Method for dual harmonisation of a portable head-up display system for bringing into conformity the display of control information of an aircraft with the outside real world, the portable head-up display system comprising: - a portable transparent head-up display D0, - a sub-system for detecting head posture DDP having a first movable tracking element D1 which is fixedly attached to the transparent display unit DO, a second fixed element D2 which is fixedly connected to the platform of the aircraft. and a means for measuring and determining the relative orientation M12 of the first movable tracking element D1 relative to a reference mark of the second fixed element D2 connected to the platform, - an inertial attitude device D3 for providing relative attitude M3t of the platform relative to a ground reference system which is connected to the Earth, - a harmonisation sub-system of the portable head-up display system to bring into conformity the display of control information on the display unit D0 with the outside real world, the harmonisation sub-system having a dual harmonisation processor and a man/system interface to control and perform the implementation of the dual harmonisation method, the dual harmonisation method being characterised in that it comprises the steps of: - carrying out (206; 306) a sequence of a whole number N greater than or equal to 3 of different targets Vi, i varying from 1 to N carried out via the display unit D0 by aligning a centred visual target pattern on any same fixed target of the outside real world, each target Vi corresponding to a different fixed position Pi of the centre of the target pattern on the display D0 and having a target vector which is determined as a function of the position Pi, and, for each target Vi, acquiring (206; 306) the corresponding measurement K̂ i of the relative angular orientation of the tracking element relative to a reference direction DDP which is fixed relative to the platform of the aircraft, then - calculating (206; 306) the relative orientation matrix M01 between the display unit D0 in the tilted position and the first tracking element D1 as the right matrix D solution of the equation system: G ^. K ^ i D ^. x → i = y → 0, for i = 1 to N, the vector y 0 designating the vector in the inertial reference system of the platform corresponding to the intended target point in the outside real world and being unknown; and the left matrix G being the relative orientation matrix M23 between the reference system of the second fixed element D2, connected to the platform of the aircraft and the reference system of the inertial attitude device D3, which is potentially incorrect but assumed to be constant as a function of time, and which, when it is unknown, requires at least four measurements K̂ i. - a portable transparent head-up display D0, - a sub-system for detecting head posture DDP having a first movable tracking element D1 which is fixedly attached to the transparent display unit DO, a second fixed element D2 which is fixedly connected to the platform of the aircraft. and a means for measuring and determining the relative orientation M12 of the first movable tracking element D1 relative to a reference mark of the second fixed element D2 connected to the platform, - an inertial attitude device D3 for providing relative attitude M3t of the platform relative to a ground reference system which is connected to the Earth, - a harmonisation sub-system of the portable head-up display system to bring into conformity the display of control information on the display unit D0 with the outside real world, the harmonisation sub-system having a dual harmonisation processor and a man/system interface to control and perform the implementation of the dual harmonisation method, - carrying out (206; 306) a sequence of a whole number N greater than or equal to 3 of different targets Vi, i varying from 1 to N carried out via the display unit D0 by aligning a centred visual target pattern on any same fixed target of the outside real world, each target Vi corresponding to a different fixed position Pi of the centre of the target pattern on the display D0 and having a target vector which is determined as a function of the position Pi, and, for each target Vi, acquiring (206; 306) the corresponding measurement K̂ i of the relative angular orientation of the tracking element relative to a reference direction DDP which is fixed relative to the platform of the aircraft, then - calculating (206; 306) the relative orientation matrix M01 between the display unit D0 in the tilted position and the first tracking element D1 as the right matrix D solution of the equation system: G ^. K ^ i D ^. x → i = y → 0, for i = 1 to N, the vector y 0 designating the vector in the inertial reference system of the platform corresponding to the intended target point in the outside real world and being unknown; and the left matrix G being the relative orientation matrix M23 between the reference system of the second fixed element D2, connected to the platform of the aircraft and the reference system of the inertial attitude device D3, which is potentially incorrect but assumed to be constant as a function of time, and which, when it is unknown, requires at least four measurements K̂ i.