APM: added XTRK_MIN_DIST option

this allows you to disable crosstrack correction on short legs

ArduPlane/Parameters.h

@@ -73,6 +73,7 @@ public:
         k_param_reset_mission_chan,
         k_param_land_flare_alt,
         k_param_land_flare_sec,
+        k_param_crosstrack_min_distance,
 
         // 110: Telemetry control
         //
@@ -237,6 +238,7 @@ public:
     AP_Float crosstrack_gain;
     AP_Int16 crosstrack_entry_angle;
     AP_Int8  crosstrack_use_wind;
+    AP_Int16 crosstrack_min_distance;
 
     // Estimation
     //

ArduPlane/Parameters.pde

@@ -126,7 +126,16 @@ const AP_Param::Info var_info[] PROGMEM = {
     // @Description: If enabled, use wind estimation for navigation crosstrack when using a compass for yaw
     // @Values: 0:Disabled,1:Enabled
     // @User: Standard
-    GSCALAR(crosstrack_use_wind, "XTRK_USE_WIND",  1),
+    GSCALAR(crosstrack_use_wind, "XTRK_USE_WIND",     1),
+
+    // @Param: XTRK_MIN_DIST
+    // @DisplayName: Crosstrack mininum distance
+    // @Description: Minimum distance in meters between waypoints to do crosstrack correction.
+    // @Units: Meters
+    // @Range: 0 32767
+    // @Increment: 1
+    // @User: Standard
+    GSCALAR(crosstrack_min_distance, "XTRK_MIN_DIST",  50),
 
     // @Param: ALT_MIX
     // @DisplayName: Gps to Baro Mix

ArduPlane/navigation.pde

@@ -189,7 +189,8 @@ static void update_crosstrack(void)
     // Crosstrack Error
     // ----------------
     // If we are too far off or too close we don't do track following
-    if (abs(wrap_180_cd(target_bearing_cd - crosstrack_bearing_cd)) < 4500) {
+    if (wp_totalDistance >= g.crosstrack_min_distance && 
+        abs(wrap_180_cd(target_bearing_cd - crosstrack_bearing_cd)) < 4500) {
         // Meters we are off track line
         crosstrack_error = sin(radians((target_bearing_cd - crosstrack_bearing_cd) * 0.01)) * wp_distance;               
         nav_bearing_cd += constrain(crosstrack_error * g.crosstrack_gain, -g.crosstrack_entry_angle.get(), g.crosstrack_entry_angle.get());