Free Route Airspace

Free Route Airspace (FRA), a new concept implemented across European airspace, is designed to eliminate
the adverse effects of air traffic, reduce fuel consumption, simplify and expand flight planning. We can model
FRA system by using graph theory, networks and geometry. We work with 2-dimensional space defined by
latitude a longitude in the horizontal part of FRA. The airspace is defined by significant points. The significant
point is given by GPS coordinates, name, and other attributes, see ^[1] and ^[2]. The FRA significant points,
are divided in:
1. FRA Horizontal Entry Point - It is a published significant point located on the horizontal boundary
of FRA from which the FRA operations are allowed. It should be located exactly on the boundary of the
relevant FRA area. This type of point is displayed with a symbol \Delta (

) and black colour in Figure 1.
2. FRA Horizontal Exit Point - It is a published significant point located on the horizontal boundary of
FRA to which the FRA operations are allowed. It should be located exactly on the boundary of the relevant
FRA area. In Figure 1, this type of point is displayed with a symbol \Delta ( ) and blue colour.
3. Combined FRA Horizontal Entry and Exit Point - It is a published significant point located on the
horizontal boundary of FRA from or to which the FRA operations are allowed. It should be located exactly
on the boundary of the relevant FRA area. This type of point is displayed with a symbol \Delta ( ) and
brown colour in Figure 1.
4. FRA Intermediate Points (I) - It is a published or unpublished point defined by geographical coordinates
or by bearing and distance via which FRA operations are allowed. The FRA concept includes other type
of significant points as well, but these are not necessary for us now. This type of point is displayed with a
symbol \ circ (º) in Figure 1. We can also see the different colours. The intermediate points can be used
for overflights and/or for arrivals to the airports and/or departures. We needed this detailed division for our
calculations, but this is not a part of this description.
We sought to answer the question of what mathematical object can be used to describe the FRA concept.
We found such an object, this object was the simple polygon with vertices Horizontal Entry or Combined
Horizontal Entry and Exit points.

Spatially Bounded Airspace Axiom

Figure 2: SEE FRA airspace polygon

We can see the graph with SEE FRA significant points in Figure 1 . All these points have the coordinates
that are expressed by longitude and latitude in a geodetic coordinate system. We identified the axiom that
determines an airspace. Each airspace is given by a simple polygon of significant entry or (entry and exit)
points.

We can see a polygon for SEE FRA airspace in Figure 2, for details see ^[4]. This polygon defines the
horizontal boundary of airspace. The characteristics of this polygon are that it does not contain holes and
loops. We were able to formulate and prove all these statements after studying an airspace’s FRA concept
and mathematical properties. We have also shown that for any set of significant points of airspace, it is
possible to construct a simple airspace polygon.