1.6.1 Most of the conventions to be used in the expression of ATS data in the messages are prescribed in the field tables on pages A3-5 to A3-29, but the conventions for the expression of level, position and route data are given below to simplify the field tables.
1.6.2 The expression of level data
1.6.2.1 Four alternative conventions are available for the expression of level data:
a)  “F” followed by 3 decimal numerics: indicates a Flight Level Number, i.e. Flight Level 330 is expressed as “F330”;
b)  “S” followed by 4 decimal numerics: indicates Standard Metric Level in tens of metres, i.e. Standard Metric Level 11 300 metres (Flight Level 370) is expressed as “S1130”;
c)  “A” followed by 3 decimal numerics: indicates altitude in hundreds of feet, i.e. an altitude of 4 500 feet is expressed as “A045”;
d)  “M” followed by 4 decimal numerics: indicates altitude in tens of metres, i.e. an altitude of 8 400 metres is expressed as “M0840”.

1.6.3 The expression of position or route
1.6.3.1 The following alternative data conventions shall be used for the expression of position or route:
a)  from 2 to 7 characters, being the coded designator assigned to an ATS route to be flown;
b)  from 2 to 5 characters, being the coded designator assigned to an en-route point;
c)  4 numerics describing latitude in degrees and tens and units of minutes, followed by “N” (meaning “North”) or “S” (South), followed by 5 numerics describing longitude in degrees and tens and units of minutes, followed by “E” (East) or “W” (West). The correct number of numerics is to be made up, where neces-sary, by the insertion of zeros, e.g. “4620N07805W”;
d)  2 numerics describing latitude in degrees, followed by “N” (North) or “S” (South), followed by 3 numerics describing longitude in degrees, followed by “E” (East) or “W” (West). Again, the correct number of numerics is to be made up, where necessary, by the insertion of zeros, e.g. “46N078W”;
e)  2 or 3 characters being the coded identification of a navigation aid (normally a VOR), followed by 3 decimal numerics giving the bearing from the point in degrees magnetic followed by 3 decimal numerics giving the distance from the point in nautical miles. The correct number of numerics is to be made up, where necessary, by the insertion of zeros, e.g. a point at 180° magnetic at a distance of 40 nautical miles from VOR “FOJ” would be expressed as “FOJ180040”.
1.7 The detail of the fields

1.7.1 The elements of data prescribed or permitted to be included in each type of field, together with a prescription of the conditions or options permitted, are shown on pages A3-5 to A3-29.
1.7.2 A key appears at the right-hand side of each of the field pages; this key permits the sequence of fields in each type of message to be followed.
1.7.3 The first field in each message type is field type 3; on the page describing field type 3 a key indicates the field type number of the next field for each message. On subsequent field pages, the field type number of the previous field is shown to permit back reference also. The Start-of-ATS-Data Signal ‘(’ is used in the key to indicate that there is no previous type of field; the End-of-ATS-Data Signal ‘)’ is used to indicate that there is no next type of field.
1.7.4 On the field pages,
elements with a fixed number of characters
are shown diagrammatically as
(three characters in this example)

elements of variable length are shown as
1.8 Accuracy in the preparation of ATS messages

1.8.1 Where the standard ATS messages are transmitted by teletypewriter channels in areas where ATC computers are known to be in use, the formats and data conventions prescribed in the field tables on pages A3-5 to A3-29 shall be adhered to rigorously.
　
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