GCS_MAVLink: send_set_position_target always sends alt-above-home

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -3928,22 +3928,26 @@ void GCS_MAVLINK::send_mount_status() const
 void GCS_MAVLINK::send_set_position_target_global_int(uint8_t target_system, uint8_t target_component, const Location& loc)
 {
 
-    uint16_t type_mask = POSITION_TARGET_TYPEMASK_VX_IGNORE | POSITION_TARGET_TYPEMASK_VY_IGNORE | POSITION_TARGET_TYPEMASK_VZ_IGNORE | \
-                         POSITION_TARGET_TYPEMASK_AX_IGNORE | POSITION_TARGET_TYPEMASK_AY_IGNORE | POSITION_TARGET_TYPEMASK_AZ_IGNORE | \
-                         POSITION_TARGET_TYPEMASK_YAW_IGNORE | POSITION_TARGET_TYPEMASK_YAW_RATE_IGNORE;
+    const uint16_t type_mask = POSITION_TARGET_TYPEMASK_VX_IGNORE | POSITION_TARGET_TYPEMASK_VY_IGNORE | POSITION_TARGET_TYPEMASK_VZ_IGNORE | \
+                               POSITION_TARGET_TYPEMASK_AX_IGNORE | POSITION_TARGET_TYPEMASK_AY_IGNORE | POSITION_TARGET_TYPEMASK_AZ_IGNORE | \
+                               POSITION_TARGET_TYPEMASK_YAW_IGNORE | POSITION_TARGET_TYPEMASK_YAW_RATE_IGNORE;
 
-    uint8_t mav_frame = loc.relative_alt ? MAV_FRAME_GLOBAL_RELATIVE_ALT_INT : MAV_FRAME_GLOBAL_INT;
+    // convert altitude to relative to home
+    int32_t rel_alt;
+    if (!loc.get_alt_cm(Location::ALT_FRAME_ABOVE_HOME, rel_alt)) {
+        return;
+    }
 
     mavlink_msg_set_position_target_global_int_send(
             chan,
             AP_HAL::millis(),
             target_system,
             target_component,
-            mav_frame,
+            MAV_FRAME_GLOBAL_RELATIVE_ALT_INT,
             type_mask,
             loc.lat,
             loc.lng,
-            loc.alt,
+            rel_alt,
             0,0,0,  // vx, vy, vz
             0,0,0,  // ax, ay, az
             0,0);   // yaw, yaw_rate