A system comprising: a) a signal generator for providing a signal, b) a splitter for splitting the signal from the signal generator and directing it to a measurement path and a compensation path, wherein: i) the measurement path comprises a measurement interferometer having a splitter for splitting the signal into a first and a second part, the interferometer being arranged to transmit the first part to a remote target, and to receive reflections therefrom, and to combine the received reflections with the second part in a measurement interferometer mixer, and ii) the compensation path comprises a compensation interferometer arranged to split its input signal into a first, delayed path, and a second, undelayed path, and to combine the first and second paths in a compensation interferometer mixer to produce an output bearing information pertaining to a phase difference between the two paths, the relative delay between the first and second paths being a predetermined time Δτ; c) sampling means (11, 16) for sampling the outputs of the measurement interferometer mixer and the compensation interferometer mixer; d) processing means for calculating a phase compensation figure by summing a number n of the Δτ-induced phase difference samples from the compensation interferometer mixer to calculate a phase change over a time n Δτ, where n Δτ is equal to n times the predetermined time Δτ, and for subtracting the calculated phase compensation figure from the output of the measurement interferometer, wherein the processing means is arranged to calculate a range L/2 to the target based upon the compensated output from the measurement interferometer, said range giving a time delay τ to the received reflections as compared to the second part, and wherein the phase compensation figure is calculated under the condition that | τ - n Δ τ | ≤ Δ τ / 2. a) a signal generator for providing a signal, b) a splitter for splitting the signal from the signal generator and directing it to a measurement path and a compensation path, wherein: i) the measurement path comprises a measurement interferometer having a splitter for splitting the signal into a first and a second part, the interferometer being arranged to transmit the first part to a remote target, and to receive reflections therefrom, and to combine the received reflections with the second part in a measurement interferometer mixer, and ii) the compensation path comprises a compensation interferometer arranged to split its input signal into a first, delayed path, and a second, undelayed path, and to combine the first and second paths in a compensation interferometer mixer to produce an output bearing information pertaining to a phase difference between the two paths, the relative delay between the first and second paths being a predetermined time Δτ; i) the measurement path comprises a measurement interferometer having a splitter for splitting the signal into a first and a second part, the interferometer being arranged to transmit the first part to a remote target, and to receive reflections therefrom, and to combine the received reflections with the second part in a measurement interferometer mixer, and ii) the compensation path comprises a compensation interferometer arranged to split its input signal into a first, delayed path, and a second, undelayed path, and to combine the first and second paths in a compensation interferometer mixer to produce an output bearing information pertaining to a phase difference between the two paths, the relative delay between the first and second paths being a predetermined time Δτ; c) sampling means (11, 16) for sampling the outputs of the measurement interferometer mixer and the compensation interferometer mixer; d) processing means for calculating a phase compensation figure by summing a number n of the Δτ-induced phase difference samples from the compensation interferometer mixer to calculate a phase change over a time n Δτ, where n Δτ is equal to n times the predetermined time Δτ, and for subtracting the calculated phase compensation figure from the output of the measurement interferometer, wherein the processing means is arranged to calculate a range L/2 to the target based upon the compensated output from the measurement interferometer, said range giving a time delay τ to the received reflections as compared to the second part, and wherein the phase compensation figure is calculated under the condition that | τ - n Δ τ | ≤ Δ τ / 2.