AP_NavEKF_Source: add ExtNav yaw source

libraries/AP_NavEKF/AP_NavEKF_Source.cpp

@@ -52,7 +52,7 @@ const AP_Param::GroupInfo AP_NavEKF_Source::var_info[] = {
     // @Param: 1_YAW
     // @DisplayName: Yaw Source
     // @Description: Yaw Source
-    // @Values: 0:None, 1:Compass, 2:External, 3:External with Compass Fallback
+    // @Values: 0:None, 1:Compass, 2:External, 3:External with Compass Fallback, 6:ExternalNav
     // @User: Advanced
     AP_GROUPINFO("1_YAW", 5, AP_NavEKF_Source, _source_set[0].yaw, (int8_t)AP_NavEKF_Source::SourceYaw::COMPASS),
 
@@ -88,7 +88,7 @@ const AP_Param::GroupInfo AP_NavEKF_Source::var_info[] = {
     // @Param: 2_YAW
     // @DisplayName: Yaw Source (Secondary)
     // @Description: Yaw Source (Secondary)
-    // @Values: 0:None, 1:Compass, 2:External, 3:External with Compass Fallback
+    // @Values: 0:None, 1:Compass, 2:External, 3:External with Compass Fallback, 6:ExternalNav
     // @User: Advanced
     AP_GROUPINFO("2_YAW", 10, AP_NavEKF_Source, _source_set[1].yaw, (int8_t)AP_NavEKF_Source::SourceYaw::NONE),
 #endif
@@ -125,7 +125,7 @@ const AP_Param::GroupInfo AP_NavEKF_Source::var_info[] = {
     // @Param: 3_YAW
     // @DisplayName: Yaw Source (Tertiary)
     // @Description: Yaw Source (Tertiary)
-    // @Values: 0:None, 1:Compass, 2:External, 3:External with Compass Fallback
+    // @Values: 0:None, 1:Compass, 2:External, 3:External with Compass Fallback, 6:ExternalNav
     // @User: Advanced
     AP_GROUPINFO("3_YAW", 15, AP_NavEKF_Source, _source_set[2].yaw, (int8_t)AP_NavEKF_Source::SourceYaw::NONE),
 #endif
@@ -412,6 +412,9 @@ bool AP_NavEKF_Source::pre_arm_check(char *failure_msg, uint8_t failure_msg_len)
         case SourceYaw::EXTERNAL_COMPASS_FALLBACK:
             compass_required = true;
             break;
+        case SourceYaw::EXTNAV:
+            visualodom_required = true;
+            break;
         default:
             // invalid yaw value
             hal.util->snprintf(failure_msg, failure_msg_len, "Check EK3_SRC%d_YAW", (int)i+1);

libraries/AP_NavEKF/AP_NavEKF_Source.h

@@ -41,7 +41,8 @@ public:
         NONE = 0,
         COMPASS = 1,
         EXTERNAL = 2,
-        EXTERNAL_COMPASS_FALLBACK = 3
+        EXTERNAL_COMPASS_FALLBACK = 3,
+        EXTNAV = 6
     };
 
     // enum for OPTIONS parameter