// check for new valid GPS data and update stored measurement if available
void NavEKF2_core::readGpsData()
{
// check for new GPS data
// do not accept data at a faster rate than 14Hz to avoid overflowing the FIFO buffer
if (_ahrs->get_gps().last_message_time_ms() - lastTimeGpsReceived_ms > 70) {
if (_ahrs->get_gps().status() >= AP_GPS::GPS_OK_FIX_3D) {
// report GPS fix status
gpsCheckStatus.bad_fix = false;
// store fix time from previous read
secondLastGpsTime_ms = lastTimeGpsReceived_ms;
// get current fix time
lastTimeGpsReceived_ms = _ahrs->get_gps().last_message_time_ms();
// estimate when the GPS fix was valid, allowing for GPS processing and other delays
// ideally we should be using a timing signal from the GPS receiver to set this time
gpsDataNew.time_ms = lastTimeGpsReceived_ms - frontend->_gpsDelay_ms;
// Correct for the average intersampling delay due to the filter updaterate
gpsDataNew.time_ms -= localFilterTimeStep_ms/2;
// Prevent time delay exceeding age of oldest IMU data in the buffer
gpsDataNew.time_ms = MAX(gpsDataNew.time_ms,imuDataDelayed.time_ms);
// read the NED velocity from the GPS
gpsDataNew.vel = _ahrs->get_gps().velocity();
// Use the speed accuracy from the GPS if available, otherwise set it to zero.
// Apply a decaying envelope filter with a 5 second time constant to the raw speed accuracy data
float alpha = constrain_float(0.0002f * (lastTimeGpsReceived_ms - secondLastGpsTime_ms),0.0f,1.0f);
gpsSpdAccuracy *= (1.0f - alpha);
float gpsSpdAccRaw;
if (!_ahrs->get_gps().speed_accuracy(gpsSpdAccRaw)) {
gpsSpdAccuracy = 0.0f;
} else {
gpsSpdAccuracy = MAX(gpsSpdAccuracy,gpsSpdAccRaw);
}
// check if we have enough GPS satellites and increase the gps noise scaler if we don't
if (_ahrs->get_gps().num_sats() >= 6 && (PV_AidingMode == AID_ABSOLUTE)) {
gpsNoiseScaler = 1.0f;
} else if (_ahrs->get_gps().num_sats() == 5 && (PV_AidingMode == AID_ABSOLUTE)) {
gpsNoiseScaler = 1.4f;
} else { // <= 4 satellites or in constant position mode
gpsNoiseScaler = 2.0f;
}
// Check if GPS can output vertical velocity and set GPS fusion mode accordingly
if (_ahrs->get_gps().have_vertical_velocity() && frontend->_fusionModeGPS == 0) {
useGpsVertVel = true;
} else {
useGpsVertVel = false;
}
// Monitor quality of the GPS velocity data before and after alignment using separate checks
if (PV_AidingMode != AID_ABSOLUTE) {
// Pre-alignment checks
gpsGoodToAlign = calcGpsGoodToAlign();
} else {
// Post-alignment checks
calcGpsGoodForFlight();
}
// Read the GPS locaton in WGS-84 lat,long,height coordinates
const struct Location &gpsloc = _ahrs->get_gps().location();
// Set the EKF origin and magnetic field declination if not previously set and GPS checks have passed
if (gpsGoodToAlign && !validOrigin) {
setOrigin();
// Now we know the location we have an estimate for the magnetic field declination and adjust the earth field accordingly
alignMagStateDeclination();
// Set the height of the NED origin to ‘height of baro height datum relative to GPS height datum'
EKF_origin.alt = gpsloc.alt - baroDataNew.hgt;
}
// convert GPS measurements to local NED and save to buffer to be fused later if we have a valid origin
if (validOrigin) {
gpsDataNew.pos = location_diff(EKF_origin, gpsloc);
gpsDataNew.hgt = 0.01f * (gpsloc.alt - EKF_origin.alt);
storedGPS.push(gpsDataNew);
// declare GPS available for use
gpsNotAvailable = false;
}
// Commence GPS aiding when able to
if (readyToUseGPS() && PV_AidingMode != AID_ABSOLUTE) {
PV_AidingMode = AID_ABSOLUTE;
// Initialise EKF position and velocity states to last GPS measurement
ResetPosition();
ResetVelocity();
}
} else {
// report GPS fix status
gpsCheckStatus.bad_fix = true;
}
}
// We need to handle the case where GPS is lost for a period of time that is too long to dead-reckon
// If that happens we need to put the filter into a constant position mode, reset the velocity states to zero
// and use the last estimated position as a synthetic GPS position
// check if we can use opticalflow as a backup
bool optFlowBackupAvailable = (flowDataValid && !hgtTimeout);
// Set GPS time-out threshold depending on whether we have an airspeed sensor to constrain drift
uint16_t gpsRetryTimeout_ms = useAirspeed() ? frontend->gpsRetryTimeUseTAS_ms : frontend->gpsRetryTimeNoTAS_ms;
// Set the time that copters will fly without a GPS lock before failing the GPS and switching to a non GPS mode
uint16_t gpsFailTimeout_ms = optFlowBackupAvailable ? frontend->gpsFailTimeWithFlow_ms : gpsRetryTimeout_ms;
// If we haven't received GPS data for a while and we are using it for aiding, then declare the position and velocity data as being timed out
if (imuSampleTime_ms - lastTimeGpsReceived_ms > gpsFailTimeout_ms) {
// Let other processes know that GPS is not available and that a timeout has occurred
posTimeout = true;
velTimeout = true;
gpsNotAvailable = true;
// If we are totally reliant on GPS for navigation, then we need to switch to a non-GPS mode of operation
// If we don't have airspeed or sideslip assumption or optical flow to constrain drift, then go into constant position mode.
// If we can do optical flow nav (valid flow data and height above ground estimate), then go into flow nav mode.
if (PV_AidingMode == AID_ABSOLUTE && !useAirspeed() && !assume_zero_sideslip()) {
if (optFlowBackupAvailable) {
// we can do optical flow only nav
frontend->_fusionModeGPS = 3;
PV_AidingMode = AID_RELATIVE;
} else {
// store the current position
lastKnownPositionNE.x = stateStruct.position.x;
lastKnownPositionNE.y = stateStruct.position.y;
// put the filter into constant position mode
PV_AidingMode = AID_NONE;
// Reset the velocity and position states
ResetVelocity();
ResetPosition();
// Reset the normalised innovation to avoid false failing bad fusion tests
velTestRatio = 0.0f;
posTestRatio = 0.0f;
}
}
}
}
// check for new valid GPS data and update stored measurement if available
void NavEKF3_core::readGpsData()
{
// check for new GPS data
// do not accept data at a faster rate than 14Hz to avoid overflowing the FIFO buffer
if (_ahrs->get_gps().last_message_time_ms() - lastTimeGpsReceived_ms > 70) {
if (_ahrs->get_gps().status() >= AP_GPS::GPS_OK_FIX_3D) {
// report GPS fix status
gpsCheckStatus.bad_fix = false;
// store fix time from previous read
secondLastGpsTime_ms = lastTimeGpsReceived_ms;
// get current fix time
lastTimeGpsReceived_ms = _ahrs->get_gps().last_message_time_ms();
// estimate when the GPS fix was valid, allowing for GPS processing and other delays
// ideally we should be using a timing signal from the GPS receiver to set this time
gpsDataNew.time_ms = lastTimeGpsReceived_ms - frontend->_gpsDelay_ms;
// Correct for the average intersampling delay due to the filter updaterate
gpsDataNew.time_ms -= localFilterTimeStep_ms/2;
// Prevent time delay exceeding age of oldest IMU data in the buffer
gpsDataNew.time_ms = MAX(gpsDataNew.time_ms,imuDataDelayed.time_ms);
// Get which GPS we are using for position information
gpsDataNew.sensor_idx = _ahrs->get_gps().primary_sensor();
// read the NED velocity from the GPS
gpsDataNew.vel = _ahrs->get_gps().velocity();
// Use the speed and position accuracy from the GPS if available, otherwise set it to zero.
// Apply a decaying envelope filter with a 5 second time constant to the raw accuracy data
float alpha = constrain_float(0.0002f * (lastTimeGpsReceived_ms - secondLastGpsTime_ms),0.0f,1.0f);
gpsSpdAccuracy *= (1.0f - alpha);
float gpsSpdAccRaw;
if (!_ahrs->get_gps().speed_accuracy(gpsSpdAccRaw)) {
gpsSpdAccuracy = 0.0f;
} else {
gpsSpdAccuracy = MAX(gpsSpdAccuracy,gpsSpdAccRaw);
gpsSpdAccuracy = MIN(gpsSpdAccuracy,50.0f);
}
gpsPosAccuracy *= (1.0f - alpha);
float gpsPosAccRaw;
if (!_ahrs->get_gps().horizontal_accuracy(gpsPosAccRaw)) {
gpsPosAccuracy = 0.0f;
} else {
gpsPosAccuracy = MAX(gpsPosAccuracy,gpsPosAccRaw);
gpsPosAccuracy = MIN(gpsPosAccuracy,100.0f);
}
gpsHgtAccuracy *= (1.0f - alpha);
float gpsHgtAccRaw;
if (!_ahrs->get_gps().vertical_accuracy(gpsHgtAccRaw)) {
gpsHgtAccuracy = 0.0f;
} else {
gpsHgtAccuracy = MAX(gpsHgtAccuracy,gpsHgtAccRaw);
gpsHgtAccuracy = MIN(gpsHgtAccuracy,100.0f);
}
// check if we have enough GPS satellites and increase the gps noise scaler if we don't
if (_ahrs->get_gps().num_sats() >= 6 && (PV_AidingMode == AID_ABSOLUTE)) {
gpsNoiseScaler = 1.0f;
} else if (_ahrs->get_gps().num_sats() == 5 && (PV_AidingMode == AID_ABSOLUTE)) {
gpsNoiseScaler = 1.4f;
} else { // <= 4 satellites or in constant position mode
gpsNoiseScaler = 2.0f;
}
// Check if GPS can output vertical velocity and set GPS fusion mode accordingly
if (_ahrs->get_gps().have_vertical_velocity() && frontend->_fusionModeGPS == 0) {
useGpsVertVel = true;
} else {
useGpsVertVel = false;
}
// Monitor quality of the GPS velocity data before and after alignment using separate checks
if (PV_AidingMode != AID_ABSOLUTE) {
// Pre-alignment checks
gpsGoodToAlign = calcGpsGoodToAlign();
} else {
gpsGoodToAlign = false;
}
// Post-alignment checks
calcGpsGoodForFlight();
// Read the GPS locaton in WGS-84 lat,long,height coordinates
const struct Location &gpsloc = _ahrs->get_gps().location();
// Set the EKF origin and magnetic field declination if not previously set and GPS checks have passed
if (gpsGoodToAlign && !validOrigin) {
setOrigin();
// set the NE earth magnetic field states using the published declination
// and set the corresponding variances and covariances
alignMagStateDeclination();
// Set the height of the NED origin to ‘height of baro height datum relative to GPS height datum'
EKF_origin.alt = gpsloc.alt - baroDataNew.hgt;
// Set the uncertinty of the GPS origin height
ekfOriginHgtVar = sq(gpsHgtAccuracy);
}
// convert GPS measurements to local NED and save to buffer to be fused later if we have a valid origin
if (validOrigin) {
gpsDataNew.pos = location_diff(EKF_origin, gpsloc);
gpsDataNew.hgt = 0.01f * (gpsloc.alt - EKF_origin.alt);
storedGPS.push(gpsDataNew);
// declare GPS available for use
gpsNotAvailable = false;
}
frontend->logging.log_gps = true;
} else {
// report GPS fix status
gpsCheckStatus.bad_fix = true;
}
}
}
// check for new valid GPS data and update stored measurement if available
void NavEKF2_core::readGpsData()
{
// check for new GPS data
if (_ahrs->get_gps().last_message_time_ms() != lastTimeGpsReceived_ms) {
if (_ahrs->get_gps().status() >= AP_GPS::GPS_OK_FIX_3D) {
// report GPS fix status
gpsCheckStatus.bad_fix = false;
// store fix time from previous read
secondLastGpsTime_ms = lastTimeGpsReceived_ms;
// get current fix time
lastTimeGpsReceived_ms = _ahrs->get_gps().last_message_time_ms();
// estimate when the GPS fix was valid, allowing for GPS processing and other delays
// ideally we should be using a timing signal from the GPS receiver to set this time
gpsDataNew.time_ms = lastTimeGpsReceived_ms - frontend._gpsDelay_ms;
// Assign measurement to nearest fusion interval so that multiple measurements can be fused on the same frame
// This allows us to perform the covariance prediction over longer time steps which reduces numerical precision errors
gpsDataNew.time_ms = roundToNearest(gpsDataNew.time_ms, frontend.fusionTimeStep_ms);
// Prevent time delay exceeding age of oldest IMU data in the buffer
gpsDataNew.time_ms = max(gpsDataNew.time_ms,imuDataDelayed.time_ms);
// read the NED velocity from the GPS
gpsDataNew.vel = _ahrs->get_gps().velocity();
// Use the speed accuracy from the GPS if available, otherwise set it to zero.
// Apply a decaying envelope filter with a 5 second time constant to the raw speed accuracy data
float alpha = constrain_float(0.0002f * (lastTimeGpsReceived_ms - secondLastGpsTime_ms),0.0f,1.0f);
gpsSpdAccuracy *= (1.0f - alpha);
float gpsSpdAccRaw;
if (!_ahrs->get_gps().speed_accuracy(gpsSpdAccRaw)) {
gpsSpdAccuracy = 0.0f;
} else {
gpsSpdAccuracy = max(gpsSpdAccuracy,gpsSpdAccRaw);
}
// check if we have enough GPS satellites and increase the gps noise scaler if we don't
if (_ahrs->get_gps().num_sats() >= 6 && (PV_AidingMode == AID_ABSOLUTE)) {
gpsNoiseScaler = 1.0f;
} else if (_ahrs->get_gps().num_sats() == 5 && (PV_AidingMode == AID_ABSOLUTE)) {
gpsNoiseScaler = 1.4f;
} else { // <= 4 satellites or in constant position mode
gpsNoiseScaler = 2.0f;
}
// Check if GPS can output vertical velocity and set GPS fusion mode accordingly
if (_ahrs->get_gps().have_vertical_velocity() && frontend._fusionModeGPS == 0) {
useGpsVertVel = true;
} else {
useGpsVertVel = false;
}
// Monitor quality of the GPS velocity data before and after alignment using separate checks
if (PV_AidingMode != AID_ABSOLUTE) {
// Pre-alignment checks
gpsQualGood = calcGpsGoodToAlign();
} else {
// Post-alignment checks
calcGpsGoodForFlight();
}
// read latitutde and longitude from GPS and convert to local NE position relative to the stored origin
// If we don't have an origin, then set it to the current GPS coordinates
const struct Location &gpsloc = _ahrs->get_gps().location();
if (validOrigin) {
gpsDataNew.pos = location_diff(EKF_origin, gpsloc);
} else if (gpsQualGood) {
// Set the NE origin to the current GPS position
setOrigin();
// Now we know the location we have an estimate for the magnetic field declination and adjust the earth field accordingly
alignMagStateDeclination();
// Set the height of the NED origin to ‘height of baro height datum relative to GPS height datum'
EKF_origin.alt = gpsloc.alt - baroDataNew.hgt;
// We are by definition at the origin at the instant of alignment so set NE position to zero
gpsDataNew.pos.zero();
// If GPS useage isn't explicitly prohibited, we switch to absolute position mode
if (isAiding && frontend._fusionModeGPS != 3) {
PV_AidingMode = AID_ABSOLUTE;
// Initialise EKF position and velocity states
ResetPosition();
ResetVelocity();
}
}
// calculate a position offset which is applied to NE position and velocity wherever it is used throughout code to allow GPS position jumps to be accommodated gradually
decayGpsOffset();
// save measurement to buffer to be fused later
StoreGPS();
// declare GPS available for use
gpsNotAvailable = false;
} else {
// report GPS fix status
gpsCheckStatus.bad_fix = true;
}
}
// We need to handle the case where GPS is lost for a period of time that is too long to dead-reckon
// If that happens we need to put the filter into a constant position mode, reset the velocity states to zero
// and use the last estimated position as a synthetic GPS position
// check if we can use opticalflow as a backup
bool optFlowBackupAvailable = (flowDataValid && !hgtTimeout);
// Set GPS time-out threshold depending on whether we have an airspeed sensor to constrain drift
uint16_t gpsRetryTimeout_ms = useAirspeed() ? frontend.gpsRetryTimeUseTAS_ms : frontend.gpsRetryTimeNoTAS_ms;
// Set the time that copters will fly without a GPS lock before failing the GPS and switching to a non GPS mode
uint16_t gpsFailTimeout_ms = optFlowBackupAvailable ? frontend.gpsFailTimeWithFlow_ms : gpsRetryTimeout_ms;
// If we haven't received GPS data for a while and we are using it for aiding, then declare the position and velocity data as being timed out
if (imuSampleTime_ms - lastTimeGpsReceived_ms > gpsFailTimeout_ms) {
// Let other processes know that GPS i snota vailable and that a timeout has occurred
posTimeout = true;
velTimeout = true;
gpsNotAvailable = true;
// If we are currently reliying on GPS for navigation, then we need to switch to a non-GPS mode of operation
if (PV_AidingMode == AID_ABSOLUTE) {
// If we don't have airspeed or sideslip assumption or optical flow to constrain drift, then go into constant position mode.
// If we can do optical flow nav (valid flow data and height above ground estimate), then go into flow nav mode.
if (!useAirspeed() && !assume_zero_sideslip()) {
if (optFlowBackupAvailable) {
// we can do optical flow only nav
frontend._fusionModeGPS = 3;
PV_AidingMode = AID_RELATIVE;
} else {
// store the current position
lastKnownPositionNE.x = stateStruct.position.x;
lastKnownPositionNE.y = stateStruct.position.y;
// put the filter into constant position mode
PV_AidingMode = AID_NONE;
// reset all glitch states
gpsPosGlitchOffsetNE.zero();
gpsVelGlitchOffset.zero();
// Reset the velocity and position states
ResetVelocity();
ResetPosition();
// Reset the normalised innovation to avoid false failing the bad position fusion test
velTestRatio = 0.0f;
posTestRatio = 0.0f;
}
}
}
}
// If not aiding we synthesise the GPS measurements at the last known position
if (PV_AidingMode == AID_NONE) {
if (imuSampleTime_ms - gpsDataNew.time_ms > 200) {
gpsDataNew.pos.x = lastKnownPositionNE.x;
gpsDataNew.pos.y = lastKnownPositionNE.y;
gpsDataNew.time_ms = imuSampleTime_ms-frontend._gpsDelay_ms;
// Assign measurement to nearest fusion interval so that multiple measurements can be fused on the same frame
// This allows us to perform the covariance prediction over longer time steps which reduces numerical precision errors
gpsDataNew.time_ms = roundToNearest(gpsDataNew.time_ms, frontend.fusionTimeStep_ms);
// Prevent time delay exceeding age of oldest IMU data in the buffer
gpsDataNew.time_ms = max(gpsDataNew.time_ms,imuDataDelayed.time_ms);
// save measurement to buffer to be fused later
StoreGPS();
}
}
}
