Theory of Operation
A. Introduction
The DME-700 measures the slant-range distance between the aircraft and ground stations.
The DME provides multiple station distance information for selected stations in the directed
mode, or all stations within DME range in free scan mode, to the flight management computer
system for high-accuracy position fixing. Units with SB 3 installed can also provide
cockpit display information (bcd output) and aural information for a selected station in the
directed and free scan modes of operation.
The DME-700 transmits coded interrogation signals (pulse pairs) to the ground station. The
ground station receives the interrogation signal and returns a coded reply signal (pulse pair)
for each interrogation. One antenna is used for both transmission and reception.
The DME-700 computes the slant-range (line-of-sight) distance to/from the ground station
as follows:
D=(T- TO)/12 359 μsThe DME provides multiple station distance information for selected stations in the directed
mode, or all stations within DME range in free scan mode, to the flight management computer
system for high-accuracy position fixing. Units with SB 3 installed can also provide
cockpit display information (bcd output) and aural information for a selected station in the
directed and free scan modes of operation.
The DME-700 transmits coded interrogation signals (pulse pairs) to the ground station. The
ground station receives the interrogation signal and returns a coded reply signal (pulse pair)
for each interrogation. One antenna is used for both transmission and reception.
The DME-700 computes the slant-range (line-of-sight) distance to/from the ground station
as follows:
Where:
D = slant range distance in nmi to or from the ground station.
T = time in μs between transmission of the interrogation pulse pair and reception of
the corresponding reply pulse pair.
TO = ground station delay between reception of DME interrogation and transmission
of reply. For X channels, TO = 50 μs, and for Y channels, TO = 56 μs.
12.359 = time in μs for RF energy to travel one nmi and return.
B. DME Channel and Frequency Allocation
There are 252 DME channels, each paired with a vortac, ILS localizer, or unpaired VHFNAV frequency. There are other channels paired with MLS ground stations. The DME-700
does not operate on these channels and will not accept tuning commands with the MLS bit
set to one. Figure 12 lists the vortac or ILS localizer frequencies for the DME paired channels
and the VHF NAV frequencies for the unpaired channels.
When a vortac, ILS localizer, or VHF NAV frequency is selected, the associated DME
ground station is automatically selected. There are 200 DME channels paired with the VHF
NAV frequencies between 108.00 and 117.95-MHz. In addition, there are 52 unpaired
channels, normally used for military TACAN ground facilities, that are between 133.30 and
135.95-MHz.
The frequency matrix, shown in Figure 12, is used to determine X-channels 1 through 126;
for example, VOR frequency 114.70-MHz is DME channel 94X. Y-channels 1 through 126
can be determined from Figure 12 by adding 0.05-MHz to each of the 0.1-MHz columns; for
example, VOR frequency 114.75-MHz is DME channel 94Y. The 126X- and 126Y-channels
total 252 DME channels available.
The DME receive frequency is 63-MHz above or below its transmit frequency for each
channel.
DME-700 Modes of Operation
The DME-700 has three basic modes: standby, directed, and free scan.
(1) Standby
In standby mode, the DME-700 is fully engaged, but no distance data is being computed.
The “no computed data” indication is shown on the indicator, and a “no computed
data” signal is transmitted to the FMCS.
The DME-700 reverts to the standby mode when it is not communicating with a ground
station. The DME can be programmed to revert to the standby mode by grounding
program pin MP5A on the rear connector. With pin MP5A grounded, the DME reverts
to standby mode if the digital tuning signal falls below 5 words/s, the tuning word
sign/status matrix indicates an invalid condition, or the word parity is incorrect. With
program pin MP5A open, the DME reverts to free scan for any of the above conditions.
The “no computed data” indication is shown on the indicator, and a “no computed
data” signal is transmitted to the FMCS.
The DME-700 reverts to the standby mode when it is not communicating with a ground
station. The DME can be programmed to revert to the standby mode by grounding
program pin MP5A on the rear connector. With pin MP5A grounded, the DME reverts
to standby mode if the digital tuning signal falls below 5 words/s, the tuning word
sign/status matrix indicates an invalid condition, or the word parity is incorrect. With
program pin MP5A open, the DME reverts to free scan for any of the above conditions.
(2) Directed
In the directed mode, the DME-700 can interrogate and provide distance information
for one to five stations.
In an aircraft that is not equipped with an FMCS, the DME-700 is operated in the directed,
single-channel mode. The station to be interrogated is selected by the crew
through the DME controller in the cockpit. In this mode, the DME-700 provides frequency,
binary distance, ident, and bcd distance information for the selected station at
a rate of once every 140 ms, about 7 times/second. The distance data output rate for
directed, single-channel mode with display is illustrated in Figure 14, view A.
In an aircraft that is equipped with an FMCS, the stations to be interrogated are selected
by the FMCS. The FMCS can request distance information for from one to five
stations. The DME-700 provides frequency, ident, and binary distance information to
the FMCS at a rate of one channel every 100 ms. The distance data output rate for the
directed mode without display is illustrated in Figure 14, view B.
If the crew desires distance information for a particular station, they can request it
through the cockpit DME controller. The FMCS selects that station as one of the directed
stations, and directs the DME to output the display distance information (bcd
output) for that station along with the frequency and bnr distance information. Only one
of the five directed stations can be selected for display (bcd distance data with audio
information available).
When none of the directed frequencies are selected as a displayed station, the DME
outputs a bcd word indicating “no computer data” (NCD) for each of the directed frequencies.
In addition, the normal frequency worked, ident, and bnr distance word with
valid distance data is also outputted. This prevents the cockpit indicator from indicating
a DME failure to the crew.
When the DME is in directed mode with a displayed station, station frequency and
distance information is provided at a rate of one station every 70 ms, with the displayed
station information provided once every 140 ms. The distance data output rate
for directed mode with display is illustrated in Figure 14, view C.
dme calc is comp. for all
for one to five stations.
In an aircraft that is not equipped with an FMCS, the DME-700 is operated in the directed,
single-channel mode. The station to be interrogated is selected by the crew
through the DME controller in the cockpit. In this mode, the DME-700 provides frequency,
binary distance, ident, and bcd distance information for the selected station at
a rate of once every 140 ms, about 7 times/second. The distance data output rate for
directed, single-channel mode with display is illustrated in Figure 14, view A.
In an aircraft that is equipped with an FMCS, the stations to be interrogated are selected
by the FMCS. The FMCS can request distance information for from one to five
stations. The DME-700 provides frequency, ident, and binary distance information to
the FMCS at a rate of one channel every 100 ms. The distance data output rate for the
directed mode without display is illustrated in Figure 14, view B.
If the crew desires distance information for a particular station, they can request it
through the cockpit DME controller. The FMCS selects that station as one of the directed
stations, and directs the DME to output the display distance information (bcd
output) for that station along with the frequency and bnr distance information. Only one
of the five directed stations can be selected for display (bcd distance data with audio
information available).
When none of the directed frequencies are selected as a displayed station, the DME
outputs a bcd word indicating “no computer data” (NCD) for each of the directed frequencies.
In addition, the normal frequency worked, ident, and bnr distance word with
valid distance data is also outputted. This prevents the cockpit indicator from indicating
a DME failure to the crew.
When the DME is in directed mode with a displayed station, station frequency and
distance information is provided at a rate of one station every 70 ms, with the displayed
station information provided once every 140 ms. The distance data output rate
for directed mode with display is illustrated in Figure 14, view C.
dme calc is comp. for all
(3) Free Scan
In the free scan mode, the DME-700 provides distance information for all stations that
are in DME range.
When in the free scan mode, the DME-700 divides the 252 DME channels into a foreground
and a background loop. The foreground loop consists of five stations, any or all
of which can be designated by the FMCS. If less than five stations are designated, the
DME fills the empty slots with the closet stations. Distance data for the five foreground
stations is provided sequentially, as illustrated in Figure 14, view D.
The background loop consists of the other 247 stations. The DME scans the background
loop, searching for station squitter pulses. If the equivalent of more than 450
squitter pp/s are received from a background loop station, the station is interrogated,
distance is calculated, and the distance data is provided to the FMCS. Distance data
for the background stations is provided at an approximate rate of one station every 2
seconds, if background station information is available. The distance data output rate
for the free scan mode with no displayed station is illustrated in Figure 14, view D.
When in free scan mode, one station can be selected by the crew as a displayed station.
The FMCS places the requested display station in the free scan for foreground
loop. The DME provides bcd distance information, along with the frequency, ident, and
bnr distance information for the selected displayed station. When none of the directed
frequencies is selected as a displayed station, the DME outputs a bcd word indicating
“no computed data” (NCD) for each of the directed frequencies, along with the normal
frequency word and bnr distance word with valid distance data. This prevents the
cockpit indicator from indicating a DME failure to the crew.
When in free scan mode with a displayed station, interrogation of the displayed station
takes priority over all the other stations. Frequency and bnr distance information is
provided for one station every 70 ms, with frequency, bnr, and bcd distance information
for the displayed station provided every 140 ms. The distance data output rate for the
free scan mode with a displayed station is illustrated in Figure 14, view E.
are in DME range.
When in the free scan mode, the DME-700 divides the 252 DME channels into a foreground
and a background loop. The foreground loop consists of five stations, any or all
of which can be designated by the FMCS. If less than five stations are designated, the
DME fills the empty slots with the closet stations. Distance data for the five foreground
stations is provided sequentially, as illustrated in Figure 14, view D.
The background loop consists of the other 247 stations. The DME scans the background
loop, searching for station squitter pulses. If the equivalent of more than 450
squitter pp/s are received from a background loop station, the station is interrogated,
distance is calculated, and the distance data is provided to the FMCS. Distance data
for the background stations is provided at an approximate rate of one station every 2
seconds, if background station information is available. The distance data output rate
for the free scan mode with no displayed station is illustrated in Figure 14, view D.
When in free scan mode, one station can be selected by the crew as a displayed station.
The FMCS places the requested display station in the free scan for foreground
loop. The DME provides bcd distance information, along with the frequency, ident, and
bnr distance information for the selected displayed station. When none of the directed
frequencies is selected as a displayed station, the DME outputs a bcd word indicating
“no computed data” (NCD) for each of the directed frequencies, along with the normal
frequency word and bnr distance word with valid distance data. This prevents the
cockpit indicator from indicating a DME failure to the crew.
When in free scan mode with a displayed station, interrogation of the displayed station
takes priority over all the other stations. Frequency and bnr distance information is
provided for one station every 70 ms, with frequency, bnr, and bcd distance information
for the displayed station provided every 140 ms. The distance data output rate for the
free scan mode with a displayed station is illustrated in Figure 14, view E.
D. Simplified Theory
The DME-700 measures distance by transmitting interrogation pulses at the selected channel
frequency, selecting the proper reply signal, and measuring the time between the interrogation
and the reply. Figure 15 is a simplified block diagram of the DME-700.
frequency, selecting the proper reply signal, and measuring the time between the interrogation
and the reply. Figure 15 is a simplified block diagram of the DME-700.
(1) DME Station Interrogation
The DME interrogates a station by transmitting an RF pulse pair of the proper frequency
and pulse-pair spacing. Station interrogation is controlled by the microcomputer
on the range processor card.
Frequency data for the selected station is received over the ARINC 429 data bus and
stored in the range processor memory. The range processor uses the frequency information
to provide a tuning word for the synthesizer.
The synthesizer consists of a vco and an SMO. The vco is a CW oscillator and amplifier
that operates in the L-band frequency range of 1025 to 1150-MHz. The vco output
provides the drive signal for the driver amplifier.
A sample of the vco output signal is applied through a frequency divider to the SMO.
The frequency divider divide ratio is controlled by the tuning signal from the range
processor. The SMO compares the divided vco signal to a reference oscillator signal
and outputs a signal that is proportional to the phase or frequency difference between
the two signals. The SMO output signal controls the frequency of the vco.
In the driver, the CW signal from the synthesizer is pulse modulated and amplified to
about 75 watts before being applied to the power amplifier. The range processor initiates
each interrogation and provides the trigger pulses for the driver modulator. The
driver also provides a local oscillator signal to the receiver to generate the receiver 63-
MHz first if frequency.
In the power amplifier, the RF pulses are shaped and amplified to a nominal 500 watts.
The range processor provides the trigger pulses for the pa modulator. The pulsed output
signal is applied through a circulator and low-pass filter to the antenna. The circulator
isolates the transmitter from the receiver. The low-pass filter prevents the radiation
of high-frequency spurious energy.
and pulse-pair spacing. Station interrogation is controlled by the microcomputer
on the range processor card.
Frequency data for the selected station is received over the ARINC 429 data bus and
stored in the range processor memory. The range processor uses the frequency information
to provide a tuning word for the synthesizer.
The synthesizer consists of a vco and an SMO. The vco is a CW oscillator and amplifier
that operates in the L-band frequency range of 1025 to 1150-MHz. The vco output
provides the drive signal for the driver amplifier.
A sample of the vco output signal is applied through a frequency divider to the SMO.
The frequency divider divide ratio is controlled by the tuning signal from the range
processor. The SMO compares the divided vco signal to a reference oscillator signal
and outputs a signal that is proportional to the phase or frequency difference between
the two signals. The SMO output signal controls the frequency of the vco.
In the driver, the CW signal from the synthesizer is pulse modulated and amplified to
about 75 watts before being applied to the power amplifier. The range processor initiates
each interrogation and provides the trigger pulses for the driver modulator. The
driver also provides a local oscillator signal to the receiver to generate the receiver 63-
MHz first if frequency.
In the power amplifier, the RF pulses are shaped and amplified to a nominal 500 watts.
The range processor provides the trigger pulses for the pa modulator. The pulsed output
signal is applied through a circulator and low-pass filter to the antenna. The circulator
isolates the transmitter from the receiver. The low-pass filter prevents the radiation
of high-frequency spurious energy.
(2) Received Signal Processing
The DME station transmitted pulse-pair signals are received by the antenna and applied
through the circulator to the receiver. The receiver selects, mixes, amplifies, and
detects the incoming signals and applies the video information to circuits in the video
processor.
The received signals from the antenna are applied to the receiver preselector, consisting
of a 2-pole filter, an RF amplifier, a 3-pole filter, and a mixer. Filter tuning signals
are received from the range processor.
through the circulator to the receiver. The receiver selects, mixes, amplifies, and
detects the incoming signals and applies the video information to circuits in the video
processor.
The received signals from the antenna are applied to the receiver preselector, consisting
of a 2-pole filter, an RF amplifier, a 3-pole filter, and a mixer. Filter tuning signals
are received from the range processor.
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