(3)
Each DME interrogator generates a suppression pulse for use internally and for suppressing the receivers of the opposite DME and ATC transponders when interrogation pulses are being transmitted. In addition, the DME interrogator accepts suppression pulses to protect its receiver when the other L-band equipment is transmitting.
(4)
Each DME ground station periodically transmits an identification signal along with distance data. One of the identification signals of the foreground station is sent to the audio management unit for flight deck audio presentation. The identification signal is also decoded and then encoded by the interrogator into two ARINC 429, 32-bit data words. Each data word contains two characters of the ident display. The data words are sent to the EIUs for display on the NDs. The DME station ident will only be displayed when a VOR station ident is not available. The DME station ident can be identified because it is displayed in smaller letters than the VOR ident display.
(5)
The AIR/GROUND DISCRETE from the air/ground relay identify flight segement legs for the nonvolatile fault memory. It also inhibits the CMC from initiating a DME interrogator self-test when the airplane is in the air.
B. Functional Description
(1) DME Modes of Operation (Fig 4)
(a) DME system modes of operation are shown in the chart on figure 4. A description of the operation of each mode is given along with the flight deck display for that mode. The conditions for each mode are as follows: 1) The STANDBY mode occurs at initial turn-on and is the
fall-back mode when no ground station pulses are present. In this mode, the transmitter is off but the receiver counts the number of pulse pairs received from the ground station. When the DME is within range of the ground station, sufficient pulse pairs will be counted by the receiver. This causes the DME to operate in the search mode.
2) In the SEARCH mode, the interrogator transmits pulse pairs and looks for ground station replies. Reply pulses are stored in memory for later analysis. Upon reception of synchronous reply pulses, the DME converts the time to distance and switches to the track mode.
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STANDBY MODE TRANSMISSION INHIBITED; RECEIVER (AND AUDIO) OPERATIVE DESCRIPTION OF OPERATION (NO COMPUTED DATA) 4 DASHES RDMI DISPLAY
SEARCH >650 PULSE PAIRS PER SECOND RECEIVED TRANSMITTER ON AND OPERATING WITH HIGH PRF 4 DASHES
TRACK THREE-OR-MORE (OUT OF 5) CORRECT PULSE PAIRS RECEIVED IN SEARCH MODE. SLANT RANGE
SCAN FIVE STATIONS OR LESS SPECIFIED IN FOREGROUND LOOP MULTIPLE STATION TUNING UP TO 15 MAY BE SCANNED IN BACKGROUND LOOP SELECTED STATION SLANT RANGE
MEMORY LOSS OF RETURN PULSES FOR 2 SECONDS COMPUTES RANGE (EXTRAPOLATED FROM PREVIOUS DISTANCE). SLANT RANGE
FAULT INTERROGATOR MALFUNCTION (DURING CONTINUOUS MONITORING OPERATION) BLANK DISPLAY
SELF-TEST INTERROGATOR TO CYCLE THROUGH THE SELF-TEST. MONITOR THE TEST PUSH AND RELEASE ONE OF THE TWO TEST SWITCHES AND WAIT FOR THE STATUS ON THE INTERROGATOR LCD. 2. 3 DASHES (NCD) SEQUENCES THRU: 1. BLANK DISPLAY 3. 0.0 NM
DME Modes of Operation
Figure 4
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The TRACK mode occurs after lock-on in the previous search
mode. Slant range distance is computed and provided to the
EIUs and FMC and is updated every 100 ms if there is no
channel designated for display or 70 ms when designated by
the FMC. The display is updated every 140 ms when the
channel selected by the pilot.
In the DIRECTED SCAN mode, the DME has the capability of
interrogating and providing distance information for one to
five frequencies.
a) The first four directed channels (called channels 1, 2, 3 and 4) are used for VOR paired DME ground stations. The fifth channel (called channel 5) is reserved for ILS or LOC paired DME ground station. Channels 3 and 4 are the primary VOR paired DME ground station pair. Channels 1 and 2 are the secondary VOR paired DME ground stations. The primary channels are predetermined by the FMC according to information in the flight deck data base. The secondary channels are used, if possible, to obtain the best DME-DME pair for position triangulation position during navigation. The interrogator provides the FMC with frequency and binary (bnr) distance data at an update rate of 70 ms.
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