A method for determining a protection radius of a vision-based navigation system, said system comprising: - at least one image sensor able to produce at least a first image at a first time and a second image at a second time; and - a processing unit coupled to each image sensor and able to implement a processing method (50, 51, 52) able to determine at least one motion value between the first time and the second time on the basis of the first image and of the second image and of a plurality of image data determined on the at least first and second images, the image data comprising coordinates of a plurality of points of interest on at least the first image and the second image, said method for determining a protection radius comprising the following steps: - a first step of determining at least one nominal uncertainty value of the at least one motion value obtained by the processing method for a set comprising the plurality of determined image data; characterised in that the method comprises the following steps: - a second step of determining i-th uncertainty sub-values of the at least one motion value determined by the processing method for an i-th subset (Si) defined by removing an i-th part of the determined image data; - a third step of determining an i-th protection radius (Di) for the i-th subset (Si) from the i-th uncertainty sub-values and the at least one nominal uncertainty value; the second and third steps being repeated for each i-th subset (Si), i varying between 1 and a defined value M; - a fourth step of determining a protection radius from the M determined i-th protection radii (Di). - at least one image sensor able to produce at least a first image at a first time and a second image at a second time; and - a processing unit coupled to each image sensor and able to implement a processing method (50, 51, 52) able to determine at least one motion value between the first time and the second time on the basis of the first image and of the second image and of a plurality of image data determined on the at least first and second images, the image data comprising coordinates of a plurality of points of interest on at least the first image and the second image, said method for determining a protection radius comprising the following steps: - a first step of determining at least one nominal uncertainty value of the at least one motion value obtained by the processing method for a set comprising the plurality of determined image data; - a first step of determining at least one nominal uncertainty value of the at least one motion value obtained by the processing method for a set comprising the plurality of determined image data; characterised in that the method comprises the following steps: - a second step of determining i-th uncertainty sub-values of the at least one motion value determined by the processing method for an i-th subset (Si) defined by removing an i-th part of the determined image data; - a third step of determining an i-th protection radius (Di) for the i-th subset (Si) from the i-th uncertainty sub-values and the at least one nominal uncertainty value; - a second step of determining i-th uncertainty sub-values of the at least one motion value determined by the processing method for an i-th subset (Si) defined by removing an i-th part of the determined image data; - a third step of determining an i-th protection radius (Di) for the i-th subset (Si) from the i-th uncertainty sub-values and the at least one nominal uncertainty value; the second and third steps being repeated for each i-th subset (Si), i varying between 1 and a defined value M; - a fourth step of determining a protection radius from the M determined i-th protection radii (Di). - a fourth step of determining a protection radius from the M determined i-th protection radii (Di).