AP_NavEKF3: add using_extnav_for_yaw

libraries/AP_NavEKF3/AP_NavEKF3.cpp

@@ -1475,6 +1475,15 @@ bool NavEKF3::using_noncompass_for_yaw(void) const
     return core[primary].using_noncompass_for_yaw();
 }
 
+// are we using (aka fusing) external nav for yaw?
+bool NavEKF3::using_extnav_for_yaw() const
+{
+    if (!core) {
+        return false;
+    }
+    return core[primary].using_extnav_for_yaw();
+}
+
 // check if configured to use GPS for horizontal position estimation
 bool NavEKF3::configuredToUseGPSForPosXY(void) const
 {

libraries/AP_NavEKF3/AP_NavEKF3.h

@@ -339,6 +339,9 @@ public:
     // are we using (aka fusing) a non-compass yaw?
     bool using_noncompass_for_yaw() const;
 
+    // are we using (aka fusing) external nav for yaw?
+    bool using_extnav_for_yaw() const;
+
     // check if configured to use GPS for horizontal position estimation
     bool configuredToUseGPSForPosXY(void) const;
     

libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp

@@ -588,6 +588,17 @@ bool NavEKF3_core::using_noncompass_for_yaw(void) const
     return false;
 }
 
+// are we using (aka fusing) external nav for yaw?
+bool NavEKF3_core::using_extnav_for_yaw() const
+{
+#if EK3_FEATURE_EXTERNAL_NAV
+    if (frontend->sources.getYawSource() == AP_NavEKF_Source::SourceYaw::EXTNAV) {
+        return ((imuSampleTime_ms - last_extnav_yaw_fusion_ms < 5000) || (imuSampleTime_ms - lastSynthYawTime_ms < 5000));
+    }
+#endif
+    return false;
+}
+
 /*
   should we assume zero sideslip?
  */

libraries/AP_NavEKF3/AP_NavEKF3_core.h

@@ -397,6 +397,9 @@ public:
     // are we using (aka fusing) a non-compass yaw?
     bool using_noncompass_for_yaw(void) const;
 
+    // are we using (aka fusing) external nav for yaw?
+    bool using_extnav_for_yaw() const;
+
     // Writes the default equivalent airspeed and 1-sigma uncertainty in m/s to be used in forward flight if a measured airspeed is required and not available.
     void writeDefaultAirSpeed(float airspeed, float uncertainty);