EFFECTIVITY
AF 121,122,157,161,183,184
CONFIG 3799 Page 13
34-55-00
Apr 25/95
E68408
DME INTERROGATOR R/T FAULT INDICATOR RESET BY GROUNDING TERMINAL A1J4 THROUGH HOLE IN RIGHT SIDE OF UNIT
ON AIRPLANES WITH CAROUSEL IV-A INERTIAL NAV SYSTEMS OR
ON 860E-3 INTERROGATORS ONLY PERFORMANCE MANAGEMENT SYSTEM DME No. 1 Block Diagram Figure 3 (Sheet 4)
E68412
EFFECTIVITY
AF 121,122,157,161,183,184
CONFIG 3 799 Page 14
34-55-00
Apr 25/95
(8)
The function of the memory unit is to store the number of clock pulses that occur between the transmission of a pulse pair and the reception of a pulse pair from the ground station. This stored number is equivalent to the distance in hundredths of a mile. Each bit counter time corresponds to one-hundredth of a nautical mile of round-trip travel time of a radio wave. The crystal-controlled timing reference, or clock, generates clock pulses at a rate of
8.091 MHz, corresponding to one-hundredth of a mile per clock pulse.
(9)
The monitor circuits, consisting of cards 7 and 8, provides output data monitor (continuous), automatic self-test monitor, and track monitor. Data monitoring is continuously accomplished and consists of checking the serial data output lines for presence of clock, word sync and DME label. Automatic self-test is accomplished upon changing channels or every 45 seconds during search or automatic standby.
(10)
Track monitor is accomplished while the DME is tracking a ground station. A secondary digital ranging circuit (within the monitor) continuously seeks synchronous targets and compares the position with the DME range gate position. In the event a synchronous target exists at a range less than the DME range gate position (indicating the DME is locked to an echo), the monitor immediately causes the DME to unlock and begin a new search cycle, allowing the ranging circuits to acquire the closer (nonecho) reply.
(11)
If, during the time the DME is tracking, the monitor ranging system does not confirm the presence of a reply within the DME range gate, it will cause the distance indicator flag to display, the R/T fault indicator on the front of the unit to change to yellow (yellow indicating failure) and one of two fault indicators located on the rear of the unit to also change to yellow. Additionally, should the monitor detect repeated system unlocks, indicating a tracking servo malfunction, it will cause the distance indicator flag to display and the R/T and both rear fault indicators to change to yellow.
(12)
The BCD distance word bits are clock-pulse synchronized and amplified into a 32-bit serial BCD word that is applied to the DME indicator. The data clock signal, and word sync signal, are also applied to the DME indicator. The 32-bit digital word is loaded into the indicator, and decoded to provide the decimal-number display.
(13)
The monitor circuits provide fault monitoring of the interrogator unit and DME antenna. The monitor circuits contain amplifier circuits that drive the fault indicators on the interrogator front panel (R/T, IND and ANT). These indicators are magnetic latching indicators, that when initially set, must be reset to remove the indication. These fault indicators enable isolation of a faulty receiver/transmitter (interrogator unit) or antenna. The IND fault indicator is not used.
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 15 AND VM ALL Apr 25/95
34-55-00
(14)
The monitor also provides a flag output signal. The flag signal is a stimulus for the flag alarm circuit which results in the DME indicator showing the shutter when a malfunction occurs in the system.
(15)
With the TEST button depressed, the distance circuits develop a synthetic reply pulse spaced to represent the functional test distance of zero nautical miles. The distance circuits measure the time between the interrogation and self-test reply pulse and apply appropriate distance information to the control circuits. The control circuits then develop the serial BCD word which is applied to the DME indicator. When the test is activated, the DME indicator displays the shutter for 1 second, followed by dashed lines for 1 second, then followed by 000.0-nautical mile readout.
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 16 AND VM ALL Apr 25/95
34-55-00
B. Functional Description (Fig. 3) (LH 014-030, 057-070, 083-099; AZ 221-299; VM 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 or 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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