When an aircraft is flying under instrument flight rules (IFR), the pilot will often have no visual reference to the ground, and must therefore rely on internal (e.g. GPS) or external (e.g. VOR) aids in order to navigate. Although in some situations air traffic control may issue radar vectors to direct an aircraft's path, this is usually done to facilitate traffic flow, and will not be the sole means of navigating to an important point, such as the position from which an aircraft commences its approach to landing.