A Bayesian lower bound is proposed as a new baseline for the positioning accuracy provided by satellite radionavigation signals in the presence of Gaussian noise and interference. Positioning accuracy is typically investigated using the Crame??r-Rao Bound (CRB), but the CRB does not always constitute a tight benchmark. The proposed bound is used to obtain a threshold on signal energy that indicates the suitability of the CRB. The threshold reveals that a large signal bandwidth cannot reliably compensate for low signal energy in order to sustain positioning accuracy.