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Nominal DME effective radiated power required
to provide –83 dBW/m power density at various
slant ranges/levels with a typical antenna
located 10.5 m (35 ft) above ground
2
Level above facility
Slant range distance from DME antenna
30 dBW
36 dBW
42 dBW
Note.— These curves are based on extensive experience
of a number of facilities and indicate the nominal effective
radiated power to assure the specified power density on a high
percentage of occasions taking into account propagation and
typical ground/aircraft installation characteristics.
0
0
Figure C-20
23/11/06 ATT C-74
Attachment C Annex 10 — Aeronautical Communications
7.3.1.2 A major potential cause of accuracy degradation encountered in the final phases of the approach and landing
operation is multipath (signal reflection) interference. DME/P FA mode minimizes these effects by using wideband signal
processing of pulses having fast rise time leading edges, and by measuring the time of arrival at a low point on the received
pulse where it has not been significantly corrupted by multipath. This is in contrast to the slower rise time pulses and higher
thresholding at the 50 per cent level used in DME/N.
7.3.1.3 Because the FA mode is used at ranges less than 13 km (7 NM), the transmitter can provide an adequate signal
level to meet the required accuracy without the fast rise time pulse violating the transponder pulse spectrum requirements.
Use of the 50 per cent threshold and a narrow receiver bandwidth in the IA mode permits an adequate but less demanding
performance to the coverage limits. The transponder determines the interrogation mode in use by the interrogation code in
order to time the reply delay from the proper measurement reference. The IA mode is interoperable with DME/N permitting a
DME/N interrogator to be used with a DME/P transponder to obtain at least the accuracy with a DME/N transponder.
Similarly, a DME/P interrogator may be used with a DME/N transponder.
7.3.2 DME/P system accuracy requirements
7.3.2.1 DME/P accuracy requirements
7.3.2.1.1 When considering the DME/P accuracy requirement, the operations that can be performed in the service
volume of the final approach mode tend to fall into one of two groups. This has led to two accuracy standards being defined
for the final approach mode:
a) accuracy standard 1: this is the least demanding and is designed to cater for most CTOL operations;
b) accuracy standard 2: this gives improved accuracy that may be necessary for VTOL and STOL operations, CTOL
flare manoeuvres using MLS flare elevation guidance and CTOL high-speed turnoffs.
7.3.2.1.2 Table C-5 shows applications of DME and typical accuracy requirements. This will assist in selecting the
appropriate accuracy standard to meet the operational requirement. The calculations are based on a distance of 1 768 m
(5 800 ft) between the DME antenna and the runway threshold. The following paragraphs refer to Table C-5.
7.3.2.1.3 It is intended that the DME/P accuracy approximately corresponds to the azimuth function PFE at a distance
of 37 km (20 NM) from the MLS reference datum both along the extended runway centre line and at an azimuth angle of 40
degrees. Also the DME/N error at the limits of MLS coverage is consistent with the 0.37 km (0.2 NM) system accuracy in
Chapter 3, 3.5.3.1.3.3. The CMN is the linear equivalent of the plus or minus 0.1 degree CMN specified for the azimuth
angle function.
7.3.2.1.4 PFE corresponds to azimuth angular error; CMN is approximately the linear equivalent of the plus or minus
0.1 degree CMN specified for the azimuth angle system.
7.3.2.1.5 The plus or minus 30 m PFE corresponds to a plus or minus 1.5 m vertical error for a 3-degree elevation
angle.
7.3.2.1.6 Flare initiation begins in the vicinity of the MLS approach reference datum; MLS elevation and DME/P
provide vertical guidance for automatic landing when the terrain in front of the runway threshold is uneven.
7.3.2.1.7 Sensitivity modification or autopilot gain scheduling requirements are not strongly dependent on accuracy.
ATT C-75 23/11/06
Annex 10 — Aeronautical Communications Volume I
Table C-5.
Function
Typical distance from
the threshold
PFE
(95% probability)
CMN
(95% probability)
Approach (7.3.2.1.3)
— extended runway centre line
— at 40° azimuth
37 km (20 NM)
37 km (20 NM)
±250 m (±820 ft)
±375 m (±1 230 ft)
±68 m (±223 ft)
±68 m (±223 ft)
Approach (7.3.2.1.4)
— extended runway centre line
— at 40° azimuth
9 km (5 NM)
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