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For the delineation of any reserved or restricted airspace volumes (TRA, TSA, CBA, D, R and P areas), the State 'Due Regard' obligation should be strategically observed so that activity in that airspace structure will not endanger non-participating aircraft operating at or near its published limits.

The establishment of a reserved or restricted airspace published boundary should always be complemented by spacing criteria. These can be part of the national air law and/or take the form of LoAs between units involved. Such rules should be as flexible as possible taking into account the efficient airspace design and operation ensuring no waste of airspace.

In defining these spacing criteria, States should ensure that safety is assured in all circumstances through:

 the definition, if so required, of specific spacing minima depending on the activities conducted in reserved airspace, with the addition of an adequate spacing volume;  the application of appropriate LoAs between civil and military units involved;

 the promulgation of the first usable IFR flight levels above/below an area in the definition of associated ATS routes.

Baseline Spacing principles

Aircraft operators must have the opportunity to submit a flight plan that does not infringe the volume of airspace associated with an active segregated/reserved airspace.

The objective is to ensure that this volume of airspace is published, including the totality of airspace to be avoided by non-participating aircraft, unless authorised by the appropriate ATS authority.

The methodologies employed to achieve the necessary spacing should be compatible with both the predetermined ATS route network and free route airspace (if applied) environments.

Delineation of the boundary of the reserved/restricted airspace

According to ICAO Annex 15 all positional data (lat/long) has to be with reference to WGS 84.

There is a need for a common reference to be applied in computation of trajectories, boundaries with regard to the earth model (great circle on a sphere, geodesic, ellipsoid). The horizontal border of a segregated airspace is specified as a sequence of segments, which can be "straight lines", "along the parallel", "arc of circle", "follow the State boundary". All digital encoding formats (ARINC 4.2.4, AIXM, etc.) are facing common problems with regard to the encoding of this data. The most common ones are mentioned here:

 In ARINC 424 straight lines are encoded as “Great Circle” with the Earth being assumed as a sphere. In modern geographical information systems, the most accurate representation of “straight lines” is a geodesic curve on the WGS 84 Ellipsoid. The difference between the two encodings can be significant when calculating the intersection between the intended aircraft trajectory and a segregated area located some 50 NM away, for example.

 “along the parallel”, or along a constant latitude, is different from a straight line. For spatial calculations they are interpolated with a certain density - replaced with a number of "straight" segments. Common rules have to be agreed for such interpolations in order for all systems to get the same calculation results.

 "arcs and circles" also require interpolation and specific projections to be used for spatial calculations. In particular “arc by centre point” is a problematic construct because it is typically over-specified and the different values (centre, radius, start/end points) need to match perfectly, which is really the case.

 “follow the State boundary” is the most problematic construct, because State boundaries are not published in the national AIP. End users use different sources of State boundaries, with different interpolations. The EAD offers a “default set” of national boundary data, but this was not yet agreed by all European States. Best solution would be to avoid using references to State boundaries and other geographical features (rivers, coastlines, etc.) in the definition of segregated areas because they are very imprecise.

 Following the aforesaid the horizontal border of reserved/restricted airspace should be described by the use of geodesic curves or lines of constant latitude.

Spacing methodology

The agreed objective requires that the necessary spacing between the participating traffic/activity inside a TRA/TSA/CBA/D/R/P be contained within the overall definition of a such reserved/restricted airspace. The extent of this spacing or buffer will be determined by the relevant authority within the State, according to the nature of the activity taking place within the airspace. This may be further influenced by whether or not the reserved/restricted airspace is permeable to GAT (e.g. coordinated tactical crossing) and has a separation service provided by the operator of that segregated airspace.

It follows therefore that non-participating aircraft (whether avoiding the segregated airspace or transiting under agreed procedure) need only know the boundary of the segregated airspace; participating aircraft (e.g. reserved/restricted airspace activity aircraft) may need to know additional details through their relevant publications.

The following methodology should be followed to determine the extent of segregated airspace:

a) Define activity

c) Choose denomination based on need for segregation: TSA and/or TRA, and D, R or P as appropriate

d) Define regulatory description

e) Add spacing volumes (lateral, vertical, time) when needed (TSA)

f) When no spacing volume needed, define separation rules and procedures

g) When airspace is too limited to integrate a designed airspace volume in the existing environment, define procedural mitigation

h) Perform supporting safety case

When establishing a CBAs it is essential to avoid different spacing elements either side of a boundary not to increase the complexity of flight planning or waste airspace. The impact of this disparity would be removed if the resultant CBA boundary was nevertheless consistent. This is why the spacing volume applied to a CBA must be harmonised across both sides of the boundary where this would otherwise lead to a non- consistent boundary.

The delineation of reserved/restricted in the upper airspace needs to be harmonised in relation to the navigation tolerance requirements, similarly for the lower airspace, but there may be differences between upper and lower airspace navigation tolerance requirements as some states have different design criteria in controlled compared with uncontrolled airspace (e.g. Class C versus Class G).

7.2.7 Guidelines for Establishment of Conditional Routes (CDR)