diff --git a/sw/airborne/math/pprz_isa.h b/sw/airborne/math/pprz_isa.h
index a5b6f0e0e7..28c397975b 100644
--- a/sw/airborne/math/pprz_isa.h
+++ b/sw/airborne/math/pprz_isa.h
@@ -46,7 +46,7 @@
 #define PPRZ_ISA_GRAVITY 9.80665
 #define PPRZ_ISA_AIR_GAS_CONSTANT (8.31447/0.0289644)
 
-#define PPRZ_ISA_M_OF_P_CONST (PPRZ_ISA_AIR_GAS_CONSTANT*PPRZ_ISA_SEA_LEVEL_TEMP/PPRZ_ISA_GRAVITY)
+static const float PPRZ_ISA_M_OF_P_CONST = (PPRZ_ISA_AIR_GAS_CONSTANT*PPRZ_ISA_SEA_LEVEL_TEMP/PPRZ_ISA_GRAVITY);
 
 /**
  * Get absolute altitude from pressure (using simplified equation).