Waves are generated in front of planetary bow shocks by a variety of plasma instabilities and they occur over a wide range of frequencies and wavelengths. In the strongly Doppler shifting environment of solar wind, multiple spacecraft observations are needed to determine unambiguously the wave properties such as frequency, phase velocity, and wavelength. The separations of the spacecraft required for these studies are restricted because the waves have finite coherence lengths. The multiple spacecraft must be located within a coherence length of each other in order that they are studying the same wave. In this study, we have used high time resolution simultaneous magnetic field data from the dual ISEE spacecraft to study the coherence lengths of upstream ULF waves. We examine the cross-correlation between ISEE 1 and 2 observations for different spacecraft separations and determine the coherence lengths for upstream 30--second waves, 3--second waves and one--Hz waves. We find that the observed coherence lengths are consistent with those estimated from the bandwidth of the spectral peak, and that these lengths vary markedly from less than 100 km to over 1 RE. In order to study all these wave phenomena, a multiple spacecraft mission such as the upcoming ESA Cluster mission would need to be capable of assuming a wide variety of possible separations. ¿ American Geophysical Union 1993