AP_AHRS: added prearm_failure_reason()

libraries/AP_AHRS/AP_AHRS.h

@@ -197,6 +197,9 @@ public:
     // Methods
     virtual void update(void) = 0;
 
+    // report any reason for why the backend is refusing to initialise
+    virtual const char *prearm_failure_reason(void) const { return nullptr; }
+    
     // Euler angles (radians)
     float roll;
     float pitch;

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -21,6 +21,7 @@
 #include <AP_HAL/AP_HAL.h>
 #include "AP_AHRS.h"
 #include <AP_Vehicle/AP_Vehicle.h>
+#include <GCS_MAVLink/GCS.h>
 
 #if AP_AHRS_NAVEKF_AVAILABLE
 
@@ -412,5 +413,14 @@ bool AP_AHRS_NavEKF::getMagOffsets(Vector3f &magOffsets)
     return status;
 }
 
+// report any reason for why the backend is refusing to initialise
+const char *AP_AHRS_NavEKF::prearm_failure_reason(void) const
+{
+    if (_ekf_use != EKF_DO_NOT_USE) {
+        return EKF.prearm_failure_reason();
+    }
+    return nullptr;
+}
+
 #endif // AP_AHRS_NAVEKF_AVAILABLE
 

libraries/AP_AHRS/AP_AHRS_NavEKF.h

@@ -124,6 +124,9 @@ AP_AHRS_NavEKF(AP_InertialSensor &ins, AP_Baro &baro, AP_GPS &gps, RangeFinder &
     // true if offsets are valid
     bool getMagOffsets(Vector3f &magOffsets);
 
+    // report any reason for why the backend is refusing to initialise
+    const char *prearm_failure_reason(void) const override;
+
 private:
     bool using_EKF(void) const;