WriteBlock(&header, sizeof(header));

    for (uint8_t i=0; i<raw.numMeas; i++) {
        struct log_GPS_RAWS pkt = {
            LOG_PACKET_HEADER_INIT(LOG_GPS_RAWS_MSG),
            time_us    : now,
            prMes      : raw.svinfo[i].prMes,
            cpMes      : raw.svinfo[i].cpMes,
            doMes      : raw.svinfo[i].doMes,
            gnssId     : raw.svinfo[i].gnssId,
            svId       : raw.svinfo[i].svId,
            freqId     : raw.svinfo[i].freqId,
            locktime   : raw.svinfo[i].locktime,
            cno        : raw.svinfo[i].cno,
            prStdev    : raw.svinfo[i].prStdev,
            cpStdev    : raw.svinfo[i].cpStdev,
            doStdev    : raw.svinfo[i].doStdev,
            trkStat    : raw.svinfo[i].trkStat
        };
        AP::logger().WriteBlock(&pkt, sizeof(pkt));
    }
#endif
}
#endif // UBLOX_RXM_RAW_LOGGING

void AP_GPS_UBLOX::unexpected_message(void)
{
    Debug("Unexpected message 0x%02x 0x%02x", (unsigned)_class, (unsigned)_msg_id);
    if (++_disable_counter == 0) {
        // disable future sends of this message id, but
        // only do this every 256 messages, as some
        // message types can't be disabled and we don't
        // want to get into an ack war
        Debug("Disabling message 0x%02x 0x%02x", (unsigned)_class, (unsigned)_msg_id);
        _configure_message_rate(_class, _msg_id, 0);
    }
}

bool
AP_GPS_UBLOX::_parse_gps(void)
{
    if (_class == CLASS_ACK) {
        Debug("ACK %u", (unsigned)_msg_id);

        if(_msg_id == MSG_ACK_ACK) {
            switch(_buffer.ack.clsID) {
            case CLASS_CFG:
                switch(_buffer.ack.msgID) {
                case MSG_CFG_CFG:
                    _cfg_saved = true;
                    _cfg_needs_save = false;
                    break;
                case MSG_CFG_GNSS:
                    _unconfigured_messages &= ~CONFIG_GNSS;
                    break;
                case MSG_CFG_MSG:
                    // There is no way to know what MSG config was ack'ed, assume it was the last
                    // one requested. To verify it rerequest the last config we sent. If we miss
                    // the actual ack we will catch it next time through the poll loop, but that
                    // will be a good chunk of time later.
                    break;
                case MSG_CFG_NAV_SETTINGS:
                    _unconfigured_messages &= ~CONFIG_NAV_SETTINGS;
                    break;
                case MSG_CFG_RATE:
                    // The GPS will ACK a update rate that is invalid. in order to detect this
                    // only accept the rate as configured by reading the settings back and
                    // validating that they all match the target values
                    break;
                case MSG_CFG_SBAS:
                    _unconfigured_messages &= ~CONFIG_SBAS;
                    break;
                case MSG_CFG_TP5:
                    _unconfigured_messages &= ~CONFIG_TP5;
                    break;
                }
                break;
            case CLASS_MON:
                switch(_buffer.ack.msgID) {
                case MSG_MON_HW:
                    _unconfigured_messages &= ~CONFIG_RATE_MON_HW;
                    break;
                case MSG_MON_HW2:
                    _unconfigured_messages &= ~CONFIG_RATE_MON_HW2;
                    break;
                }
            }
        }
        return false;
    }

    if (_class == CLASS_CFG) {
        switch(_msg_id) {
        case  MSG_CFG_NAV_SETTINGS:
	    Debug("Got settings %u min_elev %d drLimit %u\n", 
                  (unsigned)_buffer.nav_settings.dynModel,
                  (int)_buffer.nav_settings.minElev,
                  (unsigned)_buffer.nav_settings.drLimit);
            _buffer.nav_settings.mask = 0;
            if (gps._navfilter != AP_GPS::GPS_ENGINE_NONE &&
                _buffer.nav_settings.dynModel != gps._navfilter) {
                // we've received the current nav settings, change the engine
                // settings and send them back
                Debug("Changing engine setting from %u to %u\n",
                      (unsigned)_buffer.nav_settings.dynModel, (unsigned)gps._navfilter);
                _buffer.nav_settings.dynModel = gps._navfilter;
                _buffer.nav_settings.mask |= 1;
            }
            if (gps._min_elevation != -100 &&
                _buffer.nav_settings.minElev != gps._min_elevation) {
                Debug("Changing min elevation to %d\n", (int)gps._min_elevation);
                _buffer.nav_settings.minElev = gps._min_elevation;
                _buffer.nav_settings.mask |= 2;
            }
            if (_buffer.nav_settings.mask != 0) {
                _send_message(CLASS_CFG, MSG_CFG_NAV_SETTINGS,
                              &_buffer.nav_settings,
                              sizeof(_buffer.nav_settings));
                _unconfigured_messages |= CONFIG_NAV_SETTINGS;
                _cfg_needs_save = true;
            } else {
                _unconfigured_messages &= ~CONFIG_NAV_SETTINGS;
            }
            return false;

#if UBLOX_GNSS_SETTINGS
        case MSG_CFG_GNSS:
            if (gps._gnss_mode[state.instance] != 0) {
                struct ubx_cfg_gnss start_gnss = _buffer.gnss;
                uint8_t gnssCount = 0;
                Debug("Got GNSS Settings %u %u %u %u:\n",
                    (unsigned)_buffer.gnss.msgVer,
                    (unsigned)_buffer.gnss.numTrkChHw,
                    (unsigned)_buffer.gnss.numTrkChUse,
                    (unsigned)_buffer.gnss.numConfigBlocks);
#if UBLOX_DEBUGGING
                for(int i = 0; i < _buffer.gnss.numConfigBlocks; i++) {
                    Debug("  %u %u %u 0x%08x\n",
                    (unsigned)_buffer.gnss.configBlock[i].gnssId,
                    (unsigned)_buffer.gnss.configBlock[i].resTrkCh,
                    (unsigned)_buffer.gnss.configBlock[i].maxTrkCh,
                    (unsigned)_buffer.gnss.configBlock[i].flags);
                }
#endif

                for(int i = 0; i < UBLOX_MAX_GNSS_CONFIG_BLOCKS; i++) {
                    if((gps._gnss_mode[state.instance] & (1 << i)) && i != GNSS_SBAS) {
                        gnssCount++;
                    }
                }

                for(int i = 0; i < _buffer.gnss.numConfigBlocks; i++) {
                    // Reserve an equal portion of channels for all enabled systems
                    if(gps._gnss_mode[state.instance] & (1 << _buffer.gnss.configBlock[i].gnssId)) {
                        if(GNSS_SBAS !=_buffer.gnss.configBlock[i].gnssId) {
                            _buffer.gnss.configBlock[i].resTrkCh = (_buffer.gnss.numTrkChHw - 3) / (gnssCount * 2);
                            _buffer.gnss.configBlock[i].maxTrkCh = _buffer.gnss.numTrkChHw;
                        } else {
                            _buffer.gnss.configBlock[i].resTrkCh = 1;
                            _buffer.gnss.configBlock[i].maxTrkCh = 3;
                        }
                        _buffer.gnss.configBlock[i].flags = _buffer.gnss.configBlock[i].flags | 0x00000001;
                    } else {
                        _buffer.gnss.configBlock[i].resTrkCh = 0;
                        _buffer.gnss.configBlock[i].maxTrkCh = 0;
                        _buffer.gnss.configBlock[i].flags = _buffer.gnss.configBlock[i].flags & 0xFFFFFFFE;
                    }
                }
                if (memcmp(&start_gnss, &_buffer.gnss, sizeof(start_gnss))) {
                    _send_message(CLASS_CFG, MSG_CFG_GNSS, &_buffer.gnss, 4 + (8 * _buffer.gnss.numConfigBlocks));
                    _unconfigured_messages |= CONFIG_GNSS;
                    _cfg_needs_save = true;
                } else {
                    _unconfigured_messages &= ~CONFIG_GNSS;
                }
            } else {
                _unconfigured_messages &= ~CONFIG_GNSS;
            }
            return false;
#endif

        case MSG_CFG_SBAS:
            if (gps._sbas_mode != 2) {
	        Debug("Got SBAS settings %u %u %u 0x%x 0x%x\n", 
                      (unsigned)_buffer.sbas.mode,
                      (unsigned)_buffer.sbas.usage,
                      (unsigned)_buffer.sbas.maxSBAS,
                      (unsigned)_buffer.sbas.scanmode2,
                      (unsigned)_buffer.sbas.scanmode1);
                if (_buffer.sbas.mode != gps._sbas_mode) {
                    _buffer.sbas.mode = gps._sbas_mode;
                    _send_message(CLASS_CFG, MSG_CFG_SBAS,
                                  &_buffer.sbas,
                                  sizeof(_buffer.sbas));
                    _unconfigured_messages |= CONFIG_SBAS;
                    _cfg_needs_save = true;
                } else {
                    _unconfigured_messages &= ~CONFIG_SBAS;
                }
            } else {
                    _unconfigured_messages &= ~CONFIG_SBAS;
            }
            return false;
        case MSG_CFG_MSG:
            if(_payload_length == sizeof(ubx_cfg_msg_rate_6)) {
                // can't verify the setting without knowing the port
                // request the port again
                if(_ublox_port >= UBLOX_MAX_PORTS) {
                    _request_port();
                    return false;
                }
                _verify_rate(_buffer.msg_rate_6.msg_class, _buffer.msg_rate_6.msg_id,
                             _buffer.msg_rate_6.rates[_ublox_port]);
            } else {
                _verify_rate(_buffer.msg_rate.msg_class, _buffer.msg_rate.msg_id,
                             _buffer.msg_rate.rate);
            }
            return false;
        case MSG_CFG_PRT:
           _ublox_port = _buffer.prt.portID;
           return false;
        case MSG_CFG_RATE:
            if(_buffer.nav_rate.measure_rate_ms != gps._rate_ms[state.instance] ||
               _buffer.nav_rate.nav_rate != 1 ||
               _buffer.nav_rate.timeref != 0) {
               _configure_rate();
                _unconfigured_messages |= CONFIG_RATE_NAV;
                _cfg_needs_save = true;
            } else {
                _unconfigured_messages &= ~CONFIG_RATE_NAV;
            }
            return false;
            
#if CONFIGURE_PPS_PIN
        case MSG_CFG_TP5: {
            // configure the PPS pin for 1Hz, zero delay
            Debug("Got TP5 ver=%u 0x%04x %u\n", 
                  (unsigned)_buffer.nav_tp5.version,
                  (unsigned)_buffer.nav_tp5.flags,
                  (unsigned)_buffer.nav_tp5.freqPeriod);
            const uint16_t desired_flags = 0x003f;
            const uint16_t desired_period_hz = 1;
            if (_buffer.nav_tp5.flags != desired_flags ||
                _buffer.nav_tp5.freqPeriod != desired_period_hz) {
                _buffer.nav_tp5.tpIdx = 0;
                _buffer.nav_tp5.reserved1[0] = 0;
                _buffer.nav_tp5.reserved1[1] = 0;
                _buffer.nav_tp5.antCableDelay = 0;
                _buffer.nav_tp5.rfGroupDelay = 0;
                _buffer.nav_tp5.freqPeriod = desired_period_hz;
                _buffer.nav_tp5.freqPeriodLock = desired_period_hz;
                _buffer.nav_tp5.pulseLenRatio = 1;
                _buffer.nav_tp5.pulseLenRatioLock = 2;
                _buffer.nav_tp5.userConfigDelay = 0;
                _buffer.nav_tp5.flags = desired_flags;
                _send_message(CLASS_CFG, MSG_CFG_TP5,
                              &_buffer.nav_tp5,
                              sizeof(_buffer.nav_tp5));
                _unconfigured_messages |= CONFIG_TP5;
                _cfg_needs_save = true;
            } else {
                _unconfigured_messages &= ~CONFIG_TP5;
            }
            return false;
        }
#endif // CONFIGURE_PPS_PIN
        }
           
    }

    if (_class == CLASS_MON) {
        switch(_msg_id) {
        case MSG_MON_HW:
            if (_payload_length == 60 || _payload_length == 68) {
                log_mon_hw();
            }
            break;
        case MSG_MON_HW2:
            if (_payload_length == 28) {
                log_mon_hw2();  
            }
            break;
        case MSG_MON_VER:
            _have_version = true;
            strncpy(_version.hwVersion, _buffer.mon_ver.hwVersion, sizeof(_version.hwVersion));
            strncpy(_version.swVersion, _buffer.mon_ver.swVersion, sizeof(_version.swVersion));
            GCS_SEND_TEXT(MAV_SEVERITY_INFO, 
                                             "u-blox %d HW: %s SW: %s",
                                             state.instance + 1,
                                             _version.hwVersion,
                                             _version.swVersion);
            // check for F9. The F9 does not respond to SVINFO, so we need to use MON_VER
            // for hardware generation
            if (strncmp(_version.hwVersion, "00190000", 8) == 0) {
                _hardware_generation = UBLOX_F9;
            }
            break;
        default:
            unexpected_message();
        }
        return false;
    }

#if UBLOX_RXM_RAW_LOGGING
    if (_class == CLASS_RXM && _msg_id == MSG_RXM_RAW && gps._raw_data != 0) {
        log_rxm_raw(_buffer.rxm_raw);
        return false;
    } else if (_class == CLASS_RXM && _msg_id == MSG_RXM_RAWX && gps._raw_data != 0) {
        log_rxm_rawx(_buffer.rxm_rawx);
        return false;
    }
#endif // UBLOX_RXM_RAW_LOGGING

    if (_class != CLASS_NAV) {
        unexpected_message();
        return false;
    }

    switch (_msg_id) {
    case MSG_POSLLH:
        Debug("MSG_POSLLH next_fix=%u", next_fix);
        if (havePvtMsg) {
            _unconfigured_messages |= CONFIG_RATE_POSLLH;
            break;
        }
        _check_new_itow(_buffer.posllh.itow);
        _last_pos_time        = _buffer.posllh.itow;
        state.location.lng    = _buffer.posllh.longitude;
        state.location.lat    = _buffer.posllh.latitude;
        state.location.alt    = _buffer.posllh.altitude_msl / 10;
        state.status          = next_fix;
        _new_position = true;
        state.horizontal_accuracy = _buffer.posllh.horizontal_accuracy*1.0e-3f;
        state.vertical_accuracy = _buffer.posllh.vertical_accuracy*1.0e-3f;
        state.have_horizontal_accuracy = true;
        state.have_vertical_accuracy = true;
#if UBLOX_FAKE_3DLOCK
        state.location.lng = 1491652300L;
        state.location.lat = -353632610L;
        state.location.alt = 58400;
        state.vertical_accuracy = 0;
        state.horizontal_accuracy = 0;
#endif
        break;
    case MSG_STATUS:
        Debug("MSG_STATUS fix_status=%u fix_type=%u",
              _buffer.status.fix_status,
              _buffer.status.fix_type);
        _check_new_itow(_buffer.status.itow);
        if (havePvtMsg) {
            _unconfigured_messages |= CONFIG_RATE_STATUS;
            break;
        }
        if (_buffer.status.fix_status & NAV_STATUS_FIX_VALID) {
            if( (_buffer.status.fix_type == AP_GPS_UBLOX::FIX_3D) &&
                (_buffer.status.fix_status & AP_GPS_UBLOX::NAV_STATUS_DGPS_USED)) {
                next_fix = AP_GPS::GPS_OK_FIX_3D_DGPS;
            }else if( _buffer.status.fix_type == AP_GPS_UBLOX::FIX_3D) {
                next_fix = AP_GPS::GPS_OK_FIX_3D;
            }else if (_buffer.status.fix_type == AP_GPS_UBLOX::FIX_2D) {
                next_fix = AP_GPS::GPS_OK_FIX_2D;
            }else{
                next_fix = AP_GPS::NO_FIX;
                state.status = AP_GPS::NO_FIX;
            }
        }else{
            next_fix = AP_GPS::NO_FIX;
            state.status = AP_GPS::NO_FIX;
        }
#if UBLOX_FAKE_3DLOCK
        state.status = AP_GPS::GPS_OK_FIX_3D;
        next_fix = state.status;
#endif
        break;
    case MSG_DOP:
        Debug("MSG_DOP");
        noReceivedHdop = false;
        _check_new_itow(_buffer.dop.itow);
        state.hdop        = _buffer.dop.hDOP;
        state.vdop        = _buffer.dop.vDOP;
#if UBLOX_FAKE_3DLOCK
        state.hdop = 130;
        state.hdop = 170;
#endif
        break;
    case MSG_SOL:
        Debug("MSG_SOL fix_status=%u fix_type=%u",
              _buffer.solution.fix_status,
              _buffer.solution.fix_type);
        _check_new_itow(_buffer.solution.itow);
        if (havePvtMsg) {
            state.time_week = _buffer.solution.week;
            break;
        }
        if (_buffer.solution.fix_status & NAV_STATUS_FIX_VALID) {
            if( (_buffer.solution.fix_type == AP_GPS_UBLOX::FIX_3D) &&
                (_buffer.solution.fix_status & AP_GPS_UBLOX::NAV_STATUS_DGPS_USED)) {
                next_fix = AP_GPS::GPS_OK_FIX_3D_DGPS;
            }else if( _buffer.solution.fix_type == AP_GPS_UBLOX::FIX_3D) {
                next_fix = AP_GPS::GPS_OK_FIX_3D;
            }else if (_buffer.solution.fix_type == AP_GPS_UBLOX::FIX_2D) {
                next_fix = AP_GPS::GPS_OK_FIX_2D;
            }else{
                next_fix = AP_GPS::NO_FIX;
                state.status = AP_GPS::NO_FIX;
            }
        }else{
            next_fix = AP_GPS::NO_FIX;
            state.status = AP_GPS::NO_FIX;
        }
        if(noReceivedHdop) {
            state.hdop = _buffer.solution.position_DOP;
        }
        state.num_sats    = _buffer.solution.satellites;
        if (next_fix >= AP_GPS::GPS_OK_FIX_2D) {
            state.last_gps_time_ms = AP_HAL::millis();
            state.time_week_ms    = _buffer.solution.itow;
            state.time_week       = _buffer.solution.week;
        }
#if UBLOX_FAKE_3DLOCK
        next_fix = state.status;
        state.num_sats = 10;
        state.time_week = 1721;
        state.time_week_ms = AP_HAL::millis() + 3*60*60*1000 + 37000;
        state.last_gps_time_ms = AP_HAL::millis();
        state.hdop = 130;
#endif
        break;
    case MSG_RELPOSNED:
        {
            const Vector3f &offset0 = gps._antenna_offset[0].get();
            const Vector3f &offset1 = gps._antenna_offset[1].get();
            // note that we require the yaw to come from a fixed solution, not a float solution
            // yaw from a float solution would only be acceptable with a very large separation between
            // GPS modules
            const uint32_t valid_mask = static_cast<uint32_t>(RELPOSNED::relPosHeadingValid) |
                                        static_cast<uint32_t>(RELPOSNED::relPosValid) |
                                        static_cast<uint32_t>(RELPOSNED::gnssFixOK) |
                                        static_cast<uint32_t>(RELPOSNED::isMoving) |
                                        static_cast<uint32_t>(RELPOSNED::carrSolnFixed);
            const uint32_t invalid_mask = static_cast<uint32_t>(RELPOSNED::refPosMiss) |
                                          static_cast<uint32_t>(RELPOSNED::refObsMiss) |
                                          static_cast<uint32_t>(RELPOSNED::carrSolnFloat);

            const float offset_dist = (offset0 - offset1).length();
            const float rel_dist = _buffer.relposned.relPosLength * 1.0e-2;
            const float strict_length_error_allowed = 0.2; // allow for up to 20% error
            const float min_separation = 0.05;
            if (((_buffer.relposned.flags & valid_mask) == valid_mask) &&
                ((_buffer.relposned.flags & invalid_mask) == 0) &&
                rel_dist > min_separation &&
                offset_dist > min_separation &&
                fabsf(offset_dist - rel_dist) / MIN(offset_dist, rel_dist) < strict_length_error_allowed) {
                float rotation_offset_rad;
                if (offset0.is_zero()) {
                    rotation_offset_rad = Vector2f(offset1.x, offset1.y).angle();
                } else if (offset1.is_zero()) {
                    rotation_offset_rad = Vector2f(offset0.x, offset0.y).angle();
                } else {
                    const Vector3f diff = offset0 - offset1;
                    rotation_offset_rad = Vector2f(diff.x, diff.y).angle();
                }
                if (state.instance != 0) {
                    rotation_offset_rad += M_PI;
                }
                state.gps_yaw = wrap_360(_buffer.relposned.relPosHeading * 1e-5 + degrees(rotation_offset_rad));
                state.have_gps_yaw = true;
                state.gps_yaw_accuracy = _buffer.relposned.accHeading * 1e-5;
                state.have_gps_yaw_accuracy = true;
            } else {
                state.have_gps_yaw = false;
                state.have_gps_yaw_accuracy = false;
            }
        }
        break;
    case MSG_PVT:
        Debug("MSG_PVT");

        havePvtMsg = true;
        // position
        _check_new_itow(_buffer.pvt.itow);
        _last_pos_time        = _buffer.pvt.itow;
        state.location.lng    = _buffer.pvt.lon;
        state.location.lat    = _buffer.pvt.lat;
        state.location.alt    = _buffer.pvt.h_msl / 10;
        switch (_buffer.pvt.fix_type) 
        {
            case 0:
                state.status = AP_GPS::NO_FIX;
                break;
            case 1:
                state.status = AP_GPS::NO_FIX;
                break;
            case 2:
                state.status = AP_GPS::GPS_OK_FIX_2D;
                break;
            case 3:
                state.status = AP_GPS::GPS_OK_FIX_3D;
                if (_buffer.pvt.flags & 0b00000010)  // diffsoln
                    state.status = AP_GPS::GPS_OK_FIX_3D_DGPS;
                if (_buffer.pvt.flags & 0b01000000)  // carrsoln - float
                    state.status = AP_GPS::GPS_OK_FIX_3D_RTK_FLOAT;
                if (_buffer.pvt.flags & 0b10000000)  // carrsoln - fixed
                    state.status = AP_GPS::GPS_OK_FIX_3D_RTK_FIXED;
                break;
            case 4:
                GCS_SEND_TEXT(MAV_SEVERITY_INFO,
                                "Unexpected state %d", _buffer.pvt.flags);
                state.status = AP_GPS::GPS_OK_FIX_3D;
                break;
            case 5:
                state.status = AP_GPS::NO_FIX;
                break;
            default:
                state.status = AP_GPS::NO_FIX;
                break;
        }
        next_fix = state.status;
        _new_position = true;
        state.horizontal_accuracy = _buffer.pvt.h_acc*1.0e-3f;
        state.vertical_accuracy = _buffer.pvt.v_acc*1.0e-3f;
        state.have_horizontal_accuracy = true;
        state.have_vertical_accuracy = true;
        // SVs
        state.num_sats    = _buffer.pvt.num_sv;
        // velocity     
        _last_vel_time         = _buffer.pvt.itow;
        state.ground_speed     = _buffer.pvt.gspeed*0.001f;          // m/s
        state.ground_course    = wrap_360(_buffer.pvt.head_mot * 1.0e-5f);       // Heading 2D deg * 100000
        state.have_vertical_velocity = true;
        state.velocity.x = _buffer.pvt.velN * 0.001f;
        state.velocity.y = _buffer.pvt.velE * 0.001f;
        state.velocity.z = _buffer.pvt.velD * 0.001f;
        state.have_speed_accuracy = true;
        state.speed_accuracy = _buffer.pvt.s_acc*0.001f;
        _new_speed = true;
        // dop
        if(noReceivedHdop) {
            state.hdop        = _buffer.pvt.p_dop;
            state.vdop        = _buffer.pvt.p_dop;
        }
                    
        state.last_gps_time_ms = AP_HAL::millis();
        
        // time
        state.time_week_ms    = _buffer.pvt.itow;
#if UBLOX_FAKE_3DLOCK
        state.location.lng = 1491652300L;
        state.location.lat = -353632610L;
        state.location.alt = 58400;
        state.vertical_accuracy = 0;
        state.horizontal_accuracy = 0;
        state.status = AP_GPS::GPS_OK_FIX_3D;
        state.num_sats = 10;
        state.time_week = 1721;
        state.time_week_ms = AP_HAL::millis() + 3*60*60*1000 + 37000;
        state.last_gps_time_ms = AP_HAL::millis();
        state.hdop = 130;
        state.speed_accuracy = 0;
        next_fix = state.status;
#endif
        break;
    case MSG_TIMEGPS:
        Debug("MSG_TIMEGPS");
        _check_new_itow(_buffer.timegps.itow);
        if (_buffer.timegps.valid & UBX_TIMEGPS_VALID_WEEK_MASK) {
            state.time_week = _buffer.timegps.week;
        }
        break;
    case MSG_VELNED:
        Debug("MSG_VELNED");
        if (havePvtMsg) {
            _unconfigured_messages |= CONFIG_RATE_VELNED;
            break;
        }
        _check_new_itow(_buffer.velned.itow);
        _last_vel_time         = _buffer.velned.itow;
        state.ground_speed     = _buffer.velned.speed_2d*0.01f;          // m/s
        state.ground_course    = wrap_360(_buffer.velned.heading_2d * 1.0e-5f);       // Heading 2D deg * 100000
        state.have_vertical_velocity = true;
        state.velocity.x = _buffer.velned.ned_north * 0.01f;
        state.velocity.y = _buffer.velned.ned_east * 0.01f;
        state.velocity.z = _buffer.velned.ned_down * 0.01f;
        state.ground_course = wrap_360(degrees(atan2f(state.velocity.y, state.velocity.x)));
        state.ground_speed = norm(state.velocity.y, state.velocity.x);
        state.have_speed_accuracy = true;
        state.speed_accuracy = _buffer.velned.speed_accuracy*0.01f;
#if UBLOX_FAKE_3DLOCK
        state.speed_accuracy = 0;
#endif
        _new_speed = true;
        break;
    case MSG_NAV_SVINFO:
        {
        Debug("MSG_NAV_SVINFO\n");
        static const uint8_t HardwareGenerationMask = 0x07;
        _check_new_itow(_buffer.svinfo_header.itow);
        _hardware_generation = _buffer.svinfo_header.globalFlags & HardwareGenerationMask;
        switch (_hardware_generation) {
            case UBLOX_5:
            case UBLOX_6:
                // only 7 and newer support CONFIG_GNSS
                _unconfigured_messages &= ~CONFIG_GNSS;
                break;
            case UBLOX_7:
            case UBLOX_M8:
#if UBLOX_SPEED_CHANGE
                port->begin(4000000U);
                Debug("Changed speed to 4Mhz for SPI-driven UBlox\n");
#endif
                break;
            default:
                hal.console->printf("Wrong Ublox Hardware Version%u\n", _hardware_generation);
                break;
        };
        _unconfigured_messages &= ~CONFIG_VERSION;
        /* We don't need that anymore */
        _configure_message_rate(CLASS_NAV, MSG_NAV_SVINFO, 0);
        break;
        }
    default:
        Debug("Unexpected NAV message 0x%02x", (unsigned)_msg_id);
        if (++_disable_counter == 0) {
            Debug("Disabling NAV message 0x%02x", (unsigned)_msg_id);
            _configure_message_rate(CLASS_NAV, _msg_id, 0);
        }
        return false;
    }

    // we only return true when we get new position and speed data
    // this ensures we don't use stale data
    if (_new_position && _new_speed && _last_vel_time == _last_pos_time) {
        _new_speed = _new_position = false;
        return true;
    }
    return false;
}


// UBlox auto configuration

/*
 *  update checksum for a set of bytes
 */
void
AP_GPS_UBLOX::_update_checksum(uint8_t *data, uint16_t len, uint8_t &ck_a, uint8_t &ck_b)
{
    while (len--) {
        ck_a += *data;
        ck_b += ck_a;
        data++;
    }
}


/*
 *  send a ublox message
 */
bool
AP_GPS_UBLOX::_send_message(uint8_t msg_class, uint8_t msg_id, void *msg, uint16_t size)
{
    if (port->txspace() < (sizeof(struct ubx_header) + 2 + size)) {
        return false;
    }
    struct ubx_header header;
    uint8_t ck_a=0, ck_b=0;
    header.preamble1 = PREAMBLE1;
    header.preamble2 = PREAMBLE2;
    header.msg_class = msg_class;
    header.msg_id    = msg_id;
    header.length    = size;

    _update_checksum((uint8_t *)&header.msg_class, sizeof(header)-2, ck_a, ck_b);
    _update_checksum((uint8_t *)msg, size, ck_a, ck_b);

    port->write((const uint8_t *)&header, sizeof(header));
    port->write((const uint8_t *)msg, size);
    port->write((const uint8_t *)&ck_a, 1);
    port->write((const uint8_t *)&ck_b, 1);
    return true;
}

/*
 *  requests the given message rate for a specific message class
 *  and msg_id
 *  returns true if it sent the request, false if waiting on knowing the port
 */
bool
AP_GPS_UBLOX::_request_message_rate(uint8_t msg_class, uint8_t msg_id)
{
    // Without knowing what communication port is being used it isn't possible to verify
    // always ensure we have a port before sending the request
    if(_ublox_port >= UBLOX_MAX_PORTS) {
        _request_port();
        return false;
    } else {
        struct ubx_cfg_msg msg;
        msg.msg_class = msg_class;
        msg.msg_id    = msg_id;
        return _send_message(CLASS_CFG, MSG_CFG_MSG, &msg, sizeof(msg));
    }
}

/*
 *  configure a UBlox GPS for the given message rate for a specific
 *  message class and msg_id
 */
bool
AP_GPS_UBLOX::_configure_message_rate(uint8_t msg_class, uint8_t msg_id, uint8_t rate)
{
    if (port->txspace() < (int16_t)(sizeof(struct ubx_header)+sizeof(struct ubx_cfg_msg_rate)+2)) {
        return false;
    }

    struct ubx_cfg_msg_rate msg;
    msg.msg_class = msg_class;
    msg.msg_id    = msg_id;
    msg.rate      = rate;
    return _send_message(CLASS_CFG, MSG_CFG_MSG, &msg, sizeof(msg));
}

/*
 * save gps configurations to non-volatile memory sent until the call of
 * this message
 */
void
AP_GPS_UBLOX::_save_cfg()
{
    ubx_cfg_cfg save_cfg;
    save_cfg.clearMask = 0;
    save_cfg.saveMask = SAVE_CFG_ALL;
    save_cfg.loadMask = 0;
    _send_message(CLASS_CFG, MSG_CFG_CFG, &save_cfg, sizeof(save_cfg));
    _last_cfg_sent_time = AP_HAL::millis();
    _num_cfg_save_tries++;
    GCS_SEND_TEXT(MAV_SEVERITY_INFO,
                                     "GPS: u-blox %d saving config",
                                     state.instance + 1);
}

/*
  detect a Ublox GPS. Adds one byte, and returns true if the stream
  matches a UBlox
 */
bool
AP_GPS_UBLOX::_detect(struct UBLOX_detect_state &state, uint8_t data)
{
reset:
	switch (state.step) {
        case 1:
            if (PREAMBLE2 == data) {
                state.step++;
                break;
            }
            state.step = 0;
            FALLTHROUGH;
        case 0:
            if (PREAMBLE1 == data)
                state.step++;
            break;
        case 2:
            state.step++;
            state.ck_b = state.ck_a = data;
            break;
        case 3:
            state.step++;
            state.ck_b += (state.ck_a += data);
            break;
        case 4:
            state.step++;
            state.ck_b += (state.ck_a += data);
            state.payload_length = data;
            break;
        case 5:
            state.step++;
            state.ck_b += (state.ck_a += data);
            state.payload_counter = 0;
            break;
        case 6:
            state.ck_b += (state.ck_a += data);
            if (++state.payload_counter == state.payload_length)
                state.step++;
            break;
        case 7:
            state.step++;
            if (state.ck_a != data) {
                state.step = 0;
				goto reset;
            }
            break;
        case 8:
            state.step = 0;
			if (state.ck_b == data) {
				// a valid UBlox packet
				return true;
			} else {
				goto reset;
			}
    }
    return false;
}

void
AP_GPS_UBLOX::_request_version(void)
{
    _send_message(CLASS_MON, MSG_MON_VER, nullptr, 0);
}

void
AP_GPS_UBLOX::_configure_rate(void)
{
    struct ubx_cfg_nav_rate msg;
    // require a minimum measurement rate of 5Hz
    msg.measure_rate_ms = gps.get_rate_ms(state.instance);
    msg.nav_rate        = 1;
    msg.timeref         = 0;     // UTC time
    _send_message(CLASS_CFG, MSG_CFG_RATE, &msg, sizeof(msg));
}

static const char *reasons[] = {"navigation rate",
                                "posllh rate",
                                "status rate",
                                "solution rate",
                                "velned rate",
                                "dop rate",
                                "hw monitor rate",
                                "hw2 monitor rate",
                                "raw rate",
                                "version",
                                "navigation settings",
                                "GNSS settings",
                                "SBAS settings",
                                "PVT rate",
                                "time pulse settings",
                                "TIMEGPS rate"};

static_assert((1 << ARRAY_SIZE(reasons)) == CONFIG_LAST, "UBLOX: Missing configuration description");

void
AP_GPS_UBLOX::broadcast_configuration_failure_reason(void) const {
    for (uint8_t i = 0; i < ARRAY_SIZE(reasons); i++) {
        if (_unconfigured_messages & (1 << i)) {
            GCS_SEND_TEXT(MAV_SEVERITY_INFO, "GPS %u: u-blox %s configuration 0x%02x",
                (unsigned int)(state.instance + 1), reasons[i], (unsigned int)_unconfigured_messages);
            break;
        }
    }
}

/*
  return velocity lag in seconds
 */
bool AP_GPS_UBLOX::get_lag(float &lag_sec) const
{
    switch (_hardware_generation) {
    case UBLOX_UNKNOWN_HARDWARE_GENERATION:
        lag_sec = 0.22f;
        // always bail out in this case, it's used to indicate we have yet to receive a valid
        // hardware generation, however the user may have inhibited us detecting the generation
        // so if we aren't allowed to do configuration, we will accept this as the default delay
        return gps._auto_config != AP_GPS::GPS_AUTO_CONFIG_ENABLE;
    case UBLOX_5:
    case UBLOX_6:
    default:
        lag_sec = 0.22f;
        break;
    case UBLOX_7:
    case UBLOX_M8:
        // based on flight logs the 7 and 8 series seem to produce about 120ms lag
        lag_sec = 0.12f;
        break;
    case UBLOX_F9:
        // F9 lag not verified yet from flight log, but likely to be at least
        // as good as M8
        lag_sec = 0.12f;
        break;
    };
    return true;
}

void AP_GPS_UBLOX::Write_AP_Logger_Log_Startup_messages() const
{
#ifndef HAL_NO_LOGGING
    AP_GPS_Backend::Write_AP_Logger_Log_Startup_messages();

    if (_have_version) {
        AP::logger().Write_MessageF("u-blox %d HW: %s SW: %s",
                                           state.instance+1,
                                           _version.hwVersion,
                                           _version.swVersion);
    }
#endif
}

// uBlox specific check_new_itow(), handling message length
void AP_GPS_UBLOX::_check_new_itow(uint32_t itow)
{
    check_new_itow(itow, _payload_length + sizeof(ubx_header) + 2);
}