DME Testing


This material is for reference only
And should only be used as such.
Always consult the approved manuals before doing any tests.

Thomas Boston

Figure 1. Bendix/King KN 62A DME[i]


Test Frequencies
X-Channel 108.00MHz
Y-Channel 108.05MHz

These are the standard test frequencies. Other frequencies may be used to test complete range of operation. (Paired VOR frequencies are between 108.00MHz and 117.95MHz with both X and Y channels. The chart with DME/VOR frequency pairs can be found at the link below.)

Equipment Required

Figure 2. ATC 1400A[ii]

  • ATC 1400A

The ATC-1400A is a bench mounted test set, with the ability to test and calibrate DME equipment and indicators and ATC Transponders. It has in it a self-contained signal generator that can generate desired test signals from the L-Band by dialing in its VOR-paired X or Y-Channel. A DME mode is used to test DME for a variety of parameters such as range, velocity, acceleration, and output power to name a few.

  • Oscilloscope

Any oscilloscope can be used with the ATC 1400A. Whatever the technician prefers or has available will work.

Testing Procedures (both X and Y channels)[iii]

NOTE: The following four steps are to be completed before testing any parameters of the DME UUT.

1. Set up the ATC-1400A-2 and UUT as shown in figure 1, using a standard signal described by the DME manual at 108.00MHz.
2. Set POWER Switch to ON and allow 3 minute warm-up period.
3. Set DME (UUT) to stand-by, allow sufficient warm-up time and set to NORM position.
4. Press CLEAR RNG Pushbutton Switch to clear ATC-1400A-2 of any previously loaded range, velocity and acceleration information.

Figure 3. ATC 1400A-DME UUT bench test setup

Output power
For best results, select two X channels and two Y channels for testing.

1. Verify UUT transmitter frequency on XMTR FREQ MHz Display is correct. (FP1@108.00MHz = 1041 MHz [±0.07 MHz])
2. Verify UUT transmitter power on XMTR PWR WATTS Display is within Manufacturer’s specifications.
3. Set F2/P2 F1/P1 Switch to F2/P2.
4. Verify UUT transmitter power on XMTR PWR WATTS Display is within Manufacturer’s specifications.
5. Determine frequency stability of UUT by applying following formula to values in Steps 5 and 8:
100 X (1 - [FP1/FP2]) = % Frequency Stability
6. Verify Frequency Stability of UUT transmitter is within ±0.0007%.

Sensitivity (MTL)
For best results, select two X channels and two Y channels for testing.

1. Set RF Level Control switch to a level below the MTL described in the DME UUT manual (-95dBm is usually appropriate).
2. Slowly increase the RF Level control and note the level that the DME UUT is first able to obtain a distance. Verify this level with the DME manual.

Search and Track PRF
For best results, select two X channels and two Y channels for testing.

1. Connect BNC T-Connector to Oscilloscope Channel A.
2. Connect 50 Ω Load to one end of BNC T-Connector and 50 Ωcoaxial cable from other end of BNC T-Connector to XMTR Connector.
3. Connect 50 Ω coaxial cable from Oscilloscope External Trigger to SYNC Connector.
If viewing total P1 pulse is desired, remove Oscilloscope External Trigger coaxial cable from SYNC Connector. Connect Oscilloscope External Trigger coaxial cable to UUT suppression output.
4. Verify UUT transmitted pulse rise and fall time is <3.0 μs. Verify pulse width, measured between leading and trailing edges, is 3.5 μs (±0.5 μs).
5. Verify Lock/Track Mode pulse repetition frequency (PRF) is ≤30 pp/s.
6. Set DME REPLY EFFICIENCY Control to 0%. Allow ATC-1400A-2 a 15-second interval to exit from memory.
7. Verify Search Mode PRF is in accordance to Manufacturer’s specifications (≤150 pp/s).

Memory (coast) Mode

1. Set DME UUT to NORM position and wait for a lock-on condition.
2. Using a stopwatch, record elapsed time DME REPLY EFFICIENCY Control is set to 0% and Display reflects an increase of UUT PRF. Verify elapsed time is within limits described in DME manual.
3. Set RANGE/VEL/ACCEL Thumbwheels to 400 KTS.
4. Set DME REPLY EFFICIENCY CONTROL to 70%. Press CLEAR RNG Pushbutton Switch and LOAD VEL Pushbutton Switch. Allow UUT to lock-on and track.
5. Set DME REPLY EFFICIENCY Control to 0% and allow an interval of time (described in DME manual) for ATC-1400A-2 to exit from memory. Set DME REPLY EFFICIENCY Control to 70%. Verify distance, in accuracy, is ±≤0.2 NM.

Speed Circuits

1. Set DME UUT to NORM position and wait for a lock-on condition.
2. Using the LOAD RNG and LOAD VEL switches insert an appropriate range and velocity and wait for the speed circuits to set themselves (usually around 3 minutes. May be described in DME manual).
3. Verify time to station is correct using the formula d=v/t where d=distance, v=velocity and t=time.


Test Frequencies

X-Channel 108.00MHz
Y-Channel 108.05MHz

Equipment Required

Figure 4. ATC-600A[iv]

  • IFR ATC-600A[v]

The IFR 600 is a transponder/DME ramp-testing unit, with built in signal generators and modulators for frequencies in the transponder/DME range. It has variable range from 0 to 399NM and velocities possible from 50 to 2400 knots. Both X-channel and Y-channel modes are able to be tested with the flick of a switch and it has a Ni-Cad battery to provide 2 AH of battery life. It also provides for charging while connected to an AC line.

  • Remote DME antenna

A DME antenna mounted on a tripod with a coax long enough to allow it to be at the proper distance from the aircraft DME antenna and still connect to the ATC-600A while in the cockpit is required to provide the best test setup.

Testing Procedures (both X and Y Channel)[vi]

NOTE: Antenna location, aircraft location and coax length can affect results. Consult DME manual for proper testing distances and locations. Antenna location is of greatest concern as placing it too close could also result in damage to the IFR 600.

NOTE: All tests done on the DME require it to be powered up and in standby mode for a few (3) minutes to warm up. If warm up time is skipped results are not accurate.

Valid distance/Ground speed

1. Press PWR/BAT Switch to BAT.
2. Set DME to Distance Display Mode in desired range scale.
3. Set DME Frequency to 108.00.
4. Set MODE Switch to DME.
5. Set DME RANGE/VELOCITY Switch to desired velocity setting.
6. Set VELOCITY HI/LO RANGE Switch to HI (selects upper value) or to LO (select lower value).
7. Set FAST SLEW Switch and SLOW SLEW Switch to IN then to OUT (to set a desired starting range).
8. Set VELOCITY IN/OUT Switch to IN then to OUT (to track the distance toward or away from the ground station).
9. Verify DME locks on and displays the correct range and velocity in all cases.

Sensitivity (minimum trigger lever)

1. Set IDENT/50% RPLY Switch to 50% RPLY.
2. Verify 50% of all replies to the DME are deleted.
3. Verify the DME can still track and
NOTE: Deleting half of all replies to the DME checks the ability of the DME to lock-on or to track under poor signal conditions.

Transmit Power

1. Connect DME to Test Set.
2. Set FREQ/PWR Switch to PWR.
3. Verify RF peak power is displayed on FREQ/PWR Meter in kW and passes specifications in DME manual.

Alternative Tests

If no test equipment is available the only way to test the DME system is to fly with it and compare it with other known calibrated systems (such as GPS or VOR) using VOR/DME stations as waypoints for distance, velocity and time to station.

[i] American Avionics
-DME Units

[ii] Aero Express
-IFR ATC-1400A

[iii] Transponder/DME TEST SET
Operation Manual – Issue 3

[iv] Ebay
-IFR ATC-600A Transponder and DME Test Set w/ manual

[v] SAIT Polytechnic
IFR 600A Manual

Operation Manual – Issue 2