This paper discovers rules-of-thumb on how the estimation precision for an incident source’sazimuth-polar direction-of-arrival depends on the number (L) of identical isotropic sensorsspaced uniformly on an open sphere of radius R. This estimation’s corresponding Cramer–Raobounds (CRB) are found to follow these elegantly simple approximations, useful for arraydesign: (i) For the azimuth arrival angle: ; and (ii) for the polar arrival angle: . Here, denotes the number of snapshots, refers to the incident signal’s wavelength,and symbolizes the signal-to-noise power ratio