the NDBs to triangulate their position along the radial, without the need for the VOR station to have a collocated distance measuring equipment (DME). This display, along with the omni bearing indicator (OBI) for VOR/ILS information, was one of the primary radio navigation instruments prior to the introduction of the horizontal situation indicator (HSI) and subsequent digital displays used in glass cockpits. The principles of ADFs are not limited to NDB usage; such systems are also used to detect the locations of broadcast signals for many other purposes, such as finding emergency beacons. Uses Airways A bearing is a line passing through the station that points in a specific direction, such as 270 degrees (due west). NDB bearings provide a charted, consistent method for defining paths aircraft can fly. In this fashion, NDBs can, like VORs, define airways in the sky. Aircraft follow these pre-defined routes to complete a flight plan. Airways are numbered and standardized on charts. Colored airways are used for low to medium frequency stations like the NDB and are charted in brown on sectional charts. Green and red airways are plotted east and west, while amber and blue airways are plotted north and south. As of September 2022, only one colored airway is left in the continental United States, located off the coast of North Carolina and is called G13 or Green 13. Alaska is the only other state in the United States to make use of the colored airway systems. Pilots follow these routes by tracking radials across various navigation stations, and turning at some. While most airways in the United States are based on VORs, NDB airways are common elsewhere, especially in the developing world and in lightly populated areas of developed countries, like the Canadian Arctic, since they can have a long range and are much less expensive to operate than VORs. All standard airways are plotted on aeronautical charts, such as the United States sectional charts, issued by the National Oceanic and Atmospheric Administration (NOAA). Fixes NDBs have long been used by aircraft navigators, and previously mariners, to help obtain a fix of their geographic location on the surface of the Earth. Fixes are computed by extending lines through known navigational reference points until they intersect. For visual reference points, the angles of these lines can be determined by compass; the bearings of NDB radio signals are found using radio direction finder