Aircraft Operations. Volume II - Construction of Visual and Instrument ...

Part I — Section 2, Chapter 2 I-2-2-3
2.4.2 Radar fixes
Radar should not normally be the primary method of fix identification. However, where air traffic control (ATC) can
provide the service, terminal area radar (TAR) within the limitations specified in 2.4.1, “Terminal area radar” may be
used to identify any terminal area fix. En-route surveillance radar (RSR) may be used for initial approach and
intermediate approach fixes.
2.4.3 Fixes for VOR or NDB with DME
2.4.3.1 VOR/DME fixes use radial and distance information derived normally from facilities with collocated
azimuth and DME antennas. However, where it is necessary to consider a VOR/DME fix derived from separate
facilities, the fix is only considered satisfactory where the angles subtended by the facilities at the fix results in an
acceptable fix tolerance area. See Figure I-2-2-1.
2.4.3.2 Where the DME antenna is not collocated with the VOR and NDB providing track guidance, the
maximum divergence between the fix, the tracking facility and the DME shall not be more than 23 degrees.
2.4.3.3 For the use of DME with ILS, see Part II, Section 1, Chapter 1, 1.4.4, “Glide path verification check”.
2.4.4 DME
The accuracy is ± (0.46 km (0.25 NM) + 1.25 per cent of the distance to the antenna). This value is the RSS total of
minimum accuracy, monitor tolerance and flight technical tolerance, the latter two being so small as to be completely
dominated by the larger airborne value.
Note 1.— No reduction can be justified based on flight test information.
Note 2.— Tolerance values assume that published procedures will take into account slant range distance.
2.4.5 75 MHz marker beacon
Use Figure I-2-2-2 to determine the fix tolerance for ILS and “Z” markers during approach procedures.
If the facility defines the MAPt, the fixed value of zero is used (see Section 4, Chapter 6, 6.1.6.2.1, “MAPt tolerance
when MAPt is defined by a navigation facility or fix”).
2.5 FIX TOLERANCE OVERHEADING A STATION
2.5.1 VOR
Fix tolerance areas should be determined using a cone effect area based on a circular cone of ambiguity, generated by a
straight line passing through the facility and making an angle of 50 degrees from the vertical. However, where a State
has determined that a lesser angle is appropriate, fix tolerance areas may be adjusted using either of the formulae
contained in 6.4 of Part II, Section 4, Chapter 1, Appendix A. Entry into the cone is assumed to be achieved within
such an accuracy from the prescribed track as to keep the lateral deviation abeam the VOR:
d=0.2h(dandhinkm)
23/11/06