3.6. Future aeronautical communication means
3.6.2. AANET principles and properties
Figure 3.6 shows a typical use of this innovative system. As the cumulated traffic load is expected to be heavier on the inter-aircraft links, which are closer to the ground station, we concentrate our presentation on the air to ground path which is more constraining.
Figure 3.6. Typical use of aeronautical ad hoc network for air–ground communications
Regarding the feasibility of air–ground AANET-based communications, two different zones have to be considered: continental and oceanic airspaces.
These two zones exhibit different types of air traffic. In the France, as an example of continental airspace, aircrafts are numerous and fly in “all”
directions. In the oceanic airspace, for instance, in the North Atlantic Corridor, aircrafts are compelled to follow predefined tracks called North Atlantic Tracks (NATs) defined by the air control authorities on a daily basis according to weather conditions. The global study presented in the present chapter is based on two seperate datasets. The first dataset comes from the French Civil Aviation authority and consists of the trajectories of aircraft in the French sky with radar positions every 15 s. The second dataset is provided by EUROCONTROL (see OneSky website) and consists both of the known radar positions where available (with a resolution of around
10 min) and reported positions transmitted by aircraft to ATC where radar coverage is not provided (oceanic zones).
Considering the dynamicity of the air traffic, data with high time resolution data are needed in order to assess possible links between aircraft.
So, we interpolate the EUROCONTROL data in order to get positions of aircraft every 15 s. Positions are interpolated between the two closest known positions using great circle arcs of the earth (great circle arcs represent the shortest distance between two points on the surface of a sphere, called geodesic distances).
An AANET is a self-configuring, self-healing network and is based on a light ground infrastructure. The main advantage is that even if some aircrafts are outside the coverage area of the ground stations, they are nevertheless able to communicate with them using other aircrafts as relays. This type of air–ground communication can be seen as a multihop air–ground system.
Obviously, the routing protocol is quite important in ad hoc networks, especially in AANETs where we may have a highly dynamic topology because of the high speed of aircraft. As an example, in [SAK 06a] and [SAK 06b], a routing protocol is proposed for AANETs. It takes into account the relative aircraft velocity to create stable clusters. The main goal of this approach is to maximize links duration.
In order to assess the ad hoc network connectivity, a homemade tool named AeRAN (for AANETs) has been developed at ENAC labs. The software uses the obtained aircraft positions as input data for continental and oceanic airspaces as previously described as well as a file with the positions of the ground stations and the assumed communication range. The results give statistics such as the network connectivity, the ratio of aircraft connected directly or via other aircrafts to a ground station, etc. Furthermore, AeRAN allows observing how the topology and the connections dynamically evolve during one chosen day. The network connectivity has been assessed in both airspaces for several communication ranges between aircraft. In the continental airspace case, five ground stations have been positioned near the five en-route control center for the French sky. In the oceanic airspace case, the ground stations positions have been defined on islands and coasts along the tracks in order to ensure an optimal connectivity. Figures 3.7 and 3.8 show the results obtained for one day in June 2011.
Figure 3.7. Network connectivity and communications range influence (continental airspace)
Figure 3.8. Network connectivity and communications range influence (oceanic airspace)
These results allow underlining the influence of the communications range on the AANET connectivity (solid lines and left y axis) in the considered continental airspace and oceanic airspace. As expected, the connected aircraft ratio increases with the communication range up to a
point. After this point, increasing communication range does not have a significant impact on connectivity. We have intentionally also included the number of instantaneous flying aircrafts (dotted lines and right y axis), and, as expected, the network connectivity is correlated with this parameter. In the French sky, in the established continental airspace, a communication range of 150 km allows an average connectivity of 90% of aircrafts during the day, with 99% of aircrafts connecting between 6:00 and 21:00. In the oceanic airspace, this communication range should be 350 km in order to ensure an average connectivity greater than 90% in the day. These latter results are explained by the fact that the aircraft density is lower in the oceanic airspace.
The number of hops between the aircraft and ground station presents another interesting result. We assume that the path from each aircraft is defined based on the shortest path to the closest ground station, given by the Dijsktra algorithm. Of course, the number of hops to reach a ground station depends on the considered communication range. For the communication ranges previously discussed (150 and 350 km respectively), the results are given in Table 3.2. They show that even if a lot of aircrafts are directly connected to a ground station, multihop links allow connecting the major part of aircrafts that are not covered.
1 hop 2 hops 3 hops > 3 hops Continental
(comm. Range: 150 km) 29.9 42.1 23.2 4.8 Oceanic
(comm. Range: 350 km) 41.7 27.2 5.8 25.2 Table 3.2. Network connected aircraft mean ratio (%) per distance to ground station
Hence, the different results obtained regarding the network connectivity allow us to foresee the benefits of this innovative solution particularly considering its expected low cost relatively to other current solutions.