diff --git a/libraries/AP_Soaring/Variometer.cpp b/libraries/AP_Soaring/Variometer.cpp
index 112e4b8a6b..916998fc8a 100644
--- a/libraries/AP_Soaring/Variometer.cpp
+++ b/libraries/AP_Soaring/Variometer.cpp
@@ -110,7 +110,7 @@ void Variometer::update(const float polar_K, const float polar_Cd0, const float
 float Variometer::calculate_aircraft_sinkrate(float phi,
                                              const float polar_K,
                                              const float polar_CD0,
-                                             const float polar_B)
+                                             const float polar_B) const
 {
     // Remove aircraft sink rate
     float CL0;  // CL0 = 2*W/(rho*S*V^2)
diff --git a/libraries/AP_Soaring/Variometer.h b/libraries/AP_Soaring/Variometer.h
index 632fb1f818..c396973fef 100644
--- a/libraries/AP_Soaring/Variometer.h
+++ b/libraries/AP_Soaring/Variometer.h
@@ -47,13 +47,13 @@ public:
     float tau;
 
     void update(const float polar_K, const float polar_CD0, const float polar_B);
-    float calculate_aircraft_sinkrate(float phi, const float polar_K, const float polar_CD0, const float polar_B);
+    float calculate_aircraft_sinkrate(float phi, const float polar_K, const float polar_CD0, const float polar_B) const;
 
     void reset_filter(float value) { _climb_filter.reset(value);}
 
-    float get_airspeed(void) {return _aspd_filt;};
+    float get_airspeed(void) const {return _aspd_filt;};
 
-    float get_exp_thermalling_sink(void) {return _expected_thermalling_sink;};
+    float get_exp_thermalling_sink(void) const {return _expected_thermalling_sink;};
 
     float calculate_circling_time_constant();
 };