The protection level (PL) performance of an autonomous ground vehicle (AGV) due to fusing GPS signals with faulty terrestrial signals of opportunity (SOPs) is evaluated. The AGV is assumed to be equipped with receivers, which can produce a navigation solution from GPS and SOP pseudorange measurements. First, the effect of the number of SOPs on the PL reduction is analyzed. Then, the PL reduction under different assumptions of SOP fault probabilities is explored. The results show that while adding one SOP could increase the PL, adding two or more SOPs would significantly reduce the PLs. The results also demonstrate that even for highly unreliable SOPs (namely, those with fault probabilities as high as 10%), adding two or more SOPs would still reduce the PLs.