AP_Arming: add compass consistency check

Also moved the offsets and mag field limits into definitions at top of
.cpp file

libraries/AP_Arming/AP_Arming.cpp

@@ -18,6 +18,9 @@
 #include <AP_Notify/AP_Notify.h>
 #include <GCS_MAVLink/GCS.h>
 
+#define AP_ARMING_COMPASS_OFFSETS_MAX   600
+#define AP_ARMING_COMPASS_MAGFIELD_MIN  185     // 0.35 * 530 milligauss
+#define AP_ARMING_COMPASS_MAGFIELD_MAX  875     // 1.65 * 530 milligauss
 #define AP_ARMING_BOARD_VOLTAGE_MIN     4.3f
 #define AP_ARMING_BOARD_VOLTAGE_MAX     5.8f
 
@@ -256,28 +259,29 @@ bool AP_Arming::compass_checks(bool report)
 
         // check for unreasonable compass offsets
         Vector3f offsets = _compass.get_offsets();
-        if (offsets.length() > 600) {
+        if (offsets.length() > AP_ARMING_COMPASS_OFFSETS_MAX) {
             if (report) {
                 GCS_MAVLINK::send_statustext_all(MAV_SEVERITY_CRITICAL, PSTR("PreArm: Compass offsets too high"));
             }
             return false;
         }
 
-#if CONFIG_HAL_BOARD == HAL_BOARD_APM1 || CONFIG_HAL_BOARD == HAL_BOARD_APM2
-# define COMPASS_MAGFIELD_EXPECTED     330        // pre arm will fail if mag field > 544 or < 115
-#else // PX4, SITL
-#define COMPASS_MAGFIELD_EXPECTED      530        // pre arm will fail if mag field > 874 or < 185
-#endif
-
         // check for unreasonable mag field length
         float mag_field = _compass.get_field().length();
-        if (mag_field > COMPASS_MAGFIELD_EXPECTED*1.65f || mag_field < COMPASS_MAGFIELD_EXPECTED*0.35f) {
+        if (mag_field > AP_ARMING_COMPASS_MAGFIELD_MAX || mag_field < AP_ARMING_COMPASS_MAGFIELD_MIN) {
             if (report) {
                 GCS_MAVLINK::send_statustext_all(MAV_SEVERITY_CRITICAL, PSTR("PreArm: Check mag field"));
             }
             return false;
         }
-        
+
+        // check all compasses point in roughly same direction
+        if (!_compass.consistent()) {
+            if (report) {
+                GCS_MAVLINK::send_statustext_all(MAV_SEVERITY_CRITICAL,PSTR("PreArm: inconsistent compasses"));
+    }
+            return false;
+        }
     }
 
     return true;