We exploit cellular signals of opportunity (SOPs) using 5G uplink (UL) sounding reference signals (SRS) to develop an indoor positioning system utilizing unmanned aerial vehicles (UAVs) for emergency services. To facilitate a rigorous evaluation, we introduce a novel 3rd Generation Partnership Project (3GPP) compliant system-level analytical framework that meticulously captures 5 G physical layer aspects and largescale, and small-scale fading characteristics. This framework eliminates the need for exhaustive link-level simulations, making the validation process efficient. As an integral component of our proposed framework, we derive the Cramér-Rao lower bound (CRLB) for ranging and positioning errors in the presence of multipath propagation. This ensures an accurate performance evaluation, incorporating all 3GPP aspects. For the practical implementation of the proposed use case, we develop a lowcomplexity positioning algorithm: the iterative parallel projection method (IPPM). We conclude by providing insights from the evaluation and highlighting how the proposed framework aligns with the positioning requirements.