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Distance Measuring Equipment (DME) is a system that is used in aviation for navigation purposes.
The DME system consists of an interrogator on board an aircraft and a DME station on the ground. The interrogator in the aircraft transmits interrogating pulses to the DME station on the ground. The received pulses trigger the DME station to reply. The reply is received by the aircrafts interrogating DME. The time difference between transmission and reception is used to calculate the distance from the aircraft to the DME station. Because the interrogation is repeated, the information can also be used to calculate the ground speed.
DME operates in the 960 - 1215 MHz band. The interrogator transmits on a center frequency of 1025 up to 1150 MHz. In this band are 126 frequencies with 1 MHz spacing defined. The DME station replies on a frequency that is either 63 MHz lower or 63 MHz higher.
The interrogator uses unique pairs of pulses. The same paring is used for the reply. This is to differentiate the answers from other answers send by the DME station. Modern DME stations can serve up to 200 aircrafts at the same time.
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