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DME: The DME is a navigation system to measure the distance in nautical miles (NM) between a DME ground station and a aircraft. This distance is continue being displayed in the cockpit plus the speed regarding of the ground speed plus the remaining time to reach the DME ground station.
A aircraft sends continuous pulses out (interrogation) of which the DME ground station will receive. The DME ground station (transponder) sends this signal back but on a other frequency. A DME receiver in the aircraft measures the time between the transmitted signal and the received signal and translate this in the distance in NM between the aircraft and the ground station. DME has a very high degree of accuracy by operating on the line-of-sight principle. It can operate up to 199 NM with an accuracy of 3% of the distance. DME operates in the Ultra High Frequency (UHF) range between 960MHz and 1215MHz according to the ICAO Annex 10. Almost always a DME system is combined with a VOR system (then defined as VOR-DME).
GPS: Global Positioning System; a radio-navigation systems from the US Department of Defence for the US Military. DoD has decided to share the navigation system with others. GPS is also used by civil aviation (on large scale) and by the normal population.
The Nondirectional Radio Beacon (NDB) is a low or medium frequency radio beacon which transmits nondirectional signals. Normally, all aircrafts are equipped with a Automatic Direction Finder (ADF) to receive these signals. With the ADF the pilot can determine bearings on the station. When the NDB is not used in conjunction with a Instrument Landing System (ILS) it transmits a continuous three-letter identification in morse code, except during voice transmissions. The NDB uses a frequency range between 190 and 1750 kHz, according to ICAO Annex 10.
Disturbances in NDBs will result in erroneous bearing information. They can be occurred from factors as lightning, precipitation, interference from distant stations (at night). Because there is no warning in the cockpit for the pilot when erroneous bearing information is displayed, the pilot must continuously monitor the NDBs identification.
VOR: The Very High Frequency Omni-directional Range (VOR) is a very precise navigation system. A VOR transmitter on the ground transmits three-letter morse code radio signals between 108.0 and 119.95 MHz. The range varies proportionally to the altitude of the receiver. The precision of a the VOR system depends upon the aircraft equipment and ground equipment. The VOR signals from the transmitter will be disturbed if there are obstacles (like buildings or mountains) between the transmitter and the receiver. A VOR-indicator in the cockpit of an aircraft displays the angle of deviation in degrees from the magnetic north which is measured from the VOR ground station.
Mostly all VOR systems are combined with a distance measuring equipment system (DME). Now the pilot can also see his distance from the ground station and if he is flying to, or from the ground station.
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