<strong>Temporal Reference Systems</strong>
Temporal reference systems are used for items of aeronautical information that are time-related. In this context, time is used to mean both a point during a year and a point during the day, i.e., to specifically identify a unique point at which an occurrence takes place. A temporal reference system comprises a calendar and time system.
ICAO Annex 15 [Reference 4], section 126.96.36.199, requires that “For international civil aviation, the Gregorian calendar and Coordinated Universal Time (UTC) shall be used as the temporal reference system”. Consequently, it is the recommendation of this Manual that all temporal aspects for terrain and obstacle data are published using the Gregorian calendar and Coordinated Universal Time (UTC).</ul>
<strong>2. The Gregorian Calendar</strong>
As addressed above, ICAO mandates that the Gregorian calendar is used for international civil aviation.
The Gregorian calendar is the most commonly used calendar in the world and is the standard used for most transactions which occur internationally, including trade. It was first introduced on 15th October 1582.
The Gregorian calendar is specified within ISO 8601:2004 [Reference 11]. The reader is referred to this standard for more information relating to the use of the Gregorian calendar.
The UTC reference system was established by the International Bureau of Weights and Measures and the International Earth Rotation Service. It provides a basis for standard time, the use of which is legally required in most countries.
UTC provides a means of referring to a single time reference globally, i.e. it provides a time reference which is not affected by time zones and, hence, reference to a specific time using UTC will indicate a single point in time that is the same throughout the world. UTC replaced Greenwich Mean Time (GMT) as the international time reference in 1972. It should be noted that UTC and GMT are often used interchangeably, however, this is incorrect and this practice should be avoided.
The use of UTC is specified within ISO 8601:2004 [Reference 11]. The reader is, once again, referred to this standard for more information.
It should be noted that ISO 8601:2004 (section 4.2.4) requires that whenever a time is reported in UTC, it is followed immediately by a Z. For example, midday UTC would be recorded as “1200Z”.
<strong>4. Local Reference Systems</strong>
Despite ICAO Annex 15 [Reference 4] specifying the use of the Gregorian calendar and UTC, ICAO does recognise the possible need to use local systems. In particular this possibility is documented in two places within ICAO Annex 15:
<li>Section 188.8.131.52 states “When a different temporal reference system is used for some applications, the feature catalogue, or the metadata associated with an application schema or a dataset, as appropriate, shall include either a description of that system or a citation for a document that describes that temporal reference system.”</li>
<li>Appendix 1, Gen 2.1.2 states “Description of the temporal reference system (calendar and time system) employed, together with an indication of whether or not daylight saving hours are employed and how the temporal reference system is presented throughout the AIP.”</li></ul>
The use of such systems is not, however, recommended and should be avoided wherever possible.
Where an alternative system is used, it is imperative that sufficient information is provided to allow the user to transform the date and/or time from the local reference system into the global reference systems required by ICAO.
<strong>5. Time/Temporality in the Context of Terrain and Obstacles</strong>
The need to report temporal aspects for terrain and obstacles is limited in scope
and falls broadly into the following categories:
a) Start of Effectivity
The point in time at which the reported aeronautical information shall be considered as correct and in use. It is important to note the use of the words “considered as being correct”, rather than that the aeronautical information is actually correct, in reality. Why is this?
Let us take the example of a new obstacle. Typically, in a well managed environment, the intention to erect a new obstacle whose location and size may impact aviation will have been reported. This will most likely result from a request for permission to build or modify a structure. Once this permission is granted, the planned location and the size of the structure may then be reported to the necessary authorities, including aviation.
Depending upon the management processes in place, the action of reporting that permission has been granted may occur at different times. In some cases, it may be reported even though construction may not have started and, indeed, may never start. Other processes may exist, such that the report is only made once construction commences. Either way, the reported obstacle will not fully reflect the actual status on the ground, until the structure has been completed and precisely surveyed.
The start of effectivity will be published and used to indicate the point in time at which the obstacle should be considered to exist, from an operational perspective, whether it does so or not.
b) End of Effectivity
The End of Effectivity records the last point in time when the aeronautical information shall be considered and is in use, from an operational perspective. Once again, in relation to obstacles, the actual structure may have been removed before this point in time and, therefore, as with the Start of Effectivity, the effective aeronautical information may not fully reflect reality, but is considered to be operationally correct.
Some attributes of aeronautical information may only apply during certain periods. For example, an obstacle may be recorded as having lighting, but this lighting may only be in use during certain periods of time.
As may be seen, these items are, in the main, related to obstacles. Terrain is typically reported in its “as is” state, i.e. effective from the point of publication. The cases where a change to terrain is planned in advance and reported as such will be very limited.
The one exception to this statement may be where terrain is known to move on a regular basis, such as in desert areas where sand dunes may form and disappear on a regular basis. In such cases, it is foreseen that the terrain will be reported as a “highest” value, providing a fail-safe system whereby it is highly unlikely, although not impossible, that the terrain will increase above the published value, despite a shift in the conditions.
Source: Eurocontrol Terrain and Obstacle Data Manual
<a href="http://flightcrewguide.com/wiki/navigation/terrain-obstacle/horizontal-reference-systems/" title="Horizontal Reference Systems">Horizontal Reference Systems</a>
<a href="http://flightcrewguide.com/wiki/navigation/terrain-obstacle/vertical-reference-systems/" title="Vertical Reference Systems">Vertical Reference Systems</a>
<a href="http://flightcrewguide.com/wiki/navigation/terrain-obstacle/temporal-reference-systems/" title="Temporal Reference Systems">Temporal Reference Systems</a>