diff --git a/libraries/AP_Math/AP_Math.h b/libraries/AP_Math/AP_Math.h
index 72b6a46830..1d985a7dce 100644
--- a/libraries/AP_Math/AP_Math.h
+++ b/libraries/AP_Math/AP_Math.h
@@ -20,7 +20,10 @@
 #include "polygon.h"
 
 #ifndef PI
-#define PI 3.141592653589793f
+ # define PI 3.141592653589793f
+#endif
+#ifndef M_PI_2
+ # define M_PI_2 1.570796326794897f
 #endif
 #define DEG_TO_RAD 0.017453292519943295769236907684886f
 #define RAD_TO_DEG 57.295779513082320876798154814105f