Unmanned air vehicles, i.e. vehicles that do not have a physical pilot on board, can be of great interest not just for military missions but also for civil missions. Furthermore, the emergence of new data and image uptake and interpretation systems makes the number of tasks that can be carried out by this type of vehicles increase more and more in both the civil and the military or police scope.
A fundamental difference between unmanned air vehicles and manned air vehicles is that in the case of the former, in addition to the aerial system, a ground system and data link means necessary for operating the vehicle are also required. Nevertheless, there may be moments or periods which are shorter or longer in time (in some cases, corresponding to most of the duration of the flight) during which the vehicle must function autonomously. Given that during the flight or “mission” in an unmanned air vehicle it is quite probable that unexpected events or conditions (for example changes in weather conditions, wind, turbulences, mechanical problems, etc.) may occur, autonomous control of the flight becomes a complex task.
There are a large number of unmanned vehicle control systems. They are usually based on different modules responsible for different parts of the control. For example, the following general modules schematically shown in FIG. 1 may exist:                sensors 101 acquiring and transmitting data related, for example, to the state of the actuators 104, the state of the aircraft (for example, its position, altitude or orientation) and the meteorological conditions (mainly wind intensity and direction);        actuators 104 providing the position of the mechanical control elements which, in the case of an aircraft, provide the forces necessary for controlling the flight;        an estimation and navigation module 102 responsible for obtaining the state variables needed for controlling the system from the values provided by the measurement variables of the sensors;        a guidance and control module 103 providing the actuators 104 with the control variables needed for stabilizing and taking the system state variables to the desired reference values in each case; and        a mission management module 105 which, based on the available data regarding the information on the state of the aircraft it receives from the estimation and navigation module 102 and when the vehicle is flying under the control of an external operator 106, such as a ground control station, based on the instructions it receives from said external operator, provides the guidance and control module with the desired reference variables so as to fulfill certain objectives; this module normally includes means for storing data indicating a mission route comprising a plurality of mission route segments (defined, for example, by “waypoints” corresponding to the mission route).        
There are a large number of publications reflecting different aspects of unmanned air vehicle control.
For example, patent document U.S. Pat. No. 6,122,572 describes an unmanned air vehicle control system designed for the execution of a mission and having a programmable decision unit capable of managing and controlling the execution of the mission taking into account all available systems and data in the vehicle.
Patent document U.S. Pat. No. 6,349,258 relates to a method for generating, from two waypoints, a course which must necessarily pass between these two points.
Programmed unmanned air vehicles are known to fly according to a “mission route” (which can be preprogrammed) and with the capacity to calculate alternative routes in the case of incidents. For example, patent document U.S. Pat. No. 6,377,875s describes an unmanned air vehicle control system in which a safe flight route is programmed. The vehicle can be controlled remotely via radio; if communication with the control station is lost, the on-board system recalculates the route without the intervention of the control station.
However, the recalculation of the route on board the vehicle requires that the vehicle has an on-board system with sufficient capacity to recalculate the route. This may involve, for example:                the need to have fairly detailed data on the terrain (a digital terrain model);        a complex computer system with the capacity to completely recalculate the route;        a certain risk of “unpredictability” of the route which is finally chosen by the vehicle (which may involve risks and problems for aviation and/or air control in the area, for high buildings in the area, etc.);        uncertainty regarding where recovery of the vehicle will occur;        uncertainty regarding the needs of the vehicle with respect to fuel (given that in the moment of vehicle take-off, its course in the event that it has to divert from its mission route cannot be foreseen).        
An objective of the invention is to provide an alternative system for implementing alternative or auxiliary routes apart from the mission route which may involve improvements in some or in all of the aforementioned aspects.