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.
DME and TACAN ground stations always send out a certain number of pulses per second (squitter pulses and replies to requests of planes).
Since all these pulses are randomized, the interrogator uses correlation methods to find the replies to its own interrogations.