We cross correlate years of solar wind ion plasma speed and entropy observations at various sites throughout the heliosphere to determine the conditions under which one spacecraft can be successfully used as a monitor for other spacecraft. Peak correlation coefficients occur when two spacecraft lie radially aligned, although coefficients are generally enhanced during an ~2- to 3-month period surrounding alignment. Within the inner heliosphere, the prevalence of corotating solar wind streams can enable useful predictions even when the monitoring spacecraft lies on the other side of the Sun. Similarly, observations at Earth can be useful in predicting conditions in the outer heliosphere to the orbit of Jupiter or even Saturn during periods of near-radial alignment. Observations at Earth cannot be used to successfully predict conditions beyond this distance without considering steepening on the leading edge of streams or stream interactions.¿ 1997 American Geophysical Union