diff --git a/ArduCopter/Copter.h b/ArduCopter/Copter.h
index 62ba1c503d..6b8a3f786a 100644
--- a/ArduCopter/Copter.h
+++ b/ArduCopter/Copter.h
@@ -255,10 +255,8 @@ private:
         uint32_t glitch_cleared_ms; // system time glitch cleared
     } rangefinder_state, rangefinder_up_state;
 
-    /*
-      return rangefinder height interpolated using inertial altitude
-     */
-    bool get_rangefinder_height_interpolated_cm(int32_t& ret);
+    // return rangefinder height interpolated using inertial altitude
+    bool get_rangefinder_height_interpolated_cm(int32_t& ret) const;
 
     class SurfaceTracking {
     public:
diff --git a/ArduCopter/sensors.cpp b/ArduCopter/sensors.cpp
index 54968f092f..afa8c7e333 100644
--- a/ArduCopter/sensors.cpp
+++ b/ArduCopter/sensors.cpp
@@ -135,7 +135,7 @@ bool Copter::rangefinder_up_ok() const
   difference between the inertial height at that time and the current
   inertial height to give us interpolation of height from rangefinder
  */
-bool Copter::get_rangefinder_height_interpolated_cm(int32_t& ret)
+bool Copter::get_rangefinder_height_interpolated_cm(int32_t& ret) const
 {
     if (!rangefinder_alt_ok()) {
         return false;