CAPTAIN'S DME DISPLAY (TYPICAL) FIRST OFFICER'S DME DISPLAY (TYPICAL)
IB 410-499, LESS IB 410 AFTER INCORPORATION OF SB 34-2255
DME System Interface Diagram Figure 2 (Sheet 1)
EFFECTIVITY LH 011-030, 051-070, 082-099; AF 121,122,152-199; AZ 221-299;
CONFIG 3IB 406-499; SN 351-399; GN, MD, 799 Page 4AND VM ALL
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115V AC NO. 2 RADIO BUS
DME System Interface Diagram Figure 2 (Sheet 2)
EFFECTIVITY CONFIG 3 799 Page 5
AT ALL
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Oct 25/80
3. Antennas
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A. The DME antennas are conventional blade-type units which are connected to the interrogator units by coaxial cable. The No. 1 system antenna is installed on the bottom of the fuselage at station 690. The No. 2 system antenna is installed at station 830. Each antenna performs both transmitting and receiving functions for its respective DME system in the frequency range of 960 to 1215 MHz.
4. Interrogator Units
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A. Each interrogator unit provides the capability to compute the time interval between transmitted and received pulse-pair signals, convert this time difference to distance, and apply this information to the DME indicator.
B. The interrogator units are standard 1/2 ATR short assemblies which mount (side-by-side) on a shelf in the main electronics equipment center (Fig. 1).
C. On XA ALL EXCEPT LH and AZ 221-299, the front panel of each interrogator contains three fault indicators and a reset switch. The fault indicators are placarded R/T, IND, and ANT. The IND fault indicator is not used. The R/T and ANT fault indicators turn yellow to indicate a malfunction occurring in the interrogator (R/T) or the antenna unit (ANT) and are used as an aid for trouble shooting the system. The RESET switch allows the operator the option of resetting the IND and ANT fault indicators to normal (black) indication.
D. On ALL EXCEPT LH and AZ 221-299, two cable connectors (P1 and P2) at the rear of unit provide interconnection to external and internal components respectively.
E. AF 121-122, 157, 161, 183-184; two coaxial connectors (SUPPR and ANT) on the front panel connect the suppressor pulses and the interrogator. The RCVR OUT coaxial connector is not used.
5. Indicators (XA ALL EXCEPT AT)
A. The DME indicators are mechanical, digital counters which display nautical miles-to-go on three individual counter dials measuring in hundreds, tens, and units. There are two DME indicators, one for each system, contained within each pilots' radio magnetic indicator (RMI). The left-hand DME indicator in each RMI displays mileage from DME No. 1 and the right-hand indicator displays mileage from DME No. 2.
B. The indicator receives a BCD pulse signal from either or both DME systems logic in the indicator decodes this signal and converts it into a digital display showing miles-to-go to the interrogated ground station.
C. Warning flags (shutters) cover the DME indicators when the system is inoperative.
6. Indicators (AT ALL)
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A. Two dual BCD (binary-coded-decimal), seven-segment indicators are installed in the airplane (Fig. 1).
B. Each indicator provides a dual display of distance information from DME system No. 1 and No. 2, with each half of the dual indicator operating independently of the other (Fig. 2).
EFFECTIVITY LH 011-099;
AF 121-122, 152-199; AZ 221-299; IB 406-499; CONFIG 3 SN 351-399; AT, GN, MD, 01 Page 6 AND VM ALL Apr 25/95
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DME INTERROGATOR
DME No. 1 Block Diagram
Figure 3 (Sheet 1) EFFECTIVITYLH 011-013,051-056,082; AF 152-156,158-160,162-182,
CONFIG 3 185-199; SN 351-399; 799 Page 7/8 AT, GN, AND MD ALL Apr 25/95
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C. The indicator receives a BCD pulse signal from either or both DME systems. Logic in the indicator decodes this signal and converts it into a digital display showing miles-to-go to the interrogated ground station.
D. The display numerals are formed by energizing discrete segments of an incandescent, seven-segment, lamp matrix in response to the pulse-coded information. Any numeral (from 0 to 9) can be formed using a maximum of seven segments. For example: The numeral 8 uses all seven segments (Fig. 1).
7. Operation
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A. Function Description (Fig. 3) (LH 011-013, 051-056, 082; AF 152 -156, 158-160, 162-182, 185-199; IB 406-499; SN 351-399; AT, GN, AND MD ALL)
(1)
The DME interrogation signal from the airborne equipment consists of pulses of rf energy transmitted in pairs. The pulses in a pair are spaced 12 microseconds apart for the assigned 126 DME channels. A reply signal is transmitted from the ground station for each interrogation pulse-pair from the airplane equipment. The reply signal consists of pulse pairs with the same characteristics as the interrogation pulse pairs, except for rf frequency. The reply pulse rf frequency is 63 MHz above or below the interrogation pulse frequency. The exact relationship of the frequencies depends upon the particular DME channel selected. The DME system searches for synchronous pulses and provides valid distance data when locked on and tracking.
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