Copter: define limit for baro vs inav alt disparity

ArduCopter/config.h

@@ -296,6 +296,11 @@
 #define FS_GCS_ENABLED_ALWAYS_RTL           1
 #define FS_GCS_ENABLED_CONTINUE_MISSION     2
 
+// pre-arm baro vs inertial nav max alt disparity
+#ifndef PREARM_MAX_ALT_DISPARITY_CMS
+ # define PREARM_MAX_ALT_DISPARITY_CMS      100     // barometer and inertial nav altitude must be within this many centimeters
+#endif
+
 // pre-arm check max velocity
 #ifndef PREARM_MAX_VELOCITY_CMS
  # define PREARM_MAX_VELOCITY_CMS           50.0f   // vehicle must be travelling under 50cm/s before arming

ArduCopter/motors.pde

@@ -536,7 +536,7 @@ static bool arm_checks(bool display_failure)
 
     // check Baro & inav alt are within 1m
     if ((g.arming_check == ARMING_CHECK_ALL) || (g.arming_check & ARMING_CHECK_BARO)) {
-        if(fabs(inertial_nav.get_altitude() - baro_alt) > 100) {
+        if(fabs(inertial_nav.get_altitude() - baro_alt) > PREARM_MAX_ALT_DISPARITY_CMS) {
             if (display_failure) {
                 gcs_send_text_P(SEVERITY_HIGH,PSTR("Arm: Alt disparity"));
             }