trièdre orthonormé

OBS: Un trièdre : Figure formée par trois plans qui se coupent deux à deux ou par trois demi-droites, de même origine, non coplanaires.

CONT: Orthonormal trihedrons (reference frames). Orthonormal trihedron is a system of three unit vectors, which components are given in ICRS. Their length is equal to 1, and they are perpendicular to each other. To specify a reference frame it is necessary to fix two perpendicular unit vectors with their ordinal numbers. To complete the reference frame it is necessary to determine the third vector so that the resulting orthonormal trihedron will be right-handed. So to calculate the orientation of some coordinate system it is sufficient to calculate 2 elements of trihedron.