diff --git a/libraries/AP_AHRS/AP_AHRS.h b/libraries/AP_AHRS/AP_AHRS.h
index eeead3df57..1d68f1a01b 100644
--- a/libraries/AP_AHRS/AP_AHRS.h
+++ b/libraries/AP_AHRS/AP_AHRS.h
@@ -29,6 +29,7 @@
 #include <AP_InertialSensor/AP_InertialSensor.h>
 #include <AP_Baro/AP_Baro.h>
 #include <AP_Param/AP_Param.h>
+#include <AP_Common/Semaphore.h>
 
 class OpticalFlow;
 #define AP_AHRS_TRIM_LIMIT 10.0f        // maximum trim angle in degrees
@@ -571,7 +572,16 @@ public:
     // Write position and quaternion data from an external navigation system
     virtual void writeExtNavData(const Vector3f &sensOffset, const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint32_t resetTime_ms) { }
 
+    // allow threads to lock against AHRS update
+    HAL_Semaphore &get_semaphore(void) {
+        return _rsem;
+    }
+    
 protected:
+    
+    // multi-thread access support
+    HAL_Semaphore_Recursive _rsem;
+    
     AHRS_VehicleClass _vehicle_class;
 
     // settable parameters
diff --git a/libraries/AP_AHRS/AP_AHRS_DCM.cpp b/libraries/AP_AHRS/AP_AHRS_DCM.cpp
index ed0849da04..b1ae1fed28 100644
--- a/libraries/AP_AHRS/AP_AHRS_DCM.cpp
+++ b/libraries/AP_AHRS/AP_AHRS_DCM.cpp
@@ -49,6 +49,9 @@ AP_AHRS_DCM::reset_gyro_drift(void)
 void
 AP_AHRS_DCM::update(bool skip_ins_update)
 {
+    // support locked access functions to AHRS data
+    WITH_SEMAPHORE(_rsem);
+    
     float delta_t;
 
     if (_last_startup_ms == 0) {
@@ -141,6 +144,9 @@ AP_AHRS_DCM::matrix_update(float _G_Dt)
 void
 AP_AHRS_DCM::reset(bool recover_eulers)
 {
+    // support locked access functions to AHRS data
+    WITH_SEMAPHORE(_rsem);
+    
     // reset the integration terms
     _omega_I.zero();
     _omega_P.zero();
diff --git a/libraries/AP_AHRS/AP_AHRS_NavEKF.cpp b/libraries/AP_AHRS/AP_AHRS_NavEKF.cpp
index 4fd5ac22cc..efbced5b89 100644
--- a/libraries/AP_AHRS/AP_AHRS_NavEKF.cpp
+++ b/libraries/AP_AHRS/AP_AHRS_NavEKF.cpp
@@ -70,6 +70,9 @@ const Vector3f &AP_AHRS_NavEKF::get_gyro_drift(void) const
 //  should be called if gyro offsets are recalculated
 void AP_AHRS_NavEKF::reset_gyro_drift(void)
 {
+    // support locked access functions to AHRS data
+    WITH_SEMAPHORE(_rsem);
+    
     // update DCM
     AP_AHRS_DCM::reset_gyro_drift();
 
@@ -80,6 +83,9 @@ void AP_AHRS_NavEKF::reset_gyro_drift(void)
 
 void AP_AHRS_NavEKF::update(bool skip_ins_update)
 {
+    // support locked access functions to AHRS data
+    WITH_SEMAPHORE(_rsem);
+    
     // drop back to normal priority if we were boosted by the INS
     // calling delay_microseconds_boost()
     hal.scheduler->boost_end();
@@ -89,7 +95,6 @@ void AP_AHRS_NavEKF::update(bool skip_ins_update)
         _ekf_type.set(2);
     }
 
-
     update_DCM(skip_ins_update);
 
 #if CONFIG_HAL_BOARD == HAL_BOARD_SITL
@@ -366,6 +371,9 @@ const Vector3f &AP_AHRS_NavEKF::get_accel_ef_blended(void) const
 
 void AP_AHRS_NavEKF::reset(bool recover_eulers)
 {
+    // support locked access functions to AHRS data
+    WITH_SEMAPHORE(_rsem);
+    
     AP_AHRS_DCM::reset(recover_eulers);
     _dcm_attitude(roll, pitch, yaw);
     if (_ekf2_started) {
@@ -379,6 +387,9 @@ void AP_AHRS_NavEKF::reset(bool recover_eulers)
 // reset the current attitude, used on new IMU calibration
 void AP_AHRS_NavEKF::reset_attitude(const float &_roll, const float &_pitch, const float &_yaw)
 {
+    // support locked access functions to AHRS data
+    WITH_SEMAPHORE(_rsem);
+    
     AP_AHRS_DCM::reset_attitude(_roll, _pitch, _yaw);
     _dcm_attitude(roll, pitch, yaw);
     if (_ekf2_started) {
@@ -1395,6 +1406,9 @@ uint32_t AP_AHRS_NavEKF::getLastPosDownReset(float &posDelta) const
 // If using a range finder for height no reset is performed and it returns false
 bool AP_AHRS_NavEKF::resetHeightDatum(void)
 {
+    // support locked access functions to AHRS data
+    WITH_SEMAPHORE(_rsem);
+    
     switch (ekf_type()) {
 
     case 2: