diff --git a/include/gnss/sensor/sensor_gnss.h b/include/gnss/sensor/sensor_gnss.h
index d3406455faa28be57193382dbc5f1b3d54e8850e..45948231683034e035700f15e77401fb9cddbf3c 100644
--- a/include/gnss/sensor/sensor_gnss.h
+++ b/include/gnss/sensor/sensor_gnss.h
@@ -8,10 +8,10 @@
 
 namespace wolf {
 
-static char CLOCK_BIAS_KEY = 'T';
-static char CLOCK_BIAS_GPS_GLO_KEY = 'G';
-static char CLOCK_BIAS_GPS_GAL_KEY = 'E';
-static char CLOCK_BIAS_GPS_CMP_KEY = 'M';
+static const char CLOCK_BIAS_KEY = 'T';
+static const char CLOCK_BIAS_GPS_GLO_KEY = 'G';
+static const char CLOCK_BIAS_GPS_GAL_KEY = 'E';
+static const char CLOCK_BIAS_GPS_CMP_KEY = 'M';
 
 WOLF_STRUCT_PTR_TYPEDEFS(ParamsSensorGnss);
 WOLF_PTR_TYPEDEFS(SensorGnss);