AP_NavEKF3: add external nav system support

includes decoupling height source from use of external nav data

libraries/AP_NavEKF3/AP_NavEKF3.cpp

@@ -196,8 +196,8 @@ const AP_Param::GroupInfo NavEKF3::var_info[] = {
 
     // @Param: ALT_SOURCE
     // @DisplayName: Primary altitude sensor source
-    // @Description: Primary height sensor used by the EKF. If the selected option cannot be used, baro is used. 1 uses the range finder and only with optical flow navigation (EK2_GPS_TYPE = 3), Do not use "1" for terrain following. NOTE: the EK3_RNG_USE_HGT parameter can be used to switch to range-finder when close to the ground.
-    // @Values: 0:Use Baro, 1:Use Range Finder, 2:Use GPS, 3:Use Range Beacon
+    // @Description: Primary height sensor used by the EKF. If the selected option cannot be used, baro is used. 1 uses the range finder and only with optical flow navigation (EK3_GPS_TYPE = 3), Do not use "1" for terrain following. NOTE: the EK3_RNG_USE_HGT parameter can be used to switch to range-finder when close to the ground. Setting 4 uses external nav system data, but only if data is also being used for horizontal position
+    // @Values: 0:Use Baro, 1:Use Range Finder, 2:Use GPS, 3:Use Range Beacon, 4:Use External Nav
     // @User: Advanced
     // @RebootRequired: True
     AP_GROUPINFO("ALT_SOURCE", 9, NavEKF3, _altSource, 0),
@@ -1315,6 +1315,26 @@ void NavEKF3::writeEulerYawAngle(float yawAngle, float yawAngleErr, uint32_t tim
     }
 }
 
+/*
+ * Write position and quaternion data from an external navigation system
+ *
+ * pos        : XYZ position (m) in a RH navigation frame with the Z axis pointing down and XY axes horizontal. Frame must be aligned with NED if the magnetomer is being used for yaw.
+ * quat       : quaternion describing the the rotation from navigation frame to body frame
+ * posErr     : 1-sigma spherical position error (m)
+ * angErr     : 1-sigma spherical angle error (rad)
+ * timeStamp_ms : system time the measurement was taken, not the time it was received (mSec)
+ * resetTime_ms : system time of the last position reset request (mSec)
+ *
+*/
+void NavEKF3::writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint32_t resetTime_ms)
+{
+    if (core) {
+        for (uint8_t i=0; i<num_cores; i++) {
+            core[i].writeExtNavData(pos, quat, posErr, angErr, timeStamp_ms, resetTime_ms);
+        }
+    }
+}
+
 // return data for debugging optical flow fusion
 void NavEKF3::getFlowDebug(int8_t instance, float &varFlow, float &gndOffset, float &flowInnovX, float &flowInnovY, float &auxInnov,
                            float &HAGL, float &rngInnov, float &range, float &gndOffsetErr) const

libraries/AP_NavEKF3/AP_NavEKF3.h

@@ -281,6 +281,19 @@ public:
     */
     void writeEulerYawAngle(float yawAngle, float yawAngleErr, uint32_t timeStamp_ms, uint8_t type);
 
+    /*
+     * Write position and quaternion data from an external navigation system
+     *
+     * pos        : position in the RH navigation frame. Frame is assumed to be NED if frameIsNED is true. (m)
+     * quat       : quaternion desribing the the rotation from navigation frame to body frame
+     * posErr     : 1-sigma spherical position error (m)
+     * angErr     : 1-sigma spherical angle error (rad)
+     * timeStamp_ms : system time the measurement was taken, not the time it was received (mSec)
+     * resetTime_ms : system time of the last position reset request (mSec)
+     *
+    */
+    void writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint32_t resetTime_ms);
+
     // called by vehicle code to specify that a takeoff is happening
     // causes the EKF to compensate for expected barometer errors due to ground effect
     void setTakeoffExpected(bool val);
@@ -439,7 +452,7 @@ private:
     AP_Int8  _flowDelay_ms;         // effective average delay of optical flow measurements rel to IMU (msec)
     AP_Int16  _rngInnovGate;        // Percentage number of standard deviations applied to range finder innovation consistency check
     AP_Float _maxFlowRate;          // Maximum flow rate magnitude that will be accepted by the filter
-    AP_Int8 _altSource;             // Primary alt source during optical flow navigation. 0 = use Baro, 1 = use range finder.
+    AP_Int8 _altSource;             // Primary alt source. 0 = Baro, 1 = range finder, 2 = GPS, 3 = range beacons, 4 = external nav
     AP_Float _rngNoise;             // Range finder noise : m
     AP_Int8 _gpsCheck;              // Bitmask controlling which preflight GPS checks are bypassed
     AP_Int8 _imuMask;               // Bitmask of IMUs to instantiate EKF3 for

libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp

@@ -210,7 +210,7 @@ void NavEKF3_core::setAidingMode()
             // and IMU gyro bias estimates have stabilised
             // If GPS usage has been prohiited then we use flow aiding provided optical flow data is present
             // GPS aiding is the preferred option unless excluded by the user
-            if(readyToUseGPS() || readyToUseRangeBeacon()) {
+            if (readyToUseGPS() || readyToUseRangeBeacon() || readyToUseExtNav()) {
                 PV_AidingMode = AID_ABSOLUTE;
             } else if ((readyToUseOptFlow()  && (frontend->_flowUse == FLOW_USE_NAV)) || readyToUseBodyOdm()) {
                 PV_AidingMode = AID_RELATIVE;
@@ -251,6 +251,9 @@ void NavEKF3_core::setAidingMode()
             bool gpsPosUsed = (imuSampleTime_ms - lastPosPassTime_ms <= minTestTime_ms);
             bool gpsVelUsed = (imuSampleTime_ms - lastVelPassTime_ms <= minTestTime_ms);
 
+            // Check if external nav position is being used
+            bool extNavUsed = (imuSampleTime_ms - lastExtNavPassTime_ms <= minTestTime_ms);
+
             // Check if attitude drift has been constrained by a measurement source
             bool attAiding = gpsPosUsed || gpsVelUsed || optFlowUsed || airSpdUsed || rngBcnUsed || bodyOdmUsed;
 
@@ -258,7 +261,7 @@ void NavEKF3_core::setAidingMode()
             bool velAiding = gpsVelUsed || airSpdUsed || optFlowUsed || bodyOdmUsed;
 
             // check if position drift has been constrained by a measurement source
-            bool posAiding = gpsPosUsed || rngBcnUsed;
+            bool posAiding = gpsPosUsed || rngBcnUsed || extNavUsed;
 
             // Check if the loss of attitude aiding has become critical
             bool attAidLossCritical = false;
@@ -294,12 +297,14 @@ void NavEKF3_core::setAidingMode()
                 rngBcnTimeout = true;
                 tasTimeout = true;
                 gpsNotAvailable = true;
+                extNavTimeout = true;
              } else if (posAidLossCritical) {
                 // if the loss of position is critical, declare all sources of position aiding as being timed out
                 posTimeout = true;
                 velTimeout = true;
                 rngBcnTimeout = true;
                 gpsNotAvailable = true;
+                extNavTimeout = true;
 
             } else if (posAidLossPending) {
                 // attempt to reset the yaw to the estimate from the EKF-GSF algorithm
@@ -364,6 +369,16 @@ void NavEKF3_core::setAidingMode()
                 gcs().send_text(MAV_SEVERITY_INFO, "EKF3 IMU%u is using range beacons",(unsigned)imu_index);
                 gcs().send_text(MAV_SEVERITY_INFO, "EKF3 IMU%u initial pos NE = %3.1f,%3.1f (m)",(unsigned)imu_index,(double)receiverPos.x,(double)receiverPos.y);
                 gcs().send_text(MAV_SEVERITY_INFO, "EKF3 IMU%u initial beacon pos D offset = %3.1f (m)",(unsigned)imu_index,(double)bcnPosOffsetNED.z);
+            } else if (readyToUseExtNav()) {
+                // we are commencing aiding using external nav
+                posResetSource = EXTNAV;
+                velResetSource = DEFAULT;
+                gcs().send_text(MAV_SEVERITY_INFO, "EKF3 IMU%u is using external nav data",(unsigned)imu_index);
+                gcs().send_text(MAV_SEVERITY_INFO, "EKF3 IMU%u initial pos NED = %3.1f,%3.1f,%3.1f (m)",(unsigned)imu_index,(double)extNavDataDelayed.pos.x,(double)extNavDataDelayed.pos.y,(double)extNavDataDelayed.pos.z);
+                // handle height reset as special case
+                hgtMea = -extNavDataDelayed.pos.z;
+                posDownObsNoise = sq(constrain_float(extNavDataDelayed.posErr, 0.1f, 10.0f));
+                ResetHeight();
             }
 
             // clear timeout flags as a precaution to avoid triggering any additional transitions
@@ -456,6 +471,12 @@ bool NavEKF3_core::readyToUseRangeBeacon(void) const
     return tiltAlignComplete && yawAlignComplete && delAngBiasLearned && rngBcnAlignmentCompleted && rngBcnDataToFuse;
 }
 
+// return true if the filter is ready to use external nav data
+bool NavEKF3_core::readyToUseExtNav(void) const
+{
+    return tiltAlignComplete && extNavDataToFuse;
+}
+
 // return true if we should use the compass
 bool NavEKF3_core::use_compass(void) const
 {
@@ -489,7 +510,8 @@ bool NavEKF3_core::assume_zero_sideslip(void) const
 // set the LLH location of the filters NED origin
 bool NavEKF3_core::setOriginLLH(const Location &loc)
 {
-    if (PV_AidingMode == AID_ABSOLUTE) {
+    if ((PV_AidingMode == AID_ABSOLUTE) && (frontend->_fusionModeGPS != 3)) {
+        // reject attempt to set origin if GPS is being used
         return false;
     }
     EKF_origin = loc;

libraries/AP_NavEKF3/AP_NavEKF3_MagFusion.cpp

@@ -920,7 +920,7 @@ void NavEKF3_core::FuseMagnetometer()
  * usePredictedYaw -  Set this to true if no valid yaw measurement will be available for an extended periods.
  *                    This uses an innovation set to zero which prevents uncontrolled quaternion covariance
  *                    growth or if available, a yaw estimate from the Gaussian Sum Filter.
- * UseExternalYawSensor - Set this to true if yaw data from an external yaw sensor is being used instead of the magnetometer.
+ * UseExternalYawSensor - Set this to true if yaw data from an external yaw sensor (GPS or external nav) is being used instead of the magnetometer.
 */
 void NavEKF3_core::fuseEulerYaw(bool usePredictedYaw, bool useExternalYawSensor)
 {

libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp

@@ -936,6 +936,51 @@ void NavEKF3_core::writeDefaultAirSpeed(float airspeed)
     defaultAirSpeed = airspeed;
 }
 
+/********************************************************
+*            External Navigation Measurements           *
+********************************************************/
+
+void NavEKF3_core::writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint32_t resetTime_ms)
+{
+    // limit update rate to maximum allowed by sensor buffers and fusion process
+    // don't try to write to buffer until the filter has been initialised
+    if (((timeStamp_ms - extNavMeasTime_ms) < 70) || !statesInitialised) {
+        return;
+    } else {
+        extNavMeasTime_ms = timeStamp_ms;
+    }
+
+    if (resetTime_ms != extNavLastPosResetTime_ms) {
+        extNavDataNew.posReset = true;
+        extNavLastPosResetTime_ms = resetTime_ms;
+    } else {
+        extNavDataNew.posReset = false;
+    }
+
+    extNavDataNew.pos = pos;
+    if (posErr > 0) {
+        extNavDataNew.posErr = posErr;
+    } else {
+        extNavDataNew.posErr = frontend->_gpsHorizPosNoise;
+    }
+
+    // calculate timestamp
+    const uint32_t extnav_delay_ms = 10;
+    timeStamp_ms = timeStamp_ms - extnav_delay_ms;
+    // Correct for the average intersampling delay due to the filter update rate
+    timeStamp_ms -= localFilterTimeStep_ms/2;
+    // Prevent time delay exceeding age of oldest IMU data in the buffer
+    timeStamp_ms = MAX(timeStamp_ms, imuDataDelayed.time_ms);
+    extNavDataNew.time_ms = timeStamp_ms;
+
+    // extract yaw from the attitude
+    float roll_rad, pitch_rad, yaw_rad;
+    quat.to_euler(roll_rad, pitch_rad, yaw_rad);
+    writeEulerYawAngle(yaw_rad, angErr, timeStamp_ms, 2);
+
+    storedExtNav.push(extNavDataNew);
+}
+
 /*
   update timing statistics structure
  */

libraries/AP_NavEKF3/AP_NavEKF3_Outputs.cpp

@@ -111,7 +111,7 @@ bool NavEKF3_core::getRangeBeaconDebug(uint8_t &ID, float &rng, float &innov, fl
 bool NavEKF3_core::getHeightControlLimit(float &height) const
 {
     // only ask for limiting if we are doing optical flow navigation
-    if (frontend->_fusionModeGPS == 3) {
+    if (frontend->_fusionModeGPS == 3 && (PV_AidingMode == AID_RELATIVE) && flowDataValid) {
         // If are doing optical flow nav, ensure the height above ground is within range finder limits after accounting for vehicle tilt and control errors
         const RangeFinder *_rng = AP::rangefinder();
         if (_rng == nullptr) {

libraries/AP_NavEKF3/AP_NavEKF3_PosVelFusion.cpp

@@ -119,6 +119,15 @@ void NavEKF3_core::ResetPosition(void)
             // clear the timeout flags and counters
             rngBcnTimeout = false;
             lastRngBcnPassTime_ms = imuSampleTime_ms;
+        } else if ((imuSampleTime_ms - extNavDataDelayed.time_ms < 250 && posResetSource == DEFAULT) || posResetSource == EXTNAV) {
+            // use external nav system data as the third preference
+            stateStruct.position.x = extNavDataDelayed.pos.x;
+            stateStruct.position.y = extNavDataDelayed.pos.y;
+            // set the variances as received from external nav system data
+            P[7][7] = P[8][8] = sq(extNavDataDelayed.posErr);
+            // clear the timeout flags and counters
+            extNavTimeout = false;
+            lastExtNavPassTime_ms = imuSampleTime_ms;
         }
     }
     for (uint8_t i=0; i<imu_buffer_length; i++) {
@@ -284,6 +293,9 @@ void NavEKF3_core::SelectVelPosFusion()
         posVelFusionDelayed = false;
     }
 
+    // Check for data at the fusion time horizon
+    extNavDataToFuse = storedExtNav.recall(extNavDataDelayed, imuDataDelayed.time_ms);
+
     // Read GPS data from the sensor
     readGpsData();
 
@@ -291,7 +303,7 @@ void NavEKF3_core::SelectVelPosFusion()
     gpsDataToFuse = storedGPS.recall(gpsDataDelayed,imuDataDelayed.time_ms);
 
     // Determine if we need to fuse position and velocity data on this time step
-    if (gpsDataToFuse && PV_AidingMode == AID_ABSOLUTE) {
+    if (gpsDataToFuse && (PV_AidingMode == AID_ABSOLUTE) && (frontend->_fusionModeGPS != 3)) {
 
         // Don't fuse velocity data if GPS doesn't support it
         if (frontend->_fusionModeGPS <= 1) {
@@ -300,8 +312,28 @@ void NavEKF3_core::SelectVelPosFusion()
             fuseVelData = false;
         }
         fusePosData = true;
+        extNavUsedForPos = false;
 
         CorrectGPSForAntennaOffset(gpsDataDelayed);
+
+        // copy corrected GPS data to observation vector
+        if (fuseVelData) {
+            velPosObs[0] = gpsDataDelayed.vel.x;
+            velPosObs[1] = gpsDataDelayed.vel.y;
+            velPosObs[2] = gpsDataDelayed.vel.z;
+        }
+        velPosObs[3] = gpsDataDelayed.pos.x;
+        velPosObs[4] = gpsDataDelayed.pos.y;
+    } else if (extNavDataToFuse && (PV_AidingMode == AID_ABSOLUTE) && (frontend->_fusionModeGPS == 3)) {
+        // use external nav system for position
+        extNavUsedForPos = true;
+        activeHgtSource = HGT_SOURCE_EXTNAV;
+        fuseVelData = false;
+        fuseHgtData = true;
+        fusePosData = true;
+        velPosObs[3] = extNavDataDelayed.pos.x;
+        velPosObs[4] = extNavDataDelayed.pos.y;
+        velPosObs[5] = extNavDataDelayed.pos.z;
     } else {
         fuseVelData = false;
         fusePosData = false;
@@ -369,13 +401,14 @@ void NavEKF3_core::SelectVelPosFusion()
         }
     }
 
+    // To-Do: add external nav position reset handling here?
+
     // If we are operating without any aiding, fuse in the last known position
     // to constrain tilt drift. This assumes a non-manoeuvring vehicle
     // Do this to coincide with the height fusion
     if (fuseHgtData && PV_AidingMode == AID_NONE) {
-        gpsDataDelayed.vel.zero();
-        gpsDataDelayed.pos.x = lastKnownPositionNE.x;
-        gpsDataDelayed.pos.y = lastKnownPositionNE.y;
+        velPosObs[3] = lastKnownPositionNE.x;
+        velPosObs[4] = lastKnownPositionNE.y;
 
         fusePosData = true;
         fuseVelData = false;
@@ -413,7 +446,6 @@ void NavEKF3_core::FuseVelPosNED()
     // declare variables used by state and covariance update calculations
     Vector6 R_OBS; // Measurement variances used for fusion
     Vector6 R_OBS_DATA_CHECKS; // Measurement variances used for data checks only
-    Vector6 observation;
     float SK;
 
     // perform sequential fusion of GPS measurements. This assumes that the
@@ -423,17 +455,11 @@ void NavEKF3_core::FuseVelPosNED()
     // so we might as well take advantage of the computational efficiencies
     // associated with sequential fusion
     if (fuseVelData || fusePosData || fuseHgtData) {
-        // form the observation vector
-        observation[0] = gpsDataDelayed.vel.x;
-        observation[1] = gpsDataDelayed.vel.y;
-        observation[2] = gpsDataDelayed.vel.z;
-        observation[3] = gpsDataDelayed.pos.x;
-        observation[4] = gpsDataDelayed.pos.y;
-        observation[5] = -hgtMea;
-
         // calculate additional error in GPS position caused by manoeuvring
         float posErr = frontend->gpsPosVarAccScale * accNavMag;
 
+        // To-Do: this posErr should come from external nav when fusing external nav position
+
         // estimate the GPS Velocity, GPS horiz position and height measurement variances.
         // Use different errors if operating without external aiding using an assumed position or velocity of zero
         if (PV_AidingMode == AID_NONE) {
@@ -463,6 +489,8 @@ void NavEKF3_core::FuseVelPosNED()
             // Use GPS reported position accuracy if available and floor at value set by GPS position noise parameter
             if (gpsPosAccuracy > 0.0f) {
                 R_OBS[3] = sq(constrain_float(gpsPosAccuracy, frontend->_gpsHorizPosNoise, 100.0f));
+            } else if (extNavUsedForPos) {
+                R_OBS[3] = sq(constrain_float(extNavDataDelayed.posErr, 0.01f, 10.0f));
             } else {
                 R_OBS[3] = sq(constrain_float(frontend->_gpsHorizPosNoise, 0.1f, 10.0f)) + sq(posErr);
             }
@@ -480,8 +508,8 @@ void NavEKF3_core::FuseVelPosNED()
         // the accelerometers and we should disable the GPS and barometer innovation consistency checks.
         if (useGpsVertVel && fuseVelData && (frontend->_altSource != 2)) {
             // calculate innovations for height and vertical GPS vel measurements
-            float hgtErr  = stateStruct.position.z - observation[5];
-            float velDErr = stateStruct.velocity.z - observation[2];
+            const float hgtErr  = stateStruct.position.z - velPosObs[5];
+            const float velDErr = stateStruct.velocity.z - velPosObs[2];
             // check if they are the same sign and both more than 3-sigma out of bounds
             if ((hgtErr*velDErr > 0.0f) && (sq(hgtErr) > 9.0f * (P[9][9] + R_OBS_DATA_CHECKS[5])) && (sq(velDErr) > 9.0f * (P[6][6] + R_OBS_DATA_CHECKS[2]))) {
                 badIMUdata = true;
@@ -494,8 +522,8 @@ void NavEKF3_core::FuseVelPosNED()
         // test position measurements
         if (fusePosData) {
             // test horizontal position measurements
-            innovVelPos[3] = stateStruct.position.x - observation[3];
-            innovVelPos[4] = stateStruct.position.y - observation[4];
+            innovVelPos[3] = stateStruct.position.x - velPosObs[3];
+            innovVelPos[4] = stateStruct.position.y - velPosObs[4];
             varInnovVelPos[3] = P[7][7] + R_OBS_DATA_CHECKS[3];
             varInnovVelPos[4] = P[8][8] + R_OBS_DATA_CHECKS[4];
             // apply an innovation consistency threshold test, but don't fail if bad IMU data
@@ -546,7 +574,7 @@ void NavEKF3_core::FuseVelPosNED()
                 // velocity states start at index 4
                 stateIndex   = i + 4;
                 // calculate innovations using blended and single IMU predicted states
-                velInnov[i]  = stateStruct.velocity[i] - observation[i]; // blended
+                velInnov[i]  = stateStruct.velocity[i] - velPosObs[i]; // blended
                 // calculate innovation variance
                 varInnovVelPos[i] = P[stateIndex][stateIndex] + R_OBS_DATA_CHECKS[i];
                 // sum the innovation and innovation variances
@@ -580,7 +608,7 @@ void NavEKF3_core::FuseVelPosNED()
         // test height measurements
         if (fuseHgtData) {
             // calculate height innovations
-            innovVelPos[5] = stateStruct.position.z - observation[5];
+            innovVelPos[5] = stateStruct.position.z - velPosObs[5];
             varInnovVelPos[5] = P[9][9] + R_OBS_DATA_CHECKS[5];
             // calculate the innovation consistency test ratio
             hgtTestRatio = sq(innovVelPos[5]) / (sq(MAX(0.01f * (float)frontend->_hgtInnovGate, 1.0f)) * varInnovVelPos[5]);
@@ -639,16 +667,14 @@ void NavEKF3_core::FuseVelPosNED()
                 stateIndex = 4 + obsIndex;
                 // calculate the measurement innovation, using states from a different time coordinate if fusing height data
                 // adjust scaling on GPS measurement noise variances if not enough satellites
-                if (obsIndex <= 2)
-                {
-                    innovVelPos[obsIndex] = stateStruct.velocity[obsIndex] - observation[obsIndex];
+                if (obsIndex <= 2) {
+                    innovVelPos[obsIndex] = stateStruct.velocity[obsIndex] - velPosObs[obsIndex];
                     R_OBS[obsIndex] *= sq(gpsNoiseScaler);
-                }
-                else if (obsIndex == 3 || obsIndex == 4) {
-                    innovVelPos[obsIndex] = stateStruct.position[obsIndex-3] - observation[obsIndex];
+                } else if (obsIndex == 3 || obsIndex == 4) {
+                    innovVelPos[obsIndex] = stateStruct.position[obsIndex-3] - velPosObs[obsIndex];
                     R_OBS[obsIndex] *= sq(gpsNoiseScaler);
                 } else if (obsIndex == 5) {
-                    innovVelPos[obsIndex] = stateStruct.position[obsIndex-3] - observation[obsIndex];
+                    innovVelPos[obsIndex] = stateStruct.position[obsIndex-3] - velPosObs[obsIndex];
                     const float gndMaxBaroErr = 4.0f;
                     const float gndBaroInnovFloor = -0.5f;
 
@@ -814,7 +840,10 @@ void NavEKF3_core::selectHeightForFusion()
     baroDataToFuse = storedBaro.recall(baroDataDelayed, imuDataDelayed.time_ms);
 
     // select height source
-    if (_rng && ((frontend->_useRngSwHgt > 0) && (frontend->_altSource == 1)) && (imuSampleTime_ms - rngValidMeaTime_ms < 500)) {
+    if (extNavUsedForPos && (frontend->_altSource == 4)) {
+        // always use external navigation as the height source if using for position.
+        activeHgtSource = HGT_SOURCE_EXTNAV;
+    } else if (_rng && ((frontend->_useRngSwHgt > 0) && (frontend->_altSource == 1)) && (imuSampleTime_ms - rngValidMeaTime_ms < 500)) {
         if (frontend->_altSource == 1) {
             // always use range finder
             activeHgtSource = HGT_SOURCE_RNG;
@@ -865,10 +894,11 @@ void NavEKF3_core::selectHeightForFusion()
         activeHgtSource = HGT_SOURCE_BARO;
     }
 
-    // Use Baro alt as a fallback if we lose range finder or GPS
+    // Use Baro alt as a fallback if we lose range finder, GPS or external nav
     bool lostRngHgt = ((activeHgtSource == HGT_SOURCE_RNG) && ((imuSampleTime_ms - rngValidMeaTime_ms) > 500));
     bool lostGpsHgt = ((activeHgtSource == HGT_SOURCE_GPS) && ((imuSampleTime_ms - lastTimeGpsReceived_ms) > 2000));
-    if (lostRngHgt || lostGpsHgt) {
+    bool lostExtNavHgt = ((activeHgtSource == HGT_SOURCE_EXTNAV) && ((imuSampleTime_ms - extNavMeasTime_ms) > 2000));
+    if (lostRngHgt || lostGpsHgt || lostExtNavHgt) {
         activeHgtSource = HGT_SOURCE_BARO;
     }
 
@@ -899,7 +929,10 @@ void NavEKF3_core::selectHeightForFusion()
     }
 
     // Select the height measurement source
-    if (rangeDataToFuse && (activeHgtSource == HGT_SOURCE_RNG)) {
+    if (extNavDataToFuse && (activeHgtSource == HGT_SOURCE_EXTNAV)) {
+        hgtMea = -extNavDataDelayed.pos.z;
+        posDownObsNoise = sq(constrain_float(extNavDataDelayed.posErr, 0.1f, 10.0f));
+    } else if (rangeDataToFuse && (activeHgtSource == HGT_SOURCE_RNG)) {
         // using range finder data
         // correct for tilt using a flat earth model
         if (prevTnb.c.z >= 0.7) {
@@ -907,6 +940,7 @@ void NavEKF3_core::selectHeightForFusion()
             hgtMea  = MAX(rangeDataDelayed.rng * prevTnb.c.z, rngOnGnd);
             // correct for terrain position relative to datum
             hgtMea -= terrainState;
+            velPosObs[5] = -hgtMea;
             // enable fusion
             fuseHgtData = true;
             // set the observation noise
@@ -920,6 +954,7 @@ void NavEKF3_core::selectHeightForFusion()
     } else if  (gpsDataToFuse && (activeHgtSource == HGT_SOURCE_GPS)) {
         // using GPS data
         hgtMea = gpsDataDelayed.hgt;
+        velPosObs[5] = -hgtMea;
         // enable fusion
         fuseHgtData = true;
         // set the observation noise using receiver reported accuracy or the horizontal noise scaled for typical VDOP/HDOP ratio
@@ -944,6 +979,7 @@ void NavEKF3_core::selectHeightForFusion()
         if (motorsArmed && getTakeoffExpected()) {
             hgtMea = MAX(hgtMea, meaHgtAtTakeOff);
         }
+        velPosObs[5] = -hgtMea;
     } else {
         fuseHgtData = false;
     }

libraries/AP_NavEKF3/AP_NavEKF3_core.cpp

@@ -140,6 +140,9 @@ bool NavEKF3_core::setup_core(uint8_t _imu_index, uint8_t _core_index)
     if(!storedRangeBeacon.init(imu_buffer_length)) {
         return false;
     }
+    if (!storedExtNav.init(obs_buffer_length)) {
+        return false;
+    }
     if(!storedIMU.init(imu_buffer_length)) {
         return false;
     }
@@ -329,6 +332,7 @@ void NavEKF3_core::InitialiseVariables()
     ekfGpsRefHgt = 0.0;
     velOffsetNED.zero();
     posOffsetNED.zero();
+    memset(&velPosObs, 0, sizeof(velPosObs));
     posResetSource = DEFAULT;
     velResetSource = DEFAULT;
 
@@ -390,6 +394,16 @@ void NavEKF3_core::InitialiseVariables()
     memset(&yawAngDataNew, 0, sizeof(yawAngDataNew));
     memset(&yawAngDataDelayed, 0, sizeof(yawAngDataDelayed));
 
+    // external nav data fusion
+    memset(&extNavDataNew, 0, sizeof(extNavDataNew));
+    memset(&extNavDataDelayed, 0, sizeof(extNavDataDelayed));
+    extNavMeasTime_ms = 0;
+    extNavLastPosResetTime_ms = 0;
+    lastExtNavPassTime_ms = 0;
+    extNavDataToFuse = false;
+    extNavUsedForPos = false;
+    extNavTimeout = false;
+
     // zero data buffers
     storedIMU.reset();
     storedGPS.reset();
@@ -400,6 +414,7 @@ void NavEKF3_core::InitialiseVariables()
     storedRangeBeacon.reset();
     storedBodyOdm.reset();
     storedWheelOdm.reset();
+    storedExtNav.reset();
 
     // initialise pre-arm message
     hal.util->snprintf(prearm_fail_string, sizeof(prearm_fail_string), "EKF3 still initialising");

libraries/AP_NavEKF3/AP_NavEKF3_core.h

@@ -43,10 +43,11 @@
 #define MASK_GPS_HORIZ_SPD  (1<<7)
 
 // active height source
-#define HGT_SOURCE_BARO 0
-#define HGT_SOURCE_RNG  1
-#define HGT_SOURCE_GPS  2
-#define HGT_SOURCE_BCN  3
+#define HGT_SOURCE_BARO     0
+#define HGT_SOURCE_RNG      1
+#define HGT_SOURCE_GPS      2
+#define HGT_SOURCE_BCN      3
+#define HGT_SOURCE_EXTNAV   4
 
 #define earthRate 0.000072921f // earth rotation rate (rad/sec)
 
@@ -286,6 +287,19 @@ public:
     */
     void writeEulerYawAngle(float yawAngle, float yawAngleErr, uint32_t timeStamp_ms, uint8_t type);
 
+    /*
+    * Write position and quaternion data from an external navigation system
+    *
+    * pos        : position in the RH navigation frame. Frame is assumed to be NED if frameIsNED is true. (m)
+    * quat       : quaternion desribing the the rotation from navigation frame to body frame
+    * posErr     : 1-sigma spherical position error (m)
+    * angErr     : 1-sigma spherical angle error (rad)
+    * timeStamp_ms : system time the measurement was taken, not the time it was received (mSec)
+    * resetTime_ms : system time of the last position reset request (mSec)
+    *
+    */
+    void writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint32_t resetTime_ms);
+
     // called by vehicle code to specify that a takeoff is happening
     // causes the EKF to compensate for expected barometer errors due to ground effect
     void setTakeoffExpected(bool val);
@@ -572,6 +586,13 @@ private:
         uint8_t     type;           // type specifiying Euler rotation order used, 1 = 312 (ZXY), 2 = 321 (ZYX)
     };
 
+    struct ext_nav_elements {
+        Vector3f        pos;        // XYZ position measured in a RH navigation frame (m)
+        float           posErr;     // spherical poition measurement error 1-std (m)
+        uint32_t        time_ms;    // measurement timestamp (msec)
+        bool            posReset;   // true when the position measurement has been reset
+    };
+
     // bias estimates for the IMUs that are enabled but not being used
     // by this core.
     struct {
@@ -777,6 +798,9 @@ private:
     // return true if the filter is ready to start using body frame odometry measurements
     bool readyToUseBodyOdm(void) const;
 
+    // return true if the filter to be ready to use external nav data
+    bool readyToUseExtNav(void) const;
+
     // return true if we should use the range finder sensor
     bool useRngFinder(void) const;
 
@@ -949,6 +973,7 @@ private:
     uint32_t airborneDetectTime_ms; // last time flight movement was detected
     Vector6 innovVelPos;            // innovation output for a group of measurements
     Vector6 varInnovVelPos;         // innovation variance output for a group of measurements
+    Vector6 velPosObs;              // observations for combined velocity and positon group of measurements (3x1 m , 3x1 m/s)
     bool fuseVelData;               // this boolean causes the velNED measurements to be fused
     bool fusePosData;               // this boolean causes the posNE measurements to be fused
     bool fuseHgtData;               // this boolean causes the hgtMea measurements to be fused
@@ -978,7 +1003,7 @@ private:
     uint32_t secondLastGpsTime_ms;  // time of second last GPS fix used to determine how long since last update
     uint32_t lastHealthyMagTime_ms; // time the magnetometer was last declared healthy
     bool allMagSensorsFailed;       // true if all magnetometer sensors have timed out on this flight and we are no longer using magnetometer data
-    uint32_t lastSynthYawTime_ms;   // time stamp when synthetic yaw measurement was last fused to maintain covariance health (msec)
+    uint32_t lastSynthYawTime_ms;   // time stamp when yaw observation was last fused (msec)
     uint32_t ekfStartTime_ms;       // time the EKF was started (msec)
     Vector2f lastKnownPositionNE;   // last known position
     uint32_t lastDecayTime_ms;      // time of last decay of GPS position offset
@@ -1089,7 +1114,8 @@ private:
                     FLOW=3,         // Use optical flow rates
                     BARO=4,         // Use Baro height
                     MAG=5,          // Use magnetometer data
-                    RNGFND=6        // Use rangefinder data
+                    RNGFND=6,       // Use rangefinder data
+                    EXTNAV=7        // Use external nav data
                         };
     resetDataSource posResetSource; // preferred source of data for position reset
     resetDataSource velResetSource; // preferred source of data for a velocity reset
@@ -1292,6 +1318,17 @@ private:
     bool onGroundNotMoving;             // true when on the ground and not moving
     uint32_t lastMoveCheckLogTime_ms;   // last time the movement check data was logged (msec)
 
+    // external navigation fusion
+    obs_ring_buffer_t<ext_nav_elements> storedExtNav; // external navigation data buffer
+    ext_nav_elements extNavDataNew;     // External nav data at the current time horizon
+    ext_nav_elements extNavDataDelayed; // External nav at the fusion time horizon
+    uint32_t extNavMeasTime_ms;         // time external measurements were accepted for input to the data buffer (msec)
+    uint32_t extNavLastPosResetTime_ms; // last time the external nav systen performed a position reset (msec)
+    uint32_t lastExtNavPassTime_ms;     // time stamp when external nav position measurement last passed innovation consistency check (msec)
+    bool extNavDataToFuse;              // true when there is new external nav data to fuse
+    bool extNavUsedForPos;              // true when the external nav data is being used as a position reference.
+    bool extNavTimeout;                 // true if external nav measurements have failed innovation consistency checks for too long
+
     // flags indicating severe numerical errors in innovation variance calculation for different fusion operations
     struct {
         bool bad_xmag:1;