Many interesting tasks require conducting polarisation experiments with the longitudinal polarisation of the beam. Such polarisation can be achieved by accelerating a horizontally polarised beam at the outlet of the source and controlling the time of each event. A brief history of precessing polarisation is given. An algorithm for converting primary data to a form acceptable for fitting is shown. A fit of the generated events was performed. The accuracy of restoring the precession period is estimated. The preliminary requirements for the polarimeter are given.