The invention relates to a system comprising a coder that has an alternation of North and South magnetic poles separated by transitions extending along a helix of pitch p and of angle α, the magnetic track having Npp pairs of North and South poles and a polar width Lp measured along a normal to the transitions which are: Npp=πa/l and Lp=p·cos α. The invention also includes at least one sensor able to detect the rotating magnetic field in a plane perpendicular to the magnetic track and to the transitions by means of a mounting of at least two sensitive magnetic elements. The mounting being disposed at a radial reading distance from the magnetic track and being arranged to deliver signals in quadrature.