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The time taken for the transmission of an interrogation pulse by a Distance Measuring Equipment (DME) to travel to the ground transponder and return to the airborne receiver was 2000 micro-second, including time delay. The slant range from the ground transponder was:
  • A
    330 NM
  • B
    158 NM
  • C
    186 NM
  • D
    296 NM
The Distance Measuring Equipment (DME) works on a transmitting-receiving principle. The distance from the DME station (transmitter) to the aircraft (receiver) is called the slant range. This question talks about the signal being transmitted from the station and back that will take 2000 microseconds (µs). DME sends radio waves that has the same speed as the speed of light.

First convert the speed of light into nm/s. Use the following table:

1852 m/s 1 nm/s
299.792.458 m/s ?

? = (299792458 x 1)1852 = 161874 nm/s

A DME station has a natural delay in transmitting of around 50 µs. The actual transmitting/receiving time will be: 2000-50 = 1950 µs.

Convert the 1950 µs to seconds using the following table:

1 ms 0.000001 s
1950 µs ?

? = (1950 µs x 0,001 s)/(1 µs ) = 0,00195 s


The time it takes to send the signal back and forth is 0,00195 seconds. The time it takes to reach the aircraft is the time divided by 2: 0,00195/2 = 0,00097 seconds.

Use the following table:

161874 nm 1 seconds
? 0,00097 seconds
? = (161874 nm x 0,00097 sec)/(1 sec ) = 157 nm

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