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calibrated to an accuracy of 0.5 per cent or better at the specified frequency is recommended as a calibration standard.
6.6.1.2.2 This test should be made at least once a month.
6.6.1.3 Ground wave field strength
6.6.1.3.1 It is recommended that remote field strength measurements be made using a field strength indicator set
capable of checking that the power radiated from an NDB is in accordance with Chapter 3, 3.4.3.
6.6.1.3.2 This test should be carried out at least once a month.
6.6.1.4 Modulation depth
6.6.1.4.1 Chapter 3, 3.4.6.2, requires that the depth of modulation of the carrier frequency should be maintained as near
to 95 per cent as practicable. Use of an oscilloscope capable of operation at the transmitting frequency of the NDB is
recommended for this purpose.
6.6.1.4.2 This test, using an oscilloscope, should be made at least every three months. Additionally, rough checks of
modulation depth may be made frequently by using a calibrated diodevoltmeter, but these are not a satisfactory substitute for
the test using the oscilloscope.
6.6.1.4.3 Unwanted audio frequency modulation should be measured at least every three months to ensure that it does
not exceed the limits of Chapter 3, 3.4.6.5. An oscilloscope is also recommended for this test.
6.6.1.5 Fall of carrier power with modulation
6.6.1.5.1 When modulation is applied, the carrier power should not decrease by more than the amount specified in
Chapter 3, 3.4.6.4. A recommended method of ensuring this is to use, at a suitable remote site, a calibrated communication
receiver having a continuous wave IF crystal filter as an indicator of transmitter field strength and having sufficient
selectivity to reject the audio sidebands.
6.6.1.5.2 An alternative but probably less accurate method of determining the fall of carrier power would be a
comparison of the current fed into the antenna when the transmitter is modulated with that when it is not modulated.
6.6.1.5.3 This test should be done at least once a month.
7. Material concerning DME
7.1 Guidance material concerning both DME/N and DME/P
7.1.1 System efficiency
7.1.1.1 System efficiency is the combined effect of down-link garble, ground transponder dead time, up-link garble,
and interrogator signal processor efficiency. Since each of these efficiency components are statistically independent, they can
Attachment C Annex 10 — Aeronautical Communications
be computed individually and then combined to yield the system efficiency. The effect of a single component is defined as
the percentage ratio of valid replies processed by the interrogator in response to its own interrogations assuming all other
components are not present. The system efficiency is then the product of the individual components.
7.1.1.2 In computing system efficiency, the number of missing replies as well as the accuracy of the range
measurement made with the received replies should be considered. Missing replies may result from signal interference due to
garble or from interrogations being received at the transponder during a dead time period. Replies which contain significant
errors large enough to be rejected by the interrogator signal processing also should be treated as missing replies when
computing the efficiency component.
7.1.1.3 The interference rate due to garble is dependent upon the channel assignment plan, traffic loading, and the
ground transponder and interrogator receiver bandwidths. Because the FA mode has a wider receiver bandwidth than the IA
mode, it is more susceptible to interference. These factors were accommodated in the DME/P system definition and normally
do not require special consideration by the operating authority.
7.1.2 Down-link garble
Down-link garble occurs when valid interrogations at the ground transponder are interfered with by coincident
interrogations from other aircraft and results in loss of signal or errors in time-of-arrival measurement. This undesired air-toground
loading is a function of the number of interrogating aircraft in the vicinity of the serving transponder and the
corresponding distribution of interrogation frequencies and signal amplitudes received at the transponder.
Note.— Transponder to transponder garbling is controlled by the channel assignment authorities.
7.1.3 Up-link garble
Up-link garble occurs when valid replies at the interrogator are interfered with by other transponders and results in loss
of signal or errors in pulse time-of-arrival measurement. The garble can be interference from any transponder whose
frequency is within the bandwidth of the interrogator, including those on the same frequency, but with different pulse coding.
This undesired ground-to-air loading is a function of the number of transponders in the vicinity of the interrogator and the
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