diff --git a/libraries/AP_Soaring/Variometer.cpp b/libraries/AP_Soaring/Variometer.cpp
index 9cf95179d9..eb1ca5493e 100644
--- a/libraries/AP_Soaring/Variometer.cpp
+++ b/libraries/AP_Soaring/Variometer.cpp
@@ -126,7 +126,7 @@ float Variometer::calculate_aircraft_sinkrate(float phi) const
     return _aspd_filt_constrained * (C1 + C2 / (cosphi * cosphi));
 }
 
-float Variometer::calculate_circling_time_constant(float thermal_bank)
+float Variometer::calculate_circling_time_constant(float thermal_bank) const
 {
     // Calculate a time constant to use to filter quantities over a full thermal orbit.
     // This is used for rejecting variation in e.g. climb rate, or estimated climb rate
diff --git a/libraries/AP_Soaring/Variometer.h b/libraries/AP_Soaring/Variometer.h
index c8abb1193e..0678884ee9 100644
--- a/libraries/AP_Soaring/Variometer.h
+++ b/libraries/AP_Soaring/Variometer.h
@@ -78,7 +78,7 @@ public:
 
     float get_exp_thermalling_sink(void) const {return _expected_thermalling_sink;};
 
-    float calculate_circling_time_constant(const float thermal_bank);
+    float calculate_circling_time_constant(const float thermal_bank) const;
 
 private:
     const PolarParams &_polarParams;