Current automotive radar systems measure the distance, the relative velocity and the direction of objects in their environment. This information enables the car to support the driver. When objects cannot be completely separated by their distance or relative velocity, a two object direction of arrival estimator must normally be used to separate the objects. In this paper we take a close look at the Cramér-Rao bound of the two-object estimation problem. We show the influence of the objects' phase difference and their signal power. Furthermore, criterions are introduced, which can be used to optimize the average direction estimation performance of an antenna array. It turns out that there are array and angle combinations which are more sensitive to phase difference changes than others.
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