User's Manual

Figure 1-2 DME Transponder Block Diagram
Three separate signals are transmitted by the beacon as a train of pulse pairs. These signals, in order of
priority, are: identification, replies to interrogations, and squitter pulse pairs (used as fill-in pulses). This
priority system prevents any interference between the three signals in the overall pulse train.
The identification of the ground facility is important to the using aircraft; therefore, it has been assigned
first priority in the priority system. The generation of identification intelligence is a function of the RTC.
Identification is transmitted periodically in International Morse Code with the characters of the code
consisting of a periodic train of pulse pairs. Identification keying occurs approximately every 30 seconds.
When keyed, the priority logic circuit input is disabled; and the circuits will not accept any decodes from
the receiver.
The replies to an interrogation signal are second in the order of priority. Their induction into the pulse train
must be controlled (to prevent interference with the identity cycle and to establish priority over the squitter
pulses). This is accomplished by allowing them to enter the pulse train only during a time interval not
occupied by the identity cycle. This is a major portion of the time, since the identity cycle only occurs
approximately every 30 seconds. Once the receiver accepts an interrogation and decodes it, a blanking gate
is generated (the so-called dead-time gate). The dead-time gate is used to inhibit the transponder decoder
for approximately 60 µs. During this period, the decoded interrogation is delayed a predetermined amount
of time and transmitted back as a reply. The total delay from the time of a received interrogation to
transmission of a reply is typically set for 50 µs for an X-Channel DME.