DME-Distance Measuring Equipment basics
DME avionic system is a short range navigational aids and it stands for Distance Measuring Equipment.
DME instrument provides distance between Aircraft and Ground station. Ideally we need separation between the Aircraft and the DME station measured over ground. However we get the slant distance. From this Ground distance(D) is calculated as described below based on Aircraft altitude and error.
On the Ground DME has following features:
• It operates in the UHF frequency band from 960 to 1215 MHz with 100 number of channels.
• Each ground station is able to handle 100 Aircrafts simultaneously.
Figure-1 depicts DME ground station system.
In the Aircraft DME has following specifications:
As we know DME system is also referred as Integrator. It operates in UHF band. It operates in the range from 150 to 200 miles. Aircraft DME equipment displays distance in nm, GS in kts and remaining time to get to the Ground station in minutes. Figure-2 depicts DME airborne system.
Following are functional features which describes operation of the DME system:
1. Aircraft DME interrogator transmits signal on one of the channels at frequency of say 'f1'.
2. DME Ground station receives the signal and transmits the signal back on frequency f2 equal to f1+/-63MHz after delay of 50microsec.
3. Aircraft DME interrogator receives the signal, measures the actual time the signal has traveled. Using this calculates the distance of the aircraft.
DME system is rarely affected by precipitation static and thunderstorms.
• We use DME equipment for the purpose
of getting the distance D. However the system will provide slant distance(S) .
Error between actual distance D and S is less when slant distance(S) is taken larger than the Aircraft altitude.
• DME system is limited to LOS due to UHF band of operation.
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