DME Flashcards
The DME will usually be co-located with -
a VOR
Explain the principle of operation:
The method for calculating distance is quite simple, the aircraft transmitter will send a pulse and wait for a reply from the ground transmitter. Distance will equal the time taken between transmitting and receiving the pulse multiplied by the speed of light(in this case 0.162nm per μs) divided by 2(divided by two, because it will have to travel to the ground system and return to the aircraft). But how can the aircraft receiver differentiate between a pulse meant for other aircraft and those meant for itself?
The DME system comprise of two main elements, the airborne transmitter and receiver collectively referred to as the interrogator and the ground equipment which also comprise of a transmitter and receiver referred to as the transponder.
The interrogator radiates a series of pulses in all directions on the receiving frequency of the DME ground station. The pairs of pulses are transmitted continuously, but the time interval between each pair will be random. Once the second pair leaves the transmitter it sets off the aircraft receiver’s timing circuit which will then start to search for a reply. When the ground system(transponder) receives the pairs of pulses it will delay the response by 50μs and reply with the same random pulse pattern. The aircraft equipment will only accept its own random pulse pattern, and not that of other aircraft.
DME range in NM = V x (T - 50) / 2
That maximum amount of aircraft the DME can respond to before becoming saturated?
100 aircraft
How to calculate slant range -
Slant range² = Ground range² + Height²
In which frequency band do the DME operate?
UHF band
Describe TACAN:
TACAN is the military version of VOR/DME. Civil equipment cannot obtain bearings from TACAN, but it can obtain DME distances
Describe VORTAC:
A VOR co-located with a TACAN DME. Civil aircraft can use the VOR bearings and obtain distance from the TACAN DME.
The DME errors:
Nil, no errors.
