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In the DME tracking mode the interrogator transmits a relatively..
  • A
    low number of pulse pairs per second in order to obtain an accurate range measurement.
  • B
    low number of pulse pairs per second in order to avoid a transponder overload.
  • C
    high number of pulse pairs per second in order to obtain an accurate range measurement.
  • D
    high number of pulse pairs per second in order to avoid multipath effects from objects in the DME transponder’s neighbourhood.

Distance measuring equipment (DME) Interrogator

The aircraft's interrogator sends random distributed pulses to the ground station.
The ground station receives and returns theses pulses to the interrogator after the main delay at the transponder frequency (frequency offset of 63 MHz to the interrogation frequency).
The receiver in the aircraft determines the time difference (Δt) between the transmitted and received pulse.
Taking into account the main delay of the station it then uses this time to calculate the slant range to the ground station.
The distance is usually indicated in nautical miles.

The interrogator can be in three different modes (states):

  • SEARCH mode

  • TRACK mode

  • MEMORY mode

In SEARCH mode, the interrogator attempts to set up a connection to a ground station and to synchronize to this ground station.
In this mode, the pulse repetition rate can be increased up to 150 ppps (ppps = pulse pairs per second) to speed up the search process, much more than in track mode.
In TRACK MODE less than 30 interrogation Pulse-pairs per second are sent in order to avoid a transponder overload.
The aircraft interrogator locks on to the DME ground station once it recognizes a particular reply pulse sequence has the same spacing as the original interrogation sequence.
Once the receiver is locked on, it has a narrower window in which to look for the echoes and can retain lock.

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