A simple technique, exploiting a property of pseudorandum binary signalsreduces to zero the effect of linear disturbances in the estimation of asystem weighting function by cross-correlation. Errors due to quadraticdisturbances are then constant and may be minimised by choice of particularpseudorandom binary signals which are tabulated. The principal remainingerror is also constant, and an unbiased estimate of the system weightingfunction is easily obtained.