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back azimuth map/CRC indicator bit is set to ONE to indicate that this is a map/CRC word. The coding for data word B39 is
shown in Table G-5.
2.7.4.6 Procedure descriptor words specified in Appendix A, Table A-15 are defined for all approach and departure
procedures. Missed approach procedures are linked to approach procedures in the data format and hence do not require a
procedure descriptor. Procedure descriptor words for the sample data set are shown in Table G-6. It is noted that the
procedure descriptor data words cannot be fully defined until the completion of the actual assignment of the way-point data
due to the need for a “first way-point index” associated with each procedure. This item is the first way-point for the
procedure sequence. The index is generated as indicated in 2.7.3.6. It is noted that the “validity indicator” of a procedure
name (see Table G-4) is the version number of the procedure and is a value from 1 to 9.
2.7.4.7 The way-point data assignment process is in accordance with Appendix A, Tables A-15, 16 and 17. Table G-7
represents the assignment of the sample data set. The preambles, addresses and parity bits have been left out of the table.
Starting with the data word immediately after the approach procedure descriptor words, the first way-point of the first
procedure is assigned. For the sample data set, it means that data word B5 is the first word with way-point data. The next step
is to insert the data into the appropriate format. The procedures data always commence with the X coordinate of the initial
way-point. The structure of the database allows for individual data items to overlap between auxiliary data words. For
example, the first 14 bits of the X coordinate of WP 3 of procedure KASEL are transmitted in word B5. The final bit is
transmitted in word B6.
2.7.4.7.1 Because of the bit weight of the way-point coordinate least significant bit, the coded way-point coordinate
must be rounded. It is desirable to achieve a result as close as possible to the actual way-point coordinate value. Such
23/11/06 ATT G-10
Attachment G Annex 10 — Aeronautical Communications
rounding is normally performed by adding to the actual value half the weight of the LSB then performing integer division on
the result. For example, the X coordinate of WP 2 of procedure KASEL is 6 556 m (actual). The coded binary value should
be 2 561 since,
6 556 2.56
2 2 561
2.56
Integer
⎡⎢⎢⎛⎜⎝ + ⎞⎟⎠⎤⎥⎥=
⎢ ⎥
⎢⎣ ⎥⎦
For negative numbers the sign bit should be carried through the calculation.
2.7.4.8 After the X coordinate is the “Y coordinate follows” bit. This bit would be set to zero, and the Y coordinate
would not be transmitted as shown in Table G-7 for KASEL WP 2 and WP 1. As shown in KASEL WP 3, the Y coordinate
is needed and is transmitted after the “Y coordinate follows” bit.
2.7.4.9 Depending on the coding of the “Y coordinate follows” bit, the “Z coordinate follows” bit is coded after the Y
coordinate information. For procedure KASEL, WP 4 does not require the Z coordinate since it is prior to the PFAF. The Z
coordinate is also not required for WP 2 because there is a constant glide path between WP 3 and WP 1. As shown in KASEL
WP 3, the Z coordinate is needed and is transmitted after the “Z coordinate follows” bit.
2.7.4.10 The next segment/field identifier is assigned in accordance with Appendix A, Table A-17. For the identifier
following WP 2 in procedure KASEL, the value 5 indicates that the threshold way-point height is transmitted next, followed
by the way-point index of the missed approach procedure. For procedure NELSO, since the last two way-points are shared
with procedure KASEL the identifier following WP 3 has the value 3, indicating that the index for the next way-point is
transmitted next. In this case the index is 3, pointing to WP 2 of procedure KASEL. For the missed approach procedure the
identifier is set to 6, indicating that this is the last way-point in the procedure. For secondary runway procedure SEMOR the
identifier is also set to 6. In this case, however, it indicates that the virtual azimuth to way-point distance follows.
2.7.4.11 Table G-8 shows the assignment of the departure procedure way-points. The departure data start with word
B36, the procedure descriptor. The way-points data begin with word B37. Departure data are assigned using the same method
as for the approach data.
2.7.4.12 After the database is completely assigned, the CRC values may be calculated using B1-B39 and the other
required data items. Table G-9 shows the results of this calculation for the sample data set including the auxiliary A words,
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