A method for determining a protection radius of a vision-based navigation system includes at least one image sensor able to produce at least one first image at a first instant and one second image at a second instant; and a processing unit coupled to each image sensor and able to implement a processing method able to determine at least one value of motion between the first instant and the second instant on the basis of the first image, of the second image and of a plurality of image data determined on the at least first and second images; the method comprising the following steps: a first step of determining at least one nominal value of uncertainty of the at least one motion value obtained by the processing method for a set comprising the plurality of image data determined; a second step of determining i-th sub-values of uncertainty of the at least one motion value determined by the processing method for an i-th subset defined by removing an i-th part of the image data determined; a third step of determining an i-th protection radius for the i-th subset on the basis of the i-th uncertainty sub-values and of the at least one nominal uncertainty value; the second and third steps being repeated for each i-th subset, i varying between 1 and a defined value M; a fourth step of determining a protection radius on the basis of the M i-th protection radii determined.