diff --git a/libraries/AP_ExternalAHRS/AP_ExternalAHRS.cpp b/libraries/AP_ExternalAHRS/AP_ExternalAHRS.cpp
index f3f73297f1..0033b83d01 100644
--- a/libraries/AP_ExternalAHRS/AP_ExternalAHRS.cpp
+++ b/libraries/AP_ExternalAHRS/AP_ExternalAHRS.cpp
@@ -361,7 +361,7 @@ bool AP_ExternalAHRS::healthy(void) const
     return (now - last_pkt1_ms < 40 && now - last_pkt2_ms < 500);
 }
 
-bool AP_ExternalAHRS::initialised(void)
+bool AP_ExternalAHRS::initialised(void) const
 {
     return last_pkt1_ms != 0 && last_pkt2_ms != 0;
 }
diff --git a/libraries/AP_ExternalAHRS/AP_ExternalAHRS.h b/libraries/AP_ExternalAHRS/AP_ExternalAHRS.h
index 484601d179..fe4936e992 100644
--- a/libraries/AP_ExternalAHRS/AP_ExternalAHRS.h
+++ b/libraries/AP_ExternalAHRS/AP_ExternalAHRS.h
@@ -93,7 +93,7 @@ public:
 
     // accessors for AP_AHRS
     bool healthy(void) const;
-    bool initialised(void);
+    bool initialised(void) const;
     bool get_quaternion(Quaternion &quat);
     bool get_origin(Location &loc);
     bool get_location(Location &loc);