Plane: added RALLY_LIMIT_KM

this will prevent an embarressing use of a rally point from the wrong
airfield

ArduPlane/Parameters.h

@@ -96,6 +96,7 @@ public:
         k_param_waypoint_max_radius,
         k_param_ground_steer_alt,        
         k_param_ground_steer_dps,
+        k_param_rally_limit_km,
 
         // 110: Telemetry control
         //
@@ -316,6 +317,7 @@ public:
 #endif
 
     AP_Int8 rally_total;
+    AP_Float rally_limit_km;
 
     // Fly-by-wire
     //

ArduPlane/Parameters.pde

@@ -265,6 +265,14 @@ const AP_Param::Info var_info[] PROGMEM = {
     // @User: Advanced
     GSCALAR(rally_total,            "RALLY_TOTAL",    0),
 
+    // @Param: RALLY_LIMIT_KM
+    // @DisplayName: Rally Limit
+    // @Description: Maximum distance to rally point. If the closest rally point is more than this number of kilometers from the current position and the home location is closer than any of the rally points from the current position then do RTL to home rather than to the closest rally point. This prevents a leftover rally point from a different airfield being used accidentally. If this is set to 0 then the closest rally point is always used.
+    // @User: Advanced
+    // @Units: kilometers
+    // @Increment: 0.1
+    GSCALAR(rally_limit_km,          "RALLY_LIMIT_KM",    5),
+
     // @Param: ARSPD_FBW_MIN
     // @DisplayName: Fly By Wire Minimum Airspeed
     // @Description: Airspeed corresponding to minimum throttle in auto throttle modes (FBWB, CRUISE, AUTO, GUIDED, LOITER, CIRCLE and RTL). This is a calibrated (apparent) airspeed.

ArduPlane/rally.pde

@@ -53,6 +53,12 @@ static bool find_best_rally_point(const Location &myloc, const Location &homeloc
         }
     }
 
+    if (min_dis > g.rally_limit_km*1000.0f && 
+        get_distance(myloc, homeloc) < min_dis) {
+        // return false, which makes home be used instead
+        return false;
+    }
+
     return min_dis >= 0;
 }