(RDF) equipment. Plotting fixes in this manner allow crews to determine their position. This usage is important in situations where other navigational equipment, such as VORs with distance measuring equipment (DME), have failed. In marine navigation, NDBs may still be useful should Global Positioning System (GPS) reception fail. Determining distance from an NDB station To determine the distance to an NDB station, the pilot uses this method: Turns the aircraft so that the station is directly off one of the wingtips. Flies that heading, timing how long it takes to cross a specific number of NDB bearings. Uses the formula: Time to station = 60 x number of minutes flown / degrees of bearing change Computes the distance the aircraft is from the station; time * speed = distance NDB approaches A runway equipped with NDB or VOR (or both) as the only navigation aid is called a non-precision approach runway; if it is equipped with ILS, it is called a precision approach runway. Instrument landing systems NDBs are most commonly used as markers or "locators" for an instrument landing system (ILS) approach or standard approach. NDBs may designate the starting area for an ILS approach or a path to follow for a standard terminal arrival route, or STAR. In the United States, an NDB is often combined with the outer marker beacon in the ILS approach (called a locator outer marker, or LOM); in Canada, low-powered NDBs have replaced marker beacons entirely. Marker beacons on ILS approaches are now being phased out worldwide with DME ranges or GPS signals used, instead, to delineate the different segments of the approach. Naval operational uses German Navy U-boats during World War II were equipped with a Telefunken Spez 2113S homing beacon. This transmitter could operate on 100 kHz to 1500 kHz with a power of 150 W. It was used to send the submarine's location to other submarines or aircraft, which were equipped with DF receivers and loop antennas. Antenna and signal characteristics NDBs typically operate in the frequency range from 190 kHz to 535 kHz (although they are allocated frequencies from 190 to 1750 kHz) and transmit a carrier modulated by either 400 or 1020 Hz. NDBs can also be collocated with a DME in a similar installation for the ILS as the outer marker, only in this case, they function as the inner marker. NDB owners are mostly governmental agencies