Distance Measuring Equipment

From Academic Kids

Distance Measuring Equipment (DME) is a transponder-based radio navigation technology that measures distance by timing the propagation delay of UHF radio signals.

Aircraft use DME to determine their distance from a land-based transponder by sending and receiving pulse pairs--two pulses of fixed duration and separation. The ground stations are typically colocated with VORs.

The DME system has a UHF transmitter/receiver (interrogator) in the aircraft and a UHF receiver/transmitter (transponder) in the ground station. The interrogator transmits interrogation pulses to the transponder, which in reply transmits a sequence of reply pulses with a precise time delay. The DME receiver then searches for two pulses with the correct time interval between them. Once the receiver is locked on, it has a narrower window in which to look for the echoes and can retain lock. The time difference between interrogation and reply is measured by the interrogator and translated into a distance measurement which is displayed in the cockpit.

A typical DME transponder can provide concurrent distance information to about 100 aircraft. Above this limit the transponder avoids overload by limiting the gain of the receiver. Replies to weaker more distant interrogations are ignored to lower the transponder load.

DME frequencies are paired to VHF omnidirectional range (VOR) frequencies. So generally a DME interrogator is designed to automatically tune to the corresponding frequency when the colocated VOR is selected. An airplane’s DME interrogator uses frequencies from 1025 to 1150 MHz. DME transponders transmit on a channel in the 962 to 1150 MHz range and receive on a corresponding channel between 962 to 1213 MHz. The band is divided into 126 channels for interrogation and 126 channels for transponder replies. The interrogation and reply frequencies always differ by 63 MHz. The channel width is 100 kHz.

One important thing to understand is that DME provides the physical distance from the aircraft to the DME transponder. This distance is often referred to as 'slant range' and depends trigonometrically upon both the altitude above the transponder and the ground distance from it.de:Distance Measuring Equipment fr:Distance Measuring Equipment pl:Distance Measuring Equipment zh:测距仪

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