Due to vulnerabilities of Global Navigation Satellite Systems (GNSS) there is an increased interest in alternative navigation solutions, such as using signals of opportunity (SOPs). We propose a system where a mobile navigator localizes itself using two-way ranging (TWR) measurements to a stationary base station at a known location as well as time difference of arrival (TDOA) measurements from two terrestrial digital television transmitters. We investigate the feasibility of such a system by deriving the Cramér-Rao Lower Bound (CRLB) for varying noise levels and optimizing the placement of the base station, using the real-life positions of transmitters in the area around LinkÖping, Sweden. We simulate measurements and compute snapshot estimates, verifying that root mean square errors similar to the CRLB can be obtained. The results indicate that for the investigated levels of TWR noise, as long as the TDOA noise is sufficiently low it could be possible to achieve errors of a few tens of meters.