Cellular fifth-generation (5G) signals are assessed for opportunistic navigation. A carrier-aided code-based software-defined receiver (SDR) is presented, which produces navigation observables from received downlink 5G signals. These observables are analyzed to assess the performance of 5G signals for opportunistic navigation. Experimental results are presented of a ground vehicle navigating with the 5G SDR while receiving signals from two 5G base stations (known as gNBs). It is shown that over a trajectory of 1.02 km traversed in 100 seconds, the position root mean-squared error (RMSE) was 14.9 m.