A method for compensating for the effect of gravity on a ground-based instrument, which instrument is intended to operate in orbit, comprising: - measuring positions of a plurality of intended targets distributed on the instrument, in an XYZ reference coordinate system linked to the instrument, before and after turning the instrument by rotating from 0° to 180° about a predetermined axis and, on the basis of the position measurements, deducing therefrom a measurement of a position 0g of the instrument in the XYZ reference coordinate system, where g is the acceleration of the gravity; - determining intensities of compensation forces (F1, F2,..., FN) to be applied to N interface points for reaching the position 0g; - then applying the gravity compensation forces (F1, F2,..., FN) to the N interface points of the instrument, characterised in that the measurements of the positions of the intended targets are performed by photogrammetry and in that determining the compensation forces (F1, F2,..., FN) to be applied at each interface point comprises: - from the obtained position 0g, determining movements (D1, D2,..., DN) to be performed at N interface points of the instrument, where N is a whole number greater than 1; - performing photogrammetry measurements of the movements of the instrument at all the interface points after applying a single force of any intensity to only one of the N interface points, the measurements of movements being performed N times by successively applying a single force on each of the N interface points considered individually and successively one after the other; - adding all the movements measured by photogrammetry to a matrix U of dimensions N*N; and - solving a matrix system F = U -1 *D, where F is a vector N*1 comprising N unknowns relative to the forces (F1, F2,..., FN) to be applied to the N interface points for reaching the position 0g, D is a vector N*1 comprising N values (D1, D2,..., DN) relative to the movements to be performed at the N interface points for reaching the position 0g and where U -1 is the inverse of the matrix U. - measuring positions of a plurality of intended targets distributed on the instrument, in an XYZ reference coordinate system linked to the instrument, before and after turning the instrument by rotating from 0° to 180° about a predetermined axis and, on the basis of the position measurements, deducing therefrom a measurement of a position 0g of the instrument in the XYZ reference coordinate system, where g is the acceleration of the gravity; - determining intensities of compensation forces (F1, F2,..., FN) to be applied to N interface points for reaching the position 0g; - then applying the gravity compensation forces (F1, F2,..., FN) to the N interface points of the instrument, - from the obtained position 0g, determining movements (D1, D2,..., DN) to be performed at N interface points of the instrument, where N is a whole number greater than 1; - performing photogrammetry measurements of the movements of the instrument at all the interface points after applying a single force of any intensity to only one of the N interface points, the measurements of movements being performed N times by successively applying a single force on each of the N interface points considered individually and successively one after the other; - adding all the movements measured by photogrammetry to a matrix U of dimensions N*N; and - solving a matrix system F = U -1 *D, where F is a vector N*1 comprising N unknowns relative to the forces (F1, F2,..., FN) to be applied to the N interface points for reaching the position 0g, D is a vector N*1 comprising N values (D1, D2,..., DN) relative to the movements to be performed at the N interface points for reaching the position 0g and where U -1 is the inverse of the matrix U.