Copter: pre-arm check for gps hdop < 2

ArduCopter/Parameters.h

@@ -89,7 +89,8 @@ public:
         k_param_ch8_option,
         k_param_arming_check_enabled,
         k_param_sprayer,
-        k_param_angle_max,              // 34
+        k_param_angle_max,
+        k_param_gps_hdop_good,          // 35
 
         // 65: AP_Limits Library
         k_param_limits = 65,            // deprecated - remove
@@ -287,6 +288,7 @@ public:
     AP_Int8         failsafe_battery_enabled;   // battery failsafe enabled
     AP_Int8         failsafe_gps_enabled;       // gps failsafe enabled
     AP_Int8         failsafe_gcs;               // ground station failsafe behavior
+    AP_Int16        gps_hdop_good;              // GPS Hdop value below which represent a good position
 
     AP_Int8         compass_enabled;
     AP_Int8         optflow_enabled;

ArduCopter/Parameters.pde

@@ -113,6 +113,13 @@ const AP_Param::Info var_info[] PROGMEM = {
     // @User: Standard
     GSCALAR(failsafe_gcs, "FS_GCS_ENABLE", FS_GCS_DISABLED),
 
+    // @Param: GPS_HDOP_GOOD
+    // @DisplayName: GPS Hdop Good
+    // @Description: GPS Hdop value below which represent a good position.  Used for pre-arm checks
+    // @Range: 100 900
+    // @User: Advanced
+    GSCALAR(gps_hdop_good, "GPS_HDOP_GOOD", GPS_HDOP_GOOD_DEFAULT),
+
     // @Param: VOLT_DIVIDER
     // @DisplayName: Voltage Divider
     // @Description: Used to convert the voltage of the voltage sensing pin (BATT_VOLT_PIN) to the actual battery's voltage (pin_voltage * VOLT_DIVIDER). For the 3DR Power brick, this should be set to 10.1. For the PX4 using the PX4IO power supply this should be set to 1.

ArduCopter/config.h

@@ -394,6 +394,9 @@
 #ifndef FAILSAFE_GPS_TIMEOUT_MS
  # define FAILSAFE_GPS_TIMEOUT_MS       5000    // gps failsafe triggers after 5 seconds with no GPS
 #endif
+#ifndef GPS_HDOP_GOOD_DEFAULT
+ # define GPS_HDOP_GOOD_DEFAULT         200     // minimum hdop that represents a good position.  used during pre-arm checks if fence is enabled
+#endif
 
 // GCS failsafe
 #ifndef FS_GCS

ArduCopter/motors.pde

@@ -293,9 +293,9 @@ static void pre_arm_checks(bool display_failure)
 
 #if AC_FENCE == ENABLED
     // check fence is initialised
-    if(!fence.pre_arm_check()) {
+    if(!fence.pre_arm_check() || (fence.enabled() && g_gps->hdop > g.gps_hdop_good)) {
         if (display_failure) {
-            gcs_send_text_P(SEVERITY_HIGH,PSTR("PreArm: No GPS Lock"));
+            gcs_send_text_P(SEVERITY_HIGH,PSTR("PreArm: Bad GPS Pos"));
         }
         return;
     }