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back azimuth equipment. However, way-points belonging to approaches, missed approaches or departures could be
transmitted in either the azimuth or the back azimuth coverage. For example, a departure may be initiated in approach
azimuth coverage, therefore that data would be transmitted by approach azimuth. If the procedure begins in a common
coverage region the associated data can be transmitted in only one region, except where otherwise dictated by operational
requirements.
2.7.3.3 The procedures are defined by a series of way-points. The way-points are specified in a cartesian coordinate
system with X, Y and Z coordinates whose origin is at the MLS datum point. The coordinate system is illustrated in
Figure G-13.
2.7.3.4 The segments between way-points are either straight or curved. Curved segments are defined as the arc joining
two way-points, as illustrated in Figure G-14. The arc of the circle is always tangent to the preceding and following segments,
straight or circular. Final approach segments and segments pointing to the initial way-point of an approach procedure or
extending from the last flown way-point of a departure or missed approach procedure are always straight. They are
extensions to straight segments or tangents to circular segments. These straight segments would not necessarily require a
way-point at the edge of the coverage, thus way-points could be saved.
2.7.3.5 For any procedure type the coding starts with the way-point farthest from the threshold and ends with the waypoint
nearest to the runway. All way-points for approach procedures must be coded before any missed approach way-points
or departure way-points. This rule simplifies the decoding by segregating the way-points belonging to the approaches from
the others. Several procedures can share one or more way-points. When this is the case it is feasible to transmit this
information only once. The shared way-points must be the final ones for approach procedures and the initial ones for missed
approach and departure procedures. Approaches, missed approaches and departures can share data provided the data are
transmitted in the same coverage sector. When way-points are shared with a procedure previously defined in the database this
is indicated by a way-point index following a way-point. The way-point index gives the location in the database of the first
shared way-point.
23/11/06 ATT G-8
Attachment G Annex 10 — Aeronautical Communications
2.7.3.6 The way-point index is the value representing the sequential order in which the way-points are listed in the
database. It is used in the coding to indicate where the way-points for a procedure are located. A way-point index of zero in
the procedure descriptor indicates that this is a computed centre line application where no way-points are provided.
2.7.3.7 Although way-points are defined by X, Y and Z coordinates, in a variety of cases not all coordinates have to
be transmitted. Way-points located on the primary runway centre line have a Y coordinate equal to zero. The corresponding
field defining this value can be omitted by setting the “Y coordinate follows” bit to ZERO.
2.7.3.8 Whenever the Z coordinate is not necessary for path construction, data can be saved by not transmitting this
value. This is indicated by setting the “Z coordinate follows” bit to ZERO. This may apply to initial way-points preceding the
final approach fix where guidance is based on altimetry and not on a computed MLS vertical position. It may also apply to
way-points located on a constant gradient between way-points for which the Z value is defined. In this case, the airborne
equipment would compute the Z coordinate assuming a constant gradient. Missed approach and departure way-points located
in back azimuth coverage are also candidates for deleting the Z coordinate, since vertical guidance is not available. For the
back azimuth application, the Z coordinate may be transmitted for use by the airborne equipment to resolve the horizontal
position of the aircraft. This allows for a reduction of the lateral errors introduced in the conversion from the slant range and
conical back azimuth angle to X-Y coordinates.
2.7.3.9 The 3-bit field following the way-point coordinates contains the next segment/field identifier. This data item
indicates whether the next segment of the procedure is straight or curved, whether the current way-point is the last one
defined for the procedure, and whether to link the procedure to a missed approach or a shared portion of another procedure
identified by a missed approach index or next way-point index. It also indicates whether a data field for threshold crossing
height or virtual azimuth to way-point distance is appended to the way-point definition.
2.7.3.9.1 Some typical applications of the identifiers in Appendix A, Table A-17 are listed below. This list is not all
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