A rotor arrangement and method by which a time reference is provided for a rotor. The rotor includes N time of arrival features. The method includes steps to: provide a plurality of time of arrival probes spaced apart circumferentially outside the periphery of the rotor; for each revolution of the rotor, measure a time of arrival of each feature at each probe; select N time of arrival measurements at each probe; derive a best fit of the measured times of arrival measured at all the probes against angular position; and set the time reference for the next revolution of the rotor equal to the best fit at the end of the current revolution of the rotor.