diff --git a/libraries/AP_NavEKF3/AP_NavEKF3.cpp b/libraries/AP_NavEKF3/AP_NavEKF3.cpp
index 6d1172dc046..7130822bc0e 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3.cpp
+++ b/libraries/AP_NavEKF3/AP_NavEKF3.cpp
@@ -234,8 +234,8 @@ const AP_Param::GroupInfo NavEKF3::var_info[] = {
     AP_GROUPINFO("MAG_M_NSE", 13, NavEKF3, _magNoise, MAG_M_NSE_DEFAULT),
 
     // @Param: MAG_CAL
-    // @DisplayName: Magnetometer calibration mode
-    // @Description: EKF_MAG_CAL = 0 enables calibration when airborne and is the default setting for Plane users. EKF_MAG_CAL = 1 enables calibration when manoeuvreing. EKF_MAG_CAL = 2 prevents magnetometer calibration regardless of flight condition, is recommended if the external magnetic field is varying and is the default for rovers. EKF_MAG_CAL = 3 enables calibration when the first in-air field and yaw reset has completed and is the default for copters. EKF_MAG_CAL = 4 enables calibration all the time. This determines when the filter will use the 3-axis magnetometer fusion model that estimates both earth and body fixed magnetic field states. This model is only suitable for use when the external magnetic field environment is stable.
+    // @DisplayName: Magnetometer default fusion mode
+    // @Description: This determines when the filter will use the 3-axis magnetometer fusion model that estimates both earth and body fixed magnetic field states and when it will use a simpler magnetic heading fusion model that does not use magnetic field states. The 3-axis magnetometer fusion is only suitable for use when the external magnetic field environment is stable. EK3_MAG_CAL = 0 uses heading fusion on ground, 3-axis fusion in-flight, and is the default setting for Plane users. EK3_MAG_CAL = 1 uses 3-axis fusion only when manoeuvring. EK3_MAG_CAL = 2 uses heading fusion at all times, is recommended if the external magnetic field is varying and is the default for rovers. EK3_MAG_CAL = 3 uses heading fusion on the ground and 3-axis fusion after the first in-air field and yaw reset has completed, and is the default for copters. EK3_MAG_CAL = 4 uses 3-axis fusion at all times. NOTE : Use of simple heading magnetometer fusion makes vehicle compass calibration and alignment errors harder for the EKF to detect which reduces the sensitivity of the Copter EKF failsafe algorithm. NOTE: The fusion mode can be forced to 2 for specific EKF cores using the EK3_MAG_MASK parameter.
     // @Values: 0:When flying,1:When manoeuvring,2:Never,3:After first climb yaw reset,4:Always
     // @User: Advanced
     AP_GROUPINFO("MAG_CAL", 14, NavEKF3, _magCal, MAG_CAL_DEFAULT),
@@ -524,6 +524,13 @@ const AP_Param::GroupInfo NavEKF3::var_info[] = {
     // @Units: m/s/s
     AP_GROUPINFO("ACC_BIAS_LIM", 48, NavEKF3, _accBiasLim, 1.0f),
 
+    // @Param: MAG_MASK
+    // @DisplayName: Bitmask of active EKF cores that will always use heading fusion
+    // @Description: 1 byte bitmap of EKF cores that will disable magnetic field states and use simple magnetic heading fusion at all times. This parameter enables specified cores to be used as a backup for flight into an environment with high levels of external magnetic interference which may degrade the EKF attitude estimate when using 3-axis magnetometer fusion. NOTE : Use of a different magnetometer fusion algorithm on different cores makes unwanted EKF core switches due to magnetometer errors more likely.
+    // @Bitmask: 0:FirstEKF,1:SecondEKF,2:ThirdEKF,3:FourthEKF,4:FifthEKF,5:SixthEKF
+    // @User: Advanced
+    AP_GROUPINFO("MAG_MASK", 49, NavEKF3, _magMask, 0),
+
     AP_GROUPEND
 };
 
diff --git a/libraries/AP_NavEKF3/AP_NavEKF3.h b/libraries/AP_NavEKF3/AP_NavEKF3.h
index 6e18aec1a78..be259366c4c 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3.h
+++ b/libraries/AP_NavEKF3/AP_NavEKF3.h
@@ -358,6 +358,7 @@ private:
     AP_Int8  _rngBcnDelay_ms;       // effective average delay of range beacon measurements rel to IMU (msec)
     AP_Float _useRngSwSpd;          // Maximum horizontal ground speed to use range finder as the primary height source (m/s)
     AP_Float _accBiasLim;           // Accelerometer bias limit (m/s/s)
+    AP_Int8 _magMask;               // Bitmask forcng specific EKF core instances to use simple heading magnetometer fusion.
 
     // Tuning parameters
     const float gpsNEVelVarAccScale;    // Scale factor applied to NE velocity measurement variance due to manoeuvre acceleration
diff --git a/libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp b/libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp
index 8fd587c1369..3578743a7c6 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp
+++ b/libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp
@@ -45,7 +45,12 @@ void NavEKF3_core::controlFilterModes()
  */
 uint8_t NavEKF3_core::effective_magCal(void) const
 {
-    return frontend->_magCal;
+    // force use of simple magnetic heading fusion for specified cores
+    if (frontend->_magMask & core_index) {
+        return 2;
+    } else {
+        return frontend->_magCal;
+    }
 }
 
 // Determine if learning of wind and magnetic field will be enabled and set corresponding indexing limits to