The invention relates to an arrangement for determining the roll angle position of a rotating projectile, shell, missile or the like with the aid of polarised electromagnetic radiation, comprising a transmitter (Fig. 3) arranged to emit a position-determining polarised radiation in the direction towards the projectile and a receiver arranged in the projectile (Fig. 4) in order to receive the transmitted radiation. The emitted radiation consists of two components, on the one hand a component in the long-wave band (LF) and on the other hand a component in the microwave band. The microwave component comprises a pulse train in which the pulses are intended to indicate when the long-wave component is situated in a certain phase position, for example to indicate the sinusoidal long-wave component's zero cross-overs with a positive-going derivative. This takes place in such a way that a synchronising pulse is emitted by the long-wave transmitter to the microwave transmitter which transmits a short series of microwave pulses (13) as an indication to the projectile.