Copter: added LAND_SPEED_HIGH parameter

this is the descent rate for the first part of a landing. Separating
this from WPNAV_SPEED_DN allows for independent tuning of fast fwd
flight from landings

ArduCopter/Parameters.cpp

@@ -251,6 +251,15 @@ const AP_Param::Info Copter::var_info[] = {
     // @User: Standard
     GSCALAR(land_speed,             "LAND_SPEED",   LAND_SPEED),
 
+    // @Param: LAND_SPEED_HIGH
+    // @DisplayName: Land speed high
+    // @Description: The descent speed for the first stage of landing in cm/s. If this is zero then WPNAV_SPEED_DN is used
+    // @Units: cm/s
+    // @Range: 0 500
+    // @Increment: 10
+    // @User: Standard
+    GSCALAR(land_speed_high,        "LAND_SPEED_HIGH",   0),
+    
     // @Param: PILOT_VELZ_MAX
     // @DisplayName: Pilot maximum vertical speed
     // @Description: The maximum vertical velocity the pilot may request in cm/s

ArduCopter/Parameters.h

@@ -321,6 +321,7 @@ public:
         k_param_land_speed,
         k_param_auto_velocity_z_min, // remove
         k_param_auto_velocity_z_max, // remove - 219
+        k_param_land_speed_high,
 
         //
         // 220: PI/D Controllers
@@ -405,6 +406,7 @@ public:
     //
     AP_Int32        rtl_loiter_time;
     AP_Int16        land_speed;
+    AP_Int16        land_speed_high;
     AP_Int16        pilot_velocity_z_max;        // maximum vertical velocity the pilot may request
     AP_Int16        pilot_accel_z;               // vertical acceleration the pilot may request
 

ArduCopter/control_land.cpp

@@ -230,6 +230,10 @@ float Copter::get_land_descent_speed()
 #endif
     // if we are above 10m and the sonar does not sense anything perform regular alt hold descent
     if (pos_control.get_pos_target().z >= pv_alt_above_origin(LAND_START_ALT) && !(sonar_ok && sonar_alt_health >= SONAR_ALT_HEALTH_MAX)) {
+        if (g.land_speed_high > 0) {
+            // user has asked for a different landing speed than normal descent rate
+            return -abs(g.land_speed_high);
+        }
         return pos_control.get_speed_down();
     }else{
         return -abs(g.land_speed);