diff --git a/libraries/AP_AHRS/AP_AHRS.cpp b/libraries/AP_AHRS/AP_AHRS.cpp
index 6be34de803..f77ea3551c 100644
--- a/libraries/AP_AHRS/AP_AHRS.cpp
+++ b/libraries/AP_AHRS/AP_AHRS.cpp
@@ -252,7 +252,7 @@ void AP_AHRS::init()
 #if AP_AHRS_DCM_ENABLED
     dcm.init();
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     external.init();
 #endif
 
@@ -321,7 +321,7 @@ void AP_AHRS::reset_gyro_drift(void)
 #if AP_AHRS_DCM_ENABLED
     dcm.reset_gyro_drift();
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     external.reset_gyro_drift();
 #endif
 
@@ -400,7 +400,7 @@ void AP_AHRS::update(bool skip_ins_update)
     update_SITL();
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     update_external();
 #endif
     
@@ -458,7 +458,7 @@ void AP_AHRS::update(bool skip_ins_update)
             shortname = "SIM";
             break;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
         case EKFType::EXTERNAL:
             shortname = "External";
             break;
@@ -685,7 +685,7 @@ void AP_AHRS::update_EKF3(void)
 }
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
 void AP_AHRS::update_external(void)
 {
     external.update();
@@ -695,7 +695,7 @@ void AP_AHRS::update_external(void)
         copy_estimates_from_backend_estimates(external_estimates);
     }
 }
-#endif // HAL_EXTERNAL_AHRS_ENABLED
+#endif // AP_AHRS_EXTERNAL_ENABLED
 
 void AP_AHRS::reset()
 {
@@ -709,7 +709,7 @@ void AP_AHRS::reset()
     sim.reset();
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     external.reset();
 #endif
 
@@ -754,7 +754,7 @@ bool AP_AHRS::_get_location(Location &loc) const
         return sim_estimates.get_location(loc);
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external_estimates.get_location(loc);
 #endif
@@ -812,7 +812,7 @@ bool AP_AHRS::_wind_estimate(Vector3f &wind) const
         return EKF3.getWind(wind);
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external.wind_estimate(wind);
 #endif
@@ -942,7 +942,7 @@ bool AP_AHRS::_airspeed_estimate(float &airspeed_ret, AirspeedEstimateType &airs
         break;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
 #if AP_AHRS_DCM_ENABLED
         airspeed_estimate_type = AirspeedEstimateType::DCM_SYNTHETIC;
@@ -996,7 +996,7 @@ bool AP_AHRS::_airspeed_estimate_true(float &airspeed_ret) const
 #if AP_AHRS_SIM_ENABLED
     case EKFType::SIM:
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
 #endif
         break;
@@ -1033,7 +1033,7 @@ bool AP_AHRS::_airspeed_vector_true(Vector3f &vec) const
         break;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         break;
 #endif
@@ -1065,7 +1065,7 @@ bool AP_AHRS::airspeed_health_data(float &innovation, float &innovationVariance,
         break;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         break;
 #endif
@@ -1108,7 +1108,7 @@ bool AP_AHRS::use_compass(void)
         return sim.use_compass();
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         break;
 #endif
@@ -1155,7 +1155,7 @@ bool AP_AHRS::_get_quaternion(Quaternion &quat) const
         }
         break;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         // we assume the external AHRS isn't trimmed with the autopilot!
         return external.get_quaternion(quat);
@@ -1206,7 +1206,7 @@ bool AP_AHRS::_get_secondary_attitude(Vector3f &eulers) const
         return false;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL: {
         // External is secondary
         eulers[0] = external_estimates.roll_rad;
@@ -1267,7 +1267,7 @@ bool AP_AHRS::_get_secondary_quaternion(Quaternion &quat) const
         return false;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         // External is secondary
         return external.get_quaternion(quat);
@@ -1319,7 +1319,7 @@ bool AP_AHRS::_get_secondary_position(Location &loc) const
         return false;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         // External is secondary
         return external_estimates.get_location(loc);
@@ -1359,7 +1359,7 @@ Vector2f AP_AHRS::_groundspeed_vector(void)
     case EKFType::SIM:
         return sim.groundspeed_vector();
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL: {
         return external.groundspeed_vector();
     }
@@ -1383,7 +1383,7 @@ float AP_AHRS::_groundspeed(void)
     case EKFType::THREE:
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
 #endif
 
@@ -1431,7 +1431,7 @@ bool AP_AHRS::set_origin(const Location &loc)
         // simulation backend
         return false;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         // don't allow origin set with external AHRS
         return false;
@@ -1485,7 +1485,7 @@ bool AP_AHRS::_get_velocity_NED(Vector3f &vec) const
     case EKFType::SIM:
         return sim.get_velocity_NED(vec);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external.get_velocity_NED(vec);
 #endif
@@ -1520,7 +1520,7 @@ bool AP_AHRS::get_mag_field_NED(Vector3f &vec) const
     case EKFType::SIM:
         return false;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return false;
 #endif
@@ -1552,7 +1552,7 @@ bool AP_AHRS::get_mag_field_correction(Vector3f &vec) const
     case EKFType::SIM:
         return false;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return false;
 #endif
@@ -1587,7 +1587,7 @@ bool AP_AHRS::get_vert_pos_rate_D(float &velocity) const
     case EKFType::SIM:
         return sim.get_vert_pos_rate_D(velocity);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external.get_vert_pos_rate_D(velocity);
 #endif
@@ -1618,7 +1618,7 @@ bool AP_AHRS::get_hagl(float &height) const
     case EKFType::SIM:
         return sim.get_hagl(height);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL: {
         return false;
     }
@@ -1672,7 +1672,7 @@ bool AP_AHRS::get_relative_position_NED_origin(Vector3f &vec) const
     case EKFType::SIM:
         return sim.get_relative_position_NED_origin(vec);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL: {
         return external.get_relative_position_NED_origin(vec);
     }
@@ -1725,7 +1725,7 @@ bool AP_AHRS::get_relative_position_NE_origin(Vector2f &posNE) const
         return sim.get_relative_position_NE_origin(posNE);
     }
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external.get_relative_position_NE_origin(posNE);
 #endif
@@ -1780,7 +1780,7 @@ bool AP_AHRS::get_relative_position_D_origin(float &posD) const
     case EKFType::SIM:
         return sim.get_relative_position_D_origin(posD);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external.get_relative_position_D_origin(posD);
 #endif
@@ -1832,7 +1832,7 @@ AP_AHRS::EKFType AP_AHRS::ekf_type(void) const
     case EKFType::SIM:
         return type;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return type;
 #endif
@@ -1923,7 +1923,7 @@ AP_AHRS::EKFType AP_AHRS::_active_EKF_type(void) const
         ret = EKFType::SIM;
         break;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         ret = EKFType::EXTERNAL;
         break;
@@ -1957,7 +1957,7 @@ AP_AHRS::EKFType AP_AHRS::_active_EKF_type(void) const
             get_filter_status(filt_state);
             break;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
         case EKFType::EXTERNAL:
             get_filter_status(filt_state);
             should_use_gps = true;
@@ -2053,7 +2053,7 @@ AP_AHRS::EKFType AP_AHRS::fallback_active_EKF_type(void) const
     }
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     if (external.healthy()) {
         return EKFType::EXTERNAL;
     }
@@ -2064,7 +2064,7 @@ AP_AHRS::EKFType AP_AHRS::fallback_active_EKF_type(void) const
     return EKFType::THREE;
 #elif HAL_NAVEKF2_AVAILABLE
     return EKFType::TWO;
-#elif HAL_EXTERNAL_AHRS_ENABLED
+#elif AP_AHRS_EXTERNAL_ENABLED
     return EKFType::EXTERNAL;
 #endif
 }
@@ -2089,7 +2089,7 @@ bool AP_AHRS::_get_secondary_EKF_type(EKFType &secondary_ekf_type) const
             return true;
         }
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
         if ((EKFType)_ekf_type.get() == EKFType::EXTERNAL) {
             secondary_ekf_type = EKFType::EXTERNAL;
             return true;
@@ -2106,7 +2106,7 @@ bool AP_AHRS::_get_secondary_EKF_type(EKFType &secondary_ekf_type) const
 #if AP_AHRS_SIM_ENABLED
     case EKFType::SIM:
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
 #endif
         // DCM is secondary
@@ -2171,7 +2171,7 @@ bool AP_AHRS::healthy(void) const
     case EKFType::SIM:
         return sim.healthy();
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external.healthy();
 #endif
@@ -2192,7 +2192,7 @@ bool AP_AHRS::pre_arm_check(bool requires_position, char *failure_msg, uint8_t f
         ret = false;
     }
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     // Always check external AHRS if enabled
     // it is a source for IMU data even if not being used as direct AHRS replacement
     if (AP::externalAHRS().enabled() || (ekf_type() == EKFType::EXTERNAL)) {
@@ -2223,7 +2223,7 @@ bool AP_AHRS::pre_arm_check(bool requires_position, char *failure_msg, uint8_t f
         return dcm.pre_arm_check(requires_position, failure_msg, failure_msg_len) && ret;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external.pre_arm_check(requires_position, failure_msg, failure_msg_len);
 #endif
@@ -2277,7 +2277,7 @@ bool AP_AHRS::initialised(void) const
     case EKFType::SIM:
         return true;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external.initialised();
 #endif
@@ -2310,7 +2310,7 @@ bool AP_AHRS::get_filter_status(nav_filter_status &status) const
     case EKFType::SIM:
         return sim.get_filter_status(status);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external.get_filter_status(status);
 #endif
@@ -2407,7 +2407,7 @@ void AP_AHRS::getControlLimits(float &ekfGndSpdLimit, float &ekfNavVelGainScaler
         sim.get_control_limits(ekfGndSpdLimit, ekfNavVelGainScaler);
         break;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         // no limit on gains, large vel limit
         ekfGndSpdLimit = 400;
@@ -2455,7 +2455,7 @@ bool AP_AHRS::getMagOffsets(uint8_t mag_idx, Vector3f &magOffsets) const
     case EKFType::SIM:
         return sim.get_mag_offsets(mag_idx, magOffsets);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return false;
 #endif
@@ -2490,7 +2490,7 @@ void AP_AHRS::_getCorrectedDeltaVelocityNED(Vector3f& ret, float& dt) const
     case EKFType::SIM:
         break;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         break;
 #endif
@@ -2634,7 +2634,7 @@ uint32_t AP_AHRS::getLastYawResetAngle(float &yawAng)
     case EKFType::SIM:
         return sim.getLastYawResetAngle(yawAng);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external.getLastYawResetAngle(yawAng);
 #endif
@@ -2667,7 +2667,7 @@ uint32_t AP_AHRS::getLastPosNorthEastReset(Vector2f &pos)
     case EKFType::SIM:
         return sim.getLastPosNorthEastReset(pos);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return 0;
 #endif
@@ -2700,7 +2700,7 @@ uint32_t AP_AHRS::getLastVelNorthEastReset(Vector2f &vel) const
     case EKFType::SIM:
         return sim.getLastVelNorthEastReset(vel);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return 0;
 #endif
@@ -2733,7 +2733,7 @@ uint32_t AP_AHRS::getLastPosDownReset(float &posDelta)
     case EKFType::SIM:
         return sim.getLastPosDownReset(posDelta);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return 0;
 #endif
@@ -2784,7 +2784,7 @@ bool AP_AHRS::resetHeightDatum(void)
     case EKFType::SIM:
         return sim.resetHeightDatum();
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return false;
 #endif
@@ -2808,7 +2808,7 @@ void AP_AHRS::send_ekf_status_report(GCS_MAVLINK &link) const
         break;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL: {
         external.send_ekf_status_report(link);
         break;
@@ -2851,7 +2851,7 @@ bool AP_AHRS::_get_origin(EKFType type, Location &ret) const
     case EKFType::SIM:
         return sim.get_origin(ret);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return external.get_origin(ret);
 #endif
@@ -2960,7 +2960,7 @@ bool AP_AHRS::get_hgt_ctrl_limit(float& limit) const
     case EKFType::SIM:
         return sim.get_hgt_ctrl_limit(limit);
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return false;
 #endif
@@ -3027,7 +3027,7 @@ bool AP_AHRS::get_innovations(Vector3f &velInnov, Vector3f &posInnov, Vector3f &
         return sim.get_innovations(velInnov, posInnov, magInnov, tasInnov, yawInnov);
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return false;
 #endif
@@ -3053,7 +3053,7 @@ bool AP_AHRS::is_vibration_affected() const
 #if AP_AHRS_SIM_ENABLED
     case EKFType::SIM:
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
 #endif
         return false;
@@ -3095,7 +3095,7 @@ bool AP_AHRS::get_variances(float &velVar, float &posVar, float &hgtVar, Vector3
         return sim.get_variances(velVar, posVar, hgtVar, magVar, tasVar);
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return false;
 #endif
@@ -3133,7 +3133,7 @@ bool AP_AHRS::get_vel_innovations_and_variances_for_source(uint8_t source, Vecto
         return false;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         return false;
 #endif
@@ -3194,7 +3194,7 @@ uint8_t AP_AHRS::_get_primary_IMU_index() const
     case EKFType::SIM:
         break;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         break;
 #endif
@@ -3219,7 +3219,7 @@ int8_t AP_AHRS::_get_primary_core_index() const
         // we have only one core
         return 0;
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         // we have only one core
         return 0;
@@ -3267,7 +3267,7 @@ void AP_AHRS::check_lane_switch(void)
         break;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         break;
 #endif
@@ -3299,7 +3299,7 @@ void AP_AHRS::request_yaw_reset(void)
         break;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
         break;
 #endif
@@ -3371,7 +3371,7 @@ bool AP_AHRS::using_noncompass_for_yaw(void) const
 #if AP_AHRS_SIM_ENABLED
     case EKFType::SIM:
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
 #endif
         return false; 
@@ -3398,7 +3398,7 @@ bool AP_AHRS::using_extnav_for_yaw(void) const
 #if AP_AHRS_SIM_ENABLED
     case EKFType::SIM:
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
 #endif
         return false;
@@ -3441,7 +3441,7 @@ const EKFGSF_yaw *AP_AHRS::get_yaw_estimator(void) const
 #if AP_AHRS_SIM_ENABLED
     case EKFType::SIM:
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     case EKFType::EXTERNAL:
 #endif
         return nullptr;
diff --git a/libraries/AP_AHRS/AP_AHRS.h b/libraries/AP_AHRS/AP_AHRS.h
index abb36f2e13..a4bd0c3979 100644
--- a/libraries/AP_AHRS/AP_AHRS.h
+++ b/libraries/AP_AHRS/AP_AHRS.h
@@ -443,7 +443,7 @@ public:
 #if AP_AHRS_SIM_ENABLED
         SIM = 10,
 #endif
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
         EXTERNAL = 11,
 #endif
     };
@@ -800,7 +800,7 @@ private:
     void update_SITL(void);
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     void update_external(void);
 #endif    
 
@@ -1002,7 +1002,7 @@ private:
     struct AP_AHRS_Backend::Estimates sim_estimates;
 #endif
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
     AP_AHRS_External external;
     struct AP_AHRS_Backend::Estimates external_estimates;
 #endif
diff --git a/libraries/AP_AHRS/AP_AHRS_External.cpp b/libraries/AP_AHRS/AP_AHRS_External.cpp
index 0c4be3d5bd..8cdf8ecb52 100644
--- a/libraries/AP_AHRS/AP_AHRS_External.cpp
+++ b/libraries/AP_AHRS/AP_AHRS_External.cpp
@@ -1,6 +1,6 @@
 #include "AP_AHRS_External.h"
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
 
 #include <AP_ExternalAHRS/AP_ExternalAHRS.h>
 #include <AP_AHRS/AP_AHRS.h>
diff --git a/libraries/AP_AHRS/AP_AHRS_External.h b/libraries/AP_AHRS/AP_AHRS_External.h
index 4d63a07029..6ca69f73f8 100644
--- a/libraries/AP_AHRS/AP_AHRS_External.h
+++ b/libraries/AP_AHRS/AP_AHRS_External.h
@@ -21,9 +21,11 @@
  *
  */
 
-#include "AP_AHRS_Backend.h"
+#include "AP_AHRS_config.h"
 
-#if HAL_EXTERNAL_AHRS_ENABLED
+#if AP_AHRS_EXTERNAL_ENABLED
+
+#include "AP_AHRS_Backend.h"
 
 class AP_AHRS_External : public AP_AHRS_Backend {
 public:
diff --git a/libraries/AP_AHRS/AP_AHRS_config.h b/libraries/AP_AHRS/AP_AHRS_config.h
index fd56a5f6de..3b27e78606 100644
--- a/libraries/AP_AHRS/AP_AHRS_config.h
+++ b/libraries/AP_AHRS/AP_AHRS_config.h
@@ -2,6 +2,7 @@
 
 #include <AP_HAL/AP_HAL_Boards.h>
 #include <AP_InertialSensor/AP_InertialSensor_config.h>
+#include <AP_ExternalAHRS/AP_ExternalAHRS_config.h>
 
 #ifndef AP_AHRS_ENABLED
 #define AP_AHRS_ENABLED 1
@@ -15,6 +16,10 @@
 #define AP_AHRS_DCM_ENABLED AP_AHRS_BACKEND_DEFAULT_ENABLED && AP_INERTIALSENSOR_ENABLED
 #endif
 
+#ifndef AP_AHRS_EXTERNAL_ENABLED
+#define AP_AHRS_EXTERNAL_ENABLED AP_AHRS_BACKEND_DEFAULT_ENABLED && HAL_EXTERNAL_AHRS_ENABLED
+#endif
+
 #ifndef HAL_NAVEKF2_AVAILABLE
 // EKF2 slated compiled out by default in 4.5, slated to be removed.
 #define HAL_NAVEKF2_AVAILABLE 0