AC_PosControl: z-axis stopping point up to 3m above vehicle

Stopping point while descending remains at 2m for safety

libraries/AC_AttitudeControl/AC_PosControl.cpp

@@ -293,7 +293,7 @@ void AC_PosControl::get_stopping_point_z(Vector3f& stopping_point) const
             stopping_point.z = curr_pos_z - (linear_distance + curr_vel_z*curr_vel_z/(2.0f*_accel_z_cms));
         }
     }
-    stopping_point.z = constrain_float(stopping_point.z, curr_pos_z - POSCONTROL_STOPPING_DIST_Z_MAX, curr_pos_z + POSCONTROL_STOPPING_DIST_Z_MAX);
+    stopping_point.z = constrain_float(stopping_point.z, curr_pos_z - POSCONTROL_STOPPING_DIST_DOWN_MAX, curr_pos_z + POSCONTROL_STOPPING_DIST_UP_MAX);
 }
 
 /// init_takeoff - initialises target altitude if we are taking off

libraries/AC_AttitudeControl/AC_PosControl.h

@@ -16,9 +16,10 @@
 #define POSCONTROL_ACCELERATION_MIN             50.0f   // minimum horizontal acceleration in cm/s/s - used for sanity checking acceleration in leash length calculation
 #define POSCONTROL_ACCEL_XY                     100.0f  // default horizontal acceleration in cm/s/s.  This is overwritten by waypoint and loiter controllers
 #define POSCONTROL_ACCEL_XY_MAX                 980.0f  // max horizontal acceleration in cm/s/s that the position velocity controller will ask from the lower accel controller
-#define POSCONTROL_STOPPING_DIST_Z_MAX          200.0f  // max stopping distance vertically   
                                                         // should be 1.5 times larger than POSCONTROL_ACCELERATION.
                                                         // max acceleration = max lean angle * 980 * pi / 180.  i.e. 23deg * 980 * 3.141 / 180 = 393 cm/s/s
+#define POSCONTROL_STOPPING_DIST_UP_MAX         300.0f  // max stopping distance (in cm) vertically while climbing
+#define POSCONTROL_STOPPING_DIST_DOWN_MAX       200.0f  // max stopping distance (in cm) vertically while descending
 #define POSCONTROL_JERK_LIMIT_CMSSS             1700.0f // default jerk limit on horizontal acceleration (unit: m/s/s/s)
 
 #define POSCONTROL_SPEED                        500.0f  // default horizontal speed in cm/s