A navigation assistance system (1; 201) to be used on board an aircraft, comprising: - an inertial navigation module (3; 203), adapted for providing first aircraft positional information resulting at least partially from accelerometric and gyrometric measurements, - a satellite positioning module (5; 205), comprising at least one satellite navigation data receiver, said positioning module (5; 205) being adapted for determining second aircraft positional information from satellite positioning data received by said satellite navigation data receiver, said system comprising estimation means (3,9; 203, 209) adapted for estimating a first localization (L 1) of the aircraft at a given time of estimation (t e) on the basis of: - first aircraft positional information, resulting at least partially from accelerometric and gyrometric measurements at said time of estimation (t e), and - second aircraft positional information, determined from satellite positioning data received by said satellite navigation data receiver prior to said time of estimation (t e), said first localization being determined independently from any satellite positioning data received in a predetermined time interval preceding said time of estimation (t e) and including said time of estimation (t e), said predetermined time interval being equal to a time to alert, said satellite positioning module (5; 205) comprising means for validating or invalidating said satellite positioning data, depending on the precision of said satellite positioning data and/or depending on the operating mode of said satellite positioning module (5; 205), the system being characterized in that the second aircraft positional information includes an estimate of a position (p GPS) of the aircraft at least at a given time of determination (t e) and an estimate of a precision (VPL, HPL) of said position (p GPS), determined from satellite positioning data, and in that said satellite positioning module (5; 205) is adapted for comparing the estimate of the precision (VPL, HPL) of said position (p GPS) to a required minimum precision (VAL, HAL) at said time of determination (t e), and validating said satellite positioning data, at a validation time (t e +TTA) after said time of determination (t e), if the estimate of the precision (VPL, HPL) of said position (p GPS) is in accordance with said required minimum precision (VAL, HAL), the time interval between said time of determination (t e) and said validation time (t e +TTA) being less or equal to said time to alert. - an inertial navigation module (3; 203), adapted for providing first aircraft positional information resulting at least partially from accelerometric and gyrometric measurements, - a satellite positioning module (5; 205), comprising at least one satellite navigation data receiver, said positioning module (5; 205) being adapted for determining second aircraft positional information from satellite positioning data received by said satellite navigation data receiver, said system comprising estimation means (3,9; 203, 209) adapted for estimating a first localization (L 1) of the aircraft at a given time of estimation (t e) on the basis of: - first aircraft positional information, resulting at least partially from accelerometric and gyrometric measurements at said time of estimation (t e), and - second aircraft positional information, determined from satellite positioning data received by said satellite navigation data receiver prior to said time of estimation (t e), said first localization being determined independently from any satellite positioning data received in a predetermined time interval preceding said time of estimation (t e) and including said time of estimation (t e), said predetermined time interval being equal to a time to alert, said satellite positioning module (5; 205) comprising means for validating or invalidating said satellite positioning data, depending on the precision of said satellite positioning data and/or depending on the operating mode of said satellite positioning module (5; 205), the system being characterized in that the second aircraft positional information includes an estimate of a position (p GPS) of the aircraft at least at a given time of determination (t e) and an estimate of a precision (VPL, HPL) of said position (p GPS), determined from satellite positioning data, and in that said satellite positioning module (5; 205) is adapted for comparing the estimate of the precision (VPL, HPL) of said position (p GPS) to a required minimum precision (VAL, HAL) at said time of determination (t e), and validating said satellite positioning data, at a validation time (t e +TTA) after said time of determination (t e), if the estimate of the precision (VPL, HPL) of said position (p GPS) is in accordance with said required minimum precision (VAL, HAL), the time interval between said time of determination (t e) and said validation time (t e +TTA) being less or equal to said time to alert.