+ inline const char * get_nav_id() const { return nav_id.c_str(); }
+ inline int get_nav_id_c1() const { return nav_id.c_str()[0]; }
+ inline int get_nav_id_c2() const { return nav_id.c_str()[1]; }
+ inline int get_nav_id_c3() const { return nav_id.c_str()[2]; }
+ inline int get_nav_id_c4() const { return nav_id.c_str()[3]; }