Option R&S SMA-K26 enables DME modulation on the R&S SMA.
Distance Measurement Equipment (DME) is a radar system which determines the slant distance between the aircraft and the ground station. On the aircraft, the time is measured which the radio signal takes to travel from the aircraft to the ground station and back. The aircraft is equipped with an interrogator and the ground station with a transponder.

The DME channels are paired with the VOR frequencies, in the range between 1025 MHz to 1150 MHz for the interrogator, and 962 MHz and 1213 MHz for the transponder. The spacing for all channels is 1 MHz. X and Y channels differ in the spacing between the two pulses of the pulse pair and in the delay for the reply pulse.
The interrogator transmits a stream of pulse pairs with fixed duration and spacing. The ground-based transponder receives the pulse train and retransmits them after a defined delay on a frequency which is +/-63 MHz from the interrogation frequency. The airborne interrogator identifies its own stream of pulses and measures the time between the start of interrogation and response from the ground transponder to evaluate the slant distance. The distance is given in nautic miles (nm). 1 nm is 1852.01 meters and corresponds to a run time of 12.359 µs.
- DME interrogation
Pulse stream from the interrogator, X or Y channel (simulation of aircraft interrogator) - DME reply
Reply pulses from the transponder + optional ID signal, X or Y channel (simulation of ground station)
In addition, a single test pulse can be generated.
The following graph shows the timing of the pulse pairs (slant range distance = 0 nm).

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