1. A method of selecting radionavigation beacons implemented in an onboard navigation system aboard an aircraft from a list of eligible beacons at an instant t 2 , a position A(t 1 ) taken by the aircraft in a reference frame tied to the Earth at an instant t 1 prior to t 2 being known, a projection of the position of the aircraft onto the 2D terrestrial globe according to the vertical to the aircraft being dubbed the 2D terrestrial position of the aircraft, the eligible beacons being arranged on the terrestrial surface at known positions which are stored in a database with which the aircraft is equipped, a domain of employment of an eligible beacon B i defining a set of 2D terrestrial positions of the aircraft corresponding to positions of the aircraft for which a distance measurement separating the aircraft and the beacon B i is relevant, an employment zone Z X,Y (θ, t 1 ) of a pair of eligible beacons being defined by an intersection between the employment domains of the beacons B X and B Y and a geometric locus that groups together 2D terrestrial positions of the aircraft for which an angle formed by axes connecting the 2D terrestrial position of the aircraft and the positions of the beacons B X and B Y is greater than a predefined value θ, the employment zone Z X,Y (θ, t 1 ) being delimited by a boundary, said method comprising: formulating a criterion for selecting a pair of beacons (B X , B Y ) from among the beacons forming part of the list of the eligible beacons, wherein the selection criterion is based on a search for a maximum duration of membership, for instants subsequent to the instant t 1 , for which the 2D terrestrial position of the aircraft belongs to the zones of employment of all the pairs of beacons achievable from among the eligible beacons, on the basis of a given predefined trajectory of the aircraft.