A method, apparatus and computer program for improving the signal-to-noise ratio of a signal S(t), S(t) containing Signal and noise, are disclosed. A measurement of S(t) at a frequency-of-interest is obtained. Noise measurements of S(t) at one or more noise frequencies where the Signal portion of S(t) is expected to be small are obtained. The noise at the frequency-of-interest is estimated using the noise measurements at the one or more noise frequencies. The estimated noise is subtracted from the measurement of S(t) at the frequency-of-interest.